User`s manual

10.4.4 PHASE & GROUP DELAY
We used the term "Frequency Response" to refer to graphs of Fig.10.5 and
Fig.10.8. Frequency is in the x-axis in both figures. The units that respond to
frequency, y-axis, are Volt and Ohm, respectively. Both of them are complex
quantities (have real and imaginary parts) and their magnitude is shown. Doing
this we obtained a very useful piece of information but we lost the original data
(infinite numbers of different real and imaginary part can lead to the same
magnitude). How this information loss will affect your results depends on what you
are going to do with these graphs, or, better still, what the original question you
were trying to answer was. Referring to Fig.10.5. A reasonable question could have
been: how much does "A" filter attenuate a signal at 100Hz in respect to 1kHz? You
go through the graph with the marker and answer 19.3dB. If you have the IEC 651
norm you can go to the "A" filter specs and you will find this attenuation should be
19.1dB +- 0.5dB for type 0 devices. End of your job. Let’s now pose another
question referring to Fig.10.8: by how much would a 10kHz tone would be
attenuated if we wire a 10 ohm resistor in series with that woofer? We are simply
not able to give the correct answer. We need another piece of information, which is
PHASE. Fig.10.19 is the same measure of Fig.10.8 with phase curve overlaid. To
obtain it we just stored the magnitude curve and clicked on the phase button.
Audiomatica Srl MLS - Frequency Response 06/07/2001 15.35.04
CH A Ohm Unsmoothed 51.2kHz 16K Rectangular File: fig4.mlsi
100 1k 10 k 20k20 Hz
150.0
Ohm
180.0
Deg
120.0 108.0
90.0 36.0
60.0 -36.0
30.0 -108.0
0.0 -180.0
CL IO
Figure 10.19
The same principal applies to acoustic devices. Fig.10.20 shows the magnitude
response of a woofer and tweeter in a box without a cross-over network.
Audiomatica Srl MLS - Frequency Response 10/07/2001 18.50.43
CH A dBSPL Unsmoothed 51.2kHz 16K Rectangular File: wooferalone.mls
100 1k 10k 20k20 Hz
110.0
dBSPL
180.0
Deg
100.0 108.0
90.0 36.0
80.0 -36.0
70.0 -108.0
60.0 -180.0
CL IO
Audiomatica Srl MLS - Impulse Response 10/07/2001 18.48.53
CH A dBSPL Unsmoothed 51.2kHz 16K Rectangular File: tweeteralone.mls
0.00 1.6 3.2 4.8 6.4 8.0 9.6 11 13 14 16
ms
1.00
0.80
0.60
0.40
0.20
0.00
-0.20
-0.40
-0.60
-0.80
-1.00
V
CL IO
Figure 10.20 and 10.21
The two measurements were taken from the same microphone position. If we were
to we ask ourselves which is the summed output we could not answer from the
magnitude data alone. Unfortunately acoustic phase is not so easy to handle as
electric phase. We are going to base our example on the tweeter, whose impulse
response and window settings are in Fig.10.21. The procedure for the woofer would
126 10 MLS & LOG CHIRP