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Topic: Acoustic weighting curves and the human hear (Read 6854 times) previous topic - next topic
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Acoustic weighting curves and the human hear

So, I've just bought a SPL Meter.

I know about the Equal-Loudness Countours (here) and the A-B-C weightings curves (here) and I'm able to calculate the value in dBs depending on the value shown on the meter for each weightings.

What I don't understand is, why are there many weightings ? When one, for example, would want to use A and not C, or vice versa ?

I'm trying to know as precisely as possible the volume at which I'm listening to my music with headphones. I'm planning to do something like this.

I'm wondering, how one can tell which weight to use for that particular purpose ? How should I do it, play tones ?

Should I only use 1 khz, since it's the same value for all weightings ?

Pardon my ignorance on the subject, I'm kind of lost ^^

Thanks

Alan

Acoustic weighting curves and the human hear

Reply #1 –
Questions: "we are now playing signal #1 at this volume. What volume for signal#2 is equivalently loud?"
Note "equivalently", not "equally"; it depends on the purposes:
- physical pressure? Unweighted dB.
- subjective loudness? The Fletcher-Munson curves measure the equivalence curves to depends on volume. The dB(A) weighting is not far from the 40 phon curve - those are low volumes.
- loud volumes? Then the C-curve is more accurate. https://www.noisemeters.com/help/faq/frequency-weighting.asp
- distance to pain threshold?
- damage to hearing under long-term exposure? One also uses A-weighting for that.
- disturbance to sleep?

Etc etc. The Wikipedia article on the A-weighting explains the D-weighting but not the C-weighting: http://en.wikipedia.org/wiki/A-weighting#B...nd_Z-weightings
Google it.

Acoustic weighting curves and the human hear

Reply #2 –
Quote from: Aldem on
So, I've just bought a SPL Meter.

I know about the Equal-Loudness Countours (here) and the A-B-C weightings curves (here) and I'm able to calculate the value in dBs depending on the value shown on the meter for each weightings.

What I don't understand is, why are there many weightings ? When one, for example, would want to use A and not C, or vice versa ?

I'm trying to know as precisely as possible the volume at which I'm listening to my music with headphones. I'm planning to do something like this.

I'm wondering, how one can tell which weight to use for that particular purpose ? How should I do it, play tones ?

Should I only use 1 khz, since it's the same value for all weightings ?

Pardon my ignorance on the subject, I'm kind of lost ^^


A well-known collection of equal loudness curves were produced by Fletcher and Munson over 50 years ago and they are important enough that people have been trying to keep them updated:



They are explained in some detail here:

Discussion of Equal Loudness Cur ves

The bottom line is that the ear's sensitivity to various frequencies varies with SPL. This is one reason why the tone quality of an audio system seems so muted and dull when it is played at very low SPL's. To a point, louder is better.

In short to be as accurate as possible you'd want a different loudness weighting curve for every different SPL you wanted to evaluate.

Acoustic weighting curves and the human hear

Reply #3 –
Although A weightings were only intended to apply at low noise levels and become much more accurate at higher levels, almost all legal guidance and regulation uses A weighted values - at least in the UK, as far as I'm aware in the EU, and probably in most other countries. The main exception I can think of is for peak levels in noise at work regulations.

So, although C weightings are probably more representative at moderate to high listening levels, if you want to compare them to anything else, I would stick to A weightings.

You'll have to be careful about the physical set up as noise levels from headphones are heavily influenced by the physical connection/seal between the earpad and the side of your head.

Acoustic weighting curves and the human hear

Reply #4 –
Quote from: Cubist Castle on
Although A weightings were only intended to apply at low noise levels and become much more accurate at higher levels, almost all legal guidance and regulation uses A weighted values - at least in the UK, as far as I'm aware in the EU, and probably in most other countries. The main exception I can think of is for peak levels in noise at work regulations.


It is my understanding that the use of A weighting for occupational safety related measurements is used to include the fact that the ear is less sensitive to damage at lower frequencies.

Acoustic weighting curves and the human hear

Reply #5 –
Quote from: Cubist Castle on
Although A weightings were only intended to apply at low noise levels and become much more accurate at higher levels

Sorry this is obviously wrong - I meant less accurate, not more!

Quote from: Arnold B. Krueger on
It is my understanding that the use of A weighting for occupational safety related measurements is used to include the fact that the ear is less sensitive to damage at lower frequencies.

Thanks - interesting point, I'd not heard it put explicitly before but it certainly makes sense.

Acoustic weighting curves and the human hear

Reply #6 –
So, for my purposes, with a SPL Meter that can do A and C only, I should use A ?

Acoustic weighting curves and the human hear

Reply #7 –
It depends on what you're trying to measure and what (if anything) you want to compare it to.

For instance, often A is used for measuring background noise levels whereas C is used for measuring listening levels.

For comparing to safe hearing guidelines/regulations you need to use the same curve as the guidelines/regulations.

Acoustic weighting curves and the human hear

Reply #8 –
The A weighting is only accurate to about 60 dBSPL.  It is mostly used for workplace noise and public noise control. In my opinion it is the wrong application for that curve. The at the loud short term noise levels that hear damage occurs the curve is is getting pretty far off. Of course when many of theses occupational standard were put in place, I'm sure the factory owners pushed for A curves.

For noise control applications of course the people making the noise want to use A weightings. Where we run into problems is the bass is what is disturbing the people that don't want the noise. Bass also has more energy in it and more easily causes other things to resonate. For noise control I measure everything without a curve and apply the curve after if needed. Sometimes the curve masks the problem, when you really need to fix the problem. 

Back to your question.

If your goal is to protect your own hearing from damage I would use the C curve, it will give you a little more safety margin. The goal of the occupational standards is to protect your hearing so that you can still understand speech as you grow old. You more likely want to protect your ability to enjoy music the rest of your life. That is a higher standard.