Location: Ch 2: Sound
SOUND 

IN THIS CHAPTER: 

  • The Origin of Sound
  • Sound In Air
  • Media that Transmit Sound
  • Speed of Sound
  • Loudness
  • Forced Vibration
  • Natural Frequency
  • Resonance
  • Interference
  • Beats
  • The Ear

Sound is produced by a vibration of some object. When it vibrates back and forth, it moves air molecules back and forth. As these molecules move back and forth, they compress and expand, producing compressions and rarefractions. As these molecules compress and rarefy, they strike our eardrums. The bones in our eardrums vibrate back and forth, creating pressure changes in the fluids of our cochlea. As these fluids move back and forth, they stimulate tiny ear hairs that trigger impulses to be sent to our brain. Thus we perceive sound. Because sound requires molecules to vibrate, sound cannot travel through a vacuum. The pitch of a sound depends on the frequency of the sound. A high-pitched scream has a very high frequency, while a low-pitched humming has a very low frequency. Sound usually travels at about 340 m/s in room temperature air. The precise speed of sound can be calculated with Decibelsthe following formula: 331.5 + (0.6)(temperature °C). The speed of sound in a material, however, depends on the material’s elasticity, or its ability to resume its original shape. The intensity of a sound is proportional to the powers of ten. Apparent loudness, on the other hand, is a psychological sensation that is subjective to the person’s brain and is not related to the speed of sound.

   When any object is struck, it is forced into vibration. This is known as a forced vibration. When any object is disturbed, it vibrates at a special set of frequencies known as its natural frequency. Natural frequency depends on elasticity and shape of the object. Natural frequency is the frequency at which the minimum amount of energy is required to produce and sustain vibrations. Almost all objects have a natural frequency. Interestingly enough, if the natural frequency and the frequency of the forced vibration are equal, the amplitude dramatically increases and resonance occurs. Resonance, while most readily apparent with sound waves, is not restricted to wave motion.

   Sound waves can interfere with each other. When sound waves interfere, they affect the loudness of the resulting sound. If two speakers are equally distant from you, the crests of both waves will overlap, producing a louder sound. However, if the speakers’ distance are different by a half-wavelength, the crests and troughs will cancel each other out, causing a dramatic reduction in loudness. While the use of interference to cancel out loud noises is great for jackhammer operators, it is not very good for home stereo equipment. An interesting thing happens when two tones of slightly different frequency interfere with each other. The sounds seem to alternate between loud and soft. These periodic variations are known as beats. Beats can be heard when two computers beep at different frequencies. If one beeps at 300hz and the other at 306hz, the crests will be in line three times a second, producing a 3hz beat frequency. Two computers can be made to produce the correct sounds with the two BASIC scripts:
Computer 1: Computer 2:
10 cls
15 rem Generates a 300hz tone
20 sound(300, 1)
30 goto 20		 10 cls
15 rem Generates a 306hz tone
20 sound(306, 1)
30 goto 20
   In conclusion, sound is a longitudinal wave that requires particles to create compressions and rarefractions. The vibrations of any object produce sound. An object vibrating at high frequency produces a high-pitched sound and an object vibrating at low frequency produces a low-pitched sound. Sound travels fastest through elastic materials, and if a material vibrates at its own natural frequency, the sound become louder. Finally, beat frequencies are created when two tones with very close frequencies are sounded at the same time.
Location: Ch 2: Sound