20. Acoustical Perception

  1. Limits of Human Frequency Perception
    The output of an audio oscillator is amplified and sent to a Hi-Fi speaker. The frequency is varied so each person in the class can determine his/her limits of audible perception. A xylophone extending into the ultrasonic region, and a Galton whistle can also be used to test the upper limits of audible perception.
  2. Harmonics and Beats
    The outputs of two audio oscillators are fed to the x & y axes of an oscilloscope, and to a stereo amplifier and Hi-Fi speakers. The two tones sound pleasing together when the Lissajous figure on the oscilloscope assumes a simple form. The ratio of the frequencies can be determined by counting the lobes of the figure; e.g. octave, C to C', 1 : 2, third, C to E, 4 : 5; fifth, C to G, 2 : 3. Beats are illustrated by the vibrating of the figure.
  3. Decibel Scale
    Using the dB power meter on a stereo amplifier, you can illustrate what power differences sound twice as loud. (See also Science of Sound Tape, A.2.1)
  4. Pitch Perception
    The perception of pitch depends both on the frequency and the intensity of the tone. The output of two audio oscillators are fed to a stereo amplifier and two speakers. The two tones are set at different power levels, and a student volunteer with good musical pitch ability adjusts the frequency of one oscillator until both tones have the same pitcn. It will be found that when the two tones have the same pitch as determined by ear, the louder tone has about 10% higher frequency. (So the louder the tone, the lower the pitch.) The exact frequencies of the oscillators can be checked by an oscilloscope or frequency counter. See also Science of Sound Tape, A.2.1