[Arpiben]

Last reply - then I give up.
(I'll leave it to everyone else to decide who seems to be making sense here...)

1)

According to the laws of physics, at least in this universe, at 20 degrees Celsius, at sea level, at 50% relative humidity, the attenuation at 30 kHz is 0.94 dB / meter.
That means that, at six feet, the attenuation is just under 2 dB.
Unless you were referring to the laws of physics in some other universe - that's what the ones in this universe say.
(And, if you tried to record ultrasonic frequencies at a distance of a few feet, and were unable to do so, then something else must have been the cause of your inability to do so.)
I don't need to try it myself - I'll settle for believing the laws of physics - as you seem to be suggesting I should.

Here's the link to the calculator:
http://resource.npl.co.uk/acoustics/techguides/absorption/

@KeithEmo

I will let @gregorio reply more in details if he feels like.
But what you totally miss in your point 1 and previous posts is that sound pressure attenuation in outdoor does not depend solely on the atmospheric absorption coefficient!
@gregorio provided the correct estimation in various posts. I will let you find by yourself.
Note that other attenuation or damping effects have been omitted for sake of clarity ( diffraction/reflection/ground coefficient/scattering/barriers/obstacles/ gradient of temperature/humidity/etc...)

 

[analogsurviver]

@KeithEmo

I will let @gregorio reply more in details if he feels like.
But what you totally miss in your point 1 and previous posts is that sound pressure attenuation in outdoor does not depend solely on the atmospheric absorption coefficient!
@gregorio provided the correct estimation in various posts. I will let you find by yourself.
Note that other attenuation or damping effects have been omitted for sake of clarity ( diffraction/reflection/ground coefficient/scattering/barriers/obstacles/ gradient of temperature/humidity/etc...)

Although all of the above mentioneed causes of ultrasonic absorption are undoubtedly real, in practice they do not reduce the ultrasonics to as low levels as some would like us to believe.

 

[Arpiben]

Although all of the above mentioneed causes of ultrasonic absorption are undoubtedly real, in practice they do not reduce the ultrasonics to as low levels as some would like us to believe.

The 50 dB attenuation for f=30 kHz @20m/0.5m with 50% humidity in free space is indeed very correct.

 

[KeithEmo]

No.... you didn't read the details....
(We were talking about a VERY specific situation... whether ultrasonic content would be present to be recorded when recording cymbals using an overhead microphone.)

If I place a microphone six feet away from a source.
(For example, if I hang a microphone six feet over the drum set.. which was the context of the discussion.)
In direct line of sight to the source.
Then the only thing that is going to prevent that microphone from hearing the source is the absorption of the air in between.

There are no obstacles...
There are no barriers...
And I specified the temperature and humidity (the calculator I linked to allows you to enter frequency, atmospheric pressure, temperature, and relative humidity).

Various types of reflections absolutely may indeed affect the fidelity... by adding reflected or diffracted sound unequally to the intended signal.
However, since a cymbal delivers a sort of wide spectrum noise burst, rather than a pure tone, there is no possibility whatsoever of any sort of "hard null" occurring that would cancel it entirely.

No mention was made about whether we were indoors or outdoors.
No mention was made of the fidelity of the recording that would result.
(I absolutely agree that some of the factors you mentioned might result in poor fidelity or an irregular frequency response.)

@KeithEmo

I will let @gregorio reply more in details if he feels like.
But what you totally miss in your point 1 and previous posts is that sound pressure attenuation in outdoor does not depend solely on the atmospheric absorption coefficient!
@gregorio provided the correct estimation in various posts. I will let you find by yourself.
Note that other attenuation or damping effects have been omitted for sake of clarity ( diffraction/reflection/ground coefficient/scattering/barriers/obstacles/ gradient of temperature/humidity/etc...)

 

[KeithEmo]

Absolutely.
However, since we were talking about recording with a microphone located 2 meters overhead, it is also irrelevant.

[EDIT: Actually, according to the calculator, the attenuation under those conditions is slightly less than 1 dB/meter so, at 20 meters, it would be more like 19 dB.... although I'd be willing to accept a little leeway there for different models ]

The 50 dB attenuation for f=30 kHz @20m/0.5m with 50% humidity in free space is indeed very correct.

 

[analogsurviver]

The 50 dB attenuation for f=30 kHz @20m/0.5m with 50% humidity in free space is indeed very correct.

Even if it is actually correct - how many recordings of acoustic instruments are actually made from 20 m distance ?

 

[KeithEmo]

Exactly.....

In every multi-track recording situation I've ever seen the cymbals are recorded using one or more microphones...
Usually located directly over the drum set, slightly off the the side, or slightly under the cymbals...

Even if it is actually correct - how many recordings of acoustic instruments are actually made from 20 m distance ?

 

[Zapp_Fan]

Yeah, got to chime in and say that 20m is probably not a relevant distance for any recording setup except (maybe?) some unusual classical / organ stuff. Overhead mics are probably no more than 2-3m away from a cymbal most times. Free air absorption of ultrasonics is actually pretty low on the list of reasons of "why ultrasonics don't matter for music". Poor ultrasonic performance of gear, absorption by objects in the space, and low initial SPL from the instrument itself are much more important.

 

[analogsurviver]

Exactly.....

In every multi-track recording situation I've ever seen the cymbals are recorded using one or more microphones...
Usually located directly over the drum set, slightly off the the side, or slightly under the cymbals...

We could go back and fort ad nuseaum regarding the distance and consequent ultrasonics attenuation.

It all depends on the type of microphone technique - and actual setting/seating. Even binaural can be used both from 2 metres - or > 20/30/40/etc metres .

And one of the indicators of relative distance from the source to the microphone/listener is the attenuation of ultrasonics - the depth of a soundstage with bandwidth limited recording and/or reproduction will NEVER sound natural.

 

[Arpiben]

No.... you didn't read the details....
(We were talking about a VERY specific situation... whether ultrasonic content would be present to be recorded when recording cymbals using an overhead microphone.)

If I place a microphone six feet away from a source.
(For example, if I hang a microphone six feet over the drum set.. which was the context of the discussion.)
In direct line of sight to the source.
Then the only thing that is going to prevent that microphone from hearing the source is the absorption of the air in between.

There are no obstacles...
There are no barriers...
And I specified the temperature and humidity (the calculator I linked to allows you to enter frequency, atmospheric pressure, temperature, and relative humidity).

Various types of reflections absolutely may indeed affect the fidelity... by adding reflected or diffracted sound unequally to the intended signal.
However, since a cymbal delivers a sort of wide spectrum noise burst, rather than a pure tone, there is no possibility whatsoever of any sort of "hard null" occurring that would cancel it entirely.

No mention was made about whether we were indoors or outdoors.
No mention was made of the fidelity of the recording that would result.
(I absolutely agree that some of the factors you mentioned might result in poor fidelity or an irregular frequency response.)

No Keith.
You have distance! - 20 log r in Free space or Fresnel zone. It is the case for f=30 kHz at 6 feet.

 

[Zapp_Fan]

No Keith.
You have distance! - 20 log r in Free space or Fresnel zone. It is the case for f=30 kHz at 6 feet.

Are you talking about normal attenuation that occurs due to sound radiating through space? Can you explain why ultrasonics don't follow normal 'inverse square' numbers for that? Not having any need for it, I never bothered to learn how ultrasonic "audio" might vary from normal sound. I'm confused as to why you're saying there is a much greater attenuation (other than from atmosphere) than there would be with normal sound.

 

[Arpiben]

Even if it is actually correct - how many recordings of acoustic instruments are actually made from 20 m distance ?

2 meters:
~14 dB attenuation for f=30 kHz @2m/0.5m with 50% humidity in free space.

 

[Arpiben]

Are you talking about normal attenuation that occurs due to sound radiating through space? Can you explain why ultrasonics don't follow normal 'inverse square' numbers for that? Not having any need for it, I never bothered to learn how ultrasonic "audio" might vary from normal sound. I'm confused as to why you're saying there is a much greater attenuation (other than from atmosphere) than there would be with normal sound.

Ultrasonics do follow same law as any other frequency provided that they are in Far Field (greater than two wavelength distance).
Atmospheric Absorption is frequency dependent. ISO 9613 calculator provided is dealing only with such absorption.
https://www.mne.psu.edu/lamancusa/me458/10_osp.pdf

 

[Zapp_Fan]

OK, I get that, makes total sense. But then since we are comparing normal audio to ultrasonics, (the argument being... ultrasonics don't matter because they're too attenuated relative to normal sound to matter) isn't the relative atmospheric absorption the most relevant number?

I mean, yes, certainly, 14db total attenuation at 2m, but unless I'm missing something, that value would be similar at (say) 200hz also.

 

[Arpiben]

OK, I get that, makes total sense. But then since we are comparing normal audio to ultrasonics, (the argument being... ultrasonics don't matter because they're too attenuated relative to normal sound to matter) isn't the relative atmospheric absorption the most relevant number?

I mean, yes, certainly, 14db total attenuation at 2m, but unless I'm missing something, that value would be similar at (say) 200hz also.

Not exactly 20log(2/0.5) + AIr Absorption at 200 Hz ( neglectable) -> roughly 12dB vs 14dB.
( 200 Hz has a 1.5 meter wavelength, in principle the 1/r attenuation is not applicable interactions between waves are more complex. I did the above calculation just for copping with your example )