The Bioscope (Jul-Sep 1931)

THE BIOSCOPE MODERN CINEMA TECHNIQUE July 15, 1931 xii Measuring Loudness New Instrument With Many Uses In a recent issue of The Bioscope we gave an advance illustration of the latest Western Electric Sound Meter. The measurement of loudness of sounds has in the past been generally confined to audiometric methods. As the name implies, these have involved the judgment of an observer, and some standard source of noise. Such measurements, while useful, are not essentially accurate, depending upon the skill of the observer and changes in the physiological and psychological condition of the individual. Moreover, the readings so obtained are only relative, and if sounds are of different character, the readings are apt to be misleading. Measurements by instrumental means have been, in general, of no comparative value and were generally expressed in some arbitrary units selected for the particular measurements at hand. Values so obtained bore, in general, no relationship to the physiological effect ; i.e., loudness, annoy ance, or any other reaction of a human observer. Moreover, the inability to establish a common point of comparison made it impracticable to make relative judgments between any two sets of readings. Cutting Out the Human Element The Western Electric sound meter is based upon the fact that the effects of noise are primarily interpreted through the human ear. The instrument is, therefore, designed to record the effects of noise in terms comparable to the loudness sensation as judged by the ear. It consists essentially of a microphone, amplifier, weighting network, indicating meter, and necessary battery supply, contained in two cases, both together being portable by one man. The microphone picks up the sound, converting it into an electrical current which is a counterpart of the original sound. This is then amplified by a vacuum tube amplifier and actuates a meter reading the loudness of the sound directly in decibels. The decibel scale is an arbitrary one in which each unit represents approximately the smallest change in loudness that can be detected by the normal ear. Zero of this scale is near the threshold of hearing or the point at which sound becomes inaudible in a very quiet space. Since the apparent loudness of a sound of specified energy is not the same for all frequencies, the amplifier is adjusted by means of a weighting network to stimulate the sensitivity of the ear. Meter or Graphic Record as Desired At frequencies near 2,000 cycles per second, the amplifier is most sensitive and decreases in sensitivity with increase or decrease in frequency from this value. The dynamic characteristics of the meter are also adjusted to simulate the ballistic qualities of the ear. Thus the meter indicates visually to an accuracy of about one decibel, the extent to which the ear is affected by a sound. The range of the meter, in the interests of portability, is slightly less than the range of the ear. However, it is not anticipated that it will be required to measure to absolute threshold, nor sounds of actually painful intensity ; and for practical problems the range is, therefore, adequate. Where a continuous graphic record is desired, a level recorder may be substituted for the visual indicating meter. The re corder can be adjusted to produce a graphic record of sound covering any period of time. The substitution of this device for the visual indicator alters the dynamic characteristics of the meter, since mechanical limitations preclude any possibility of a recording device fluctuating as rapidly as noise. However, if peak noise levels are not required, the level recorder is extremely satisfactory. The meter alone merely indicates loudness , and gives no information relative 1 o the character or frequency distribution, which is equally important in any noise study. The frequency distribution may, however, be determined by the meter in conjunction with an analyser attachment. Either broad band analysis to deteimine general characteristics of noise, or sing’e frequency analysis for an exact determination of individual components may be made. Can be Made to Discriminate Obviously, the uses to which the meter can be put are manifold ; surveys of noise conditions, determination of acceptable levels in various types of buildings, compliance of equipment with specified levels, are typical of one field of employment. Studies of loudness of other sounds, such as music, with relation to the performer or with relation to the architecture of the auditorium, represent another type of application. The control of uniformity of manufactured products by Curtain Controllers Resistances Dimmers, etc. / \ * :• • • V \* # 0 VA v « ***•••„ f§ /. tb 5 kw. Dimmer (100-Stud). THE ELECTRO-CONTROL Co. Manufacturers , Riste’s Place, Barker Gate, Nottingham. Telephone: 41184. Telegrams: 41184. means of acoustic measurements is itself a wide and distinctly useful field. Certain limitations in the use of the noise meter must be observed, since it cannot discriminate between different sounds but indicates the total loudness of all sounds picked up by the microphone. Consequently, if it is desired to measure a sound not appreciably louder than others striking the microphone, it is generally' necessary' to stop all sources other than the one creating the sound to be measured. In some instances it is possible to measure sound not appreciably higher or even less than the surrounding or background noise by use of the analyser, providing the sound differs considerably in frequency composition from the background noise. The analy'ser is adjusted to pass the frequencies of which the sound is chiefly composed and reject all other frequencies. Consequently7, sounds of relatively high level, providing they do ‘not fall within the band passed by the analy'ser, will not affect the meter reading. Other limitations include the acoustic conditions surrounding the measurements, which must be fully accounted for to avoid errors. Abuse of a Good Thing By DAVID ROBSON As a result cf my article, “ The Show of Shows — 1931,” published recently in The Bioscope, it has come to my knowledge that the practice of running talking pictures on variable speed is more common than might be supposed. I refer, of course, to the abusers of Western Electric sound system. Therefore, it would appear that one of the greatest beneficiaries known to talking picture apparatus is being grossly abused by thoughtless people. The unit to which I refer is the Motor Control Cabinet, a decided advantage over all other methods for the precise regulation of speed of electrical motors. Lentil I saw for myself the instance I quoted I would not have believed that such a practice would be tolerated. So sure was I that the Western Electric Company did not know of the facts that I promptly' wrote them, and gave them what information I had. To think that exhibitors would purposely destroy the greatest refinement over all other reproducing sets by cutting out the control of speed to suit temporary box-office requirements, seems preposterous. Any projectionist who aids and abets such an order by carrying it out must surely be a weak-kneed nincompoop. But this stupid trickery must surely have its effect on box-office receipts, especially when it becomes evident to the patron that he can hear better at the opposition. Answer to Prayer Scorned Many projectionists will undoubtedly' remember only too well the troubles of the “ silent ” days — when we were in repeated conflict with the musical directors over irregular projection speed. Even the face of the best speed indicator would be made to appear a liar by the musical director should his music fail to pan out to the cue given on the “ suggestion ” sheet without a repeat. How we prayed in those days for that “ something ” that could not be disputed in the form of dead right speed. Now that prayer has been answered, the Motor Control Cabinet is our indisputable reply. In those days a musical director would earn anything from seven to twenty pounds a week for a four and a-half hour day. For moral interpretation only, let me explain that it would be better for us, as projectionists, to look well after what we hold, lest history repeats the sad story.