Click for the PAforMusic home page
  Amplifiers and

Back to PAforMusic Home


Introductory Notes

First a couple of points of clarification:

On this site, when we talk about 'speakers' we always mean the complete enclosures (or cabinets) containing the drivers (that is, the parts that actually make the sound) along with any other associated parts such as crossovers and protection components. We never mean just the drivers.

On this page, by 'amplifiers' we mean any equipment (or internal part of equipment) that is able to produce an electrical signal of sufficient strength to directly operate a speaker driver. In the context of PA systems, such amplifiers are distinguished by the term 'power amplifiers'.

Amplifiers and speakers need to be considered together because the behaviour and performance of each of these components of the system is heavily dependent on the other. In any particular system, the selection of these two components therefore needs to be well co-ordinated.

One approach to achieving this co-ordination is to use powered speakers; these are speakers that incorporate their own amplifier(s) within the speaker enclosure. Such designs have long existed in the case of backline equipment, but are now also increasingly common for front-of-house (FOH) speakers and floor monitors. However, this approach is not always optimal and so arrangements continue to be used in which unpowered speakers are used with separate amplifiers. Unpowered speakers are more commonly referred to as 'passive' types.

Types of Speakers

Speakers can be categorised into the following six types, according to their use. Most of these may be either passive or powered types (see above).

  • Backline
    Intended for reproduction of the sound of a single instrument (e.g. a guitar, electric bass or keyboard), primarily for the benefit of the musician playing that instrument. In some situations, some members of the band may benefit from the sound produced by the backline of certain other members, however they may also have difficulty accommodating this sound, especially when the backline is loud and/or the band members are positioned close together. From a sound engineering perspective, in order to achieve optimal control over the mix heard by the audience it is generally preferable if backline levels are kept as low as possible. Ideally, each backline speaker should be just adequately loud enough to meet the needs of the relevant individual musician. (To enable the musicians to hear each other adequately, either floor monitors or in-ear monitors, or a combination of both, are usually employed.)

    When a backline speaker and its amplifier(s) are combined into a single powered unit then it is referred to as a combo. When the amplifier is separate from the speaker then the amplifier is called a head, because it is usually placed on top of the speaker. However, in either case band members most usually refer to their complete backline arrangement simply as an 'amp'.

    The amplifiers used for backline speakers, whether integral or separate, are different from the amplifiers used for the other three types of speakers, in that backline amplifiers provide pre-amplification (pre-amp) facilities, to allow the direct connection of the low-level high-impedance signal that is obtained from most types of instrument − particularly guitars and basses. Furthermore, such amps are provided with facilities to allow musicians to adjust the characteristics (e.g. the tone) of the amplified sound produced by their instrument. The facilities provided by the pre-amp vary according to the type of instrument that the amplifier is intended for, and from model to model, but will usually include tone adjustment controls as a minimum.

    Backline speakers (as well as their amplifiers) vary according to the type of instrument that the speaker is intended for, because different instruments produce sounds with different frequency characteristics. Guitar speakers emphasise the mid-range frequencies, bass speakers are specially designed to handle bass-dominant frequencies and keyboard speakers usually have a frequency range approaching that of a full range speaker.

  • Front-of-House − Full Range (Tops)
    Usually consists of one or more woofers (bass drivers) and one or more horns (treble, or HF drivers) in the same enclosure. When powered, (i.e. contains its own amplification within the same enclosure), it may contain a single amplifier to drive both the woofer(s) and horn(s), or it may contain separate amplifiers for the woofer(s) and horn(s) (called a bi-amped speaker). Although described as 'full range', these speakers are usually limited in their deep-bass response − typically 3 dB down at some point in the region of 45 to 100 Hz. Therefore subs (also know as bass bins) are often additionally required, to cover frequencies below that point, and the 'full-range' speakers are then referred to as 'tops' because they are commonly situated on top of the bass bins − either placed directly or on poles or stands.
  • Front-of-House − Line Arrays
    Increasingly popular for medium to large venues and large outdoor systems, these are flown systems consisting of several individual speaker units whose angles (and sometimes their phasing) are arranged so as to give optimal coverage of the desired audience area. Often the individual units are identical, and may themselves contain different types of drivers to cover most of the required frequency range. However, they are generally unable to provide adequate high-powered deep bass performance and so, as with conventional 'full range' tops, floor-standing subs are often additionally required.
  • Front-of-House − Subs / Bass Bins
    Consists only of one or more woofers, covering the sub-bass frequencies (below the frequencies covered by the tops). These speakers are usually driven by their own separate amplifier(s), which are supplied with the sub-bass frequencies by an active crossover. This crossover also protects the tops from the sub-bass frequencies. As sub-bass frequencies are essentially non-directional, the bass-bins are usually operated in mono mode − even when the tops operate in stereo.
  • Monitor
    Intended for the benefit of the performers, these are generally either of a moderately powerful floor-standing wedge-shaped design, or are low-powered stand-mounted units. On large stages, much larger side-fill (or cross-fill) speakers are sometimes used. Increasingly, in-ear monitoring (IEM) is being used in place of monitor speakers, or to reduce the number of them required.
  • Public Address
    Whilst the types of speaker listed above are frequently used together as parts of a PA system serving a sound reinforcement function, public address speakers serve an entirely different purpose. They are primarily intended to allow spoken announcements to be made throughout a building or an arena, or at outdoor events. They are frequently also used to play background music ('musac') between announcements. These applications are distinguished from sound reinforcement PA systems by operation at relatively low volume levels, the use of very many small speech-optimised speakers to provide coverage over relatively large areas, and compromises in sound quality made for sensible economic and practical reasons.

    For building and arena applications the speakers and their cabling are usually permanently installed, while for short-duration outdoor events they are usually mounted on poles. As good sound dispersion for speech and high efficiency are usually the priority over the high-quality reproduction of music, exponential horn speakers are a common choice. Bass response is usually heavily compromised. Public address speakers are frequently of the '100 volt line' variety, which enables a large number speakers, installed over a wide area, to be conveniently and cost-effectively connected in daisy-chain style to a single amplifier that provides a 100 volt line output. In cases where a public address system is intended to enable emergency evacuation or other safety-critical announcements to be made, high integrity equipment, cabling and power arrangements must be employed.

Power Ratings

The quoted power handling capacity of a speaker, measured in watts (abbreviated "W"), is a figure to be treated with some caution. This is for two main reasons:

Firstly, it tells you only how much electrical power the speaker is able to accommodate without sustaining physical damage and/or causing undue distortion of the sound. This gives only a very rough guide as to the sound level (volume) that it is capable of producing, because the sound level obtained from a given number of watts of power depends upon the sensitivity of the speaker, which varies significantly between different models.

Secondly, because of the fluctuating power level of most sound signals, the proper specification of power handling capacity requires a standardised way of measuring the power level that a particular speaker can accommodate. Unfortunately there are several different methods of measurement in use, so when looking at the power rating quoted for speakers you need to be certain of which method is being used, and compare only "like with like". The most common methods are:

  • RMS or "continuous" power. This refers to a sustained average level of power, and is the most useful figure. (The term "RMS" is strictly inappropriate here, though its use is very widespread. The term is used because it refers to the power calculated by multiplying the RMS value of the voltage by the RMS value of the current. The correct term is "continuous average sine wave power".)
  • Programme power (DIN 45573, or IEC 268-5), sometimes called 'music power'. This figure attempts to take into account the dynamics of a typical sound signal, and for a speaker will typically be around twice the RMS value.
  • Peak power (or PMP, peak music power). This refers to the equipment's ability to handle very short duration peaks in the signal, and can be anything from 2 to 20 times the RMS value. For professional speakers, a figure of 4 times the RMS value is often quoted. There is no agreed standard for how this value should be measured, so it is of little real use and is often best ignored.

These different measurement methods must also be taken into account when matching the power rating of a speaker with the output power rating of the amplifier that is going to drive it. (When looking at amplifier rating plates, be careful not to confuse the mains input power requirement with the audio output power rating.) Assuming that the same power rating methods are being used for both the speaker and the amplifier, one possible approach is to ensure that the speaker power handling capacity is amply adequate to produce the highest sound level required (taking into account the speaker's sensitivity), and then choose an amplifier whose output power rating into the overall speaker impedance to be driven (see below) is around 30-50% higher than the total power handling capacity of the speakers to be connected to it. (This is a very basic approach − a more thorough one is detailed in Amplifier and Speaker Selection for required SPL, later on this page.)

For example, if two 100 W RMS rated speakers having a combined impedance of 4 ohms are to be connected to a mono amplifier, then the amplifier output power rating into 4 ohms should be around 260 to 300 W RMS if the maximum useful sound level is to be safely obtained from the speakers. The reason for the 30-50% margin is that the short-term power handling capacity of a speaker is well in excess of its RMS value, whereas that of a high-quality amplifier is usually only marginally above its RMS value. So to be able to utilise the available capacity of the speaker fully, without risk of speaker damage due to the amplifier being driven into overload during short-term peaks in the sound level, it is necessary to use an amplifier with an RMS output power capability that is considerably greater than the total RMS capacity of the speakers that are to be connected to it.

The down-side of this is that it is possible to slightly over-drive the speakers, but this should be a rare occurrence because if the speakers are correctly rated then over-driving them would produce a sound level higher than what you required. In any case, most speakers will tolerate a degree of short-term over-driving without difficulty. If this is a concern, you can either take care to keep a close eye on the amplifier power meters, or you can start by choosing speakers with a rating somewhat higher than you expect to need (but this may increase the cost significantly). Alternatively you can buy powered speakers, in which case the manufacturer has made these difficult decisions for you!

Speaker Sensitivity

The sensitivity of a speaker is a measure of the sound pressure level (SPL) that it will produce for a given amount of electrical input power (or voltage − see below). It is usually quoted in dB SPL at an input power of one watt, measured at a distance of one metre directly in front of the speaker, for a given type of programme signal. The type of signal is important because the sensitivity of a speaker varies with frequency. A typical sensitivity figure would be 100 dB SPL @ 1W @ 1m, which corresponds to an efficiency of approximately 10%.

The sensitivity figure can be used 'in reverse' to estimate the speaker input power required to achieve a particular SPL at a given distance, using the inverse square law. For example, suppose that we require a maximum SPL of 112 dB at a distance of 8 metres from a speaker having the sensitivity figure quoted above. From the inverse square law, the SPL will be 118 dB at 4 metres, 124 dB at 2 metres and 130 dB at 1 metre. Therefore, the maximum power input required to the speaker is 30 dB greater than 1 W, which is 1 kW. (For a more comprehensive explanation and example, see Amplifier and Speaker Selection for required SPL later on this page.) However, this is only an estimate because many complicating factors such as room acoustics and grazing have not been taken into account. Of course, if the particular speaker being used is unable to handle this power level (see Power Ratings above), then several can be used together − but the maximum power input to them will still need to total 1 kW. (Refer to the Impedance section below for information regarding the connection of several speakers to a single amplifier.)

In practice, an amplifier supplies a controlled voltage, not a controlled power, and the power taken by the speaker depends upon its impedance, which varies with frequency. Therefore it is becoming increasingly popular to specify speaker sensitivities in voltage-related terms. For nominally 8 ohm speakers, the value is specified in dB SPL @ 2.83 V @ 1m, 2.83 volts being the voltage required for a power of 1 W in 8 ohms. (For 4 ohm speakers the voltage required is 2 V.) So, the sensitivity value specified in this new way is effectively equivalent to a value specified in the old power-related way.


For a general definition of impedance, see its glossary entry. The impedance of a speaker is important for three main reasons:

  • It affects the amount of power that the speaker will draw from an amplifier. If speaker 'A' has half the impedance of speaker 'B', then speaker 'A' will draw twice as much power from an amplifier as 'B' − assuming that the amplifier input level and the amplifier settings remain the same. This of course also assumes that the amplifier can cope with the lower speaker impedance value and that the amplifier's output power rating is not exceeded.
  • It governs the number of speakers that may be connected to a single amplifier (assuming the usual simple approach to connecting multiple speakers, which connects them in parallel). This is because the total load impedance presented to the amplifier output is the impedance of a single speaker divided by the number of them (assuming all the speakers have the same impedance), and amplifiers are designed to operate into load impedances within specified limits − typically 2 to 15 ohms. (We are here referring to a single-channel (mono) amplifier, or to one channel of a multi-channel amplifier − regardless of how many speaker sockets it has for the connection of speakers to that channel.) Exceeding the specified maximum impedance will not usually harm the amplifier − you will simply not be able to achieve the rated output power of the amplifier. However, by connecting an impedance lower than the specified minimum, you risk permanent damage to the amplifier (with possible consequential damage to the speakers). Where four or more speakers must be connected to a single high-power amplifier, a 'series-parallel' method of connection is sometimes used to avoid the load impedance becoming too low.
  • It affects the gauge (thickness) of the speaker cables required, and their maximum length, in order to avoid an unacceptable loss of power in the cables, cable overheating or an unacceptably low damping factor. Lower impedance speakers will require heavier gauge and/or shorter cables.

It should be noted that the impedance value quoted for a speaker is a nominal value, and in practice the value varies with frequency. The minimum value is often of particular interest; this is likely to be around three-quarters of the nominal value, so an 8 ohm speaker is likely to have a minimum impedance of around 6 ohms.

A special case of speaker impedance arises in the case of so-called '100 volt line' speakers, which have a much higher impedance than conventional speakers. This allows many of them, located over a large area (such as throughout a public building), to be connected to a common amplifier using moderate gauge cable. The amplifier used must be one that has a 100 volt line output. These speakers are generally of low power rating (5 to 50 watts RMS each), and are usually equipped with a means to adjust the power level drawn from the 100 volt line by each speaker. As the usual application of this arrangement is for speech announcements and/or 'musac', the sound quality of these units is often not high − particularly as regards bass response.


The Basics

Bridging is a technique to improve the matching between the impedance of a speaker (or the overall impedance of several interconnected speakers) and the optimum load impedance of the available power amplifiers, so as to increase the maximum amount of power that the amplifiers can provide to that speaker(s). It is most useful when it is required to use more of the power-handling capacity of the speaker(s), or more of the power-providing capability of the amplifiers, than could be achieved with a simple non-bridged connection of the speaker(s) to the amplifier.

In a bridged arrangement, two identical amplifier channels are driven by signals of opposite polarity, and the speakers are connected between the "hot" (i.e. non-earthy) terminals of their output connections. This is properly referred to as a 'bridge-tied load' (BTL) connection − or, less formally, as 'bridging the amplifiers' − and effectively doubles the voltage available to the speakers. Some models of amplifier have a switch to select this mode of operation; it is sometimes additionally necessary to use a different speaker output connector of the amplifier, or to 'manually' arrange the appropriate connections to its output terminals.

WARNING: The output voltage of bridged high-power amplifiers can be high enough to cause electric shock. The speaker cable used must be suitable for the voltage and current supplied by the amplifier. Speakers and/or amplifiers can be seriously damaged by inappropriate use of bridging, or by making incorrect connections.

The following lists provide some basic guidance regarding when it might be appropriate to use bridging, but they are not exhaustive.

Reasons NOT to bridge

  • The two amplifier channels to be used are not identical.
  • The amplifier handbook does not explicitly state that its channels may be bridged, or does not state the maximum output power levels that may be obtained in bridged mode.
  • The amplifier handbook does not give clear guidance on how to connect the speaker(s) to the amplifier outputs to achieve bridged operation, or does not specify the resulting drive polarity.
  • The voltage at exposed amplifier/speaker terminals would result in an unacceptable shock hazard.
  • The speaker cable or connectors to be used are unsuitable for the voltages and/or currents that would result from bridging.
  • The power level available from the bridged amplifiers places the connected speakers at too great a risk of overload damage.
  • The overall impedance of the single load is below the minimum specified for the amplifier when used in bridged mode. (Typically that minimum value will be twice the minimum load impedance permissible for the individual channels when used in non-bridged mode.)
  • Reliability / system integrity considerations override other factors, determining that the two amplifier channels should be used independently of each other for that reason.

Reasons to bridge

None of the conditions in the above list apply, and:

  • The power level required by a particular single-circuit load exceeds what a single channel of the available amplifier is able supply into that load; the total available power from both channels is needed.
  • Or, more precisely, the impedance of the available single-circuit load is too high to enable the required power level to be obtained from a single channel of the amplifier, and the power available from each channel (into an impedance equal to half the load impedance value) is about half of the total power level required.

In More Detail...

Bridging is possible only when each channel of the amplifier is capable of driving an impedance of one-half of the overall impedance of the connected speaker load. It should only be employed if approved by the amplifier manufacturer and if the speakers are suitably rated, otherwise serious damage to the amplifier or speakers may occur.

When a speaker (or a set of interconnected speakers) is connected to a single amplifier, the full power rating of the amplifier can be delivered to the speakers only if the (combined) impedance of the speakers is that which the amplifier is optimised to drive. For example, if the (overall) impedance of the speakers were four times the optimal value for an amplifier, then it would only be possible to obtain about a quarter of the amplifier's rated output. This is because the amplifier would be unable to provide sufficient voltage to force through these speakers the maximum amount of current that the amplifier is capable of supplying. So, assuming that the speakers are suitable for handling the full rated power of the amplifier, the speakers would be very under-utilised.

However, by using two identical amplifier channels driven by signals of opposite polarity, and by connecting the speakers between the "hot" (i.e. non-earthy) terminals of the outputs of the two channels, the voltage available to the speakers is doubled. Since (for a given impedance) the power obtained is proportional to the square of the applied voltage, this arrangement increases the power supplied to the speakers by a factor of four − which would nicely compensate for the problem with our example speakers whose impedance is four times too high. In this example, we have obtained the power rating of just one of the amplifier channels − but at the cost of using a second channel (typically, both halves of a 2-channel unit).

Since, in this example, we needed two amplifier channels to get the rated power of just one of them, each channel was clearly only working at half-power. The speakers were fully powered but the channels were only half-loaded, which is still not a satisfactory situation. So, we can connect a second speaker (or set of speakers), of the same impedance and power rating as the first, to our bridged channels − and double our total output power. Now, both channels and both (sets of) speakers are fully utilised.

From a technical perspective, the reasoning behind this is that, in a bridged configuration, the load impedance "seen" by each channel is half of the actual connected load. In our example the load was four times too large, so even in the bridged arrangement each channel still "saw" twice its optimum load impedance. Therefore, only half of the available current was being taken and so only half the power was being supplied. By connecting a second set of speakers, the total actual load became twice (instead of four times) the optimal load for a single channel. So now, in the bridged configuration, each channel sees its optimal load and therefore is able to deliver its full rated power output.

To recap using some example figures, suppose we have a pair of 16 ohm speakers, each suitable for being driven by a 100 W amplifier, but our pair of 100 W amplifier channels are both optimised for connection of 4 ohm speakers. If we simply connect one speaker to each channel, we can only get a maximum of 25 W (or perhaps a little more) from each channel − a total of 50 W. But if we bridge the channels and connect both of the speakers (in parallel, as usual) to the bridged output, then we have connected an 8 ohm load in a way that causes each channel to see half of it − the amplifiers' optimal value of 4 ohms. So each channel is able to deliver its rated 100 W and we get a total of 200 W (four times what we got without bridging!) shared equally between the two speakers. (Put another way, we have effectively created a single 200 W amplifier with an output optimised for 8 ohms − and connected 8 ohms to it.)

Note, however, that bridging is not a way to get more out of an amplifier than it was designed for. It's just a way of summing the power capability of two identical amplifier channels in a way that effectively creates a single amplifier with an optimum load impedance which is double that of each individual channel.

Thought we were finished? Well let's just briefly look at this another way. Suppose we have a single really powerful speaker (or a set of interconnected speakers that for some reason it's just not practical to split up). We don't have an amplifier powerful enough to drive this speaker to capacity, but we do have a pair of identical channels of a lower power rating. Can we use bridging to help in this situation? As we saw above, it's all a question of impedances. The first thing to check is the minimum acceptable load impedance for the two individual channels. For a bridged arrangement to be acceptable, the impedance of the connected speaker (or set of speakers) must be no less than twice this figure because, when bridged, each channel will "see" only a half of the actual connected load impedance. The next thing is to determine if there's enough advantage to be gained from bridging to make it worthwhile. To do this we compare how much power the channels can provide into this halved impedance (when both amplifiers are driven), as compared to how much can be provided into the full speaker impedance with just one of them driven. (For two-channel amplifiers in which a common power supply is used for both channels, the available output power with both channels driven can be significantly less than that available when using just one channel − check the manufacturer's specifications.)

To see how this approach works in practice, let's look at some example figures. Suppose we have a single 4 ohm speaker suitable for driving by an 800 W amplifier, but what we have is a "300+300 W" two-channel amplifier. First we check the amplifier specifications to see if it's channels can drive 2 ohm loads (half of 4). So far so good. Now let's say that further checks show that when both channels are driving 2 ohm loads (the bridged situation), the maximum output is 250 W per channel (a total of 500 W), whereas if we connect our 4 ohm load to just one channel of the amplifier (and don't use the other channel), the amplifier can deliver 350 W on that one channel. So, in this example, bridging has given us only an extra 150 W of drive power into our speaker, an increase of just 1.5 dB − possibly because this amplifier was optimised for 4 ohm loads. Unless you're desperate for that extra 1.5 dB, or bridging is very easy to do (see the next paragraph), it probably wouldn't be worth it.

Some two-channel amplifiers have a switch that configures the pair of channels into bridging mode, which is far more convenient than having to arrange for it yourself. But if you have to do it yourself, you will need a means of inverting the polarity of the signal fed to one of the two channels (easy if the amplifier accepts a true balanced input − just swap the 'hot' and 'cold' conductors of the input connection − pins 2 and 3 of an XLR). You will also need some specially wired cables to allow your speakers to be connected between the 'hot' connections of the outputs of the two channels. Beware though, not all amplifiers are suited to bridging − check the manufacturer's information to avoid possible damage to your expensive equipment!

Frequency Range

Audio frequencies are generally considered to fall into the following ranges (or "bands"):

  • Sub-bass − Below 80 Hz
  • Bass − 80 Hz to 250 Hz
  • Mid-range − 250 Hz to 2.5 kHz
  • HF − Above 2.5 kHz

The boundary frequencies between the ranges are somewhat arbitary, and you will find other similar figures quoted. Sometimes the mid-range band is divided into upper mid and lower mid bands.

The range of frequencies that a speaker system must handle depends very much on the application. Speakers intended only for announcements in public buildings, for example, may only require a range of 200 Hz to 10 kHz, whereas good quality reproduction of music requires at least 80 Hz to 15 kHz. Good response down into the 'sub-bass' region will allow the music to be felt as well as heard, and good response up towards the upper limit of human hearing (20 kHz) will add increased clarity and crispness to the sound.

A speaker that is capable of handling frequencies across the bass, mid and HF ranges is known as a "full range" speaker, and these are commonly available up to about 800 W RMS power handling. However, where a wide frequency response is required at much higher powers than this, one approach is to use separate speakers for each frequency range, each speaker being specifically designed for a particular band and being driven from an amplifier that is fed from an active crossover unit. In this latter arrangement, the crossover frequencies must be set appropriately for the speakers being used, and the relative levels of the amplifiers must be properly adjusted. Another common approach for high-power systems is to use a flown line array system for the majority of the frequency range, in conjuction with 'subs' to cover the sub-bass.

It is important to understand that the distribution of audio power is not uniform across the frequency spectrum − most of the power falls into the bass and lower-mid ranges. As you look at higher and higher frequencies, less and less power is present. This has important consequences in the design of speaker and amplifier systems − especially when separate speakers are used for the different frequency bands. For example, a 5 kW 3-band system might be made up as follows:

  • 3 kW bass
  • 1.6 kW mid-range
  • 0.4 kW HF

Some caution is needed in interpreting the frequency range figures quoted by speaker manufactures and suppliers. The norm is to quote the frequency at which the sensitivity has reduced to 3 dB below frequencies around the centre of the band for which it is intended (i.e. the power level of the reproduced sound has halved). However, sometimes "usable" frequencies are quoted − which may mean a reduction of as much as 10 dB (i.e. one tenth of the sound power) at the quoted frequency, compared with the speaker's mid-band sensitivity.

Amplifier Classes

To describe the various possible modes of internal operation, reference is sometimes made to the 'class' of a power amplifier. This is a design parameter, and rarely impacts significantly on how the amplifier is used within a system, but for completeness and technical interest this section gives a brief explanation of the most common classes encountered. Some of these are not relevant to PA applications, but are still listed.

As a starting point, it is necessary to understand that all power amplifiers have at least two 'output devices' (usually transistors of some kind, though valves are sometimes still encountered) − one device supplies the current to the speaker(s) on positive excursions of the waveform, and the other device supplies the current on the negative excursions. (High power amplifiers have several devices operating together to perform each of these two functions, but this has no bearing on the amplifier class.)

Simply put, the class of the amplifier is a description of how the two power output devices are utilised to supply the required output current, particularly how they are coordinated in their operation in order to achieve the crossing over of the output voltage waveform from positive to negative and vice versa. The amplifier class has a bearing on the linearity and the efficiency of the amplifier. 'Linearity' relates to distortion levels − good linearity means less distortion (and vice versa). 'Efficiency' relates to how much power is wasted as heat in the amplifier − high efficiency means that little power is wasted, meaning less mains power is required and that the amplifier can be much smaller and lighter, and requires less cooling.

ClassBrief descriptionComment
A Both devices pass some current continuously, during both positive and negative excursions. Or, only a single output device is used, that passes some current continuously. Poor efficiency.
B At the point where one device starts to pass current, the other ceases to. Good efficiency but poor linearity around the cross-over region.
AB One device starts to pass current just before the other ceases to, i.e. they are both 'on' for a small part of the waveform cycle. Moderate efficiency with good linearity.
AB+B A hybrid configuration, using one pair of output devices operating in class AB and another pair operating in class B. Improved efficiency with good linearity.
C One device ceases to pass to current before the other starts to, i.e. they are both 'off' for a part of the waveform cycle. Generally used only for radio-frequency power amplification.
D The devices operate in switching mode, either fully 'on' or fully 'off'. They are never both 'on' at the same time. In audio applications, this class is now used to refer to what was originally called class S (see below).
[Note that D does not stand for 'digital', and Class D amplifiers should not be referred to as digital amplifiers.]
Very high efficiency.
E Normally has only a single output device, driven by rectangular pulses. Not generally relevant to audio applications.
F Similar to class C, but can operate at harmonic frequencies. For radio-frequency use.
G Uses two or more different internal DC voltages to supply the output devices, the higher voltage(s) being used only when required by the presence of high signal peaks. Improved efficiency.
H A further development from class G, in which the internal DC supply voltage to the output devices is arranged to track the signal being amplified. However, confusingly, the term 'class H' is also sometimes used to describe class G amplifiers (as defined above). Further improved efficiency.
S A switching amplifier (as per class D) that is arranged to provide a normal audio output by the addition of a filter to remove the switching transients. Now generally called class D, in audio applications Very high efficiency.

Speaker Components

Enclosures (Cabinets)

All speakers require some kind of enclosure to house the drivers. Enclosures are usually either:

  • Traditional vinyl-covered or 'carpet'-covered wood construction.
  • Moulded plastic construction, offering a lighter weight and improved weather-resistance.

However, in the case of full-range, bass and mid-range speakers, the enclosure doesn't simply house the drivers − it has a significant effect on the sound produced. This is primarily because of the damping effect of the air trapped within the enclosure, which is often modified by carefully designed 'porting' arrangements − one or more holes which may be fitted with internal tuning tubes or ducts.

Bass Drivers (Woofers)

These are large, heavy units, usually with 12", 15" or 18" diameter cones. As a general rule, the larger the diameter the lower the frequencies that can be reproduced, and the heavier the magnet the greater the power handling capability.

It is important to understand that the operation of a bass driver is heaviliy influenced by the speaker enclosure − its dimensions, construction and porting arrangements. Therefore, when replacing faulty bass drivers, it is essential to fit the correct type − the low-frequency response or power-handling capability of a speaker cannot usually be improved simply by fitting a higher-spec driver.

HF Drivers (Horns)

HF drivers fall into the following categories:

  • Dynamic
  • Piezo (or Electrostatic)

Piezo drivers are capable of little power output in comparison with dynamic ones, and so are used only in low power equipment, generally less than 200 W overall system power.

Because the amount of power at high frequencies in an audio signal is much less than the overall power of the signal, HF drivers are used which have a power rating much lower than the bass drivers. Therefore, the HF drivers are more prone to damage by overload than are the lower-frequency drivers in the speaker system. Although some systems provide some measure of internal overload protection for the HF drivers, in order to prevent the possibility of serious damage it is important to avoid:

  • High frequency feedback − especially for prolonged periods
  • Driving the HF drivers from an overdriven (i.e. distorting) amplifier
  • High-level impulsive sounds, e.g. loud crackles due to faulty cables, making alterations to cabling on an in-use channel, etc.

Crossovers and Built-in Amplification

The crossover is the component that separates the signal into a number of frequency ranges, usually two or three. There are two types of crossover:

  • Passive: A device that is internally connected between the input connector(s) and the drivers of a passive full-range speaker, or between a single internal amplifier and the drivers of a powered full-range speaker. The crossover is arranged to supply the high frequency part of the signal to the HF driver(s), the low frequency part to the bass driver(s) and (when the crossover separates the signal into three parts) the mid-range part to the mid-range driver(s). It requires no power source of its own.
  • Active: A device that is connected between the mixer and the input of the amplifiers, there being a separate amplifier for each frequency range into which the signal is split by the crossover. Therefore, this type will be used as an internal component of a speaker only if the speaker is of the powered full-range type and contains multiple amplifiers. When an active crossover is used external to the speakers, the speakers connected to each amplifier must be suitable for the relevant frequency ranges and the crossover frequencies must be adjusted to suit them. The amplifier that is connected to the HF output of the crossover feeds the HF speakers or drivers, the amplifier connected to the LF output of the crossover feeds the bass speakers or drivers) and (when the crossover separates the signal into three parts) the amplifier connected to the mid-range output of the crossover feeds the mid-range speakers or drivers. An active crossover requires its own power source; in the case of powered speakers this will be the same mains supply that powers the built-in amplifiers.

Note that in the case of a powered full-range speaker, either a passive or an active crossover may be included.

Amplifier and Speaker Selection for required SPL

There are many factors which may influence the choice of speakers and amplifiers for a particular situation − here we are just considering sound levels and power ratings.

Note that, unless stated otherwise, all the sound levels and power levels and ratings that we refer to here are 'continuous average' values (often referred to as 'RMS' values, although this is not strictly correct). Do not substitute 'programme power', 'music power' or 'peak power' values (see Power Ratings above). For brevity we will just use the term 'continuous' here rather than 'continuous average'.

To illustrate the principles clearly, we will initially consider a single full-range speaker connected to a single amplifier channel. In summary, the procedure is:

  • Select a speaker that is able to deliver the required sound level (SPL) at the required distance.
  • Calculate the power input that the speaker needs in order to produce the required sound level.
  • Select an amplifier that is able to deliver the power level required by the speaker.
We will now break this procedure down into more detailed steps:
  1. Determine the furthest distance from the speaker at which members of the audience will be located.
  2. Decide on the maximum continuous sound level that is needed at that furthest distance.
  3. Calculate the continuous sound output level that must be produced by the speaker at 1 metre from it, in order to give the maximum sound level needed at the furthest required distance. (This can be done approximately using the inverse square law, provided that natural reverberation does not dominate at that distance.)
  4. Check that the sound level will not be excessively high for audience members nearest to the speaker − if so, reconsider the location of the speaker or audience and go back to step 1.
  5. Select a speaker of the desired type that is able to provide the required continuous sound output level at 1 metre, and obtain its sensitivity figure and impedance value from the specifications.
  6. Using the speaker's sensitivity figure, calculate the continuous power input that the speaker requires in order to produce the desired continuous sound output level at 1 metre. (Note that sensitivities that are specified in dB SPL @ 2.83 V, for an 8 Ω speaker, or in dB SPL @ 2 V, for a 4 Ω speaker, are essentially equivalent to the more traditional dB SPL @ 1 W figures.)
  7. Decide on the amount of headroom required, above the maximum continuous value, to handle the peaks and transients of the programme material in question (taking into account any limiting that is applied to the programme signal).
  8. Using the continuous power value and the headroom figure, calculate the power required during the peaks and transients, and check that this value of peak power can be handled by the selected speaker.
  9. Select an amplifier that is able provide an output power level, into that speaker's impedance, that is at least as high as the value required during the peaks and transients, but not so high as to risk the speakers being easily overdriven. (The likelihood of overdriving will depend to some degree upon the skill-level of the sound engineer.) For multi-channel amplifiers, be sure to use the 'all channels driven' figure.
  10. Take care that the amplifier headroom intended to cater for peaks and transients is not abused by driving the speaker for any appreciable time at a level greater than its continuous power rating.

Example Selection Procedure

  1. The furthest distance from the speaker at which any of the audience will be located is 16 m.
  2. The maximum continuous sound level needed at 16 m is 95 dB SPL.
  3. The inverse square law specifies a 6 dB increase for each halving of the distance, so, ignoring the effect of room acoustics (such as reverberation and absorption), a continuous sound output level of 119 dB SPL must be produced by the speaker at 1 metre from it, in order to give the 95 dB SPL needed at 16 m.
  4. The audience members nearest to the speaker are 4 m from it, so the maximum sound level for them is 107 dB SPL (a 6 dB decrease for each doubling of the distance). We will assume that this is judged to be OK.
  5. The selected speaker is the JBL JRX115, which has a maximum continuous sound output level of 122 dB SPL (128 dB SPL peak) at 1 metre, just above the required 119 dB SPL. Its sensitivity figure is 98 dB SPL @ 1W @ 1m and its impedance is 8 Ω.
  6. The power input that is required to produce 119 dB SPL from this speaker at 1 metre is 119 − 98 = 21 dB above 1 W, which is 126 W.
  7. The headroom required, above the maximum continuous value, to handle the peaks and transients of the programme material in question, taking into account limiting applied to the programme signal, is judged to be 4 dB.
  8. The power required during the peaks and transients is 4 dB above 126 W, which is 316 W. The JRX115 can handle 1000 W peaks and so is easily OK in this respect.
  9. The selected amplifier is the Crown K1, which is able to provide an output power level of 350 W (per channel) into an impedance of 8 Ω. This is higher than the 316 W required during the programme peaks and transients, but not so high as to risk the speakers being easily overdriven. However, the headroom available from this amplifier is only just over the required 4 dB. If more headroom were required then the Crown K2 could safely be used (with care), as this provides a maximum output power of 500 W per channel into 8 Ω (giving a headroom of 6 dB above 126 W).
  10. The continuous power rating of the JRX115 is 250 W. Take care that the available headroom is not abused by driving the speaker for any appreciable time at greater than 250 W.
When using multiple speakers, you must take into account which part of the audience they are covering (determined by the speakers' location and orientation):
  • If the speakers are covering different parts of the audience, then each speaker and the sound levels in its covered area can be considered separately, as above. (But be aware that the sound levels may be higher in any areas of coverage overlap.)
  • If several identical speakers are covering the same part of the audience, then the power that is needed to produce the required sound level is split evenly between the speakers. So, in the above example, if two JRX115 speakers are used then the continuous power input required to each speaker is 126 W / 2 = 63 W, and the peak value required is 316 W / 2 = 158 W.

Multiple Speakers

When connecting more than one speaker to an amplifier channel, there are two additional considerations:
  • You must ensure that the amplifier's minimum load-impedance figure is complied with. For example, if the minimum load-impedance specified for the amplifier is 2 Ω, and the impedance of each speaker is 8 Ω, then no more than four of these speakers may be connected to each channel of the amplifier (assuming the normal parallel connection method).
  • The maximum possible total output power of the amplifier channel (which will be distributed evenly between the connected speakers, provided that they have the same impedance), will be dependent upon the overall load impedance on that channel. For example, an amplifier channel might have a maximum output of 250 W into a single 8 Ω speaker, but if two of these speakers are connected (giving an overall load impedance of 4 Ω), then the maximum output into this overall load might be 400 W, i.e. a maximum of only 200 W into each speaker.


Front-of-House Speakers

Each front-of-house speaker should be positioned pointing directly towards the part of the audience to be covered by that speaker.

An important aspect to consider when choosing speakers is the angle, both vertical and horizontal, over which the sound needs to be dispersed. This will depend entirely upon the placement of the speaker in relation to the audience, and must be considered in conjunction with the number of speakers to be used. In general, short throw units have a wide dispersion angle, whilst long throw units have a narrow one. When the dispersion angle required to cover the audience is very large, the use of trapezoidally-shaped speakers can be of considerable benefit, as several of these may be neatly positioned side by side, in an arc formation, to give a very wide angle of coverage.

Typical support methods for front-of-house speakers are:

  • Floor standing.
  • Stand mounted − requires a stand-mounting hole in the base of the speaker (called a 'top hat').
  • Wall mounted, using brackets or a short right-angled pole into the stand-mounting hole.
  • Flown from the ceiling or a gantry, typically using steel wires or chains.

Whichever method is used, safety is always an important consideration, as speakers are heavy and can cause serious injury if they fall.

Monitor Speakers

Monitor speakers are usually floor-standing wedge-shaped units (floor monitors), or are small stand-mounting units. On large stages side-fill (or cross-fill) monitors are also used, and occasionally monitors are flown above the front edge of the stage, appropriately angled downwards to point towards the relevant performers.

All wedge-shaped monitors should be correctly angled up from the horizontal; that is, they should have an angle which projects the sound towards the performers's head when he or she is located at their usual distance from the monitor. This means that on a large stage where the distance from the monitor may be several metres, a sound projection angle of 30 from the horizontal may be appropriate, whereas when the monitor is very close an angle of 60 from the horizontal may be more suitable. To accommodate this, some monitor speaker enclosures are designed to enable placement at one of several different angles.

In general, monitor speakers for vocalists should be placed so as to minimise the risk of feedback. This means placing them such that the sound from the monitors reaches the microphone at the angle at which the microphone is least sensitive − its "null point". This angle depends upon the polar response of the microphone you are using. For a cardioid microphone, the null point is on the rear axis, so the monitor should point directly at the back of the microphone. For a super-cardioid type, there are null points at about 55, measured from the rear axis towards the front, on each side. A single monitor can be placed pointing at this angle at just one side of the microphone, but the vocalist will usually be happier with two monitors, one on each side, so that they are able to hear the monitor sound equally with both ears. With a hyper-cardioid microphone, the null points are at about 70 on each side of the rear axis, and the monitors (again, ideally two of them) should be placed accordingly.

Monitor speakers for musicians should be placed close to the musician concerned and be pointed towards them. They should preferably point away from the audience and not point towards any microphones (unless they aim at the microphone's null-point − see above).

Go to the top of this page.

This page last updated 30-Apr-2017.