NEMA is protecting us from self-induced deafness

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

13 December 2009

The new environmental regulations state that the noise from any device should not interfere with normal conversion 30 metres away from the source. The question that arises is how one can tell whether the sound from their radio, for example, is disturbing people outside the house.

For the most, part we shall have to rely on our common sense to decide on that; however, if you like listening to loud music, you may have to get yourself a sound level meter. You can buy a decent one for under $100 from the Internet.

Even with such a device, you will still have the problem of how to actually carry out the measurement. Ideally, you should go outside with the gadget, 30m from your sitting room. But we can mix a little common sense with science to simplify the process.

First of all, it is common knowledge that the farther away you are from a source of noise, the softer it sounds. The reason for this is simple: the greater the distance, the greater the area over which the sound energy is distributed; therefore, the lower the intensity (that is, energy or power per unit area).

If the distance from the source doubles, the area goes up by a factor of four and therefore the intensity drops to a quarter of the original. This is equivalent to 6dB reduction. Similarly, when the distance is increased from one metre to 30m, the sound intensity goes down by a factor of 900, or about 30dB.

This means that if you are listening to your radio at a moderate 65dB, the intensity of the sound will drop to a barely noticeable 35dB when listened at 30m from your house... and this is assuming that the walls do not block the sound. When these are taken into consideration, we can knock of another 5dB or so leaving only 30dB.

Let me reiterate: the sound of your radio playing at a moderately high level is barely discernible by a person standing 30m from your house.

In order to interfere with normal conversation, the sound heard at 30m away would have to be at least 60dB. Working backwards, it turns out that you would have to be playing the radio at an ear-shattering 95db + 5dB (to account for the walls) = 100dB - enough to make you deaf in about 10 minutes!

Thus it appears that the National rules are not only protecting our neighbours from the nuisance of our sound equipment: they are also protecting us from self-induced deafness!

Finally, I need to clarify a point I made two weeks ago regarding decibels. I wrote that for noise to sound twice as loud, it has to be raised by about 10dB. Several mathematically inclined readers have written to dispute that statement. They point out that doubling of intensity is equivalent to only 3dB.

Well, both of us are right. The confusion arises because we are talking about two different things: perceived loudness and measured intensity. I was referring to the former while these readers are talking about the latter.

A 3dB change in sound level is just barely discernible while a 10dB increment is perceived as a doubling of loudness...even though it represents a ten-fold increment in intensity.