Hi Raimund--
1 meter it is. Your take on distance might be a little looser from
what I interpreted. As the goal is to establish file saturation for a
stimulus that is as close to 64 dBA that a poor person can afford
($50 meter accuracy of +/- 2dB :-) ), distance seems to have the
potential to swing the reference significantly. For example, some of
the mic Hz response tests I read used .5 meter in anechoic chambers
and in contrast I would normally use 4 or more meters for testing
localization and Hz response outdoors. Those examples compute to a
difference in amplitude of 18 dB.
http://www.sengpielaudio.com/calculator-distance.htm Of course, the
distance of low Hz weighted background ambience is effectively to the
sound horizon in the field, but we are trying to simulate this
diffuse source with a speaker that is much closer. Rob D.
= =
At 3:59 PM +0000 11/30/10, Raimund wrote:
>Hi Rob,
>
>Besides the (minor) potential errors due to reflections off the
>environment, the distance does not matter for your calibration
>procedure. This is true as long as you place the microphone and the
>SLM at exactly the same spot (and keep the distance from the sound
>source fixed).
>
>If you however wish to quantify the noise that is emitted by a
>particular sound source, then it is important to specify also the
>distance (1 meter is a commonly used value such measurements)
>because the sound level will of course change according to that.
>
>Regards,
>Raimund
>
>--- In
><naturerecordists%40yahoogroups.com>
>Rob Danielson <> wrote:
>>
>> Hi Bob, Raimund, Greg and Prataps--
>>
>> For several reasons including the ones Bob notes, we know that mic to
>> speaker distance will make a difference in the critical step of 64 dB
>> amplitude calibration. I looked through three SLM manuals and
>> couldn't find anything about what distance to measure isolated, loud
>> sound sources. (In most applications, measurements are taken where
>> background levels are perceived). Doesn't 1 meter replicate usual mic
>> sensitivity test conditions? I looked for a while could not find a
>> succinct specification, but the distances were all on the order of 1
>> meter. At this short distance, if I place the sound source and mics
>> at 7 feet above thick grass, reflection impacts should be negligible-
>> especially considering my $50 Nady ASM-2 meter is +/- 2dB! I noticed
>> that some of the more expensive meters we've discussed are +/- 1.5dB.
>>
>> It would also be interesting to play the same sound samples in one's
>> sound studio (same speaker) at 1 meter just to see what differences
>> crop up.
>>
>> Its #4 on my current test list. I was thinking of using AT4022's and
>> my SD744T and record level "60.5 dB" at a distance of 1 meter if
>> anyone wants to get started. Rob D.
>
>
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