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Discussing Decibels

Recently we discussed decibels and found that they are just a way of expressing the ratio of two power levels. They do not say how much power, but only how one power level compares with another. The usefulness of this may not be instantly obvious but the following examples should clear any doubts.

If we have a musician making a sound and an amplifier reproducing it, providing the amplifier has sufficient power handling capability to avoid distortion, we would not normally be interested in the exact power level of the music signal, but only in the way the output is related to the input. If we boost the treble we want to know how much it has been boosted compared with the 'no boost condition'. Even when we wish to know about background noise, the absolute value of sound level due to background noise is of less use than knowing how loud it is compared with the signal. In each case we have the ratio between two power levels which could be expressed in dB.

In our discussion we found that if two power levels are the same we could express the ratio as 0dB. If one is ten times bigger it is +10dB, if 100 times bigger it is then +20dB et cetera. Similarly, if it is 10 times smaller, that is —10dB,or, if 100 times smaller,that is -20dB and so on. Another figure worth remembering is that 3dB is a power ratio of 2. +3dB is then twice as much power and -3dB is half as much.

The following list is an explanation of some of the expressions used in manufacturers' data — there are others but once the more common ones are understood most of the others can be worked out by using common sense and sometimes a little intuition.

Bandwidth — 30Hz TO 16KHz (-3dB)

This means that the equipment in question has a frequency range over which it will work. If you use signals of very high or very low frequencies the output power for a given size input will fall as you get towards the end of the band. The points at which the level will have fallen to half power (-3dB) are 30Hz in the low and 16K Hz at the high end.

The response at the ends of the band does not usually cut off sharply, so only by deciding how much fall off can be tolerated can you decide how much of the band can be used. Sometimes -0.5dB or -1dB points are quoted. If a band width figure is given without saying how much fall off in power is accepted as being within the band then the -3dB points are implied.

Treble Control — +13dB @ 10KHz

This means that frequencies in the high treble (10KHz) can have their power level changed from 20 times the normal power (+13dB) to 1/20th of the normal power (-13dB) by the treble control. Signals of frequencies nearer the centre of the audio band will be boosted or cut by proportionally smaller amounts.

Signal Level = 0dBm

This is a case where you must know the convention before it means anything. 0dBm means a power level of 1 milliwatt (1/1000 watt) to a signal level of 0.775 volts.

Most pre-amplifiers are designed to feed this signal level out and most slave amplifiers are designed to give full output when 0dBm signal level is fed in.

Signal to Noise Ratio 86dB

This means that the 'noise power', when there is no signal present, is 86dB below the maximum power output that the equipment will give. 86dB is a power ratio of 400,000,000 to 1; that is a 400 watt amplifier with signal to noise ratio of 86dB will give 1/1000,000th of a watt noise power output.

S/N ratios of less than 50dB become objectionable.

Sound Level of 80dBA

The convention here, 0dB, is the threshold of hearing, which has been chosen as a sound pressure level of 0.0002 dynes per cm2. The sound level is the number of decibels above this reference.

The 'A' means that the measuring instrument used has a frequency response which has been modified to account for the ear's uneven sensitivity to all frequencies.

Sound levels of greater than 95dB above 0.0002 dynes/cm2 can cause permanent ear damage.

You will also find dB used to:-

(i) define the directional properties of microphones, i.e., how much signal output the mike gives with the sound source behind it compared with that with the sound source in front.

(ii) the sound distribution patterns from speaker boxes.

(iii) gains of amplifiers and many other things.

Once you understand dB's most of these are self-evident but one or two (for example, transistor noise figures) are not. However, you are unlikely to meet these in normal music circles.

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Jeff Beck

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Working Abroad

International Musician & Recording World - Copyright: Cover Publications Ltd, Northern & Shell Ltd.


International Musician - Jul 1975

Donated & scanned by: Mike Gorman

Feature by Bruce Gibbs

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> Jeff Beck

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