Specifications

EAW Smaart 6 Operation Manual Concepts, Glossary, and Bibliography
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2.1.10 Coherence
The coherence for any two FFT frames The Smaart 6 Coherence display represents a
complex mathematical function used to determine the coherence between the two sig-
nals. Smaart displays coherence values as a percentage from 0-100, rather than as a
number from 0-1 because we have found that many people find it more convenient to
think of it as a score than as a probability. The coherence for any two FFT frames mea-
sured using the same input parameters is always 1. It is only when two signals are av-
eraged over some period of time that nonlinear issues appear so the Coherence feature
is disabled when the number of averages is set to 1. Overall coherence tends to decrease
as the number of averages increases.
Additional factors that can adversely affect the coherence of transfer function data in-
clude delay between the two signals, insufficient energy in the reference signal at a given
frequency, acoustical influences (i.e., reflections and reverberation), and ambient or
electrical noise. Nonlinear processors such as compressors and limiters in the measure-
ment signal path can also have a negative influence on coherence and should therefore
be bypassed for Frequency Response and Impulse Response measurements.
2.1.11 Signal Alignment
When performing Frequency Response measurements, it is essential that the measure-
ment and reference input signals be aligned in time. To make a meaningful comparison
of two signals, the transfer function calculation must measure the same piece of each
signal at the same time. With most analog audio equipment, this is not an issue and
compensation is not normally required. However, all digital signal processors have
some propagation delay, as does sound travelling through air to a microphone.
Before a meaningful Frequency Response measurement of a DUT/SUT with propagation
delay can be made, the delay must be found and compensated for. This process requires
finding the delay in the measurement signal and adding it to the reference signal. Smaart
6’s Delay Locator and Internal Delay functions make this process easy (see page 52).