More Power Response

The great Kung Fu movies taught us; power is useless without control. The power generated by a loudspeaker is the end result of an entire sound reinforcement system. Controlling this power by sending it to a paying audience, instead of the rafters, can reduce the truck pack and may even provide good sound in poor venues. Some great ideas have been developed to provide this control in large-scale systems; just don't lose sight of its importance when using smaller loudspeaker systems.

The control over this power has become the defining characteristic in the most recent loudspeaker array developments; the line array and the shaded array. Both depend on achieving the correct physical orientation of the loudspeakers and using sophisticated DSP techniques to extend the directivity of the loudspeakers to very low frequencies. Those big, lumbering lows are always the troublemakers. Able to leap tall horns and walk through walls, bass has the wavelengths to make simple physical measures difficult. Instead, it takes some tricky moves in processing the audio feeds to multiple drivers to bring these bad boys of the audio spectrum into line.

If we look at the on-axis response from a typical two-way sound reinforcement loudspeaker, staring right down the HF horn, we can often achieve a nice flat output across the audio spectrum. However, if we look at the directivity of this loudspeaker, we will find that it will be omnidirectional at low frequencies and the directivity will increase (narrowing the coverage angle) until it becomes unidirectional at high-frequencies. If the bass goes in all directions but we measure a flat frequency response at the front of the device, then the total output from the loudspeaker must have more low-frequency energy than high-frequency energy.

From the perspective of the acoustical consultant, this is the difference between the on-axis frequency response (that is featured in the product brochure) and the power response (that is featured in the reverberation of the venue). I have been in numerous venues that provided excellent acoustics for listening to music that was unamplified but were described as muddy or boomy when a sound reinforcement system was used. The difference was power response.

The designers of most high-quality acoustic instruments consider the balance between the total sound from the instrument and that which is directed out at the audience. The right balance leads to an instrument that is sought after for its tone and clarity. For instance, the audience hears the mixture of the total sound from the piano bouncing off the floor, the upstage wall, and the ceiling in combination with the sound bouncing off the lid. A great piano must not overpower the upper register that depends on the lid reflections with the low notes that are also emitted from under the soundboard and bounce off the stage floor.

The same can be said for good loudspeaker designs. The power response must be balanced and not simply ignored to achieve a flat on-axis response. Poor designs may be acceptable in some background music applications, where no one will listen with a critical ear (or at least to the point of complaining). Loudspeakers with a large imbalance between on-axis and power response may even be acceptable in highly absorptive acoustical conditions, such as fuzzy clubs, cushy theatres and outdoors venues. However, in situations where the sound radiating from (or bending around to) the sides and rear of the loudspeaker will bounce off hard surfaces and eventually reach the listener, it becomes important to consider the power response of the loudspeaker.

The on-axis sound from the loudspeaker will drop in level at a rate of 6 dB (3 dB with true line arrays) with each doubling of distance from the device but the off-axis sound becomes part of the diffuse (reverberant) sound that builds up to be an almost constant volume throughout the venue. This inevitably leads to listeners being seated where the diffuse energy is louder than the direct energy. These listeners have bought seats that are served by the loudspeaker’s bass-heavy power response and not that fantastically flat on-axis response. This gives the distinct impression to the uninformed (yet, often paying) listener that the venue has poor, boomy acoustics.

The solution to this dilemma is to pick the right option for the venue: Option A -- loudspeakers that have an equally flat power response and on-axis response through the use of frequency shading (including some line arrays), or very, very large horns to control those big wavelengths; or Option B -- use loudspeakers with an imbalance between power and on-axis response in venues that do not reflect the off-axis energy back into the audience. Of course, selecting Option B will also impact the sound that reaches the stage mics, which are beside or behind the loudspeaker(s). It can be a dilemma. Diffuse your power wisely.

United Entertainment Media Inc.
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New York, NY, 10016
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FAX 212-378-2160
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