Loudness and pitch of sound are totally unrelated

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

08 March 2009

Last week’s article seems to have created some confusion. Several readers have pointed out that it is obvious that the sound of an approaching car will get louder as it get closer while that of a receding one will get softer. As one person put it: “What’s the point of giving this simple observation a complex name?”

Well, let me clarify: Most of the time, people describe sound in terms of its loudness. Indeed, when some one says that a music system has “good sound”, they probably mean that it can get very loud.

Loudness is normally expressed in decibels (dB). Zero dB is the lowest level that can be detected by any human ear. However, this doesn’t mean that there is no sound; No! It only means that no human being can hear it!

The loudest level that can be handled by a human ear is about 130dB. Anything above that will break the eardrum and lead to permanent deafness. As explained a few years ago (May 2006), this is a strange scale: “An increase of 10dB is perceived in the ear as a doubling of the loudness. Thus 30dB sounds twice as loud as 20dB; 70dB is twice as loud as 60dB and so on.”

The second important characteristic of sound is the tone, or pitch. This is the frequency of the sound wave, that is, how many times it beats per second. One beat per second is called a hertz (Hz)

The best human ear (e.g., that of a baby) can hear between 20Hz and 20,000Hz. By the time one gets as old as I am (middle aged), they will have entered enough matatus and discotheques playing very loud music that the range of hearing is reduce to about 100Hz to 15,000Hz.

The Doppler Effect alluded to last week affects the tone of the sound; not the loudness. To measure it accurately, one needs to compare the frequency in three situations: first, when riding in the car; second, while standing on the roadside with the car approaching; and third at the roadside after the car passes by and is driving away. All measurements must be made with the car moving at the same fixed speed.

It is then observed that the frequency (tone) detected from the roadside while the car is approaching is higher than that measured when riding on the vehicle.

On the other hand, the frequency detected from the roadside while the car is driving away is lower than that measured when riding on the vehicle. This is the meaning of Doppler Effect. It has nothing to do with loudness.

Obviously however, as the car approaches, its sound will get louder (because it is getting nearer) and as it drives away, the sound gets softer. This should not be confused with the change in frequency.

Finally, it is important to note that loudness and pitch are totally unrelated – one does not affect the other!