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Sound

Sound. Production of Sound. Sound is produced by a vibrating object A loudspeaker has a membrane or diaphragm that is made to vibrate by electrical currents. Musical instruments such as gongs or cymbals are some examples of vibrating sources of sound.

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Sound

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  1. Sound

  2. Production of Sound • Sound is produced by a vibrating object • A loudspeaker has a membrane or diaphragm that is made to vibrate by electrical currents. • Musical instruments such as gongs or cymbals are some examples of vibrating sources of sound. • The human voice is a result of vibrations of the vocal cords, two membranes located in the throat.

  3. Brass instruments produce their sound as a result of vibration of the lips of the performer (similar to voice). • Reed instruments have a thin wooden strip that vibrates as a result of air blown across it. • In flutes, recorders, organ pipes or whistles, air is blown across an opening in a pipe. • Stringed instruments have a wire string that is set into vibration.

  4. Detection of Sound • The ear is an amazing sound detector. • Not only can it detect sound waves over a very wide range of frequencies, it is also sensitive to an enormous range of sound intensities. • The ear is a very complex organ. • The interpretation of sounds by the brain is even more complex and not totally understood.

  5. Anatomy of the Ear

  6. The ear is not equally sensitive to all frequencies. • Most people cannot hear frequencies below 20 Hz or above 16000 Hz. • Most are more sensitive to frequencies between 1000 and 5000 Hz. • Older people are less sensitive to frequencies above 10000 Hz.

  7. Relative Intensity • Intensity is measured with acoustical instruments and does not depend of the hearing of the listener. • At 1000 Hz, the intensity of the faintest audible sound (the threshold of hearing) is 10-12 W/m2. (I0) • We compare the intensity of sound to this threshold of hearing by setting up ratios. The ratio of the intensities yield an expression call relative intensity.

  8. Relative intensity is given by the equation: • β (relative intensity) is measured in decibels (dB). I0 is the intensity of the threshold of hearing (10-12 W/m2). I is the intensity of the sound in question. Decibels is a dimensionless unit.

  9. A decibel is actually 0.1 of a bel. A term that is named after Alexander Graham Bell (inventor of the telephone)

  10. Human Perception • A change in the sound power level of 1 dB is just barely perceptible. However, this 1 dB change represents a change in intensity of 26%.

  11. Effect of temperature on speed of sound • The speed of sound is affected by the temperature of the air. • @ 0o C, the velocity of sound is 331.5 m/s • For every degree increase you will ADD 0.6 m/s/oC. • For every degree decrease you will SUBTRACT 0.6 m/s/oC. • Ex: What is the speed of sound @ 7.0 oC?

  12. The Doppler Effect • The Doppler Effect is the change in wavelength and frequency caused by motion of objects. • Sound from moving objects are perceived at different frequencies depending on whether the distances between the source and the listener are increasing or decreasing. • If the distance between the objects is decreasing, the frequency is perceived to be higher. • If the distance between the objects is increasing, the frequency is perceived to be lower.

  13. Resonance • In sound applications, a resonant frequency is a natural frequency of vibration determined by the physical properties of an object. This same basic idea of physically determined natural frequencies applies throughout physics in mechanics, electricity and magnetism, and even throughout the realm of modern physics. Some of the implications of resonant frequencies are:

  14. 1. It is easy to get an object to vibrate at its resonant frequency, hard to get it to vibrate at other frequencies. • 2. A vibrating object will pick out its resonant frequencies from a complex excitation and vibrate at those frequencies, essentially “filtering out” other frequencies present in the excitation. • 3. Most vibrating objects have multiple resonant frequencies.

  15. Picking out Resonant Frequencies • If you just whack a mass on a spring with a stick, the initial motion may be complex, but the main response will be to bob up and down at its natural frequency. The blow with the stick is a complex excitation with many frequency components, but the spring picks out its natural frequency and responds to that.

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