Measurements on the speakers

[Jean-Marc Valin]

Jim Gettys wrote:
> (Some of) the microphones in B2 are really lousy: B3 should improve the
> situation on the mikes quite a bit.
>                          - Jim

>From my experience, even cheap electret microphones (<<$1) tend to
perform much better than loudspeakers, both in terms of frequency
response and harmonic distortion. The only thing I think is important --
if it's not the case already -- would be to use figure-of-eight
directional microphones so that the loudspeakers end up being in the
(theoretical) null of the microphone. That would minimise the direct
speaker->mic path.

	Jean-Marc

> 
> On Fri, 2007-05-11 at 11:19 +1000, Jean-Marc Valin wrote:
>> Hi,
>>
>> I just made some impulse response and distortion measurements on the
>> speaker->microphone path. The measurements are shown in figures
>> http://people.xiph.org/~jm/olpc_distortion.png
>> http://people.xiph.org/~jm/olpc_distortion2.png
>>
>> Both figues have the same data. The only difference is the frequency
>> axis: first figure has the frequency of the harmonic on the horizontal
>> scale, while the second one has the frequency of the fundamental that
>> caused it. e.g. for a 1 kHz tone, you need to read the harmonics at 2,
>> 3, 4, 5 kHz in the first figure, while for they're all aligned at 1 kHz
>> in the second one. (second figure is easier to read)
>>
>> The main things that strike me from these figures is that there are
>> resonances in the fundamental around 3-6 kHz and which cause quite a bit
>> of harmonic distortion. The distortion is also especially significant
>> because it happens at a "normal" volume. It's measured using a sine
>> sweep from 40 Hz to 24 kHz as explained in:
>> http://pcfarina.eng.unipr.it/Public/Papers/134-AES00.PDF
>> The sine waves were nearly full amplitude and the alsamixer volume for
>> Master and PCM were both set to 61.
>>
>> In case anyone's interested, the raw data is available at
>> http://people.xiph.org/~jm/response and requires the paper above to
>> interpret. I'm still not sure what can (or can't) be done to improve the
>> situation. Any thoughts? Oh, and I was just wondering, what kind of
>> microphone is the BT2 using (omni, figure-of-eight, cardioid, ...)?
>>
>> Cheers,
>>
>> 	Jean-Marc
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