<text><span class="style10">coustics (3 of 4)</span><span class="style7"></span><span class="style10">The human ear</span><span class="style7">The ear is an extremely sensitive detector. Its threshold of hearing corresponds to an intensity of sound of 10-12 watts per square meter (W m to the power of 2): this is a measure of the energy impinging on the ear, and is known as the </span><span class="style26">threshold intensity</span><span class="style7">. The loudest tolerable sound is about 1 W m to the power of 2. This range is enormous, and so a logarithmic scale, to the base 10, is used. The original unit was the </span><span class="style26">bel</span><span class="style7">, named after Alexander Graham Bell (1847-1922), the Scottish inventor of the telephone.The bel is graduated using a logarithmic scale, but as the bel is rather a large unit, the </span><span class="style26">decibel</span><span class="style7"> (db) is more normally used (1 bel = 10 db). If threshold intensity is at 0 db, then a sound at ten times threshold intensity is 10 db, one at a hundred times threshold intensity is 20 db, one at a thousand times threshold intensity is 30 db, and so on. This means that the value of 1 W m to the power of 2 is at 120 db above threshold.The ear canal resonates slightly to sounds with frequencies of about 3200 Hz. The human ear is most sensitive in the range 2500-4000 Hz. Even then, only about 10% of the population can hear a 0 db sound and then only in the 2500-4000 Hz region. The response of the ear is not linear, i.e. there is no direct relationship with the intensity of the sound it detects. Sensitivity is related to frequency: it de creases strongly at the lowest audible frequencies, but less so at the highest.The audible range of the normal human ear varies with age. It is usually about 20-20 000 Hz in the mid-teens. For someone 40 years old, the upper limit is more likely to be 12 000-14 000 Hz. At the lower hearing threshold, the pressure fluctuations from the sound wave are about 3 x 10-10 of atmospheric pressure. The eardrum (called the </span><span class="style26">tympanic membrane</span><span class="style7">) vibrates at very low velocities - about 10 cm (4 in) per year. This may seem strange, given that it can vibrate at frequencies of up to 20 000 Hz (cycles per second); however, the low velocity is explained by the fact that the detected displacement of the air molecules adjacent to the eardrum each time it moves is less than the typical atomic radius (about 10-10 m). The human ear is an astonishingly sensitive detector and so it is not surprising that constant overload will bring deterioration in its performance.</span><span class="style10">Characteristics of tones</span><span class="style7">There are three main characteristics of the notes played by musical instruments. </span><span class="style26">Loudness</span><span class="style7"> would seem to be the most simple, but it is complicated by the non-linear response of the ear. At 100 Hz and 10 000 Hz the hearing threshold is about 40 db compared to the 0 db at 2500-4000 Hz. Thus the concept of loudness is not dependent just on the energy reaching the ear, but also on frequency.</span><span class="style26">Pitch</span><span class="style7"> is closely related to frequency. If the frequency of vibration is doubled the pitch rises by one octave. In general, the higher the frequency the higher the pitch.Sounds created by musical instruments are not simple waveforms, but are the result of several waves combining. This complexity results in the </span><span class="style26">tone quality</span><span class="style7"> or </span><span class="style26">timbre</span><span class="style7"> of a note played by a particular musical instrument. Even a `pure' note may contain many waves of different frequencies. These frequencies are </span><span class="style26">harmonics</span><span class="style7"> or </span><span class="style26">multiples</span><span class="style7"> of the fundamental or lowest frequency, which has 2 nodes and 1 antinode, and is called the </span><span class="style26">first harmonic</span><span class="style7">. The </span><span class="style26">second harmonic</span><span class="style7"> has 3 nodes and 2 antinodes. The wavelength is halved and the frequency is doubled. The </span><span class="style26">third harmonic</span><span class="style7"> has 4 nodes and 3 antinodes. The wavelength is one third of the original wavelength, and the frequency has tripled. Different instruments emphasize different harmonics. Musical synthesizers are able to mimic instruments by mixing the appropriate harmonics electronically at various amplitudes.AS</span></text>
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<layer>background</layer>
<id>23</id>
<text>ΓÇó WAVE THEORYΓÇó OPTICSΓÇó EARTHQUAKESΓÇó SEEING AND HEARINGΓÇó MEDICAL TECHNOLOGYΓÇó SEEING THE INVISIBLEΓÇó WHAT IS MUSIC?</text>
