Electronic demonstrations for the teaching of acoustics

Abstract

Courses in elementary acoustics taught at many universities provide introductions to the following six topics: physical acoustics, environmental acoustics, the physiology of hearing, auditory psychophysics, musical acoustics, and the nature of human speech. Electronic demonstrations can be used to enhance instruction in each of the six areas. For example, the modules of an electronic music synthesizer together with an audio system and conventional electronic instrumentation form a versatile laboratory for experiments in sound. Among the demonstrations which can be done are the following. Wave interference can be demonstrated by extracting the separate signals from a stereo recording of signals A + B and A − B. Tone and noise bursts can be used to find the speed of sound, echo and reverberation times. Filter banks, set according to mean audiometric presbycusis data, can show the effects of aging on the frequency response of human hearing. Gently nonlinear electronic devices aid the recognition of combination tones normally generated by the inner ear. Analog delay lines can create repetition pitch from a source of white noise. Sine wave signals from many oscillators can be summed and varied to study the perception of pitch of an inharmonic tone complex like that of a chime. The phase variation of a single rectangular wave with pseudorandom pulse width modulation simulates chorus effect. Much of the effect of nonlinear mode mixing in conventional musical instruments can be simulated by patched of linear electronic components, transient generators and voltage controlled signal processing modules. Three resonant filters allow the synthesis of vowel sounds. Frequency shifted or ring modulated speech sounds can exhibit the importance of vowel formant transitions. Intelligibility experiments with chopped speech serve as an introduction to redundancy, autocorrelation and information theory. We note that one does not pursue this informal experimenting with signal processing equipment for long before one discovers effects which are hard to understand. Further pursuit can lead to new results. For example, our intelligibility experiments with chopped speech suggest that redundancy, measured in spoken English text agrees with that found by Shannon for written text.