A.2.1 Science of Sound Record

This record, produced by Bell Telephone Laboratories, (Folkways FX6136) is available on cassette tape.  Single cuts can be played to the class.

1. How we hear

Sound vibrations, compression and rare fractions: The Science of Sound has been recorded eight decibels lower than usual so that illustrations of acoustic phenomena can be presented with loudness approximately proportional to their level in nature. Adjust the volume of your phonograph so that the announcer sound as if he were speaking to you in a normal conversation voice. Then, orchestral music and certain other sound effects that would normally be louder than a human voice will sound louder.

2. Frequency

The frequency range of the average human ear, sweep tone from 30 to 15,000 PCs.: This recording and the phonograph equipment on which you are playing it are, even more than your ear, limited in their ability to reproduce faithfully very lost and very slow sound vibrations. However, with high-fidelity equipment, you may be able to hear vibrations between about 50 and 12,000 cps. The sweep frequency tone is recorded according to the recommendations of the RIAA. Playback equipment that is equalized for the RIAA curve and operating correctly will reproduce all tones from your loudspeaker with approximately equal power. Differences in the apparent loudness of the various frequencies are due to the characteristics of the human ear. These pitch versus loudness characteristics have been charted by Fletcher and Munson and can be seen in most standard textbooks on sound and acoustical engineering.

3. Pitch

How pitch depends chiefly on frequency and is some extent on loudness the subjective nature of pitch: the mel scale. the standard reference for 1000 mels is a 1000 cps tone at 40 decibels above 0.0002 dynes per square centimeter. the mel scale tones on this record are presented at greater intensities so that they can be heard more easily.

4. Intensity

Measuring sound intensity; standing waves; the decibel: The average human ear has a maximum range of about 130 decibels between the threshold of hearing below which no sound can be heard, and the threshold of feeling, above which sound intensity becomes uncomfortable. Since the range of ordinary phonographs is limited we have not attempted to demonstrate an intensity range of more than about 40 decibels.

5. The Doppler Effect

Two instances of the Doppler Effect: (1) observer moving, sound source stationary;(2) observer stationary, sound source moving. (the sounds of racing cards were recorded at the 1956 International Sports Car Grand Prix, Watkins Glen, N.Y.).

6. Echo and Reverberation

Speech accompanied by echo from reflecting surfaces at 500 ft., 200 ft., and 50 ft: How speech sounds in rooms with long, moderate, and short reverberation times. This demonstration was produced in Bell Telephone Laboratories by means of magnetic tape delay devices in combination with a reverberation chamber. So that delay differences could be heard easily, the echoes are recorded somewhat louder than generally experienced. An attempt is made, however, to present the loudness of an echo in accordance with the length of the echo path.

7. Delay Distortion

Speech and music with some frequencies delayed in transmission: This demonstration was simulated by using a delay system with multiple recording heads located around a rotating magnetic disc. the sound is split into two bands by high-pass and low-pass electronic filters that have a cross-over frequency of 3000 cps. (The music is a trumpet fanfare by Semmier.)

8. Fundamentals and Overtones

The fundamental; overtones; harmonics: The lowest frequency present in a sound is called the fundamental, frequencies high than the fundamental are called overtone frequencies. A special tone generator developed at Bell Telephone Laboratories was used to produce the various tones used in this demonstration.

9. Quality

The effect of overtones on sound quality: A factory whistle, a soprano, and a piano are compared with and without overtones. Special low-pass filters with unusually steep slopes of cutoff (150 decibels per octave) were used in this demonstration to eliminate some overtones. Attenuation in the stop band is 55 decibels or greater.

10. Filtered Music and Speech

Music and speech with various frequency ranges eliminated: The filters that were used in this demonstration have sharp cutoffs. 250 decibels per octave. Attenuation in the stop bands was 55 decibels or greater. Wedding Day at Troldbeugen by Greig is played by the Bamberg Symphony Orchestra.