> Greg O. wrote
> I need to have a firm grasp on the concept of the thermal noise of air an=
d how it affects microphones.
I now have about a 12" stack of acoustic texts on my desk. =C2=A0Beranek's =
"Acoustics" was most helpful. =C2=A0Morse and Ingaard's "Theoretical Acoust=
ics" gave me a 3-aspirin headache.
Beranek gives calculated values for the real part of the impedance seen by =
a disc on the end of a cylinder. =C2=A0That sounds like a microphone to me.=
=C2=A0The real part of the impedance is the resistance, and that's what we=
need to calculate the noise. =C2=A0For a 1-inch microphone diaphragm, I ca=
lculate that the noise amounts to about -20 dB SPL. That is, the noise of t=
he air is 28 dB quieter than an Audio Technica 4032 (but the 3032 diaphragm=
is smaller than 1 inch) and about 15 dB quieter than a Bruel and Kjaer 417=
Subject: =C2=A0But that doesn't do much to point us towards making a microphone
hat can measure the thermal noise of the air. =C2=A0It's the noise from the=
air that is trapped _behind_ the diaphragm in a conventional microphone th=
at limits how quiet it can be. =C2=A0Along with all of the other noise sour=
I hope that helps. =C2=A0I can show the details of the calculation if anyon=
e would like to see it. =C2=A0This venue doesn't seem like the place!
On Friday, June 13, 2014 11:10 PM, "Gregory O'Drobinak =
l.net [naturerecordists]" <> wrote:
What a lovely memory to have, being there in the mountains of India!
i wish we could not only hear the recording of that soundscape as you perce=
ived it, but also to see a good picture of what the space was like.
Perhaps in the future, we can look forward to such things [Mike Rooke, you =
have much work to do. ++].
Please take a look at the Gras microphone that i posted and its data sheet.
It claims that the lowest measurable SPL level is limited by the thermal no=
ise of the microphone, which is spec'd at -2 dB SPL.
This implies that one can break the 0 dB SPL barrier as Miike has proposed.
But it will certainly take quite a lot of creative thinking and work to mak=
e this happen.
Gras claims to have it already, but we need an affordable solution that is =
compact and adaptable to a large variety of recording arrays.
I'm not sure that I understand the idea of random thermal noise causing a 3=
dB hit on the self noise of a mic.
Eric B., can you please help us out here?
Also, I need to have a firm grasp on the concept of the thermal noise of ai=
r and how it affects microphones.
Somehow there must be a way to convert thermal noise power into dB SPL at t=
he microphone diaphragm.
I suspect that this involves some integration of the noise power over some =
distance as a function of the area and compliance of the microphone diaphra=
Can anyone with a solid background in acoustics help us out here?
BTW, sorry to burden all of the nature recordists out there with all of thi=
s crazy tech.
But, ultimately this will benefit all of us, assuming that there will still=
be some pristine natural habitats left to record.
Thanks & BR,
From: " [naturerecordists]" <=
Sent: Friday, June 13, 2014 6:15 PM
Subject: [Nature Recordists] Re: 3032 capsules on PIP
> Recall that 'dB' is a relative logarithmic measure that is useless withou=
t some reference level to refer to. Again, dB is always relative!
> So when you say 'dBA' or 'dB-anything it is always relative to some absol=
ute reference value.
This is just laziness when what is meant is dB(SPL) which is usually taken=
to mean as referred to 20 =C2=B5Pa (rms) as you said.
> Oh, let's now address the thermal noise of the air.
> I suspect that in absolute measurement terms, the thermal noise of the ai=
r is very much lower than the absolute value of 20 uPA. Like, way lower.
> Can someone please help out and provide the equations to calculate the 't=
hermal noise of the air' in absolute terms, i.e., in uPa?
> Please give a good description as to what all of this means, too.
The general thermal noise equation is:
P =3D kB T Delta f
P =3D power
kB =3D Boltzmann's constant
T =3D absolute temperature
Delta f =3D frequency bandwidth
My physics is 50 years old and I'm struggling to turn this into dB(SPL) but=
note that any thermal noise is proportional to frequency at the rate of 3dB=
per octave. This means that a weighting curve must be specified to turn thi=
into as audible noise threshold. Unfortunately no suitable curve is
recognised at 0 dB(SPL). In particular the phon is defined with the
40dB(SPL) weighting curve which is nothing like the threshold curve. See:
Note in particular, the threshold curve which goes down to -6dB(SPL) so kee=
ears should hear thermal noise.
I claim to have heard the thermal noise in air in my youth when I could hea=
very high frequencies. At some frequency the rising thermal noise met my
falling frequency response and I heard a hiss. It was in an exceedingly
quitet grassland in the mountains of South India and I could hear my heart=
thumping away noisily, the blood coursing through my ears, and faitly in th=
background, a high pitched hiss.