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isophonic curves. how to use them?

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isophonic curves. how to use them?

Postby raffaele1994 » Tue Oct 27, 2020 11:45 am

Hello!!
The db indicated in the fletche and munson diagram start at 0 db.
instead in the daw from -69.
what is the difference?
how can the graph be used to obtain sounds with exactly the same perception?
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Re: isophonic curves. how to use them?

Postby Hugh Robjohns » Tue Oct 27, 2020 12:26 pm

raffaele1994 wrote:The db indicated in the fletche and munson diagram start at 0 db.

The decibel (dB) is just a way of expressing a ratio between two values on a logarithmic scale. When used to express absolute values the term must be used with a suffix to define the relevant frame of reference.

The Fletcher-Munsun 'equal loudness' curves relate to the Sound Pressure Level (SPL), where 0dB SPL is taken to be the nominal 'threshold of hearing' for humans at a frequency of 1kHz.

And rather than starting at 0dB SPL, you'll notice that all F-M charts actually start at -10dB SPL because the actual 'threshold of hearing' is about -6dB SPL at the resonant frequency of the ear canal between 3 and 4 kHz.

This bizarre anomaly comes from the simple fact that frequency-related measurements are historically referenced to 1kHz... So the charts were normalised for our sensitivity at 1kHz, even though it's more sensitive two octaves higher!

instead in the daw from -69.

Again, references are all important. The meters on the DAW are referenced to the maximum output level from a D-A converter (and an A-D converter for that matter), which is noted as dBFS (decibels relative to Full Scale).

So the top of the meter is 0dBFS, and it counts down in negative numbers with lower signals. In your case the meter apparently goes down to -69dBFS, but some stop at -50dBFS, some at -90dBFS, and some at -144dBFS... In practice few acoustic recordings have a noise floor lower than -70dBFS, so your DAW's compromise is an appropriate one.

what is the difference?

The first illustrates how the sensitivity of the ear changes with acoustic sound pressure level and frequency. The second indicates the peak level of signals within your DAW's recording environment.

The two are normally completely unrelated... But it is possible to align your DAW's meter to a specific sound pressure level so that you have a reference listening volume when mixing. Most professional studios work this way as it helps to ensure consistency.

You can read up on the process here:

https://www.soundonsound.com/techniques/establishing-project-studio-reference-monitoring-levels

how can the graph be used to obtain sounds with exactly the same perception?

Same perception as what? How we perceive sounds depends on their frequency and sound pressure level (as well as their source direction and the context). You have no control over how loud or quiet someone listens to your music. All you can do is mix it so that it sounds good at a 'normal' listening volume for that genre and for most of your audience.
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