Speaker/Listener Locations

Modeling, the Boundary Effect, Soundstage, etc

In the last installment we learned that room dimensions call out resonant modes that can influence the bass response of a room. We learned that is impossible to avoid them, but that they can be controlled. There are three things we should incorporate into the design in order to accomplish this goal: (1) room dimensions that distribute the modes evenly; (2) incorporate appropriate acoustic treatments; and (3) place speakers and listeners in areas that will not exasperate them. Understand that room dimensions will have an audible effect on frequencies around 300 Hz. and below (the lower half of a piano), but acoustic treatment and speaker/listener location can affect all frequencies. In order to better understand this, try a simple experiment: While talking out loud, gradually walk into the corner of a room and notice how resonant your voice becomes. This is frequency reinforcement of room modes and reflections. Try pronouncing “shshshshsh” into the flat palm side of your hand while slowly bringing it up to your nose. What you are observing is the tone changing because of the sound reflecting back and interfering with the original sound. These are constructive and destructive phase anomalies- the same principle experienced with the low frequency room modes, only at higher (shorter) wavelengths.

Not only do we experience changes in tonality because of these reflections, but also changes in spatial cues. If we see the image of someone closing a cupboard on the screen, the sound should be located in that space of the soundstage. If we have a reflection from the speaker on the left wall, the sound image will appear to be more left than it was intended to be and no longer synchronized with the visual image on the screen. The reflection pulls the sound image more to the left because our brain processes both the original signal and the reflected signal as one event. When the sound is longer in duration, like a musical instrument, and you have such a reflection, the tonality can become altered to the extent that you may find it difficult to distinguish the difference between a Gibson 335 and Fender Stratocaster. These same effects are experienced when a listener or speaker is too close to a wall. These distortions add up to an unconvincing experience and you are not even aware of what you’re missing. Remember that the hierarchy is: set-up, calibration, room acoustics and equipment. Great equipment cannot sound great in a poor environment.

In order to become fully immersed in a movie, we have to remove all audio and visual distortions. We have to make the room disappear. How do you do that? You hire a specialist who uses his knowledge and computer modeling to design your room. The room dimensions must be designed to distribute modes as evenly as possible within the given space and still serve its purpose and budget. The room must be designed so that noise from the theater doesn’t escape to bother others elsewhere in the house, and at the same time, household appliances, footfalls, outside traffic, etc. does not distract those in the theater. Listeners and speakers must be located where they will avoid problematic modes so that certain low frequencies are not overly loud or soft. Speakers and listeners must be positioned proportionally so that a large, solid soundstage can occur. Reflections from each speaker on each surface should be treated with absorption or diffusion. Other acoustic treatments should be engineered to control reverberation times and room modes. HVAC should be modeled so that it is not heard and yet keeps the room a comfortable temperature. The screen and projector should be placed and sized for the best resolution possible. The video should integrate seamlessly with the audio. These are the basics for a great movie experience. A performance with dynamics, tonal balance, imaging and soundstage better than you’ve experienced at any movie house. Just sit back, relax and loose yourself in the movie.

Next topic: Screen Location, Size and Type – How to have a better picture for less $.

Norman Varney is the owner of A/V Room Service Ltd.