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Proper Loudspeaker Placement: How To Avoid Lobes and Nulls

Not every issue can be resolved with that handy equalizer...

By Pat Brown January 8, 2016

Excessive frequency notching can ruin the sound of the system. Why filter out sound that needs to be there?

3. Conclude that humans aren’t all that sensitive to narrow notches in the spectrum, so the comb filters are just something that we can live with.

This is rationalizing the problem and is simply not true. It’s usually the explanation provided by someone who is responsible for the problem in the first place!

4. Get out the old one-third octave real-time analyzer. You can’t see the comb filters on it.

For many years, audio professionals did not have high-resolution analyzers that could identify arrival time problems. The system response looked fine on a one-third octave analyzer, but it still sounded bad.

Today’s analyzers are vastly more powerful and can reveal much more about the nature of a sound problem.

5. Inform the owner that the current loudspeaker placement has created some problems that cannot be “corrected” electronically. The only real solution is to relocate the existing loudspeakers or redesign the array.

Figure 4: Placing a greater physical distance between the loudspeakers.

Unfortunately, the sad reality is that only the last option is likely to fix the system.

An acoustic comb filter is a symptom of a more significant problem. When two loudspeakers are placed in close proximity, the resultant distance offset will cause “lobing” in the speaker’s radiation pattern.

Lobes can be described as “fingers” of sound pressure “maximums” in the three-dimensional space surrounding the array.

The fingers are separated by nulls or axis of minimal sound pressure level. The fingers typically cause problems with microphones, since a mic is likely to feedback when it is placed within a lobe.

Figure 5: Use of aggressive pattern control to reduce the overlap.

The nulls cause problems for the audience, since parts of the audio spectrum that are critical for speech intelligibility (understanding the words) are cancelled at some listener’s seats.

When a series of these lobes and nulls exist, the visual representation of the frequency response at one listener position will resemble the teeth of a comb, with a sequence of peaks and valleys.


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About Patrick

Pat Brown
Pat Brown

Principals, Synergetic Audio Concepts
   
Pat & Brenda Brown lead SynAudCon, conducting audio seminars and workshops online and around the world. For more information go to www.prosoundtraining.com.
http://www.prosoundtraining.com

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