Digital Sound & Music: Concepts, Applications, & Science, Chapter 4, last updated 6/25/2013
Figure 4.26 RT60 reverberation time of small chamber music hall with curtains on some of the walls
220.127.116.11 Frequency Levels and Comb Filtering
When working with sound in acoustic space, you discover that there is a lot of potential for
sound waves to interact with each other. If the waves are allowed to interact destructively –
causing frequency cancelations – the result can be detrimental to the sound quality perceived by
Destructive sound wave interactions can happen when two loudspeakers generate
identical sounds that are directed to the same acoustic space. They can also occur when a sound
wave combines in the air with its own reflection from a surface in the room.
say there are two loudspeakers aimed at you, both generating the same sound.
Loudspeaker A is 10 feet away from you, and Loudspeaker B is 11 feet away. Because sound travels
at a speed of approximately one foot per millisecond, the sound from Loudspeaker B arrives at your
ears one millisecond after the sound from Loudspeaker A, as shown in
millisecond of difference doesn’t seem like much. How much damage can it really inflict on your
Let’s again assume that both sounds arrive at the same amplitude. Since the position of
your ears to the two loudspeakers is directly related to the timing difference, let’s also assume
that your head is stationary, as if you are sitting relatively still in your seat at a theater. In this
case, a one millisecond difference causes the two sounds to interact destructively. In Chapter 2
you read about what happens when two identical sounds combine out-of-phase. In real life, phase
differences can occur as a result of an offset in time. That extra one millisecond that it takes for
the sound from Loudspeaker B to arrive at your ears results in a phase difference relative to the
sound from Loudspeaker A. The audible result of this depends on the type of sound being
generated by the loudspeakers.