Understanding the new legal noise limits

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

29 November 2009

In a nutshell, Legal Notice Number 61 of 2009 says “no more shouting or producing loud noises unnecessarily”. But if you feel that you have to, then you must get a license from the National Environmental management Authority (NEAM).

The notice has given the sound level limits for various places and different times. Motor vehicles, for example are not allowed to emit sound that is louder than 84 decibels (dB); residential areas shall have no more than 50dB of noise during the day and 35dB at night; and so on.

The first thing that comes to mind is: what is a decibel and how large is it?

The ear is only able to detect sound by comparing changes in the power transmitted by the waves. If the power increases by a factor of ten, we get a sense that the loudness has doubled.

Now a sound power increase of a factor of ten is called one bel. If you increase the level by a second factor of ten, the end result will be 100 times increment power (10 x 10); but the ear will only hear a second doubling therefore this becomes two bels; another factor of ten brings it to 1,000 times the power and it is now three bels; and so on.

Clearly, one bel is a very large quantity thus it is more convenient to use fractions of the unit. This is in the same way we might find a metre to large and therefore resort to centimetres.

Now, in the metric system of measurement, there are special names given to fractions and multiples of ten, hundred, thousand, and so on. For example, the “centi” in centimetre means a one hundredth fraction of a metre…no wonder there are 100cm in one metre. “Milli” in millimetre indicates a thousandth fraction…thus there are 1,000mm in one meter and, consequently, 10mm in one centimetre (1,000 divided by 100).

In this system, a fraction of one tenth is designated the name “deci”. Thus a decibel is a tenth of a bel; in other words, there are ten decibels in one bel. Therefore, if the sound power is increased by a factor of ten, that is equivalent to a 10dB increment. A factor of 100 is NOT 100dB! It is just 20dB (remember, 2bels).

Now that explains what decibels are; but how large are they? The onset of complete, dead, silence is designated zero dB. The fact that no one can hear it does not mean there is no sound! A whisper is about 15dB; normal conversation is 60dB; a car horn about 110dB; and the car alarm siren produces about 120dB.

Even though the difference between 110dB and 120dB seems small (just 10dB), the car alarm is twice as loud as the horn. Remember, 10dB equals one bel, equals doubling of sound level.

Prolonged exposure to sound that is louder than 85dB can cause hearing loss. Presumably, this is the reason why NEMA put the limit for motor vehicles at 84dB. The 60dB for commercial zones is probably intended to ensure that the noise wouldn’t interfere with normal conversation.

How can you tell that the noise has exceeded the safe 85dB limit? If you have to raise your voice when talking in order to be heard, then the noise in your environment is higher than 85dB