User Guide

Sound System Design Reference Manual
theory allows us to make simple calculations
regarding the behavior of sound in rooms and arrive
at results sufficiently accurate for most noise control
and sound system calculations.
Going back to our model, consider what
happens when the sound source is turned off.
Energy is no longer pumped into the room.
Therefore, as a certain amount of energy is lost with
each reflection, the energy density of the sound field
gradually decreases until all of the sound has been
absorbed at the boundary surfaces.
Figure 5-6 gives a simple picture of this in
idealized form. In the left graph, the vertical axis
represents total sound energy in the room and the
horizontal axis represents some convenient time
scale. From the instant the sound source is turned
on, the total energy in the room increases until it
gradually levels off at a steady state value. Once this
balance has been achieved, the sound source is
turned off and the total energy in the room decreases
until all of it has been absorbed. Note that in this
idealized picture, growth and decay are exponential
functions. The curve is exactly the same as the
familiar graph of the charging and discharging of the
capacitor.
5-6
Figure 5-6. Idealized growth and decay of sound energy in an enclosure
Figure 5-7. Actual chart recordings of decay of sound in a room