Quantifying and Measuring Sound

Decibels

The uneven sensitivities of the human hearing system leads to measurement by ‘decibel’ on a logarithmic scale which is progressively ‘squashed’, rather than being a uniform scale. It happens that this is also the way that our hearing perceives sound energy or strength.

So the simple energy or pressure measurements of sound are converted to sound level values in decibel(dB) which are easier numbers for humans to understand andrelate to. Extra-terrestrial beings, or even your cat, might wellprefer the unconverted values!

Sound levels in decibels start with a zero at the threshold of hearing which is the weakest sound that the average human ear can detect Typical effects of sound levels and changes in sound levels are shown in the illustrations. Remember that there is a distinct difference between a change in energy and a change in our idea of loudness.

A change in sound level of + or – 10 dB is a useful figure to remember as it makes difference of approximately twice as loud, or half as loud. We have to say ‘approximately’ as the experience also depends on individual hearing, on the background noise and on the exact frequencies involved.

For example, there may be a proposal to increase the averages ound level of your environment from 60 dB to 70 dB. This seems a relatively small change, after all the scale runs from 0 to 140 but it will make the environment twice as noisy.

The same idea applies to reducing noise. If the manufacturers of a certain machine can reduce the sound level from 90 dB to 80 dB then the machine will sound approximately half as loud as before.

Sound Level meters

It is relatively simple to make instruments which can measure sound waves and display the results. Most sound meters detect sound with a microphone which converts the sound energy into electrical signals that can then be processed by electronic circuits.

Human hearing changes sensitivity with frequency, so if wemeasure noise at different frequencies, we will get different values. The most complete measurement of a particular noise, such asfrom your washing machine, is given by separate readings of thesound level (in decibels) at standard bands of frequency and showing the results in a list or on a chart.

A Weighting

We often wish to measure a noise with a single measurement and this is done by allowing the sound level meter listen to all the frequencies at one time, in the same way as the ear. An electronic circuit in the meter then emphasises the middle frequencies, justas the hearing system does. The results are usually quoted as dB(A) and the ‘A’ is an important tag telling us how the measurement was made.

Measurements in dB(A) are widespread and convenient but you must remember that they are a compromise and tell us nothing about the frequency content of a noise. The ear can tell the difference between a washing machine and a telephone buzzer because their sounds have a very different structure. But such different sounds may give identical readings in dB(A).