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gallahad
09/29/09

In reply to Mphony Is the AV Receiver I Want to Have
Off-shoot question from a non-audio person... why does the default volume for receivers (0db), higher than what is a usual good volume? From the few I've had experience with, the usual good volume was around -20db and -25db.
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robots199
09/29/09

@gallahad: We went over this below...see bottom post.

READ BEFORE YOU POST!
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KefkaticFanatic
09/29/09

@robots199: That still doesn't explain what he's asking.
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valkilmerisawful
09/29/09

@gallahad: 0dB is the an averaged level of acceptable distortion for incoming signals. For example, going too far above 0dB, the outgoing analog signal becomes distorted and can suffer more from peaks and interference, albeit measurable by only a few percent. Go WAY beyond and the signal distorts further, probably by the same logarithmic scale as the dB system (that, I do not know).

Thus, the 0dB level is less of a measure of volume, like a car receiver, and more like a measure of the signal quality. Naturally, this translates to volume directly, but the purpose of having a 0dB default was to identify the signal strength with an low, acceptable distortion.

Was that your question? Anyone else have something to add? I'm no expert, so anyone is free to correct me, I love learning :)
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burninator
09/29/09

@KefkaticFanatic: I find it moderately humorous that Robots199 would lecture you on the subject of superfluous comments, as he currently has more then half the comments on this particular board.
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spannu
09/29/09

@valkilmerisawful: A decibel is just a ratio of the logarithms of two other measures, so it doesn't have to relate to distortion or power of a sound wave.

The displayed "volume" on a receiver just compares output volume to some reference volume which varies from device to device (usually related to the detection threshold for human ears), so the calculated dB may be different.

The reason why dB is used at all is because human ears can hear such a wide range of volumes that using logarithms compresses the range, and using dB should give an indication as to whether the sound is close to the minimum or maximum sound we can perceive.
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The Lab
09/29/09

@burninator: Agreed. When one violates his own decree, foolish he looks. So sayeth Yoda Confucius.
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valkilmerisawful
09/29/09

@spannu: See post by edd9000 below which expands upon my response. Yes, dB is relative. It is a relationship to the source input, and yes, it does measure "power" and "intensity". This is the logarithm of the ratios, not the ratio of the logarithms. Power and intensity only ever mean anything by comparison. I digress: the question was, "Why is 0dB the default for home AV equipment?" Because distortion is concurrently increased as the intensity is amplified. This is inherent to all signal processing. So 0dB is the maximum level at which the signal distortion is acceptable, even tho it's not likely audibly noticeable until you exceed 10 or 15dB above the 0dB reference. Hopefully we have answered that sufficiently (again, see below as well).
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