Full-Range Loudspeakers in PA Systems

What it is

A Full-range Loudspeaker is a single cabinet in which one or more individual drivers cover the whole audio frequency range (or, at least, most of it).

In other words, a full-range loudspeaker is a box with one or more individual drivers in it. The drivers may cover different frequency ranges (separated by one or more crossovers) or may all cover the same frequency range (the Bose 802 is an example of this).

What it does

It converts variations in voltage (from an amplifier's output) into variations in sound pressure.

How it works

Generally, each individual driver covering any frequency range below about 3kHz works in the same way: the driver consists of a cone or diaphragm, attached to a coil which is suspended in a fixed magnetic field. Alternating current driven by the voltage from an amplifier's output runs through the coil, creating an electromagnetic field in the coil. The reaction between this and the fixed magnetic field causes the coil - and the attached cone or diaphragm - to move forwards or backwards. The amount and direction of current through the coil affect the amount and direction of coil (and cone or diaphragm) movement. The cone or diaphragm compresses the air in front of it as it moves forwards, and rarefies the air in front of it as it moves backwards. The variations in air pressure are recognisable as sound.

Very high frequency drivers in some budget loudspeakers use piezoelectric transducers instead of coils and magnets. The amplifier's voltage is delivered to a piezoelectric element, which distorts (by bending inwards or outwards) when voltage is applied to it. The amount and polarity of the voltage directly affects the amount and direction of distortion. The distortion of the element compresses the air in front of it as it bends outwards, and rarefies the air in front of it as it bends inwards. The variations in air pressure are recognisable as sound.

Drivers are generally mounted in full-range cabinets in one of two ways:

• Front-loaded. The driver is mounted on the cabinet's front baffle. This means the cabinet can be relatively compact, and relatively inexpensive to manufacture. These advantages are obtained at the expense of efficiency and pattern control: horn-loaded drivers are more efficiently coupled to the air they have to move, and a simple cone is less able to control the way sound radiates from it at all frequencies in its range. Most front-loaded full-range speakers have one or more holes (ports) in the front baffle, to increase output at the lower end of the speaker's frequency range. The size and number of the holes (and length of any tubes, if the holes have tubes in them) take account of the internal dimensions of the cabinet and the driver's low frequency resonance. Do not replace drivers in a ported enclosure with anything other than the original type of driver. Also, do not make holes in sealed (unported) enclosures: the drivers are not designed for it, and at high power you are more likely to destroy the driver than to improve low frequency output.

• Horn-loaded. The driver is mounted at the back of a horn. This increases the cabinet's size, weight, and manufacturing cost. These disadvantages are offset by achieving greater efficiency and better pattern control: the driver can be precisely coupled with the air in front of it, yielding a substantial improvement in output (6dB is not uncommon). Also, the horn controls the way the sound radiates, and very even frequency dispersion over more closely determined horizontal and vertical axes can be achieved.

Most budget to mid-priced speakers use front-loaded bass and midrange drivers. Almost all high-frequency drivers are horn-loaded. In practice, the use of horns in this way means that frequencies from around 1kHz upwards (depending on the horn, the driver and the crossover point) usually have reasonable pattern control. As this upper range includes the frequencies that are important for intelligibility, using a front-loaded driver for lower frequencies is less of a disadvantage.

How do you use it?

Many speakers don't have much of a manual (although most will give at least a description of their nominal dispersion angles). See the Speaker Position section for slightly more detailed guidelines about where to put them. The main things to bear in mind are:

• Point the speakers at the audience. If you think of it as a floodlight that dispenses sound rather than light (keeping its dispersion angles in mind) it will give you some idea of what to do. Point it at the audience, not at the side walls, floor or ceiling. In most small to medium-sized live productions you will have one speaker either side of the stage (or performance area, if there isn't a stage). Generally you will improve the sound in a number of ways if you angle the speakers inwards slightly (as a guideline, try aiming the speakers so that imaginary lines from their centres cross about three-quarters of the way to the back of the audience area). This will give better cover to the front part of the audience, and reduce the level of reflection from the side walls. As a development of this, it will also help if you raise the speakers and angle them downwards. If the speakers are quite high up (preferably at least 3 metres) and angled downwards towards the back of the audience, the higher frequencies will appear more similar in volume throughout the audience area. If the speakers are at head-height, what sounds clear at the back may sound harsh at the front.

• Do not point the speakers at microphones. Keep speakers forward of any microphone positions, especially when angling speakers inwards (see above).

Do you need one?

Only if you need a PA system.

What sort do you need?

In selecting speakers, you should consider what sort of music they will be used for, how loud you need them to be (NOT how much power they can handle!), and what sort of room they will usually be used in. If they will always be used in the same room, consider getting speakers with a wide area of horizontal cover (90º or more) if the room is short and wide, or with a shallower angle (60º or so) if it is long and narrow. For most other rooms, and for general-purpose use, 70º to 90º will be suitable. Vertical patterns between 40º and 60º are common, and will be OK in most venues. A vertical pattern greater than 60º is generally unnecessary, and may cause problems in some settings.

Speakers in live sound need to fulfil three main functions:

1. They need to be loud enough for the size of room they are intended to cover;

2. They need to spread sound evenly to all parts of the listening area;

3. They need to stop sound from spreading to areas they are not intended to cover (walls, floor, ceiling and - most of all - the stage or performance area).

With these points in mind, it is also advantageous if they

• Have a fairly flat frequency response

• Sound OK at all volumes

• Sound similar in tone at all volumes.

Listening is a good way to find out what they sound like. Try to arrange a demonstration, preferably in a dedicated room (not a shop) in which the walls, floor and ceiling are covered with deadening material, such as heavy curtains, acoustic tiles or carpet. When listening, make sure you get to hear the speakers at high volume (most speakers, even very cheap ones, sound OK at low volume), and move around in front of them to find out whether they sound very different off-axis. Good speakers will sound fairly similar through quite a wide arc (general-purpose speakers should cover a forward angle of somewhere between 70 and 100 degrees), and the higher frequencies will fall off fairly quickly beyond their angle of cover. Poor speakers will be prone to very noticeable changes in tone as you move across in front of them, and will often have a very narrow area of upper mid or high-frequency reproduction directly in front of them (this is known as "beaming").

The type of material and combination of instruments they will need to reproduce will determine which type of speaker(s) will be most suitable. Most loudspeakers sold as single "full-range" units are two-way passive enclosures (that is, they have one low-frequency and one high-frequency driver, and a passive crossover mounted inside the cabinet). The most common varieties use 10", 12" or 15" low-frequency drivers, and a constant-directivity high-frequency horn. Most of them use "bass reflex" cabinets, which have one or more holes or tubes in the front baffle to improve the low-frequency response.

Although they are "full-range", these are not generally suitable for reproducing low-frequency sounds (e.g. kick drum and bass guitar) at high volume. If you want to amplify the whole band, a multiple-enclosure system with separate sub-bass speakers would be a better choice.

For other purposes (vocals, acoustic guitar, low-level background music or speech reinforcement) a single pair of "full-range" speakers is usually adequate. Speakers with larger low-frequency drivers (15" rather than 10" or 12") have better low-frequency response, and generally higher acoustic output. Speakers with smaller low-frequency drivers have less tendency to "beam" at mid-frequencies, and this - combined with better response in the upper-midrange - means they can usually afford a higher crossover frequency. This, in turn, means it is generally possible to use a 1" compression driver (rather than 1.4" or 2"), and the sound of these - generally described as "more hi-fi" - is preferred by some listeners.

It is also useful to read and understand loudspeaker specifications (a subject on which a great deal of misinformation is available).

The main parameters include:

• Bandwidth. This is sometimes described as Frequency Range. The range between highest and lowest frequencies a speaker can usefully reproduce. Typically (but not invariably) this is given as the points at which the speaker's output has fallen by 10dB from its nominal output. Frequency Response (see below) is sometimes given instead.

• Frequency Response. This is stated as a range containing a margin of error (for example, 60Hz - 15kHz +/−3dB, which would mean that given a "flat" signal, the speaker's acoustic output varies above and below its average by less than 3dB between 60Hz and 15kHz). Some manufacturers use graphs as well. This tells you a bit more about how the speaker responds within its range, as well as what its range would be if tighter definitions applied. Note, however, that this is seldom constant at all volumes: a very flat response at 1 Watt may not be quite so flat at 500 Watts. If something is described as "frequency response" but the margin of error isn't given, assume it really means "bandwidth".

• Response Pattern (this may also be called Directivity or Dispersion). Up to a point, this tells you the angles (Horizontal and Vertical) over which the speaker produces its response. Although this is often presented simply (e.g. 90ºH x 60ºV), there are a couple of points to bear in mind:

1. Generally the limits are taken as the points at which measured output has fallen by 6dB, but some manufacturers use minus 10dB limits instead. If the specifications don't tell you what levels apply (e.g. −6dB, 90ºH x 60ºV) you can guess, but you won't know.

2. The response pattern is not the same at all frequencies. However, a relatively recent design development (the Constant Directivity Horn) means that most high-frequency horns have a broadly similar response pattern over most of their designated frequency range. To achieve this, the horn forces the highest frequencies to spread outwards further than they would naturally (their intensity on axis is reduced as a result, and additional filtering is sometimes employed in the crossover to compensate for this). The response pattern of a CD horn can be relatively well-defined, and is broadly representative within its stated range.

The same control cannot be achieved at lower frequencies, however, or with any front-loaded driver. Where a response pattern is stated for a single full-range cabinet, that pattern can only be assumed to apply to the high-frequency horn. Nevertheless, this is useful information. The high-frequency horn usually covers the most important frequencies for intelligibility.

• Sensitivity. Sensitivity describes the relationship between power and acoustic output. It is usually stated as the sound pressure level (dB SPL) at one metre on-axis with a (nominal) 1 Watt signal. However:

• Manufacturers use different methods to measure output. AES, IEC or EIA specifications mean the speaker has been tested to a replicable standard, and can be compared with any other speaker measured using the same standard. If the manufacturer doesn't say what standard was used (or give any other details), assume the speaker's true performance is less impressive than its claimed performance.

• The result of the test depends on whether the speaker was tested in free-space or half-space conditions (most - but not all - manufacturers test speakers in half-space conditions, which will give a higher reading).

• The measurement is normally taken from a position on-axis directly in front of the speaker. A speaker that "beams" (concentrating its output on-axis in front of it) might produce a higher efficiency reading than one with a broader and more even coverage, while the one with broader coverage might nevertheless have a higher overall acoustic output. Beaming tends to become an issue where the sound wavelength is less than the cone diameter, so readings at 1kHz (a wavelength of about 13") from a 15" driver might be misleading. Some (e.g. Audio Engineering Society standard) measurements are made at a distance that takes account of the driver diameter, using a signal that covers its entire bandwidth.

• Sensitivity (or efficiency) is an expression of how easily the speaker converts electricity into cone (and hence air) movement. This is a product of two main elements:

1. Motor efficiency. This is largely determined by the number of turns in the speaker coil and the strength of the fixed magnetic field (this is also affected by the distance between the coil and the magnet, increasing output as the distance is reduced). More turns, a stronger magnet and a narrower gap increase efficiency (up to a point), but more wire, better magnets and finer tolerances raise the cost substantially. More wire also increases the mass of the coil (making it harder to move, see next paragraph), and a stronger (which usually means bigger and heavier) magnet increases the total weight of the box.

2. The freedom of the coil and cone to move. Less mass (i.e. fewer coil turns and/or a lighter cone) and more elastic cone suspension increase freedom of movement. However, although a lighter cone and more elastic suspension may be great with a 1 Watt test signal, a lightweight cone may physically distort at high power (causing audible distortion of the sound). El Cheapo's Bargain Basement speaker may look the same on paper as the Costalot Premium Wallet Killer: 96dB/1W/1m. This may be because the Costalot Premium Wallet Killer has a very rigid cone (which won't flex when double its rated power is applied to it), and compensates for the extra weight of this by having a magnetic gap machined to very fine tolerances. El Cheapo's Bargain Basement speaker, on the other hand, has a cone made out of copier paper. The two speakers may sound reasonably similar at 1 Watt. At full power, however, El Cheapo's speaker may make the bass guitar sound like someone revving a bus with a loose exhaust. If you are planning to spend money on it, try to make sure you hear it at full power first.

In practice, most cones are made from paper or cardboard, which is treated to provide greater rigidity. Low frequency driver cones are usually heavier and more rigid than those in higher frequency drivers, while most drivers dealing with frequencies above about 1kHZ are compression drivers, utilising a strong lightweight (often titanium) diaphragm.

• Power Handling. How much juice you can run into a speaker before the coil insulation melts or the cone breaks.

There are two principal causes of loudspeaker failure: overheating (too much current) and over-excursion (too high a voltage).

Overheating happens when the amplifier's continuous average output is too high. Speakers are very inefficient (more of the power is converted into heat than into sound), and there are practical limits on how much heat a coil can dissipate. If heat is added faster than the coil can get rid of it, the speaker will fail as soon as its thermal limit is reached. Typically, a 4" coil can't get rid of heat much faster than 500 watts can produce it, so claims that a driver can handle "2,000 Watts RMS" are likely to exaggerate its capacity. If the amplifier's average output is higher than the speaker's capacity, the first thing to fail is usually the insulation on the wires of the voice coil, which melts (either filling the magnetic gap, causing the speaker to seize, or allowing adjacent turns of the coil to short together). If the speaker doesn't seize completely at this stage, other factors (the coil rubbing against the magnet, or increased current resulting from partial shorting of the coil) are likely to accelerate overheating. If it gets hot enough, the wire itself can burn through, breaking the current flow and permanently silencing the speaker.

Over-excursion happens when the amplifier's peak output is too high, forcing the coil to move further forward or back than it is designed to move. This can cause the coil to move out of the gap completely, or can tear or buckle the cone. Sometimes the speaker will continue to produce sound, but there will usually be some audible indication (rattling, hissing, or loss of level) that something has gone wrong.

How much power it can handle is is no indication of what the speaker will actually sound like or how loud it will be at that power (see Sensitivity, above). It is merely an indication that the coil won't burn out, and the cone won't rip.

Any accredited test of continuous power-handling is carried out in a similar way: a continuous signal with a measured average voltage is fed to the loudspeaker for a predetermined test period (usually at least one hour). If the speaker survives, the signal voltage is increased, and the test repeated. When the speaker fails, its continuous power-handling capability is taken to be the level at which it last survived for the whole of the test period. The power is calculated from the input voltage and the speaker's impedance, using the formula P=(V^2)/R, where P is the power in Watts, V is the voltage in Volts, and R is the impedance in Ohms. Generally, one or more of the following descriptions of power handling is given (in all measurements the thing actually measured is voltage, not power):

• Continuous RMS. "RMS" stands for "Root Mean Squared", and derives from a method used to calculate the average value of A.C. voltage. Because A.C. voltage is negative half the time and positive half the time, the "true" average is zero, regardless of the actual voltage in each half of the wave cycle. Squaring each measurement means that both positive and negative measurements yield a positive value (plus x plus = plus, minus x minus = plus). The square root of the average squared value is the RMS value, a positive number that is representative of the average voltage. While this is perfectly valid with A.C. voltage (which has positive and negative values), it isn't properly used to describe average power (apart from anything else, power cannot have a negative value). Nevertheless, it is used by some manufacturers to describe the average amount of power a speaker can handle. Where the figure given for "RMS power" derives from a test, a measurement often used is the RMS voltage of a 1 kHz sine wave (although there is no absolute certainty about what was measured unless the specifications tell you). This may not test the driver at its weakest point (a 15" driver might handle 1 kHz easily, but overheat at 60 Hz). Also, in a sine wave the ratio between average and peak values is always the same: the square root of 2, or 1.414. In an audio signal the difference is not constant. View any specification giving an "RMS" power value with suspicion.

• AES. The speaker is tested using Audio Engineering Society standards. Like the "Continuous RMS" rating, this aims to tell you the average long-term capacity of the speaker. Unlike the "Continuous RMS" rating, it always derives from a stringent replicable test. The object of the test is to establish the long-term capacity of a speaker under continuous load. The AES test uses band-limited pink noise (the crest factor - the ratio between average and peak levels - is defined, as well as the bandwidth), over a sustained period (generally two hours). The test signal takes account of the speaker's operating bandwidth, and the rating also takes account of the speakers minimum impedance (often as low as half its nominal impedance, see below), so calculating power from the measured voltage results in something pretty close to the actual power (rather than some theoretical value based on the speaker's nominal impedance). Of measures in current use, this is probably the most representative of how well the speaker does what it is claimed to do.

• EIA. A similar test to the AES test, this time as defined by the Electronic Industries Alliance.

• IEC. The International Electrotechnical Commission variation on the same theme. This test is not bandwidth-limited.

• Program Power. This term has no specific meaning. Supposedly it is the amount of power the speaker can handle during a "typical program", where actual power and frequency-content vary constantly. Because the power is varying, the coil has greater opportunity to get rid of any excess heat, so in the longer term the speaker can handle more power without melting. Typically, the stated program power will be double whatever the average power-handling capacity was. Of course, there is no such thing as a typical program (is it typical Dido or typical Iron Maiden?). Do not treat "Program Power" as if it means anything.

• Peak power. This is the amount of power the speaker can take for very brief periods (meaning milliseconds) at a time. In a continuous signal, the relationship between peak and average power is about 2:1 (in a sine wave the peak voltage is 1.414 - the square root of 2 - times the average voltage, which doubles the amount of power produced in a given load). Most speakers will briefly handle signals with greater peaks (the peak rating is typically four times the continuous power rating). The difference between 300W and 1200W might seem a lot, but next time you are near a mixer have a look at how big 6dB - the difference in decibels - looks on the meters. Peak power is not representative: typical concerts last more than a few milliseconds (unless you treat peak as if it means continuous).

• PMPO. Peak Music Power Output. A speaker with a PMPO rating isn't even worth a hollow laugh.

• Output Capability. How loud it is at full power (this is related to sensitivity and power handling). This is no indication of what it sounds like at full power: it might be loud like a chainsaw is loud. How loud it can possibly be depends on how much noise a cone moving backwards and forwards can possibly make, and using front-loaded drivers you won't get far beyond 130dB (some of the best front-loaded units get to about 134dB, which is impressive, but you won't get a pair of them in your local music shop for £125.00). There are two ways of determining a speaker's maximum output:

• Measure it. You use a calibrated sound pressure level meter to measure it.

• Calculate it. You take the output at 1W, and add the difference in decibels between 1W and the speaker's peak power-handling capability. For a "500W RMS" speaker which produces 97dB/1W/1m, the "peak output" will be 97 + (10 x log2000) decibels, or 130dB. Most speakers will exhibit power compression at the limits of their performance ("peak" is, by definition, the absolute limit), so this is likely to flatter its performance by at least one or two decibels.

If the peak output is measured, then whatever figure is given is the peak output. If the specifications don't say it is measured (or how it is measured) then it probably isn't measured, but calculated, and should be viewed with a bit more scepticism.

• Impedance. Impedance is the A.C. equivalent of resistance, and uses the same unit of measurement (the Ohm, symbol Ω). Lower impedance increases the load on the amplifier.

Although most loudspeakers have a stated nominal impedance, it is important to recognise that - unlike resistance - this is not a fixed static value: at some frequencies a speaker's actual impedance may be much higher (or - more importantly - much lower) than its nominal impedance*.

It is important that the combined load of the loudspeakers does not exceed the capacity of the amplifier driving them. Although many modern amplifiers claim to be able to handle 2Ω loads, it is not generally recommended to have a combined nominal impedance of less than 4Ω on any single amplifier channel. At best, lower impedances mean that cable impedance is proportionately higher (reducing an amplifier's damping factor, and increasing the proportion of power loss in the cable). At worst, your amplifier will shut down or burn out during a show.

Using identical speakers in parallel, the combined impedance is equal to the impedance of a single speaker divided by the number of speakers. Using different types of speaker at the same time on the same amplifier channel is not recommended.

Single speakers usually have a nominal impedance of 4Ω or 8Ω. However, in applications where it may be advantageous to drive a larger number of speakers from a single amplifier channel (high frequency clusters are one such application), 16Ω - or higher - impedances may be found.

Generally, a single amplifier channel will comfortably drive one 4Ω speaker, two 8Ω speakers, or four 16Ω speakers.

* Let us take, as an example, a speaker which - the literature tells us - has an impedance of 8Ω, and a continuous power-handling capacity of "200W RMS". The power-handling value comes from testing the speaker with a 40V RMS 1kHz sine wave. P = (V^2)/R = (40^2)/8 = 1,600/8 = 200W.

However, at 1kHz the actual impedance of the speaker is 10Ω (slightly higher than its nominal impedance). So the actual power going into the speaker on test is not 1,600/8 but 1,600/10 = 160W.

When we get our speaker out on the road, we drive it with an amplifier that delivers 200W into 8Ω. When our keyboard player hits that big bass chord in the final bar of that number with all the big bass notes in it, the speaker dies. Why?

At 40Hz (bottom E on a bass guitar), the speaker's actual impedance is 4Ω (half its nominal impedance). At 40V RMS we are driving it with 40^2/4 = 1,600/4 = 400W. It only handled 160W in its power-handling test.

Fortunately, our power amplifier will handle loads of 2Ω, so it will survive driving two of these "8Ω" speakers in parallel.

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