Herman R Silbiger <hsilbiger@attmail.att.com> (05/04/91)
I worked for some 18 years at (AT&T) Bell Labs in the Speech Transmission Quality Group. My group was responsible for the measurement of telephone transmission quality, and we devised may subjective tests of telephone transmission parameters to measure this quality by subjective tests. One factor which affects perceived quality is what telephone people call sidetone, and psychoacousticians call auditory feedback. In normal speech, some of the acoustic energy of your voice reached the ear through the air, and some internally. The perception of your own speech is one of the cues you use to control your speech level. When one ear is covered with a receiver cap, part of the acoustic feedback path is missing. The paramaters of the network in the telset feed back some electrical energy from the transmitter (microphone) to the receiver (earphone) to replace the lost acoustic energy. This actually can (and was sometimes) used to control the user's speech level. However, mainly it is desirable to replace some of the acoustic sidetone by electrical sidetone for a more natural experience. People have become so used to this feature that they experience as phobe as "dead" if the sidetone is missing. Since it was so desirable to control sidetone exactly, acoustic sidetone through the handset had to be controlled. Handsets before the "G" handset (the handset on the original "500" set only had a narrow path in the handle, and did not transmit much acoustic energy. The receiver in those sets also had a closed back. With the the 500 set and the G handset came the U1 receiver, which was of the ring armature type. The back of the U1 is open. More modern molding techniques and plastics made a lightweight, hollow handset possible. The combination of the U1 receiver and the G handset provided a high acoustic sidetone level, and severely reduced the perceived transmission quality. Thus, the cotton ball was inserted into the handle to control the amount of acoustic feedback. The size and weight of the cotton ball were tightly specified by Western Electric. Herman "Golden Ears" Silbiger