When Up Is Down

Would you buy more power amps and loudspeakers just to reduce the volume onstage? As counter-intuitive as this may seem, there are a number of loudspeaker manufacturers that are now selling loudspeaker systems that take this basic approach. It is not a new idea to use line arrays or one loudspeaker driven out of polarity with another to modify the polar response of an array. What is new is the ability to buy and install systems where the bulk of the work in achieving this end has already been done for you.

Directivity at mid and high frequencies is usually a matter of horn design but at low frequencies the problem is just too big to tackle with a horn in all but the largest venues. Instead, a form of "beam steering" can be used to achieve control over the low-frequency directivity of the array. This requires supplying a different drive signal to each low-frequency driver (or symmetrical pair of drivers) in the array. The end result is a reduction in low-frequency levels from the FOH system, onstage. Leakage from the FOH system into mics has become more obvious in recent years as in-ear monitoring has reduced or eliminated (leakage from) the onstage loudspeakers.

Achieving good coupling of the acoustic output from an array of low-frequency drivers is primarily a matter of spacing. As the inter-driver spacing increases with the number of devices, the destructive interference is also on the increase. This results in frequency-dependent lobes being created in direct relation to the wavelengths within the driver's bandpass. Murphy's Law predicts that these will be worst at the downstage centre location of the mic with the most gain. Good horn and array design can help control this effect (by minimizing coverage overlap) in mid and high frequency bands but low frequencies require a different approach.

The convergence of: decreasing power amplifier $/watt ratio; and the availability of DSP-based signal processing, has made the "beam steering" a practical consideration even in smaller venues. This approach has been used in large-venue arrays designed by Acoustic Dimensions (and others) for some years. Craig Janssen of Acoustic Dimensions has worked with EAW to develop their Tuned Dipolar Array Technology and Vance Breshears of Sound Technology Consultants (formerly of Acoustic Dimensions) has been working with Renkus-Heinz in the development of their TRAP-Tripolar System. Both of these manufacturers are taking the frequency-shaded-array approach. This is achieved by rolling-off the upper end of the woofer's frequency range relative to its location in the vertical plane of the array. The woofers nearest the centre the entire low-frequency band right up to the crossover frequency of the mid-range units. The woofers that are further away (top and bottom of the vertical line) are only receiving the low-frequencies that are within one-half wavelength relative to the spacing of the drivers. This minimizes the overlapping coverage in the upper-bass region that causes cancellation effects in the listening area while allowing pattern control over the lowest frequencies where the woofers act as a line array. The implementations differ between manufacturers so check out the manufacturer's web sites for more detailed information. Vance Breshears will be presenting a paper on this topic at the AES in San Francisco, this fall.

The latest powered subwoofer from Meyer Sound Labs also uses multiple drivers in a single cabinet to achieve pattern control at low frequencies. The PSW-6 is claimed to achieve over 15 dB of pattern control from 27 Hz to 125 Hz from a single loudspeaker. The loudspeaker uses two forward-firing 18" drivers, two forward-firing 15" drivers, and two rear-firing 15" drivers. Essentially, the rear firing drivers are canceling the low-frequency output that wraps around from the front of the cabinet. However, it is more a matter of modifying the frequency response and relative arrival time of the signal fed to each driver that achieves the cardioid polar response.

Directivity control of low frequencies is now possible in a variety of system configurations, from large-scale horns to "beam steering" of an array through the use of signal processing. Check out the array designs from Renkus Heinz and Eastern Acoustic Works, and take a listen to the new cardioid pattern PSW-6 sub-woofer from Meyer Sound Labs. These are good examples of using more power to achieve less off-axis output. You may yet find a gig where the best way to fix a complaint of "It's too loud!" will really be to TURN (some of) IT UP!

Eastern Acoustic Works: http://www.eaw.com
Meyer Sound Labs: http://www.meyersound.com
Renkus Heinz: http://www.renkus-heinz.com/

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