2009-01-13_GENERAL DOCUMENTS - M1984101 (11)

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Figure 9 The simple sound meter is a device which nxawres sound pressure and dis- plays a reading on soaae type of analog cats digital meter. (see Figure 9) Sir" we will be interested in sound levels, within The ranee of human hearing, our mace should cover that entire range. Figatc 10 shows the range of sound Pressures, in linear (micro pascals) v well as logatidtmic (e(B), of human hearing. h would be difficult, if nor impossible eo construct a maer that would read from 200 to 200,0M,0W in linear units. Fur- dtamort, the human mind cannot ac- curately perceive magnitudes such as these. By way of example a new dollar bill is .009 thick. (see Figure 11) Almost everyone has a mental picture of the rhickrT of a dollar bill- We can all es mare The thickness of a pile of ten one dolar bills or for that matter, even one hundred dollat bulls To perceive magnitudes of numbers greater than that starts m become difficult. Do you know the Thickness of one million dollar bilk' if you guessed 330 feet you are c r- tect and also rather unusual- So it is with sound tirmsurernents. Represenmazive Noise Lasts dB L og srichatic linear Sound Studio 20 dB 200 micro pascals Quiet Office 40 dB 2,000 muzo pascals Conveasadon (3 ft) 60 dB 20.000 min pascals Noisy restaurant 80 dB 200,000 micro pascals Printing prey 100 dB 2,0W,0A tnU= pastak 50 HP siren (100 fr) 120 dB 20,000,000 micro pascals let plane . 140 dB 200,000,000 micro pascals Figure 10 The human ear is an amazing in- scrumem with resolution fat in excess of most scienrifc instruments. The weakest sound intensity that the ear can detect is about- 20 micro pxcA;, the highest intensity sound which the ear on per- cei-, without pain, is approximately 100 minion micro pascak. A Taitzo pascal is a unit of pleasure (ace adden- dum). If u,e we= to work with sound in micro pascals, we would encounter a very large unmanageable numbs. To simplify eomputadom. and reduce The size of numbm encountered in work, a unit called a daibel is used. The dedbd is a logarithmic Term which is used in order co reduce The magnitude of numbers- Fipm 10 illusm es the d6, m logarithmic sole compared To the linear b min pascal sole for a variety of sound sitvatiom- Our minds have difficulty relating very large numbers rherefom to solve dais problem the decibel. stir is used. Arswh- very useful aspect of the dB sm4 is that it gives a much better approximation to dx human perception of relative loudness than the pascal- scale since The - rocts to The percentage change in level which corresponds to The decibel state. 0Te decibel is The same Tdarivc clung[ evcryvrhete on th< scale. DOLL.' BILLS S7 S100 Fagure 1 }