User`s manual

response can be obtained from the magnitude response by calculation. Another
kind of phase (we promise it is the last one), is Excess Phase. This is the algebraic
difference between true phase, as in Fig.10.22, and minimum phase. It is exactly
what we need to separate the time of flight from the devices own phase response.
We won’t use excess phase directly here but a post process of it, Excess Group
Delay. Fig.10.25 is the excess group delay of our tweeter vs. frequency.
Audiomatica Srl MLS - Frequency Response 10/07/2001 18.48.53
CH A dBSPL Unsmoothed 51.2kHz 16K Rectangular File: tweeteralone.mls
100 1k 10k 20k20 Hz
110.0
dBSPL
7.50
ms
100.0 6.00
90.0 4.50
80.0 3.00
70.0 1.50
60.0 0.00
CL IO
Figure 10.25
It was obtained by selecting Excess in Drop Down Menu. This graph represents the
distance of the sound source from the microphone vs. frequency. As long as the
distance is constant the system is minimum phase and we are in the position of a
well defined acoustic center. If you recall from previous paragraphs we have reliable
data down to 200Hz because of the time windows. As we deal with a tweeter we
will consider the 2k-20k frequency range where the marker reads a constant
2.01ms. We will use this value to operate a time shift that removes the sound flight
time. This is accomplished from the Processing Tools Dialog, selecting Time shift
and typing the value we found as in Fig.10.26.
Figure 10.26
Clicking OK we can finally display the processed phase, minimum phase and
magnitude response of our device in Fig.10.27. We used the term "processed
phase" here and this is not casual.
128 10 MLS & LOG CHIRP