Subwoofer Crossovers  529

It is also possible to design a 4th + 4th order bandpass subwoofer, but there is a general consensus that the transient response is so poor that any improvements, such as in bass extension, are irrelevant.

Isobaric Subwoofers

In an isobaric subwoofer there are two drive units acoustically in series, driven by the same electrical signal. One version is shown in Figure 18.1f. At low frequencies the two cones move together and are equivalent to a single drive unit with twice the cone mass. This reduces the resonant frequency to about 70% of what it would have been with one drive unit of the same type working in a sealed box with the same volume. The same cutoff frequency can be achieved with half the box volume, though the volume of the chamber between the two drive units must be added; the air volume in this chamber has no acoustic function beyond coupling the drive units, so it is desirable to make it as small as the physical constraints allow.

At higher frequencies cone break-up will occur, and the drive units no longer act as one unit. Other configurations are possible with the drive units mounted to front-to-front instead of front-to-back; this requires the phase of one drive unit to be reversed. There seems to be a general feeling that isobaric loudspeakers are a new idea, but this is not so. Once again, the technique is older than you might think.

It was introduced by Harry F. Olson in the early 1950s.

Dipole Subwoofers

The concept of the dipole subwoofer is quite different from the other subwoofer configurations we have looked at. They all aspire to be monopole radiators—ideally a single point source of sound that has the same polar response in all directions. A large area source or a multiple source causes reinforcements and cancellations in the sound pressure level that cause major amplitude response irregularities. To do this the rear radiation from the drive unit must be dealt with in some way, either by suppressing it (sealed boxes) or getting it into phase with the front radiation to reinforce the output (ported boxes and transmission lines).

A dipole subwoofer does neither of these things; instead the rear radiation is allowed out into the room without modification. Typically one (or quite often, more) large-diameter drive unit is mounted on a relatively small baffle, as in Figure 18.1g; this means that the bass will begin a 6 dB/octave roll-off relatively early, because of cancellation around the edges of the baffle. To counteract this, drivers with a high Qts are sometimes used to give a peaky underdamped response that lifts the overall response before roll-off; adding dipole equalisation to an active crossover is a far better approach, as it gives complete controllability and eliminates the need for special drivers that may be hard to get. This usually consists of a low-frequency shelving characteristic, where the amount of boost plateaus at low frequencies to avoid excessive cone excursions. More details are given in Chapter 14 on equalisation.

When the path around the baffle edge from front to rear of the drove unit equals a wavelength, front and back radiation are in phase and will reinforce, giving a 6 dB peak in the response; the frequency at which this happens can be simply calculated from the baffle dimensions. However, reinforcement only happens at a single frequency when the baffle is circular, so all paths are of equal length, and this