<text><span class="style10">coustics (4 of 4)</span><span class="style7"></span><span class="style10">THE DOPPLER EFFECT</span><span class="style7">The Doppler effect (or Doppler shift) - first described by the Austrian physicist C.J. Doppler (1803-53) - is valid for all waves. It is more often noticed in acoustics and is particularly noticeable in the sirens used for emergency-service vehicles. The intensity and pitch of the siren seems to rise as the vehicle is approaching, then diminishes as it moves away.This is explained by the fact that, as an observer moves towards a sound source, the pressure oscillations are encountered more frequently than if the observer were stationary. Thus the source seems to be emitting at a higher frequency. Conversely, if the observer is moving away from the source, the frequency seems to decrease. It also applies if the source is moving and the observer is stationary.Horseshoe bats emit beams of sound at constant frequency; the frequency from different species of horseshoe bat varies in the range of about 40-120 kilohertz (kHz: 1 kHz = 1000 Hz). Applying the Doppler effect, the frequency of the echo returning to their ears after reflection from an object depends on whether the bat and object are getting closer together or moving apart. The bats seem able to observe Doppler shifts of less than 1%. The nostrils are spaced a quarter of a wavelength apart so that along the normal - or perpendicular - to a line between the nostrils the waves are in phase and thus of maximum total amplitude. Thus the emitted sound is beamed straight in front.The Doppler effect is also observed in optics, and has proved to be of profound significance in our understanding of the universe. If a stellar spectrum is compared with an arc or spark spectrum of an element present in the star, then its spectral lines may be displaced. A shift to the red end of the spectrum - to the longer wavelengths - means that the star is moving away from the Earth. The US astronomer Edwin Hubble (1889-1953) studied the red shift in various galaxies, and determined that their velocity of recession was proportional to their distance from the Earth. He thus confirmed that the universe appears to be expanding.</span><span class="style10">SONIC BOOMS</span><span class="style7">If the velocity of the source of the sound wave is greater than the velocity of sound - 300 m s-1 (1080 km/h, or 670 mph) in the upper troposphere (up to 10 km / 6 mi above the Earth's surface) - then the wavefront produced is not spherical but conical. Different wave crests bunch together, forming a shock wave. A supersonic plane (traveling faster than the speed of sound) produces such a shockfront, causing a loud bang (a sonic boom) and large pressure variations.</span></text>
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<text><span class="style10">he greater the plane's</span><span class="style7"> velocity exceeds the velocity of sound, the narrower the angle of the cone. If an airplane is particularly long, a double boom may be heard, as both the front and tail of the plane generate shockfronts.</span></text>
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<text>ΓÇó WAVE THEORYΓÇó OPTICSΓÇó EARTHQUAKESΓÇó SEEING AND HEARINGΓÇó MEDICAL TECHNOLOGYΓÇó SEEING THE INVISIBLEΓÇó WHAT IS MUSIC?</text>
