Metric Halo SpectraFoo - Manual (Page 70)

The Transfer Function
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This provides us with the key to understanding how to use the coherence. The coherence tells us where the
measurement that we have made is valid. It is basically a measure of the quality of the measurement at the
corresponding frequency.
In terms of correcting the response of acoustic systems, the coherence tells us which notches or dips in the
response curve are Equalizable and which are due to nulls in the measurement. If the coherence is low at a
dip in the response curve, we will not be able to use an equalizer to fix it.
If the null is due to the specific position of the measurement microphone, we can generate a better averaged
response by making measurements at a variety of microphone positions and then averaging the measurements.
This technique is described in more detail later.
On the other hand, if the coherence remains low for a variety of microphone positions, the cancellation of the
source signal is not due to a specific reflection, but is actually part of the system response. In order to correct
these types of problems, either the configuration of the system must be changed (e.g. moving the position of
the speakers or adjusting the relative time delay of different clusters in the system) or the configuration of the
acoustic space will have to be changed (removing reflections, adding diffusion, etc.).
About the Response Curves
If the SUT is a simple processor (like an equalizer) the power response tells us the effect of the processor on
the signal. Hopefully the response matches the expected response. In any case, the measured response is the
actual response of the processor.
If the SUT is a sound reinforcement system, the expected (or more to the point, desired) response of the SUT
is that of a wire. The system should, ideally, reproduce the signal exactly without any coloration. No real
system will do this, but often the problems in a sound system are correctable. The data provided by the transfer
function allows us to determine:
1. What parts of the system are correctable with equalization.
2. What equalization is required to correct the system.
As described above, the coherence allows us to determine what is correctable with equalization. The power
response tells us what equalization is required to correct the system. The phase response can help determine
what type of equalization will best solve the problem and also help determine if other phase changing com-
ponents (like the crossovers) of a system are causing problems. It will also help you determine if the drivers in
a speaker (or a cluster of speakers) are properly time-aligned (as shown before).
The basic system configuration that we will use to measure and correct an acoustic reproduction systems is:
Figure 4.29: Measurement and Correction setup