Large Auditorium Research Articles

The Use of Loud Speakers in Groups

The number and strength of loud speakers, their relative location, and the radiated sound spectrum must be examined relative to acoustical principles. It follows that uniform distribution throughout a room is not only uneconomical but does not lead to best audibility. In view of interfering sounds, formulae are developed allowing computation of the required number and strength of loud speakers and their most favorable location, and the intensity for each observer. It is further shown how the absorption in air is to be considered. Using different sound spectra in loud speakers, there arises a greater total loudness than would be computed on a straight energy basis. Very large auditoriums do not follow the generally accepted acoustical laws. Short examples show the practical use of the formulae.

The Acoustics of Large Auditoriums<!--<xref ref-type="fn" rid="fn1-10.5594_J01344">*</xref>-->

Extremely large auditoriums present acoustical difficulties which do not readily yield to the customary methods of analysis and correction. This is illustrated by measurements of the time of reverberation, made in the Madison Square Garden, New York, N. Y., which revealed a considerable discrepancy between theoretical expectations and the times actually measured throughout the frequency range. At 500 cycles, for example, analysis of the auditorium indicated a decay period of 35.5 seconds, whereas the time actually measured by the spark chronograph reverberation meter was only 7.6 seconds. On the basis of the measured time, 47,000 square feet of one-inch rock wool were installed. This material was distributed in a manner calculated to suppress undesirable discrete reflections as well as to reduce the general reverberation time. The result was a reduction in the measured time to 3.5 seconds and the complete elimination of acoustic difficulties. Present reverberation formulas do not possess sufficient generality to justify application to enclosures which are extremely atypical in size or shape. Until such formulas are developed, reliance must be placed on actual measurements.

The Lapel Microphone

AT public meetings and at lectures, the microphone and loud speaker is often a great boon to both the speaker, who speaks practically in his natural voice, and to the audience, who hear him clearly. There are some, however, who forget that they are addressing a stationary microphone and move about freely, with the result that sometimes they can scarcely be heard. To get over this difficulty, the Bell Telephone Company has invented the lapel microphone, a description of which is given in its Record for January. The microphone is only about an inch in diameter and weighs one and one-half ounces. A thirty-foot length of flexible cord provides the connexion to the amplifier of the public address system. The diaphragm is made of thin aluminium in the shape of a cone of sufficient stiffness to cause it to vibrate as a unit throughout the required frequency range. A rubber covering for the microphone eliminates the disturbing noise which would otherwise result from rubbing against the speaker's clothing, or would be picked up through the clip which is provided for attaching it to the clothing. The best place to attach the clip is to the lapel of the speaker's coat. This device has been used very successfully in large auditoriums in America. It should prove useful to speakers who depend on gestures for effective delivery or who need to turn to explain lantern slides or use a blackboard.

The Acoustics of Large Auditoriums

Laws of acoustics known to apply to smaller volumes may not always be taken as true for large auditoriums. For example, computed reverberation times are radically different from actual measured times. In the case of Madison Square Garden, 6,200,000 cubic feet in volume, the calculated time of 35 seconds at 500 cycles contrasts with a measured time of 7.65 seconds. Use of the formula recently proposed by Knudsen reduces the discrepancy, but insufficiently. At 2000 cycles the computed time was 13 seconds; whereas, the measured value was 5.4 seconds. Some general possible causes of the anomaly are suggested.

The Hearing of Speech in Auditoriums

A summary is given of the quantitative effects of loudness, reverberation, noise and the shape of an auditorium upon the hearing of speech in auditoriums. The average loudness of speakers is found to be about 50.7 db in small auditoriums (27,200 cu. ft.) and 45.7 db in large auditoriums (240,000 cu. ft.). The loudness of spech in large auditoriums is therefore at a critically low level, so that slight disturbances from reverberation or noise suffice to make hearing conditions unsatisfactory. The percentage articulation in an auditorium is ascertained by means of the formula, Percentage Articulation = 96k1kτknks, where k1, kτ, kn and ks are the reduction factors owing to diminution of loudness, reverberation, noise, and shape of auditorium, respectively. Methods and data are given for determining these reduction factors. A study of the combined effects of loudness and reverberation yields the optimal times of reverberation for the hearing of speech in auditoriums of different sizes. Thus, the optimal time for a room of 25,000 cu. ft. is found to be .80 second. The optimal time increases to .90 second for an auditorium of 100,000 cu. ft., and to 1.00 second for an auditorium of 400,000 cu. ft. It is shown that artificial amplification of speech is imperatively needed in auditoriums larger than 400,000 cu. ft., and is desirable in auditoriums larger than 100,000 cu. ft. A series of curves indicates the probable percentage of speech-articulation in auditoriums varying in size from 25,000 cu. ft. to 1,600,000 cu. ft. and having times of reverberation between .5 and 8.0 seconds. The results of the investigation make it possible to assign a quantitative rating to the acoustic quality (for the hearing of speech) of any auditorium, or to determine the physical requirements of an auditorium which is to attain a specified degree of excellence.

THE HEARING OF SPEECH IN AUDITORIUMS

A summary is given of the quantitative effects of loudness, reverberation, noise and the shape of an auditorium upon the hearing of speech in auditoriums. The average loudness of speakers is found to be about 50.7 db in small auditoriums (27,200 cu. ft.) and 45.7 db in large auditoriums (240,000 cu. ft.). The loudness of spech in large auditoriums is therefore at a critically low level, so that slight disturbances from reverberation or noise suffice to make hearing conditions unsatisfactory. The percentage articulation in an auditorium is ascertained by means of the formula, Percentage Articulation = 96k1kτknks, where k1, kτ, kn and ks are the reduction factors owing to diminution of loudness, reverberation, noise, and shape of auditorium, respectively. Methods and data are given for determining these reduction factors. A study of the combined effects of loudness and reverberation yields the optimal times of reverberation for the hearing of speech in auditoriums of different sizes. Thus, the optimal time ...

The Acoustics of Buildings1

IN view of the examples of acoustically defective halls which abound in our towns and cities, it is the more surprising to find that the fundamental principles of architectural acoustics were clearly appreciated nearly a century ago in Groat Britain by a number of workers. Then, as now, it was realised that the two defects most frequently met with in large auditoriums are (1) echoes and (2) excess of reverberation; that is, the tendency of an arrested sound to persist unduly by multiple reflection at the boundaries, owing to their deficient absorptive properties.

Methods of High Quality Recording and Reproducing of Music and Speech Based on Telephone Research1

This paper deals with an analysis of the general requirements of recording and reproducing sound without appreciable distortion. The storing or recording of sound requires, first, a mechanical system which will respond faithfully to the sound waves which are to be recorded. Then there is required some material in or on which this sound may be recorded and an intervening system which permits the sound waves to make the record in this material. In the usual case, and in that which is particularly discussed, there is a mechanical system which will vibrate in response to the sound which is to be recorded and directly through some mechanical linkage, or less directly through an electrical linkage, drives a cutting mechanism which will impress a wax record. The amount of power available to operate the recorder directly from the sound in the recording room is so small as to make the use of high quality electrical apparatus with associated vacuum tube amplifiers of very distinct advantage over the acoustic method. Where the question of reproduction is concerned, the same two alternatives mentioned for recording present themselves, namely, direct use of power derived from the record itself vs. the use of electro-mechanical equipment with an amplifier. In this case, however, the situation is materially different since the power which can be drawn directly from the record is more than sufficient for many uses. It is, therefore, generally simpler to design one single mechanical transmission system than it is to add the unnecessary complications of amplifiers, power supply and associated circuits. In cases where music is to be reproduced in large auditoriums, the power which can be drawn from the record may be insufficient and some form of electrical reproduction using amplifiers becomes necessary. The paper points out, at length, how many of the heretofore unsolved fundamental problems of sound recording and reproduction have been readily solved by the application of a detailed knowledge of telephone transmission theory. The advances which have been effected in telephone transmission theory and in related electrical measuring apparatus in the last few years, have been so great as to surpass previous knowledge of mechanical wave transmission systems. The result is, therefore, that mechanical transmission systems of the type here considered, and perhaps other types, can be designed more successfully if they are viewed as the analogs of electric circuits. A detailed analysis is here made of the analogies between electrical and mechanical systems in the voice frequency range and a discussion of the resulting mechanical design is presented.

Acoustics of large auditoriums

Acoustics of large auditoriums

Audible electric signals in industrial plants, and acoustical engineering

The paper is a plea for a wider use of acoustic signals in industrial plants. It is shown that much time is wasted by the officials, experts, and important employees in trying to locate each other. With loud acoustic signals installed throughout a plant, in the shops, yards, and offices, it becomes possible to locate any of a considerable number of men instantly, using a simple code call for each. Such calls could be given only imperfectly by pushing a button, and for this reason a special code calling instrument has been developed, which closes electric contacts automatically, after having been set for a desired combination. The actual conditions and the needs for acoustic signals are discussed in application to steel mills, shipyards, textile mills, printing establishments, coal mines, construction jobs, etc. It is shown that in addition to code calls, audible signals and particularly electric horns are used with advantage as extensions to telephone bells in noisy places, and also as warning signals for various purposes. The general scope of the forthcoming art of acoustic engineering is then discussed, and its importance is shown in design of theaters, churches, and large auditoriums; also in the installation of fog signals. The problem of measurement of sound intensity is then taken up and the available means and devices are described. It is also shown that sound waves may be directly photographed under proper conditions and the laws of their propagation studied. Attention is called to the importance of experimental and theoretical study of the modes of vibrations of diaphragms used in acoustic devices.