User`s manual

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.
Audiomatica Srl MLS - Impulse Response 07/07/2001 10.15.18
CH A dBSPL Unsmoothed 51.2kHz 16K Rectangular File: grid.mls
0.00 1.1 2.3 3.4 4.6 5.7 6.8 8.0 9.1 10 11
ms
0.50
0.40
0.30
0.20
0.100
0.00
-0.10
-0.20
-0.30
-0.40
-0.50
V
CL IO
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.
10 MLS & LOG CHIRP 125