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648 RICHARDSON'S BLUEBOOK OF PROJECTION
recording and frequency test records should also be available.
The high-quality sound recording serves to check the equipment not only for generally good response but also for such particular troubles as flutter. Piano recordings show up flutter very plainly, and are often used for this purpose. The frequency recordings are used with an output meter or volume indicator. For this purpose it is best to use a reel containing a number of frequencies, all recorded at the same level. The volume control is not changed while this reel is run. The output volume is measured by the meter, and should be the same for all frequencies that the system is intended to reproduce. In some systems, however, the response at certain frequencies is intentionally made low or higher ; this may be done to compensate for acoustic conditions of the theatre or for other reasons. We have seen that the amplifier of Figure 139 included arrangements for varying the frequency response. In such cases the test reel and the output meter are used to determine when the desired modification has been obtained, and periodically thereafter to see that the response of the system has not changed.
The output meter or volume indicator used with the test reel for this purpose is really a disc-type-rectifier a. c. voltmeter, calibrated to read directly in decibels. As already stated, any voltmeter of the same type built to operate within the same range of power can be used instead by means of a simple chart, furnished with the instrument, to convert volts into decibels.
Another instrument that is always useful, and with some types of equipment indispensable, is a combination circuit-tester tube-tester of the kind used by radio repair men. These instruments are made in quantity production for radio dealers, and hence are not excessively costly. The later models are capable of checking the large tubes and high-voltage circuits used in sound equipment. A tester of this kind often contains only a single meter but a large number of switches. By operating the switches in accordance with instructions the same meter will read a wide range of d. c. volts, a wide range of d. c.