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A Sonic Primer: When The Volume Knob Goes To The Right

The basics of audio principles and sound system design in layman's language.

At this point, the reader may sum up that a good loudspeaker layout will provide adequate loudness to those farthest from the sound source while not causing discomfort to those closest to the sound source. So how do we achieve this? The key lies in how high we position the loudspeaker.

Figure 5 – A flown loudspeaker.

The ideal recommendation would be to fly the loudspeaker as illustrated in Figure 5. In our past experiences working with small houses of worship, some may be able to retain a contractor/system integrator to fly the loudspeaker safely and do the wiring/cabling. D-I-Y is certainly not recommended due to the safety factor. By raising the loudspeaker and orienting the front projection axis toward the intended farthest audience, it creates a smaller difference in relative distance between the front and back row audience to the loudspeaker. Positioning the loudspeaker this way will result in a more even sound coverage.

The small-scale sound system, where loudspeakers can practically only be placed on a pole can pose a limitation. If the stage/platform is raised, putting the loudspeaker stand on the stage can help to raise the loudspeaker height. To further improve the loudspeaker aiming, practitioners can also experiment using pole mounting with a built-in down-tilt feature. Figure 6 is an illustration from QSC K series where users have a choice of placing the loudspeaker straight or with a slight down-tilt on a pole. This can be done just by rotating the pole mount socket under the loudspeaker.

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Figure 6 – Pole mounted loudspeaker with and without down-tilt.

How About Subwoofer(s)?

The same principle applies to subwoofer. By locating a subwoofer under the stage or in the front-side of the stage, the front row audience may receive louder bass relative to the back row due to the distance. Since the low frequency distribution is heavily affected by the room/walls and flying a subwoofer can be an expensive addition, it is generally safer to keep the subwoofer on the floor. The uneven distribution of low frequencies is not a major concern because the human hearing system is less sensitive to low frequency. For a small-scale sound system, having an evenly distributed loudness in the voice range is much more important than fighting to have an even bass throughout the audience area.

A Long Room With Low Ceiling Height

In general, if the back row (farthest audience to the loudspeaker) is less than three times to the front row (closest audience to the loudspeaker), the level difference between front and back row may still be OK. When a small-scale sound system is deployed in a long room with a low ceiling height, the loudspeaker’s throw distance to the back row can be more than three times that of the throw distance to the front row. This layout isn’t preferred and delayed loudspeaker(s) are needed. Delayed loudspeakers refer to additional loudspeaker(s) that are deployed ‘in the middle’ of the audience area. This loudspeaker layout is also typically known as a zoned distributed system, and is also applicable for pole-mounted loudspeakers.

Side note: a loudspeaker’s required throw distance typically refers to the distance between the loudspeaker and the intended farthest audience to cover.

Figure 7 – A distributed system.

In Figure 7, the reader can notice that the throw distance ratio of each loudspeaker to its farthest/closest audience is less than 3:1. Each loudspeaker’s throw distance is illustrated by the green lines. However, there is a caveat: the front loudspeaker(s) will also spray sound to the back rows. Therefore, the half-back audience will receive sound from two sound source locations. This can create an interference problem which may degrade speech intelligibility or sound quality. The loudspeaker(s) that are deployed ‘in the middle’ of the audience area must be delayed (hence the name: delayed loudspeaker or delay ring). The delay time can be added via a digital signal processor (DSP) or by what is commonly known as loudspeaker management system (LMS) processor. The detail of the DSP setup is beyond the scope of this article, but we hope this information can direct the audio practitioners to the right direction when thinking about loudspeaker layout.

What Was Being Said?

Speech intelligibility refers to whether the messages played back from the sound system can be understood well enough for the listener to re-tell. Several factors play a part in this. Although a small-scale sound system is usually deployed in a small to medium room, the amount of the absorptive materials plays a key role. When the sound source stops and does not decay fast enough, the room is usually referred as “live” or “echoic” or in a more technical term: reverberant. This happens due to the lack of absorptive materials such as carpet, acoustical ceiling tiles, seat cushions, large cloth decorative banners, etc. Let’s discuss this from a different perspective to understand the proper loudspeaker selection.

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Figure 8 – Reading in a large dark room.

Figure 8 shows an effort to read in a large dark room. The darkness will not help the reading effort. Then a small bulb is turned on as illustrated in Figure 9. Can you read in this condition? Well, yes and no. When you get closer to the bulb, then it will aid the reading effort. However, if you are standing at the other far end of the room, reading may still not be possible. The next option is using a spotlight. Figure 10 illustrates the use of spotlight at some distance away. This latter arrangement can aid the reading effort if the reading location is inside the light beam coverage of the spotlight.

Figure 9 – A small bulb is turned on to aid the reading effort.
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Figure 10 – A spotlight is turned on at some distance away to aid the reading effort

Side note: reverberation time is the time needed for sound to decay 60dB after it stops playing. When a reverberation time is longer than 2 seconds, it can degrade speech intelligibility.

Loudspeakers have some degree of sound focusing, or in a more technical term: directivity. A loudspeaker with low directivity will radiate sound everywhere, whereas a high directivity loudspeaker will focus the sound dispersion (typically at mid to high frequencies) to its front side.

A typical trapezoidal loudspeaker is usually a point source type. Point source type refers to a loudspeaker that disperses sound like a sphere that gradually expands. When the size of a point source loudspeaker is large, it may incorporate a large horn. The large horn can help to increase the directivity of the loudspeaker. The use of a point source is shown in the top pictures of Figure 11.

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Figure 11 – Bottom pictures: Using high directivity loudspeakers (column loudspeakers) near the front-side of the stage; Top pictures: using regular trapezoidal loudspeakers (point sources) flown in the peak of the ceiling. Yellow rectangles show the loudspeaker locations. The left pictures show the direct sound coverage on the walls and audience areas. The right pictures show the direct sound coverage on the ceiling. The sound level prediction is rated high to low in the following order: yellow – pink – red – cyan (light blue).

Point source loudspeakers were simulated in the 3D computer model software as if they’re installed near the peak of the ceiling. The pink and red-colored surfaces show where the majority of the sound hit the boundaries. The reader can notice that the ceiling is receiving a lot of direct sound (Figure 11, top right picture). Some examples of typical point source loudspeakers can be seen in Figure 12.

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Figure 12 – Family of point source loudspeakers, courtesy of JBL.
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