Projection engineering (Jan 1932-Mar 1933)

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JANUARY, 1932 Page 19 absorption required to give a flat curve are given in the left portion. In other words, if a balanced condition is desired in this theatre the absorption must be added in largest amounts at low frequencies. In the last analysis of the acoustic problem, the final judge which must be satisfied is the human ear. It is well known that the ear is limited in tonal range or frequency. It is also limited in the range of intensities. The very faintest sound is sensed at the "threshold of audibility" while the very loudest sound is sensed at the "threshold of feeling." Both the frequency and intensity ranges can be represented on a chart by a certain area as shown in Fig. 6. A smaller area can be used to represent average or normal speech. The — -Jo© TO Zooo dJ»i — *> MtOOLir FfiLgQ. " ' ! I '-' ijj * -lobsy — *■'* rotate Fig. 7. Records of street noises. average extends from 200 c.p.s. to 8,000 c.p.s. for the important components. The speech area, then, will extend from 200 to 8,000 along the frequency axis and about 25 decibels along the intensity axis. The speech area can be further divided into the vowel and consonant portions. The vowels occupy the region of low frequency and high intensity while the consonants are found in the region of high frequency and low intensity. Taking the total speech power as 10 microwatts for average speech (p. 67, Fletcher, "Speech and Hearing"), the speech intensity is 1 microwatt per square centimeter at a distance of onehalf inch from the mouth of the speaker, since the power is divided over the 10 sq. cm. of surface of the hemisphere whose radius is .5 inch. The intensity of 1 microwatt per sq. cm. is taken as zero level. If sound continues to radiate into unconfined space the intensity will diminish according to the inverse square of the distance. At a distance of 12.5 feet the intensity will be 50 db. lower than the "initial level" which is taken at a distance of .5 inch bv definition. Fig. 6. Frequency and intensity ranges. On the auditory chart the effect of the listener moving from .5 inch to 12.5 feet can be represented as a 50 db. vertical drop of the speech area. When the listener moves to a distance of 125 feet a further drop of 20 db. is experienced. At this distance, the speech area is then touching the minimum audibility level. Some idea of what this means in terms of the dimensions of a theatre can be gained by the curve and sketch shown in the left portion of Fig. 6. It will be seen that if a person spoke on the stage with a speech power of 1 microwatt, it would be just audible in the rear of the theatre, assuming that no reflections took place from the walls, ceiling, and floor. If there is noise present a portion will be masked. In the shaded portion of the auditory sensation area is represented the space occupied by a 30 db. level of noise which has its components below 500 c.p.s. The masking extends well up in the higher frequency range as has been determined at the Bell Telephone Laboratories. If this noise were present as is frequently the case, the speaker could not be heard in the remote portion of the balcony. In other words, the one microwatt voice with an initial intensity level of — 10 db. would have to be raised 30 db. to 1,000 microwatts to give an initial level of -+20 db. This would insure a — 50 db. level at a distance of 125 feet without the aid of reflections. In the actual theatre the reflections raise the level of sound, immensely. For the purpose of comparison the intensity levels have been calculated for different absorption conditions. For an average coefficient of absorption of 0.10 the intensity level of approximately 50 db. below 1 microwatt per sq. cm. (zero level) would be established by a sustained sound. The nature of the room noise which is ever present should be known with some degree of accuracy. Not only the intensity should be known, but its frequency distribution as well. Fig. 7 •nves a record of the street noise which filtered into the theatre which we have been considering as typical. The electrical circuits were arranged with special filters so that the entire frequency range was divided into three bands — low, middle, and high. The distribution of the noise is fairly uniform over the three bands when picked up on the street. After finding its way into the lobby the middle and high frequencies are attenuated somewhat as can be seen in the left half of the traces. A special switching device automatically switches the circuits over to another microphone located in the foyer. This switching is accomplished in much less time than it takes the sound to travel from one microphone to the other. The traces from the noise in the foyer show an entirely different distribution of energy over the three bands. In the foyer, which is really a part of the orchestra itself, the noise is almost entirely made up of frequencies below 500 c.p.s. This fact is important in determining the characteristics of any acoustical treatment which is prescribed for noise abatement. The type of auditorv masking which it causes is Fig. 8. Traces made in an acoustically treated theatre.