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Re: Responses to ambient sound recording

Subject: Re: Responses to ambient sound recording
From: "Rob Danielson" danielson_audio
Date: Tue Nov 30, 2010 7:25 am ((PST))
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.

At 12:40 PM +0000 11/30/10, Avocet wrote:
>>  Is there an opinion about whether the
>>  speaker should be set-up at 1 meter?
>The mic to speaker placing could be critical. At 1 metre you would
>still have phase relationships with different parts of the speaker
>cone, box resonances, and reflex ports if any, but ground reflections
>would be minimised.
>As an overall check on resonances and reflections try recording a slow
>sweep tone across the whole range. My guess is that this will be very
>The mics will also be picking up a ground reflection of the speaker
>depending on the ground surface. It may well be useful to baffle off
>the direct sound, reflecting it into the air, to see what sort of
>reflections they are picking up. Soft grass is a good absorptive
>surface for most frequencies.
>To minimise standing wave patterns mic testing is usually done with
>warble tones which is why frequency response curves are usually
>smoothed out. I've got a combination bandwidth limited 800Hz to 1200
>Hx white noise and warble at:
>I made this with Audacity 1.3 which has an easy Equalization function.
>This will copy exactly end to end or you can use loop play. You can
>also shift the frequency t o save recreating bandwidtht limited
>noise and/or warble tones each time.
>>  I guess we'll see if the 1000 Hz tone produce a different RMS A
>>  measurement and explore the significances. Rob D.
>I've also got a test tone which I originally devised for adjusting
>tape azimuth in the field, avoiding false peaks as you get with pure
>tone. It is very phase sensitive and is based on 440HZ (musical chord
>A major) with subharmonics and harmonics from 44Hz to LPF rolloff.
>This will also copy end to end or loop play.
>BTW if you want to see mp3 artifacts try compreessing brinitone.
>David Brinicombe
>North Devon, UK
>Cogito cogito ergo cogito sum - Ambrose Bierce


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