Comment by hprotagonist | Hacker News Reader

Comment by hprotagonist | Hacker News Reader

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We often measure loudness using a unit called the decibel (named after Alexander Graham Bell).

Not really. Decibels themselves are unitless: we just have to use them because the magnitude of sound pressure levels varies across a real large range so it’s very useful to have a compressive scale.

Pedantry ahoy!

1. The unit of loudness is the phon. Loudness is not independent of frequency: https://en.wikipedia.org/wiki/Phon

2. sound intensity is measured as a sound pressure level (SPL https://en.wikipedia.org/wiki/Sound_pressure) in dB: re 20muPa in air, 1 muPa in water. It is a frequency independent measure, though there are perceptual weighting scales (a- and b-weightings are common) to normalize things for occupational exposure limits.

Sound intensity and sound loudness are related but not at all the same. Loudness is a percept, intensity is a physical measure of a system.

I can play a 100 kHz tone at extremely high sound pressure levels and you will never hear it. It’s not loud, to a human. It’s probably quite loud if you’re a bat.

Pedantry ahoy! part II:

Sound intensity is a measure of power (power through a unit area), and is useful for determining the distribution of sound emissions (directivity) from a source.

Sound pressure level, is, well, a measure sound pressure. Sound pressure is related to sound intensity providing you know the directivity and distance to the source.

This is a nitpick of s nitpick, and SPL is often used interchangeably with the word intensity in lay documents. But if you're reading an acoustic journal article or text book and see the word intensity, just be aware that they are likely not discussing pressure level.

Thanks for the clarification. Learned something new today =)

What I've always wondered though: can loud sounds outside the frequency range that is audible to us cause hearing damage?

So, say we have a 120dB 60kHz audio source, would that be dangerous for our hearing? Would we be able to sense it (although not hear it)

"It is known that exposure to noise of a certain frequency does not induce a higher TS at that frequency, but rather at 0.5 to 2 octaves above that frequency, depending on the species"[1] ("auditory threshold shift (TS)").

[1] https://www.frontiersin.org/articles/10.3389/fnagi.2015.0000...

A meta observation: When the internet was new, and then the web, there was a puzzle of how society might adapt to information becoming more easily accessible than it had been. Bomb making instructions was a commonly used example. Society responded narrowly, by for instance outlawing such instructions. There's an entwined puzzle around understanding information. If science education someday fails less pervasively, how might society adapt? It's perhaps not a near-term challenge, but maybe something to think about.

Bomb making intructions are not illegal however.

From my understanding too damage the hearing, the sound would have to resonate part of the ear enough to cause damage and if any part resinated, we would hear it. You may get cases in which two inaudible frequencies could produce a harmonic that we could hear, though unsure of any examples in this instance.

You may well find though that some frequencies, which not audible by the human ear, may have some effect upon another part of the body. Though I do hope we don't end up having to carry bats in canary cages everytime we go down to the shops.

But another way to look at this is - if there was some way sound could be used to harm somebody in any way, then it would have been weaponized. With that in mind - https://en.wikipedia.org/wiki/Sonic_weapon

See also: harmonic distortion.

60kHz isn't really an audio source, at that frequency it would (or used to be, anyway) be classified as an RF source.

Marconi operated in frequencies even lower, 14-30kHz, at many thousands of watts from Alexanderson alternators[0]. Dogs were not happy near these stations.

https://en.wikipedia.org/wiki/Alexanderson_alternator

edit: added link, lowered frequency range.

> 60kHz isn't really an audio source, at that frequency it would (or used to be, anyway) be classified as an RF source.

It would not be classified as an RF source. Frequency does not redefine the medium. Sounds waves above 20KHz are usually referred to as ultrasound, but it is still sound. More specifically, it is still a compression wave in matter.

EM waves are a completely different medium. I presume you are conflating the two through the existence of this machine, the mechanical nature of it's source of oscillation no doubt created compression waves in the air of the same frequency as a side effect (Pretty sure Dogs don't have RF receivers). Whereas now we use solid state oscillators, and so your dog doesn't care about your various RF devices.

I conflate the two because I was temporarily nuts. Doh!

Of course ultrasound exists, and can exist in air up to the GHz range (not for very far).

Anyway, sorry for misinfo and thanks for the correction.

No worries. It's probably not helped by the fact that RF starts at around 20 KHz in EM at the same frequency where human hearing range ends in sound waves. So it's fairly easy to read a bunch of statements about frequency ranges and then mix up mediums, especially when EM contains a bunch of seemingly very different things, light, RF, xrays etc.

Also a number of different specifications for how it is measured.

https://auphonic.com/blog/2012/08/02/loudness-measurement-an...

> I can play a 100 kHz tone at extremely high sound pressure levels and you will never hear it

You won't hear it, but it will hurt like hell when your proteins start to coagulate.

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