Characteristics of Sound

  1. pitch, frequency; measured in cycles per second
  2. wavelength = velocity/frequency

    [if the velocity of sound in air is 600 feet per second = 18,288 cm/sec. and the frequency is 440 cycles per second (a standard A note) the wavelength is 18,288 cm/sec divided by 440 cycles per second = 41.6 cm = 16.4 inches.]

  3. intensity, amplitude; measured in decibels, db

pure sound: a pure wave, a sine wave

Real Audio: A mixture of frequencies (a mixture of wavelengths).

This "Wow Cool Man" from Bart Simpson is made up of many, many sine waves of sound! We could separate them into their components using a sound filter-just what a recording studio does with recording artists.

The Power of Sound

  1. Hi-Fi set outputs are measured in watts of power.

  2. Energy per second = power = watts.

  3. The intensity of sound is measured in watts per square centimeter.

  4. The faintest sound heard by a human ear = 0.000,000,000,000,000,1 watts = 10^-16 watts.

  5. The loudest, tolerable sound heard by a human ear = 0.000,1 watts = 10^-4 watts.

  6. This is a range of 10,000,000,000 = 10^10 . Need a convenient way to deal with such a large range.

  7. Compare the intensities with the faintest sound heard.

  8. faintest: faintest/faintest = 10^-16/10^-16 = 1

  9. whisper: 100 x 10^-16/faintest = 10^-14/10^-16 = 100

  10. auto: 100,000 x 10^-16/faintest = 10^-11/10^-16 = 100,000

  11. pain: 10^-4/faintest = 10^-4/10^-16 = 1,000,000,000,000

This is still too large a range. What power of ten would give these numbers?

  1. faintest/faintest = 1, or 10^x = 1. x must be zero.
  2. whisper/faintest = 100, or 10^x = 100. x must be 2.
  3. auto/faintest = 100,000, or 10^x = 100,000. x must be 5.
  5. pain/faintest = 1,000,000,000,000, or 10x = 1,000,000,000,000. x must be 12.

Multiply this power of 10 by 10 is the definition of the decibel, db.

  1. faintest: 1, x = 0. times 10 0 db
  2. whisper: 100, x = 2. times 10 20 db
  3. auto: 100,000, x = 5. times 10 50 db
  4. pain: 1,000,000,000,000, x = 12. times 10 120 db

Next: The Ear-detector of sound. The midpoint of hearing is determinedby the length of the ear canal from the outside part of the ear to the ear drum. This length is about 3 cm in humans. The fundamental wave length of a sound wave is 4 times this length.

The ear hears a broad range of sounds as shown in the next diagram. Hearing ranges from about 20 cycles per second to just under 15,000 cycles per second