Digital Sound & Music: Concepts, Applications, & Science, Chapter 4, last updated 6/25/2013
explain the loss of perceived sound level. Even if human bodies are absorbing some of the
sound, the sound arriving at the ears directly from the loudspeaker, with no intervening
obstructions, arrives without having been dampened by absorption. It’s the reflected sound that
gets quieter. Also, most theater seats are designed with padding and perforation on the underside
of the seat so that they absorb sound at a similar rate to a human body. This way, when you’re
setting sound levels in an empty theatre, you should be able to hear sound being absorbed the
way it will be absorbed when people are sitting in those seats, allowing you to set the sound
properly. Thus, absorption can’t be the only reason for the sudden drop in sound level when the
listeners fill the audience. Temperature is also a factor here. Not only is the human body a good
absorber of acoustic energy, but it is also very warm. Fill a previously empty audience area with
several hundred warm bodies, turn on the air conditioning that vents out from the ceiling, and
you’re creating a temperature gradient that is even more dramatic than the one that is created
outdoors at sundown. As the sound wave front travels toward the listeners, the air nearest to the
listeners allows the sound to travel faster while the air up near the air conditioning vents slows
the propagation of that portion of the wave front. Just as in the outdoor example, the wave front
is refracted upward toward the cooler air, and there may be a loss in sound level perceived by the
listeners. There isn’t anything that can be done about the temperature effects. Eventually the
temperature will even out as the air conditioning does its job. The important thing to remember is
to listen for a while before you try to fix the sound levels. The change in sound level as a result
of temperature will likely fix itself over time. Modifying and Adapting to the Acoustical Space
An additional factor to consider when you're working with indoor sound is the architecture of the
room, which greatly affects the way sound propagates. When a sound wave encounters a surface
(walls, floors, etc.) several things can happen. The sound can reflect off the surface and begin
traveling another direction, it can be absorbed by the surface, it can be transmitted by the surface
into a room on the opposite side, or it can be diffracted around the surface if the surface is small
relative to the wavelength of the sound.
Typically some combination of all four of these things happens each time a sound wave
encounters a surface. Reflection and absorption are the two most important issues in room
acoustics. A room that is too acoustically reflective is not very good at propagating sound
intelligibly. This is usually described as the room being too “live.” A room that is too
acoustically absorptive is not very good at propagating sound with sufficient amplitude. This is
usually described as the room being too “dead.” The ideal situation is a good balance between
reflection and absorption to allow the sound to propagate through the space loudly and clearly.
The kinds of reflections that can help you are called early reflections, which arrive at the
listener within 30 milliseconds of the direct sound. The direct sound arrives at the listener
directly from the source. An early reflection can help with the perceived loudness of the sound
because the two sounds combine at the listener’s ear in a way that reinforces, creating a
precedence effect. Because the reflection sounds like the direct sound and arrives shortly after
the direct sound, the listener assumes both sounds come from the source and perceives the result
to be louder as a result of the combined amplitudes. If you have early reflections, it's important
that you don’t do anything to the room that would stop those early reflections such as modifying
the material of the surface with absorptive material. You can create more early reflections by
adding reflective surfaces to the room that are angled in such a way that the sound hitting that
surface is reflected to the listener.
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