Digital Sound & Music: Concepts, Applications, & Science, Chapter 4, last updated 6/25/2013
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that a frequency response graph is generated, using the same hardware and software. The
impulse response graph (or simply impulse response) has time on the x-axis and amplitude of
the audio signal on the y-axis. It is this graph that helps us to analyze the reverberations in an
acoustic space.
An impulse response measured in a small chamber music hall is shown in Figure 4.23.
Essentially what you are seeing is the occurrences of the stimulus signal arriving at the
measurement microphone over a period of time. The first big spike at around 48 milliseconds is
the arrival of the direct sound from the loudspeaker. In other words, it took 48 milliseconds for
the sound to arrive back at the microphone after the analysis software sent out the stimulus audio
signal. The delay results primarily from the time it takes for sound to travel through the air from
the loudspeaker to the measurement microphone, with a small amount of additional latency
resulting from the various digital and analog conversions along the way. The next tallest spike at
93 milliseconds represents a reflection of the stimulus signal from some surface in the room.
There are a few small reflections that arrive before that, but they’re not large enough to be of
much concern. The reflection at 93 milliseconds arrives 45 milliseconds after the direct sound
and is approximately 9 dB quieter than the direct sound. This is an audible reflection that is
outside the precedence zone and may be perceived by the listener as an audible echo. (The
precedence effect is explained in Section 4.2.2.6.) If this reflection is to be problematic, you can
try to absorb it. You can also diffuse it and convert it into the reverberant energy shown in the
rest of the graph.
Figure 4.23 Impulse response of small chamber music hall
Before you can take any corrective action, you need to identify the surface in the room
causing the reflection. The detective work can be tricky, but it helps to consider that you’re
looking for a surface that is visible to both the loudspeaker and the microphone. The surface
should be at a distance 50 feet longer than the direct distance between the loudspeaker and the
microphone. In this case, the loudspeaker is up on the stage and the microphone out in the
audience seats. More than likely, the reflection is coming from the upstage wall behind the
loudspeaker. If you measure approximately 25 feet between the loudspeaker and that wall,
you’ve probably found the culprit. To see if this is indeed the problem, you can put some
absorptive material on that wall and take another measurement. If you’ve guess correctly, you
should see that spike disappear or get significantly smaller. If you wanted to give a speech or
perform percussion instruments in this space, this reflection would probably cause intelligibility
problems. However, in this particular scenario, where the room is primarily used for chamber
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