Nature, Measure and Quality of Sound

Sound level is measured in units of decibel (dB). Here deci means one-tenth and bel is the level of sound. The term Bel is after the name of inventor of telephone, Alexander Graham Bel.

It is a unit which compares the levels of power of two sources. Two power levels P1 and P2 are known to differ by n decibel if

n = 10 log10 P2/P1

Here P2 is the sound which is measured while P1 is a reference. Normally, the reference is a sound which is just audible. For average human ears, the whisper is about 30 decibel. The normal conversation is about 65 decibels while a jet plane taking off makes a noise of about 150 decibel. Beyond 85 decibels, sound is damaging and can lead to temporary loss of hearing. Prolonged exposure to noise can cause permanent hearing loss.

It is not advisable to play band in a marriage procession near hospitals where patients can suffer because of noise. Noise raises the blood pressure and causes anxiety. Even if we don't realize, constant noise causes tension. Crackers during festivals are also harmful as they not only pollute air but also create noise.

Loudness and Pitch

Sound from a metallic tumbler on hitting with a metallic spoon is higher in pitch than sound from a pitcher when hit with a wooden stick. The voice of females is generally higher in frequency than male voice. However, voice is not just one frequency. It is a mix of many frequencies, some of which are multiples (called harmonics) of the same frequency called fundamental note for the person.

Voice is not just one frequency. It is a mix of many frequencies, some of which are multiples (called harmonics) of the same frequency called fundamental note for the person.

A flute produces a higher pitch (smaller wavelength, larger frequency) when all holes are open. When all holes are closed, it produces the largest wavelength.

Decibel Meter

Considering the effect of sound on human health, it becomes necessary to develop an instrument to measure loudness of sound. The Decibel meter makes use of a special crystal called Piezo electric crystal. This has a quality that when subjected to pressure, it generates electrical voltage. In a Decibel Meter, a combination of a mic and piezoelectric crystal is used.

Sound causes the diaphragm to vibrate and press the crystal and an electrical voltage generated is measured giving an idea of the sound level. This voltage can be converted into digits using calibration and displayed. Thus, one can estimate noise from fire crackers, vehicles and machines and monitor so that people are not exposed to noise above certain level.