John H wrote in message
...
Rod Speed wrote

John H wrote

If any of them answer my original question,
how to convert bels to dBA, I missed it.

There cant be a simple factor. Read the pdf, it spells out why.

I read it and still don't know why. What did I miss this time?

Basically dBAs allow for the ear's perception of sound, so
there can never be a simple factor between bels and dBA.

Your first link says "[it is] not possible to convert
between sound power level and sound pressure level."

Precisely.

I think that would be true only if the sound source
isn't emitting power equally in all directions.

You're wrong. There is MUCH more involved than just that.

Wrong how?

Because dBA allows for the ear's perception of sound.

Sound pressure readings are used to
calculate sound power, are they not?

Thats only PART of whats involved.

If it's known that the sound source is
emitting power equally in all directions,

Which no real world device like a hard drive ever does.

only ONE reading from any direction is needed because
readings from all other directions would be the same.

And they aint with real world devices like a hard drive.

If one pressure reading at distance x is enough to
calculate power, it follows that knowing the power
is enough to calculate pressure at any distance.

Only with a theoretical device that behaves
different to real world devices like hard drives.

Of course you need to know the air pressure (assume sea
level) and not have any reflections (acoustic test chamber).

All irrelevant with a real world device like a hard drive that
doesnt even radiate the sound equally in all directions.

You must have noticed the acoustic damping on the Barra drives.

So what is the "MUCH more" you mention?

The A in dbA indicates that it ALSO allows for the ear's perception
of sound, AND thats an entirely separate issue to the MIND'S
perception of noises which may well not even be measurable in
terms of sound pressure levels, particularly with very intermittent
sounds, but which are what the user objects to with a particular drive.

Kindly give me the DETAILS if you know them.

Already did. You just ignored them. Just like
you did with what is spelt out in that pdf too.

If it is, I bet 10dBA/bel at 1 meter will be close.

You've just lost your bet.

The Quantum I mentioned was close to 10dBA per bel, 3.6 bels and 32 dBA.

And those ratios are entirely a result of the PARTICULAR
noises that particular drive happens to emit and arent
useful for a different drive which is different on that detail.

That should be obvious from the different
ratio you get with that fan you cited.

Why would another drive be much different?

Because the noises produced vary quite significantly.

In spades when comparing say a hard drive and a fan.

I'd guess that the SPL "gain" (any deviation from what equal
power in all directions would be) for a drive mounted 1 meter
in front of a SPL meter would be similar for all drives.

Guess again. And thats an entirely separate issue to the A part of dBA.

In other words if one drive has x dBA/bel,
they probably all have about the same.

Fraid not. Most obvious when comparing that hard drive with that fan.

I'm just guessing though.

And havent managed to understand the basics.

Some noises are much more irritating than others at
the same sound pressure level. I understand that.

You claim you do, but when you still want a simple ratio between
bels and dBAs, you clearly havent managed to grasp the problem.

But what I was asking about is how to figure out what that
sound pressure _IS_ when only the sound power is given.

Not even possible, as should be obvious from the very
different ratios you get with that hard drive and that fan.

And that still leaves the entirely separate problem that
stuff like occasional deliberate head moves wont even
be measurable in SPL terms, but are what the user
considers makes a particular drive particularly noisy.