Book - School of Science and Technology

Sound control 459SoundIt is necessary first to consider the mechanism by which sound is transmitted. Vibrationsat a point A in Figure 16.19 set in motion the molecules of adjacent air such that a seriesof waves of compression and rarefaction radiate from the source spherically in alldirections, a part reaching the ear at point B where the eardrum is set in like motionand the brain interprets the sensation as a sound, pleasant or otherwise. A noise may bedefined as an unpleasant sound, or one that is overloud, or is monotonous or erratic.The speed of sound depends on the properties of the medium, but may be taken at331.46 in/s at 0 C and 101.325 kPa (NTP).FrequencySound from point A will most likely be of more than one frequency. The deepest notewhich the human ear can detect is about 15 Hz, and the highest about 20 000 Hz.Frequencies are divided into octaves, each octave being double that of the lower one.The frequency of middle C is 262 Hz; thus C of the octave above is 524 Hz and so on.Figure 16.20 shows standard frequency bands. Most sound sources set up harmonics, i.e.higher frequencies which are multiples of the fundamental.Approx.scale of wave length (mm)7200 3600 2400 1200 600 300 150 75 38Standard octave bands (H Z)(new preferred frequencies)63 125 250 500 1000 2000 4000 8000Mid-bandfrequency1 2 3 4 5 6 7 8 Band noFrequency43.5 87 174 353 706 1413 2825 5650 11300limitsPiano keyboard27.5 Hz 50 HzMains hum164 HzBig Ben262 HzMiddle C400 HzBBC TVtuningsignal480 Hz 575 Hz1000 HzGMT timesignal pips3520 Hz 10015 HzTV scanningwhistleAir raid sirens440 HzPitch standardFigure 16.20 Standard octave bands