Research methods applied to the inspection of automotive parts for noise-producing causes are analyzed by the author, who notes the increasing tendency toward the use of sound-measuring instruments and discusses first the units of sound intensity and loudness. The dyne per square centimeter is a convenient size of unit for measuring the pressure amplitude of sound-waves, since 1 dyne per sq. cm. lies within the range of amplitudes at which the ear normally functions, being approximately that at one's ear when listening to conversation.
In calibrating at high frequencies, the thermophone is used. It consists of a small strip of thin platinum or gold a few centimeters long and about 1 cm. wide through which an alternating current of desired frequency is sent. The formula for the thermophone gives the “r. m. s.,” or root mean square, pressure amplitude in dynes in terms of such easily measured quantities as the current through the strip, the resistance of the strip and some constants of the air.
Loudness as perceived by the ear is not a simple function of the intensity as measured by an instrument. Explanations of this fact are made and instruments for sound-intensity measurements are then described, the most successful methods involving the use of electrical apparatus. Analysis is made of the influence of the room in which the sound measurements are taken, as well as of frequency determination and filtering, the last being the exclusion from the measurements of sound that it is not desired to investigate. In conclusion, it is stated that although the paper emphasizes the difficulties of sound measurement, the difficulties actually exist and must be dealt with before progress can be made.
Main features of the discussion following the paper include questions and answers relating to whether different types of material affect the pitch of the noise produced; the result of sounding-box effect on transmission cases; and whether the frequency of noise or the intensity of noise is the more important.
An increasing tendency toward the use of sound-measuring instruments in the inspection of automotive parts for noise, has existed during the last few years. This has been due to the demand of the buying public for quieter cars and to the recent rapid development of instruments suitable for such inspection work. This very timely development of sound-measuring instruments was carried on mostly by the Bell Telephone Laboratories, to whom the automotive engineers are indebted for aid on this baffling problem. That the ear is at best but a very rough check on noise when used
continuously in routine inspection work is admitted by most people who have had experience. Of course, the ear is the final judge as to the loudness which can be tolerated, but once these limits have been set they can be maintained within reasonably close bounds only by the use of instruments. One company found that on Mondays, after the inspector had had a quiet day at home, the percentage of rejections for noise was unusually high. This percentage gradually tapered off to Saturday, the inspector's hearing having been dulled by a week of factory noise and continual listening to the objectionable sound.