Speakers For Reverberant Spaces

by Richard Honeycutt

This article investigates this often used sales pitch “line arrays are the speaker design of choice for reverberant spaces”.

One of the persistent dangers of sound-system design by the marginally qualified is confusing rumors that “everybody knows” are true (which often began as convincing sales pitches) with real information. One such rumor that has been around for several years now is “line arrays are the speaker design of choice for reverberant spaces”. Like all oversimplifications, this one is often not true.

Let’s remember what line arrays do well: they provide good vertical control, and they give wide horizontal coverage. (Yes, a couple of designs do provide some horizontal control as well, but at present, these are in a distinct minority.) The vertical control is credited for “keeping excess sound from exciting the reverberant field”, and thus improving intelligibility. Let’s compare three respected professional speaker systems: the Bose MA12 line array, the Renkus-Heinz STX7/94 combination horn/direct radiator, and the Danley SH96 all-horn system. Figure 1 shows the frequency-dependent Q of each of these speakers, based upon the manufacturers’ published data.

Figure 1 shows the frequency-dependent Q of each of these speakers.

Figure 1

Naturally all of them have lower Q at lower frequencies, where the speaker size is a smaller fraction of the wavelength. Since Q depends upon both horizontal and vertical coverage angle, the speakers that employ horns have a higher Q at frequencies where the HF horn controls the directivity.

Now let’s see what the calculated %ALcons looks like for each of these speakers. The calculations are based upon a 241,164-cf room, a listener distance of 60 ft, and these RT’s:

Figure 2 - Graph showing calculated %ALcons

Figure 2

While not really a nightmare, this is certainly a challenging room in terms of intelligibility, and it is not out of the ordinary for some churches. The calculations assume that the M factor is unity (omni microphone and uniform distribution of absorption). This is really conservative, since if the speaker is aimed correctly and an audience is present, much more of the “first-hit” sound will be absorbed by the audience than by the walls, increasing Ma. Also, usually, a directional microphone will be used, increasing Me. An absorptive rear wall in the room will also increase Ma. All of these factors will reduce %ALcons by the same percentage for each speaker. In each case, only one speaker is used (N=1). Looking at the intelligibility performance of these speakers, we find:

Graph showing the intelligibility performance

Figure 3

Note that among these three speakers, the all-horn speaker performs best (lowest %ALcons), and the horn/direct radiator hybrid is second, with the line array third. If we look just at the 500-2000-Hz octave bands, the %ALcons scores are:

  • SH-96 7.37
  • STX7/94 7.43
  • MA12 8.43

This is not a lot of difference, but it makes the point that once again, the rumor-mongers bite the dust.

Three points should be mentioned:

  • (1) The nominal directivity of the STX7/94 is 90X40; while the nominal directivity of the SH-96 is 90X60. These are not identical, but are as close as I could come using these manufacturers’ standard products.
  • (2) The rooms for which line arrays are often the optimal choice are wide rooms that are not very deep. In such a room, other types of speakers may well require multi-speaker clusters, and the game then changes.
  • (3) If a line array is chosen, it will often be mounted near ear height, necessitating a very absorptive rear wall, which, in turn, will help intelligibility by improving Ma.

Richard A. Honeycutt developed an interest in acoustics and electronics while in elementary school. He assisted with film projection, PA system operation, and audio recording throughout middle and high school. He has been an active holder of the First Class Commercial FCC Radiotelephone license since 1969, and graduated with a BS in Physics from Wake Forest University in 1970, after serving as Student Engineer and Student Station Manager at 50-kW WFDD-FM.  His career includes writing engineering and maintenance documents for the Bell Telephone System, operating a loudspeaker manufacture company, teaching Electronics Engineering Technology at the college level, designing and installing audio and video systems, and consulting in acoustics and audio/video design. He earned his Ph.D. in Electroacoustics from the Union Institute in 2004. He is known worldwide as a writer on electronics, acoustics, and philosophy. His two most recent books are Acoustics in Performance  and The State of Hollow-State Audio, both published by Elektor.