Sound and Vibration Basics

Sound Level Meters

Why are good sound level meters expensive?

Sound level meters have been around for many years. In the beginning they were bulky and didn't do very much. They were effectively a microphone, an A-weighting filter and a voltmeter with a 1 second [SLOW] time constant to slow down the meter fluctuations, so you could average them by eye.

Modern sound level meters are much smaller and process the data in more and more complex ways, but the accuracy of the measurements still rely on the quality of the microphone. Measurement microphone manufacturers go to great lengths to ensure the microphone responds uniformly to sound waves approaching from above, from the side or straight ahead.

This is not too difficult at say 256 Hz or middle C on a piano where the wavelength of the sound is very long compared with the size of the microphone. Even at 4 times this frequency, reasonable quality microphones can accurately follow the signal if it is coming from directly ahead or only a few degrees off centre. But at 10 times the frequency only the best microphones are still responding equally and if we want to measure to the upper limits of hearing, say 20,000 Hz or nearly 100 times higher than middle C, then a remarkable microphone [Type 1 sound level meter] is required.

Well before you get to these frequencies the wavelength of the sound wave are comparable with the dimension of the microphones and all sorts of horrible and complex sound fields defeat only the very best microphones from faithfully supplying the 'voltmeter' with an accurate voltage proportional to the original sound wave.

Some factories produce millions of microphones at 1 penny each or less for use in cheap audio products, buyer beware. But Class 1 measurement microphones are very expensive, labour intensive to produce and the manufacturing process, including the required 'ageing' can take months.

The measurement microphone can be compared to the Hi-Fi loudspeaker, one converts sound waves into electrical signals and the other converts electrical signals back into sound waves. A good quality speaker system includes woofers, mid-range speakers, tweeters and cross-over networks in an attempt to cover the range of hearing up to 20,000 cycles per second [20 kHz]. Few manage it without 'colouration' and they cost hundreds of pounds. We expect and British Standards demand that sound level meter microphones to do the same thing, so we should not be surprised if measurement microphones also cost hundreds of pounds.

It follows therefore that you can use the latest computer based sound level meter but the results will only be as good as the microphone that converts the original sound wave into an electrical signal.

See also : Integrating Sound Level Meters and a range of Certified Sound Level Meters