CLIOwin 7 PCI User's Manual - Audiomatica

Now things look much better and this is almost the anechoic response of the speaker.
However nothing comes for free. The low frequency part of the response seems quite
optimistic for such a little speaker. The price we paid in setting the impulse tail to 0 is
that we lost information on the lower part of the spectrum. The transition frequency
between meaningful and meaningless data is calculated as 1 divided by the selected
impulse length. In our case we selected a 6.8ms long impulse. 1/0.0068=147Hz right?
Wrong. We have to remember the first 2 ms of the impulse, which is the time the sound
takes to reach the microphone and hence does not carry any information. We could
have selected the impulse as in Fig.10.18 without affecting the frequency response at
all however phase response would have been greatly affected.
0.50
V
CLIO
0.40
0.30
0.20
0.100
0.00
-0.10
-0.20
-0.30
-0.40
-0.50
0.00 1.1 2.3 3.4 4.6 5.7 6.8 8.0 9.1 10 11
ms
Figure 10.18
The right calculation is 1/(0.0068-0.002)=208.33Hz. In our room the smallest
dimension is floor to ceiling. This is indeed the most frequent case. This dimension is
however 4m. The best location for the speaker would have been at 2m both from the
floor and the ceiling. The second consideration is microphone distance. The further away
it is, the more you have to subtract from the impulse length due to sound travel time
to the microphone. In practice we do not encourage distance below 70cm for complete
speaker measurement and you should increase to 1m for bigger ones. However single
driver measurement can take advantage from a reduced distance.
Chapter 10 - MLS 119