In the third part of our PA series, Paul White looks at the importance of loudspeaker enclosure design.

Most loudspeakers are of little use without an enclosure or cabinet. High-frequency and hi-fi style mid-range drivers are designed to work without needing any enclosure space behind them, but in PA applications, the mid and low frequency drivers need enclosures, and high frequency drivers often need horns to increase their efficiency. The reason is fairly evident when you consider what would happen if you tried to use a loudspeaker driver on its own as shown in Figure 1.

Whenever a loudspeaker cone moves forward, it compresses the air directly in front of it, while the pressure of the air behind the cone is reduced; when it moves back, the opposite happens. In the absence of an enclosure, there is nothing to prevent the high-pressure air in front of the cone from flowing around the edge of the speaker into the low-pressure region at the rear. In other words, instead of the loudspeaker driver projecting acoustic energy into the room, much of the energy is wasted.

The lower the frequency, the more time the air has to move around the edge of the speaker before the cone changes direction, which means that at bass frequencies, virtually all the energy is wasted pumping air from the front of the driver to the back and vice versa. In effect, instead of generating a sound wave that can be projected into the room, the driver is creating a localised pocket of turbulence with radiated sound as a bi-product.

What's needed is some way to stop the air sneaking round the sides of the driver. One method is to mount it on a baffle as shown in Figure 2. Here we have a driver mounted in a cutout at the centre of a large, flat plate or baffle. If the baffle is large enough compared to the wavelength of sound at low frequencies, the air won't have time to move from the front of the baffle to the back before the cone changes direction. As a result, the acoustic efficiency is much improved, especially at low frequencies. If you look at the way the sound is projected, it's evident that this arrangement works as a dipole. In other words, a lobe of sound is projected from the front of the baffle and an equal and opposite lobe is created at the rear, rather like a figure-of eight mic in reverse. If you were to stand edge-on to the baffle, the front and rear sounds would cancel out leaving a dead spot.

Assuming that no sound actually passes through the baffle, all the sound generated by the rear of the speaker cone is wasted as far as the listener in front of the speaker is concerned. A huge baffle isn't a practical way of mounting loudspeakers for live performance, so the next logical step is to fold the baffle into the shape of a box, then use absorbent material inside the box to soak up the unwanted power from the rear of the driver. This is known as an 'infinite baffle' enclosure.

In the design of an infinite baffle, the sound only comes out of the front (although at low frequencies, sound tends to become almost omnidirectional because of the long wavelengths involved). However, the box may need to be fairly large in order to obtain a good low frequency response, and because half of the sound energy is being absorbed inside the box, the acoustic efficiency is not great. Whenever the front of the cone couples directly with the air in the room rather than feeding a horn flare, it is known as a direct radiator.

PA APPLICATIONS

For PA applications, it is usually desirable to generate as much acoustic energy as possible from the smallest practical enclosure. This usually means choosing an electrically efficient driver and mounting it in a so-called ported or vented enclosure, an example of which is shown in Figure 3. The type of enclosure shown is a direct radiating, ported system where the port causes the box to resonate at a certain frequency, much as a milk bottle will resonate at a fixed note when you blow over its neck. If the enclosure/port is tuned so that this 'note' occurs where the natural bass end of the speaker starts to roll off, the bass response can usefully be extended downwards by making constructive use of the energy that is normally absorbed inside the box.

The frequency of resonance depends on the volume of air inside the box, the dimensions of the port, and the mechanical 'springiness' of the loudspeaker. A potential problem still exists due to sound reflecting from the internal walls of the enclosure. To combat this, enclosures are often lined with acoustic wadding or other damping material. Large amounts of porous damping materials affect the speed of propagation of sound within the enclosure and thereby influence the frequency at which the enclosure resonates. Because of this, the effect of introducing damping material on enclosure tuning has to be considered at the design stage. Furthermore, care must be taken over the choice of enclosure parameters — tuning the enclosure to a too narrow frequency band will result in a peaky, coloured low frequency response.

As a general rule, the larger the driver diameter, the larger the volume of enclosure it will require, although enclosure size is also a function of the mechanical properties of the driver. You can't, for example, take an enclosure designed to work with one kind of 12-inch speaker and expect it to work properly with a different model of 12-inch speaker.

HORN LOADING

Even with the benefits of porting, direct radiating loudspeaker systems are relatively inefficient. Loudspeaker cones are very effective at pushing hard against small volumes of air, but what we actually want is to move a large volume of air over a shorter distance. Transferring energy from a moving driver cone to the air in an efficient manner involves mechanical matching, and a significant improvement in matching can be obtained by placing the driver at the end of an exponential horn as shown in Figure 4. Horns are regularly used to increase the efficiency of high frequency units, but they may also be used with mid and bass drivers.

A horn can be thought of as the mechanical equivalent of a gearbox, matching the capabilities of an engine to the needs of its load. Not only does a horn improve the mechanical coupling between the loudspeaker cone and the air, it also helps direct the sound over a narrower area, enabling designers to construct enclosures with well-defined coverage patterns. This is vitally important when it comes to using multiple enclosures in an array, as the greatest efficiency is produced when the overlap between adjacent enclosures is kept to a minimum. Loudspeaker arraying is a complex subject and will be covered later in this series.

Short horns with square or rectangular section flares are often used in the design of mid-range PA loudspeakers, because they are simple to construct from plywood, and they can be used to help project sound to the back of a large auditorium. More advanced designs use materials such as fibreglass to create more precisely shaped horns with no sharp corners. JBL's latest system, the HLA Series, extends to mounting movable horn-loaded enclosures inside a lightweight tubular aluminium framework so that the sound source can be re-aimed without having to move the whole box. This is potentially useful when constructing large arrays.

The shape and size of a horn dictates the frequency down to which it will work properly. If frequencies below this 'cutoff frequency' are fed to the driver, there is a real risk of driver damage; because the cone is no longer 'loaded' by the air in the horn, it will tend to move too far — rather like a car engine over-revving when the wheels spin. For this reason, electronic high-pass filtering is generally included in the system to prevent potentially damaging low frequencies from reaching the drivers.

STACKING THE ODDS

For effective operation, the diameter of the horn mouth should be at least a quarter of the wavelength of the lowest note to be reproduced. At 50Hz, for example, the wavelength is approximately 20 feet, so a horn with a diameter of at least five feet would be required. Furthermore, sub-bass systems capable of going down to 25Hz would need to be 10 feet across — clearly impractical in a touring rig!

Getting a sufficiently large horn aperture to work effectively at low frequencies might seem like an impossible requirement to meet. The solution is to stack several low frequency boxes; acoustically, this is the same thing as using one very large enclosure. With large concert systems, large numbers of enclosures can be stacked to provide very efficient low frequency arrays, but even in smaller systems, stacking two or four bass boxes together can produce a few welcome extra dBs of low end.

FOLDED HORNS

We now know that with horn-loaded designs, the lower the frequency, the larger the horn, but horn length also affects efficiency. As a rule, the longer the horn, the better the mechanical matching of the driver to the air in the room. For deep bass frequencies, a straight horn would be impractically long and bulky, so enclosures are often designed which resemble a horn that has been folded to make it physically more compact. While internal reflections would compromise the performance of such a horn at high frequencies, the long wavelengths involved mean that folded low frequency horns can work almost as well as straight horns.

There are several methods of designing a folded horn bass enclosure, some of which make use of the sound from both sides of the cone, and some of which don't. Several variations of the folded horn are shown in Figure 5.

PRACTICAL SYSTEMS

Back in the '70s and early '80s, a typical concert PA was likely to comprise a stack of horn-loaded bass enclosures, or bins as they were often called, stacks of mid-range, horn-loaded boxes, and separate high frequency, horn-loaded compression drivers perched on top. Constant directivity horns were also introduced during this period. Specially calculated flare shapes were used to produce a relatively consistent directivity pattern over the entire frequency range covered by the horn. Constant directivity horns are currently very popular, and it's also possible to design these for different degrees of directivity in the horizontal and vertical planes, thus enabling system designers to produce enclosures that provide the optimum sound coverage when arrayed.

The various enclosures in such a PA system are fed from racks of high power amplifiers, usually located close to the loudspeakers so as to minimise cable lengths. The amplifier is fed from an active crossover and safety limiters are invariably needed to prevent the power amplifiers from being driven into clipping. The problem with having all the enclosures separate is that setting up takes a long time, and a great deal of expertise is required to array the enclosures in the most effective way. There are also mechanical stability problems with multi-box systems.

PROGRESS

In recent years, we've moved away from totally separate boxes to integrated systems comprising multi-way, full-range enclosures supplemented by separate sub-bass enclosures. The full-range enclosures are often three-way affairs combining direct horn-loaded (or direct radiating in smaller systems) bass and mid-range units with compression-driven high-frequency horns. As with older PA systems, these full-range enclosures are generally fed from separate power amplifier racks, but the crossover units are now more sophisticated and are often system specific. Most of these will include their own limiters. More sophisticated DSP-based 'processor' type electronic crossovers may combine the roles of crossover, protection limiting, system specific EQ, time alignment delay, and other parameters.

Although the performance of a full-range enclosure can be impressive, most touring systems use separate sub-bass enclosures. These may be slung from the ceiling in carefully angled arrays, stacked beneath the stage or at the sides of the stage beneath the full-range enclosures.

This may seem pretty heady stuff if you're playing pub gigs using whatever you can get into the back of a hatchback, but many of the modern gigging systems bear a strong resemblance to a scaled-down concert system. For example, compact full-range systems often include built-in power amps and active crossovers — JBL's very portable EON system being a typical example (see review in November issue). Similarly, there are several systems on the market where compact full-range enclosures can be augmented by relatively compact sub-bass enclosures to produce concert sound in miniature at smaller venues.

Next month, I'll be looking at the practical side of using a typical small gigging PA system.