Volume measured in -dB?
Discussion
slf2012 said:
Can someone explain why the volume of a stereo amplifier is measured from a negative dB reading rather than from 0?
Obviously, I know dB is the measurement of noise, but why when the volume is quite high, is it still a negative dB level?
It's pretty confusing stuff and despite studying sound engineering my memories of the exact details is hazy...Obviously, I know dB is the measurement of noise, but why when the volume is quite high, is it still a negative dB level?
Basically, the 0 in 0dB Full Scale refers to the maximum level in a digital system before clipping occurs. So it's not an exact 'amount' of loudness, IYSWIM.
But the short answer is that say, a plane taking off, would be a +ve number of dB but a track you were mixing would be in -ve figures.
It's also a logarithmic system and there are different dB scales in play which confuses things further.
I'm going to stop there before I start to veer into being-desperately-wrong territory and get mocked by those more knowledgeable!
slf2012 said:
Can someone explain why the volume of a stereo amplifier is measured from a negative dB reading rather than from 0?
Obviously, I know dB is the measurement of noise, but why when the volume is quite high, is it still a negative dB level?
It's the level relative to the amplifier's [very loud] reference level.Obviously, I know dB is the measurement of noise, but why when the volume is quite high, is it still a negative dB level?
Some amps will have a user setting which enables the volume to be displayed as relative (as above) or absolute (on a 0-99 scale, for instance).
Just to add to the above post, movies are mixed at a reference level of 85db with 20db headroom for peaks on the main channels and 30db headroom on the LFE channel. When you do the auto set-up on a receiver it tries to set your speakers accordingly.
On a stereo amp it's done because the designer thought it looked nice, as there's no reference level for music, so the 0db doesn't refer to any standard.
On a stereo amp it's done because the designer thought it looked nice, as there's no reference level for music, so the 0db doesn't refer to any standard.
The link posted by "ReaderScars" explains it pretty well, but with increasing decibel levels - in other words 0dB refers to a given out output in watts, +3dB means a higher wattage etc. which is inverse to your volume control.
In simple terms - 0dB on your volume control is (typically) full power output for a given (signal) input voltage - but it is by no means a calibrated measure.
The output power is measured in to a dummy load (usually 6 ohms) across the speaker terminals.
So, for example if your amplifier is rated at 200w into 6 ohms, it might require 1v RMS across the input terminals to produce 200w when the volume control is set to 0dB.
However, the volume control calibration is unlikely to be absolute - e.g. -3dB may not necessarily be exactly -3dB and so on.
All audio recordings are made using calibrated equipment.
Digital audio is recorded using a dBFS scale, where 0dBFS might equal 1vRMS (and maximum recording level).
As you will see in the Wiki article, various organisations have different ways of interpreting reference levels.
See WikiPedia for an explanation of this.
Analogue recording uses two standard levels, where 0dBV is set to equal 1vRMS and 0dBu is set to equal .775vRMS (775mVRMS)
There are various decibel measurements, db, dBu, dBV and dBm.
In simple terms - 0dB on your volume control is (typically) full power output for a given (signal) input voltage - but it is by no means a calibrated measure.
The output power is measured in to a dummy load (usually 6 ohms) across the speaker terminals.
So, for example if your amplifier is rated at 200w into 6 ohms, it might require 1v RMS across the input terminals to produce 200w when the volume control is set to 0dB.
However, the volume control calibration is unlikely to be absolute - e.g. -3dB may not necessarily be exactly -3dB and so on.
All audio recordings are made using calibrated equipment.
Digital audio is recorded using a dBFS scale, where 0dBFS might equal 1vRMS (and maximum recording level).
As you will see in the Wiki article, various organisations have different ways of interpreting reference levels.
See WikiPedia for an explanation of this.
Analogue recording uses two standard levels, where 0dBV is set to equal 1vRMS and 0dBu is set to equal .775vRMS (775mVRMS)
There are various decibel measurements, db, dBu, dBV and dBm.
Edited by TonyRPH on Thursday 17th March 14:59
Here is a thread about 'reference level'.
Going by an absolute reference level relies on at least three things.
1) The amp has an accurate dB readout on the volume control.
2) The audio part of the video has been mixed with the correct levels.
3) How accurate the tone calibration is in your receiver (if you even have it)
As I understand it, (reference) levels will even vary between Dolby and THX material - so theoretically you would need to maintain different calibrations for both.
In an ideal world, every DVD/BD disc would contain some kind of setup procedure, just like that built into the amp, to establish accurate levels.
Going by an absolute reference level relies on at least three things.
1) The amp has an accurate dB readout on the volume control.
2) The audio part of the video has been mixed with the correct levels.
3) How accurate the tone calibration is in your receiver (if you even have it)
As I understand it, (reference) levels will even vary between Dolby and THX material - so theoretically you would need to maintain different calibrations for both.
In an ideal world, every DVD/BD disc would contain some kind of setup procedure, just like that built into the amp, to establish accurate levels.
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