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This week, we talk about focused redirection and scattering of sound to improve your listening experience.

We conclude the discussion of Flutter Echo/Flutter Tones in Home Theater as featured In Home Theater Acoustics Vol. 5 of a five-part series from Home Theater magazine, written by ASC founder & president Art Noxon, PE Acoustical.
~Enjoy!

Diffusion of Flutter

In addition to positioning the speaker to weakly stimulate the distracting flutter tones, another element of acoustics can be brought into the battle and put to good use. Diffusers are devices or surfaces that scatter sound. The home theater ambience speakers are located high on the sidewalls and directed to illuminate the upper outside areas of the front and back walls. The first idea about scattering sound tends to be directed to these areas. Why not add a curved or otherwise irregular surface to these areas of direct illumination?

As it is, we can hear the flutter tone that comes from the ambience speaker because its multiple reflecting wavefront not only shuttles back and forth between the front and back walls, but the wavefront expands while doing so. What we hear is the expanding edge of the flutter echo circuit. Now if we add diffusion to the end walls, we will certainly reduce the time that the flutter tone is sustained because the diffusers are redirecting some of the flutter energy away from the flutter circuit at each reflection. This redirected energy is not absorbed but scattered more fully into the room. That means that the listener is getting an even stronger flutter tone signal than before. Not only does the listener hear the expanding edge of the flutter echo, but now additionally hears the scattered sound off the diffuser. Ironic as it seems, adding diffusers to the end walls is a trade-off treatment with mixed results. The flutter tone becomes louder but shorter-lived. It is a change, but is it an improvement? Better, worse, or merely different, this now is something for you to decide for yourself.

Let’s look at another technique. The flutter echo runs back and forth along the length of the room, hugging the upper sidewall/ceiling corner. Sound-scattering devices can be placed along the upper sidewalls of the room. Again, sound is depleted from the flutter echo circuit. As energy from the flutter echo is redirected into the room, the flutter echo lifetime is reduced. However, this time the scattering takes place between the end wall reflections and not in lumped reflections off the end walls.

These deflectors can be slightly angled down so as to not only kick the reflection to the side, but also downwards. After all, the listener is nearer the floor than the ceiling. Such deflectors are sometimes called ambience kickers in the professional world of recording studios. Another aspect in the setup of these kickers is their spacing. Just as the regular timing of end wall reflections manifests itself to us as a flutter tone, regular timing of reflections off the deflectors can also create a flutter tone. Additionally, we don’t want to place the deflectors so that their signal arrives at the same time as any of the regular flutter echo signals. In such a case, the work accomplished would be minimally different from that by diffusors on the end walls.

Clearly, we won’t want the deflector to be located the same distance towards the front of the room as the distance the ambience speaker is to the rear wall. This would give the same timing to both reflections being received at the listener’s position. The side scattering deflector has to either be in front of or behind this position. Since the ambience speaker is located about 38 percent off the back wall, the ambience kicker should avoid the location of 76 percent off the rear wall. As a first guess, we could locate it almost halfway between, about 52 percent off the rear wall. This produces two new reflections spaced out between the timing of the end wall reflections. The strength of these reflections will be similar to the end wall reflections because of the longer distances involved.

Another deflector could be placed about halfway between the ambience speaker and the rear wall. This one will produce a reflection that arrives somewhat before the rear wall reflection and helps to fill in that big time gap. How many other such ambience kickers can be installed is not so easily predicted. The side fill they produce and its value to the listener belong, in a large degree, to the listener’s taste and judgement.

The sonic impact produced by upper sidewall diffusers is quite different on two levels. First, the scattering reflections are distributed all around the listener rather than coming from just in front of and behind the listener. This more diffuse “source” of the ambience signal seems to promise to be more supportive and involving for the surround sound effect. Second, is the relief provided due to multiple reflections that crop up in between the end wall reflections. These intermediate reflections spoil the perception of the otherwise clear and distinct end wall reflections. The result is that distributed, upper sidewall deflectors produce a signal that masks out the flutter tone. The result is a lively, diffuse, and colorless ambience signal.

Conclusion

Over the last two sections, the dipole ambience speaker has been shown to best be placed about 38 percent of the room length off the back wall, and 20 percent of the room height down from the ceiling. Located directly above it there needs to be a bass trap good through 100 Hz. Along the upper sidewalls there should be distributed a set of ambience kickers. Attend to these details and the ambience speakers can safely play into your. room without inducing coloration or distracting distortions. Only then can the true shading and hue of the signal on the ambience sound track be heard.

Review all 3 parts of this series.