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Author
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Topic: Decibel ranges?
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Steve Guttag
We forgot the crackers Gromit!!!
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Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 04-02-2004 07:19 PM
The whole decible thing is actually more complicated than has been listed here.
For instance...Randy did real well until he gave you the formula. with the 10 multiplier. That works for power measurements but not voltage and most other forms of dB.
At its heart, dB is merely a unit of measure that compares one value to another on a log scale (common log, base-10)
Normally when someone refers to some level, there is a suffix attached to the dB value...such as dBc which denotes that it is a sound pressure level measured against a "C" weighted scale (mimics the pass band of the range of human hearing from 20Hz to 20KHz). Another popular form is dBa which also is a sound pressure level measurement but using the A-weighted scale that not only accounts for the passband of human hearing but the typical human's frequency response. You will typically find residual noise measurments based on the dBa scale where as overall program levels referenced to the dBc scale.
There are tons of other scales and that little suffix will often be a clue as to what it is you are measuring (what the reference is and what the units are). Common ones are dBm, dBv and dBV.
More often than not the multiplier is actually 20 such that when one is making a dB calculation it will take the form of 20Log(measured value/reference value). The reason for the difference in multipliers has to do with how logs work and the relationship of voltage, current and power.
Some simple rules to remember for working with voltage. Doubling the voltage equals a 6dB increase. Likewise, halving the voltage will result in a 6dB decrease. So if you have a measurement of 300mV...and you raise the level to 600mV...you raised it 6dB. e.g. 20Log(.6/.3)= +6dB.
With power there is only a multiplier of 10...if you double the power you only add 3dB. But another nifty rule comes up with this...10 times more power results in a gain of 10dB which sounds about 10 times louder. Normally you don't get to have such a linear feel for sound like that.
The next thing to realize for cinema and other sound is log addtion. It is a more complicated subject for the faint of math but will let you understand how adding surround levels such that if you have 85dBc from the left surround and 85dBc from the right surround will yield a combined SPL of 88dBc.
So, in the above examples...I've shown how voltages (or currents) compare on the dB scale, how sound pressure levels compare since they have those special suffixes to the dB scale.
As others have pointed out...you could have a volume knob that has "0dB" when all of the way up and negative dB numbers as you rotate it counter-clockwise all of the way down to "-infinity."
What does it mean when the volume knob (or some call it the attenuator) to say -20dB? Just that you have attenuated the signal, any signal, by 20dB. Its reference is only to maximum and not to some finite number (except maybe when the device clips or otherwise runs out of power rails). If you were to input say a 1KHz tone into your device with the knob at 0dB and note its voltage at the output, then lower the volume knob until it read -10dB...you should be able to predict what the output voltage is using the above formulas. Then, you could take say pink noise and an SPL meter and connect your device up to an amplifier and speaker and do the same experiment measure the pink noise at 0dB...(set your pink noise generator so the level is comfortable)...note what it is...say 75dBc...then lower the volume knob to -10dB and the SPL meter should agree and say 65dBc...presuming your inherent room noise is not contributing a significant amount.
Class dismissed.
Steve
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