Digital Sound & Music: Concepts, Applications, & Science, Chapter 4, last updated 6/25/2013
would never overlap with another. In the real world, loudspeaker coverage is very difficult to
control. We discuss this further and demonstrate how to compensate for comb filtering in the
video tutorial entitled "Loudspeaker Interaction."
Comb filtering in the air is not always the result of two loudspeakers.
The same thing can happen when a sound reflects from a wall in the room and
arrives in the same place as the direct sound. Because the reflection takes a
longer trip to arrive at that spot in the room, it is slightly behind the direct
sound. If the reflection is strong enough, the amplitudes between the direct and
reflected sound are close enough to cause comb filtering. In really large rooms,
the timing difference between the direct and reflected sound is large enough
that the comb filtering is not very problematic. Our hearing system is quite
good at compensating for any anomalies that result in this kind of sound
interaction. In smaller rooms, such as recording studios and control rooms, it’s
quite possible for reflections to cause audible comb filtering. In those situations, you need to
either absorb the reflection or diffuse the reflection at the wall.
The worst kind of comb filtering isn’t the kind that occurs in the air but the kind that
occurs on a wire. Let’s reverse our scenario and instead of having two sound sources, let’s switch
to a single sound source such as a singer and use two microphones to pick up that singer.
Microphone A is one foot away from the singer, and Microphone B is two feet away. In this
case, Microphone B catches the sound from the singer one millisecond after Microphone A.
When you mix the sounds from those two microphones (which happens all the time), you now
have a one millisecond comb filter imposed on an electronic signal that then gets delivered in
that condition to all the loudspeakers in the room and from there to all the listeners in the room
equally. Now your problem can be heard no matter where you sit, and no matter how much you
move your head around. Just one millisecond delay causes a very audible problem that no one
can mask or hide from. The best way to avoid this kind of problem is never to allow two
microphones to pick up the same signal at the same time. A good sound engineer at a mixing
console ensures that only one microphone is on at a time, thereby avoiding this kind of
destructive interaction. If you must have more than one microphone, you need to keep those
microphones far away from each other. If this is not possible, you can achieve modest success
fixing the problem by adding some extra delay to one of the microphones. This changes the
phase effect of the two microphones combining, but doesn’t mimic the difference in level that
would come if they were physically farther apart.
188.8.131.52 Resonance and Room Modes
In Chapter 2, we discussed the concept of resonance. Now we consider how resonance comes
into play in real, hands-on applications.
Resonance plays a role in sound perception in a room. One practical
example of this is the standing wave phenomenon, which in an acoustic space
produces the phenomenon of room modes. Room modes are collections of
resonances that result from sound waves reflecting from the surfaces of an
acoustical space, producing places where sounds are amplified or attenuated.
Places where the reflections of a particular frequency reinforce each other,
amplifying that frequency, are the frequency’s antinodes. Places where the
frequency’s reflections cancel each other are the frequency’s nodes. Consider
this simplified example – a 10-foot-wide room with parallel walls that are good sound reflectors.