Understanding Acoustics - Part One


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Understanding Acoustics - Part One

  1. 1. Part One of two:In this tutorial we will consider aspects of: • Sound Generation and Propagation • Sound Perception • Properties of Sound
  2. 2. • Sound may be defined as any pressure variation (in air, water or other elastic medium) that the human ear can detect.• Sound may be wanted e.g. Metallica in concert, your lover’s whisper!• Sound may be unwanted (noise) e.g. Metallica in concert! a dripping tap, a car alarm at 3pm.
  3. 3. • Sound is a longitudinal wave that consists of alternating regions of greater-than-normal pressure (compressions) and less-than-normal pressure (rarefactions).
  4. 4. • The air molecules do not travel with the sound wave - they simply move from side to side in the same plane that the sound wave is travelling in.• Generally, the variation in pressure must take place between 20 times per second and 20,000 times per second for us to perceive it as sound.• Below this range is infrasonic, above, ultrasonic.
  5. 5. • Vibrating object - disturbs surrounding air• (Does not even have to vibrate) Two hands clapping cause a disturbance of the air around the hands: the hands are the source of the sound• The local region of air has increased energy caused by the motion of the air molecules – This energy spreads outwards in sound waves
  6. 6. • Waves can propagate through other materials, so we can hear in the water and we can also hear conversation in another room even though the door is closed (speed is different for different propagating media)• there is no sound propagation if there is no propagating medium (sound can not propagate in vacuum). So can’t hear you scream in space!!!• Some (or all) of the sound energy may be lost in the propagating medium (sound absorption)
  7. 7. • For us humans the sound receiver is the ear!• Our sense of sound is greatly determined through frequency, timbre, your age and many other factors.
  8. 8. Here are some quantities/qualities to describe sound:Objective properties• Frequency• Amplitude• Waveshape (spectrum, frequency content) (more from the point of the sound source)Subjective properties• Pitch• Loudness• Timbre (more from the point of the sound receiver)
  9. 9. • The number of wave disturbances (compressions or rarefactions but usually measured as the number of wave crests) that pass a given point in a specific amount of time• This is the inverse of the period• Expressed in Hertz (Hz) - number of cycles per second i.e. 1 Hz = 1 cycle/1 sec = 1/1sec = sec -1
  10. 10. • Amplitude (A) - maximum excursion of a particle of the medium from the particles undisturbed position • Wavelength (λ) - distance along the wave between two successive equivalent points, such as two crests or two troughs• Period (T) - time required for the wave to travel a distance of one wavelength