Paul White visits the secret world of loudspeaker drivers where death, burning, and the rending of limb from limb are commonplace occurrences!

One of the most important components of any PA system is the loudspeaker, yet it is often one of the least understood. Basically, the fundamental job of any loudspeaker is to convert electrical energy to acoustic energy (within the human hearing spectrum), as accurately as possible and as efficiently as possible. Young people with undamaged hearing can usually perceive sound in the 20Hz to 20kHz range so this is generally accepted as being the frequency range over which a well-designed audio system should operate, but the laws of physics conspire to make it very difficult for a single loudspeaker to cover the entire audio spectrum. Physically small loudspeakers may well be able to accurately cover the mid and high frequencies, but they are incapable of generating the high levels of bass and lower midrange required in most PA applications.

To reproduce low frequencies effectively, where we may be dealing with wavelengths of 40 feet or longer, it is necessary to move a lot of air, which is why conventional bass speakers have larger diameters than high-frequency or mid-range speakers. Another general rule is that larger diameter drivers tend to require larger cabinets, which is why bass bins are so big and heavy.

While large diameter loudspeakers can be designed to reproduce low frequencies quite effectively, their high moving mass and large physical dimensions make them relatively poor at reproducing higher frequencies so in most practical systems, the audio range is shared between speakers of different sizes so that each can handle the section of the audio spectrum with which it is most comfortable.

A circuit known as a crossover is used to send just the relevant part of the audio spectrum to each speaker. Depending on the size of the system, the spectrum may be divided into between two and four frequency bands. In a two-way system, the bass is handled by a so-called woofer and the treble by a tweeter or High Frequency (HF) driver, whereas in larger systems, there may be one or more additional speakers handling the mid-range. Not surprisingly, these are called mid-range drivers.

Before looking more closely at how these various loudspeakers work together, it is useful to examine the mechanics of their operation as this provides clues to their strengths and limitations.

DRIVERS

A chassis loudspeaker without a box or cabinet is called a driver. Virtually all drivers (with the exception of small piezo electric tweeters, or the electrostatic speakers used in specialist hi-fi systems) work on the same electric motor principle invented during the dawn of radio. Designs have been improved over the years, and new materials have been developed, but the basic principle remains the same.

The familiar 'cone' loudspeaker comprises a stiff cone of doped paper or synthetic material suspended in a rigid cage or chassis by means of a flexible surround, as shown in Figure 1. The narrow end of the cone is fixed onto a parallel-sided tube or former, onto which is wound a coil of thin wire; this coil is positioned in a slot between the poles of a powerful magnet. Whenever a current is passed through the coil, a force is set up between it and the magnet, causing the cone of the driver to move either backwards or forwards from its neutral position, depending on the polarity of the electric current. This is exactly the same principle as an electric motor, except the motion developed is linear rather than rotational. If the electric current passed through the coil is an audio signal, then sound is produced.

The driver is fed from the output of a power amplifier, whose job it is to intensify the small audio signal from a mixing desk or other source. A dynamic microphone works in almost the exact opposite way to a loudspeaker, so what happens in practice is that the tiny movements of the microphone diaphragm are re-created on a much larger scale by the loudspeaker cone.

From Figure 1, it is evident that the cone can only move so far before it reaches a physical limit, where either the suspension will permit no more travel or the voice coil will be driven out of the gap. As the cone approaches these limits, its movement becomes restricted, with the result that the physical movement of the cone is no longer exactly proportional to the electrical input. The result is distortion — distortion is defined as any difference between the input signal and the output signal other than in amplitude (level). In reality, all systems distort to some extent, but providing the amount of distortion is kept small, the human hearing system perceives the sound as being accurate or clean. Audible levels of distortion result in a fuzzy or unclear sound where musical details and vocal intelligibility tend to become lost.

LIMITATIONS

A loudspeaker cone can only reproduce the input signal faithfully if it behaves as a rigid piston pushing against the air — if it bends or vibrates, distortion will occur. Much research goes into cone shape and materials to reduce distortion, but some distortion is inevitable. After all, the basic principle of a cone driver is very crude, and all the movement comes from magnetic forces pushing against the voice coil. This places enormous mechanical stresses on the components, and very advanced adhesives are required to stop the speaker cone assembly from literally shaking itself apart. The more power handling is required, the greater these mechanical stresses become.

Another major physical limitation on the power handling of a loudspeaker is the heat generated in the voice coil — loudspeakers are relatively inefficient devices, so most of the amplifier power fed into them is converted into heat. Because any voice coil has an electrical impedance, the more power that's put in, the more the coil heats up. If more heat is being generated than can be dissipated, the temperature will continue to increase and the coil will eventually burn out. Before burnout occurs, however, the increased temperature of the voice coil and magnet system will reduce the loudspeaker's efficiency — a phenomenon known as power compression. For this reason, high power loudspeakers need to be designed with cooling very much in mind. In recent years, several imaginative solutions have been implemented, including Volt's distinctive PA driver with heavy cast spokes in front of the cone.

BASS DRIVERS

As we now know, a driver required to deliver high levels of bass needs to have a large diameter and must be able to move over as large a distance as is practical so as to move the maximum amount of air. The larger the cone, the more rigid it has to be to prevent bending or deforming under load, and that in turn results in a heavier cone assembly. The mass of a large diameter voice coil capable of handling the high powers encountered in PA bass applications is also significant. Simple mechanics tells us when we try to accelerate a large mass, it requires far more energy than accelerating a small mass, and in the case of a bass driver, it's not just the cone that's being accelerated, but also the air in contact with the cone. The faster you try to move the cone, the higher the inertial resistance to that movement, and in audio terms, this means the higher the frequency, the harder the cone is to move. This is one reason why large bass drivers cannot faithfully reproduce high frequencies.

Another reason why large bass drivers can't reproduce high frequencies effectively is their diameter. If you imagine an 18-inch diameter bass driver that has, somehow, been built with an infinitely light, infinitely strong cone, and an infinitely powerful magnetic motor, it might seem that it should be able to function as a high frequency driver. However, even if these impossible criteria were achievable, the driver performance would still be compromised by its very geometry. The diameter of the cone would simply be too large compared with the wavelengths of the high frequency sounds being reproduced. While this doesn't affect the ability of the driver to project sound forward, what happens if you move slightly to one side?

Referring to Figure 2, you can see that sound originating from the side of the cone nearest you will reach you before sound from the other side, and the same is true of all the points in between. The result is phase cancellation, and the further you move away from the main axis of the loudspeaker, the more serious this becomes. The practical outcome is that large diameter drivers tend to concentrate their high frequency output into a relatively narrow beam, whereas lower frequencies are propagated with a much wider dispersion angle. The higher the frequency, the more serious this 'beaming' effect. This not only prevents the system from providing an even audience coverage, it also invites feedback as it is highly likely that a reflection from one of these tightly beamed, high frequency sounds will find its way back into a stage microphone.

Yet another problem occurs at high frequencies, because the cone of the driver ceases to behave as a perfect piston, and instead starts to vibrate or break up. The term 'loudspeaker breakup modes' describes the way the cone assembly vibrates or ripples under load and does not imply mechanical failure. Indeed, guitar speakers are designed to break up in certain ways so as to produce a characteristic sound coloration. Some hi-fi speakers use complex damping or bracing to minimise distortion due to cone break-up. This in turn tends to increase the mass of the cone, resulting in a further decrease in efficiency. In PA applications, high efficiency is important, not just to minimise the amplifier power needed, but perhaps more importantly, to minimise the amount of power dissipated as heat inside the loudspeaker drivers.

MID-RANGE AND BEYOND

As you may have grasped from the previous section, the usual solution to driver beaming problems is to use progressively smaller diameter loudspeakers to handle the higher frequencies. In other words, midrange drivers are smaller than bass drivers and high frequency drivers are smaller still.

Mid-range drivers are usually cone drivers, similar in construction to bass drivers, but physically smaller. High frequency drivers, on the other hand, need to be so small that the direct radiating area may be only one or two square inches. While this is fine for domestic hi-fi applications, such a driver would be incapable of providing the level necessary for live sound. What's more, you can't simply mount a dozen or so dome tweeters on the front of a cabinet, because then you get the same phase cancellation problems you'd get with a single, large diameter driver. In a properly designed PA system, each high frequency speaker radiates into its own section of space, producing minimum overlap with other high frequency drivers in other cabinets — that's one of the reasons speaker cabinets are arrayed at an angle to each other. Although the use of hi-fi style dome tweeters for high power PA use is ruled out, there is some constructional similarity between a hi-fi tweeter and a PA high-frequency driver, so it's worth looking at how hi-fi speakers are constructed.

Hi-fi tweeters often employ small, dome-shaped diaphragms in place of cones, and these may be fabricated from a variety of materials including plastic, treated fabric, and even metals such as aluminium or titanium. The diaphragm is driven by a voice coil arrangement similar to that employed in the cone loudspeaker.

In PA applications, horn-loaded tweeters are almost always used because of their higher efficiency. These comprise a compression driver, not unlike an industrial grade hi-fi tweeter, driven into a flared horn. Using a horn has two main advantages: firstly, it significantly increases acoustic efficiency by matching the driver to the air in the room more efficiently (think of it as an acoustic gearbox); secondly, the horn shape controls the directivity or angle of coverage of the sound, enabling it to be focused more precisely. It is also possible to feed a single horn from two or more compression drivers by using a kind of Y manifold shape at the rear of the horn, a technique sometimes used where very high power levels are needed. Figure 3a shows both a conventional, direct radiating tweeter; 3b shows a compression-driven horn.

A more recent development is the constant directivity horn, where the profile of the horn is designed to prevent the lower end of the high frequency spectrum from being radiated over too wide an angle (with a non-constant directivity system, the dispersion angle narrows as the frequency increases). This tends to make the lower end of the high frequency too loud when compared with the higher end (because it's being concentrated over a smaller angle), so some filtering has to be introduced in the crossover circuitry to compensate. This subject will be covered in more depth later in the series.

Horns are also often employed to increase the efficiency, and to control the directivity, of mid-range drivers. Because of the larger dimensions of the horn, it will probably be built from wood or fabricated from glass fibre.

CROSSOVERS

So far, we've learned that bass driver cones move relatively slowly, but over quite large distances, whereas high frequency driver diaphragms move more quickly over a more limited distance. High frequency drivers, therefore, have to be protected from potentially damaging, low-frequency signals outside their range. Similarly, bass and mid drivers must be prevented from receiving frequencies higher than they are designed to reproduce, not because they will be damaged, but to prevent high frequency beaming. Furthermore, most drivers are only capable of producing a flat frequency response and low distortion over a specific part of the audio spectrum, so feeding a driver with frequencies it was never designed to handle will seriously compromise the quality and accuracy of the overall sound. That's where the crossover comes in.

At its simplest, a crossover is a series of passive electrical filters, comprising resistors, capacitors, and inductive coils wired between the amplifier output and the drivers. Such systems are invariably located inside the speaker cabinets themselves. In a three-way speaker system comprising bass, mid, and tweeter units, the bass speaker would be fed via a so-called low-pass filter — a filter that only allows frequencies below a certain limit through. This ensures that the bass speaker never has to deal with frequencies higher than those it was designed to reproduce.

On the other hand, a mid-range speaker has both upper and lower limits of operation, so it has to be fed by both high and low-pass filters to ensure that it receives only midrange frequencies. Frequencies that are too low will cause damage and distortion, whereas higher frequencies will again result in beaming and coloration.

HF drivers are fed via a high-pass filter, ensuring they receive frequencies only above a certain limit. Figure 4 shows how the three crossover bands are arranged so that one filter slopes away as the next one rises, ensuring a smooth transition from one driver to the next.

The type of crossover described is known as a passive crossover, because the only circuitry needed comprises passive filters. Such designs are generally reliable and cost-effective, but they are best suited only to low power applications. Some of the amplifier power is absorbed by the passive crossover circuit, resulting in a loss of efficiency. Unless all three drivers are equally efficient at turning electrical energy into acoustic energy (which is most unlikely), the more efficient drivers have to be fed with attenuated or reduced signals to bring them down to the level of the least efficient driver in the system. In other words, you have to deliberately waste power to get a flat frequency response across the audio spectrum. An accurate system must have a flat frequency response, which simply means that all frequencies within the audio range are treated equally, rather than some being amplified more than others.

Because of the restrictions of passive circuitry, the filter characteristic of a passive filter can't be made particularly steep without wasting an enormous amount of power. A filter doesn't simply block all frequencies beyond its cutoff point, rather it has a sloping response that reduces frequencies beyond the cutoff point by so many dBs per octave. The more dBs per octave, the sharper the response of the filter is said to be.

Simple passive filters usually have slopes of 6 or 12dB per octave. In practical terms, this means that the drivers still receive significant amounts of power outside their ideal range. For example, with a 6dB per octave filter, the signal voltage is only halved for every octave you go beyond the cutoff point, whereas a 24dB per octave filter will reduce the signal to one-sixteenth of the original for each octave beyond the cutoff frequency. As well as making sure each driver covers only its designated frequency range, a steep filter also minimises the area of overlap between drivers handling adjacent frequency bands. This is usually desirable, because a wide overlap can lead to phase problems as both drivers struggle to deliver a slightly different version of what's going on in the crossover's overlap region.

ACTIVE CROSSOVERS

PA as we now know it was revolutionised by the introduction of the active crossover — a device that allows an audio signal to be separated into different frequency bands before the power amplifiers. This enables separate power amplifiers to be used to drive the bass, mid, and high frequency loudspeakers, and because the filtering occurs before the amplifiers, no amplifier power is lost in the crossover circuit. Figure 5 shows a simple three-way passive crossover system, while in Figure 6, the same three-way speaker system is driven from an active crossover system feeding separate power amplifiers. Because the crossover circuits deal with low level audio signals rather than the massive amounts of power needed to drive loudspeakers, the filters can be built using active electronic circuitry, permitting much greater flexibility and precision in design. What's more, the system performance is no longer restricted by the driver with the least efficiency — now the amplifier powers and gains can be optimised by providing more power to less efficient speakers. This gives the designers far more flexibility when selecting drive units for use in a system, and the ability to design steeper filters helps minimise the amount of 'out of band' signal that each driver receives.

Active crossovers considerably reduce the risk of driver damage, because an overload in one part of the audio spectrum won't necessarily cause an overload in the other areas. With typical pop music programme material, the energy at the bass end of the spectrum far exceeds that at the high end, and in a passive system, this can pose a danger to the tweeters. Imagine what happens when something like a dance record is played too loud so that the amplifier is driven into clipping distortion. Every time a loud bass note or drum beat is played, the amplifier clips, producing square waves rich in high frequency harmonics. These pass through the crossover in just the same way as legitimate high frequency signals, so they reach the tweeter. If these artificially generated harmonics are high enough in level and long enough in duration, they may easily cause mechanical damage, or result in voice coil burn out through overheating.

Looking at the same scenario in an active system where the crossover comes before the amplifiers, an overload at the bass end will be confined to the bass amplifier — the mid-range drivers and tweeters still receive clean signals from their own amplifiers.

Active crossovers constitute an important subject area in their own right, so they'll get a slot of their own later in this series. In the meantime, next month we will concentrate on loudspeaker cabinets, which come in a variety of shapes and sizes for different applications.