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#1 of 4 Greg Bax

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Posted October 29 2003 - 01:30 AM

Can anyone tell me why the decibels on the receiver go down as the volume goes up? I was under the impression that the larger the dB number the louder the sound. For example 100dB would be louder than 50dB. But it appears that on the receiver the dB number goes down as the volume goes up.

#2 of 4 Vince Maskeeper

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Posted October 29 2003 - 03:22 AM

Because it's a scale of potential-- without getting to technical- the volume knnob is measured in attenuation: how much it is reducing the signal.

If you think of those numbers on the volume knob as being prefixed with a negative- it might be easier to picture in your head.

The way to picture the signal flow in your head is that your receiver is actually 2 stages: a preamp and an amplifier, seperated by a volume knob. If the volume knob was not there- The preamp supplies a FULL LEVEL signal to the AMP-- and the amplifer amplifies it and makes it REALL LOUD.

However the volume knob can regulate, specifically can reduce, the amount of signal flowing between the two. So the volume knob works kinda like a faucet knob- the water pressure is in the pipe, running up to the faucet knob-- and by opening it- you let the water out, up to the full pressure amount in the pipe... If you were to put a scale on the faucet knob- you could label it in terms of how much it was reducing the water flow, so at the "higest" level- when you get the most water, it would be labelled 0, as it would be allowing the full amount to pass from the pipe...

The idea being that 0 (which is probably the marking when the knob is all the way up on the receiver) means the full signal from the preamp is being passed onto the amplifer.

So if you think of it as a negative in terms of it's reduction of signal, at the lowest level, it is at -infinity, and as you turn it up it is -60, -50, -40 etc...

Hope that helps

-V
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#3 of 4 Wayne A. Pflughaupt

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Posted October 29 2003 - 09:55 AM

Greg,

You have to first understand that the dB scale has meaning in all kinds of scientific measurement -- from sound, to electrical or mechanical power, to voltage, and so on.

Quote:
I was under the impression that the larger the dB number the louder the sound.
That would be correct if the volume control was indicating sound pressure level. But if you think about it, it would be an impossible task to assign a particular value of dB/SPL to a volume knob, because high-efficient speakers produce more SPL at a given setting than medium-or low-efficiency speakers.

Thus the calibration of the volume knob has nothing to do with volume level. As Vince indicated, “0dB” at the top of the scale is the reference for the pre-amp’s maximum output. In other words, the calibration scale is dBV (voltage), not dB SPL.

In the simplest terms, the “backwards” scale means “dB before maximum gain.” This is why the calibration on the volume knob is given as -dB. Voltage can always be given as a negative figure; obviously sound pressure level cannot.

For more reading on this topic (with other good analogies from Vince), you might want to peruse this thread.

Regards,
Wayne A. Pflughaupt

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#4 of 4 ChrisWiggles

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Posted October 29 2003 - 10:34 AM

Those are two, very good answers.

But if that went a little over your head, basically 0 is your reference point. Below that, is negative dB, and above is positive. So the numbers are *actually* going up, but they are just becoming less negative, so it looks backwards if you don't notice the - sign.