Acoustic Tests

Figure 38. A thermal receiver.

A telephone transmitter can be tested with some degree of reliability by placing the transmitter in a sound-proof box that is acoustically treated so that negligible reflection occurs from the inner walls. A small loudspeaker and the microphone of a sound-level meter are also placed in the box. The microphone is placed adjacent to the transmitter under test and is used to measure the sound level as the sound frequency is varied, so that the level can be maintained constant. The transmitter under test is provided with normal direct current, and the signal output is measured with a vacuum-tube voltmeter, or by other suitable means.

A telephone receiver can be tested with a fair degree of reliability in a similar box. In this instance the receiver and the microphone of the sound-level meter are placed in the box, and the receiver is driven with an oscillator operating at suitable level. The sound-level meter gives an indication of the output at various frequencies. An oscilloscope and a wave analyzer also are useful in studying the performance of a transmitter or a receiver.

If a special acoustic laboratory is not available, the frequency response of a loudspeaker can be measured out-of-doors in a location free from reflecting objects. A simple method is to measure the intensity of the radiated sound with a sound-level meter. For such tests a beat-frequency oscillator provides an excellent source because the frequency is continuously variable and the loudspeaker can therefore be studied for any decided resonant points which often exist and might be missed if a point-to-point test were made. If measurements must be made in an untreated room, the reflected waves will cause points of maximum and minimum intensity to exist within the room; also, the location of these points will vary with frequency.