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In other words, given a magical room with walls that produce no vibration and transmit zero vibration from the outside, and nothing on the inside except room temperature air, what would be the noise level in dB SPL (sound pressure level) from the thermal motion of the air itself? (Similar to the noise floor of electronics being determined by thermal noise in the conductors.) What is the quietest possible anechoic chamber?

For reference: Sound pressure is defined as the root-mean-square value of the instantaneous pressure, measured in pascal = N/m². SPL is the same number, but expressed in decibels relative to 20 µPa.

(I assume that it has a white spectrum, but I could be wrong. Thermal noise in electronics is white, but other types of electronic noise are pink, and blackbody thermal radiation has a bandpass spectrum.)

Here's an explanation in the context of underwater acoustics. Not sure how this applies to air:

Mellen (1952) developed a theoretical model for thermal noise based on classical statistical mechanics, reasoning that the average energy per degree of freedom is kT (where k is Boltzmann’s constant and T is absolute temperature). The number of degrees of freedom is equal to the number of compressional modes, yielding an expression for the plane-wave pressure owing to thermal noise in water. For non-directional hydrophones and typical ocean temperatures, the background level due to thermal noise is given by:

NL = −15 + 20 log f (in dB re 1 µPa)

where f is given in kHz with f >> 1, and NL is the noise level in a 1 Hz band. Note that thermal noise increases at the rate of 20 dB decade−1. There are few measurements in the high-frequency band to suggest deviations from the predicted levels.

Citation is R. H. Mellen, The Thermal-Noise Limit in the Detection of Underwater Acoustic Signals, J. Acoust. Soc. Am. 24, 478-480 (1952).

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I'm not totally confident on this, at least not enough to make it an answer. But sound is due to pressure differences -- an isolated system with nothing but air at equilibrium will not have any sound because the pressure is uniform. Even if the gas is really hot and pressure is 100atm or something. –  tpg2114 Apr 29 at 20:33
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@tpg2114 But even though the average pressure is constant, the pressure at a point in the middle of the room will vary randomly over time due to the thermal motion of the molecules, which are constantly moving and colliding with each other. Calculating the noise floor may require defining a sensor membrane with finite surface area for the particles to hit or something, I don't know. –  endolith Apr 29 at 20:36
    
But I think that's the crux of the issue -- pressure isn't defined in any way other than a continuum (average) property. And if the gas is in equilibrium, we know the continuum properties are independent of space and time. So "pressure" fluctuations due to collision should average out to be zero over a volume which allows us to define "pressure." At least that's my take on it but I'm not positive so I leave these as comments only :) If somebody confirms this, he/she is free to use it as an answer! –  tpg2114 Apr 29 at 20:46
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@tpg2114 "pressure isn't defined in any way other than a continuum (average) property" Then sound doesn't exist, since it always averages out to atmospheric pressure. –  endolith Apr 29 at 22:56
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I was thinking low gas density vs molecule hitting the mic, now Maxwell–Boltzmann distribution: at 25C He has an avg speed of 422m/s which can then according to Pressure is Confined Kinetic Energy be computed to pressure, similarly the variance of speed will give the RMS acoustic noise. Beyond the calculations then the question is whether this noise should be weighted for audible range and whether sensitivity of membrane are achievable. –  oberron May 4 at 8:38

2 Answers 2

Every molecule in the air undergoes something like 5 billion collisions per second. There are about $6.022×10^{23}$ molecules in a mole of gas (for oxygen at normal pressure, about 16 litres). An small room is something like 50,000 litres. That means you'd have of the order of $1.5×10^{38}$ collisions per second. Even if they were all in step (and they're not), you would need rather special equipment to hear that noise. Even listening to 1 molecule would require a microphone sensitive to 5 GHz. A good human ear only goes up to 20 kHz, bats and dolphins to about 150 kHz.

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This doesn't answer the question, just gives some side data. The question asks for some estimation of noise level. –  Ruslan Apr 30 at 9:13
    
Why would it need to be sensitive to 5 GHz? Is the spectrum of the noise not white? –  endolith Apr 30 at 16:15
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This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. –  DavePhD Apr 30 at 16:40
    
I wrongly assumed the question was specifically about sound. On re-reading it, I realised that it could also be about microwave radiation, in which case my answer is not really relevant. –  hdhondt May 1 at 0:03
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@hdhondt: Why would it be above the audible range? Thermal noise in other contexts is white up to a very high frequency, and then drops off. –  endolith May 4 at 0:25

Apparently the search term I was missing was "Brownian motion". With that, I found several leads. They contradict each other somewhat, but I can at least post a partial answer:

Geisler - Sound to Synapse: Physiology of the Mammalian Ear:

Estimates for the first of these sources, the pressure fluctuations due to the Brownian motion of air molecules impinging on the eardrum, are about 2 µPa (−20 dB SPL), when the frequency bandwidth relevant for the detection of a 3 kHz tone is included (Harris. 1968). Calculations using this number suggest that the behavioral thresholds of humans for 3 kHz tones are not limited by this Brownian motion, but that those for the most sensitive of cats may approach it (Green. 1976)

Dallos - The Auditory Periphery Biophysics and Physiology:

By assuming a 1000-Hz bandwidth, Harris computed that the Brownian motion of air molecules generates a mean pressure fluctuation of 1.27×10−5 dyne/cm2 [−24 dB SPL]. The usually accepted value of sound pressure corresponding to free-field listening threshold is 18 dB above the pressure level of thermal fluctuations. Thus one can immediately see that Brownian motion of air molecules is certainly not the limiting factor of our hearing sensitivity.

These both cite this paper that I don't have access to:

  • Harris, G. G. Brownian motion in the cochlear partition. J Acoust. Soc. Am. 44: 176-186, 1968

But there's another available with more details:

Harris - Brownian motion and the threshold of hearing:

We can avoid the calculation of the Brownian noise at the eardrum by using the Brownian noise in a free field and comparing that with the minimum audible field (MAF) instead of the minimal audible pressure (MAP).

If we use frequency limits of 2500 Hz and 3500 Hz. we obtain a root mean square (rms) pressure fluctuation of 98 db below 1 dyne/cm2 [−24 dB SPL]. The MAF2 is about 80 db below 1 dyne/cm2 at 3000 Hz. This is 18 db above the estimate of Brownian noise. It seems clear from this calculation that Brownian noise in the air is not a limiting factor to the threshold of hearing.

2.5 kHz to 3.5 kHz is not the total bandwidth that would be picked up by a microphone, though.

Yost & Killian - Hearing Thresholds:

By making some assumptions about the acoustic energy present in the Brownian motion of air molecules, it can be shown that a sound presented at 0 dB SPL is only 20-30 dB more intense than that being produced by Brownian motion

So −20 to −30 dB SPL.

Howard & Angus - Acoustics and Psychoacoustics:

At 4kHz, which is about the frequency of the sensitivity peak, the pressure amplitude variations caused by the Brownian motion of air molecules, at room temperature and over a critical bandwidth, correspond to a sound pressure level of about −23 dB. Thus the human hearing system is close to the theoretical physical limits of sensitivity. In other words there would be little point in being much more sensitive to sound, as all we would hear would be a ”hiss” due to the thermal agitation of the air!

I would still like to know:

  • How this is derived
  • What the spectrum is, and if it's different from the violet spectrum in water, why?
  • What the 20 Hz-to-20 kHz and A-weighted values are
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