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of the plans. This would mean that the site, in the case of a new building, would be selected with the probable conditions as to noise in mind. The noise of heavy traffic, of street cars, or of elevated railways, being transmitted in part by way of the earth is almost impossible to eliminate. If the recording room is to be included in an old bulding, careful consideration should be given to its location with reference to machinery or other existing sources of noise. The difficulties of successful sound insulation in a building already constructed are frequently extremely great.
Having admitted the acoustical problem as of prime importance in the studio or stage layout, the designer should undertake to secure definite quantitative data on the various specific problems involved. In a field as new as is the recording of sound in connection with motion pictures, there will have to be considerable experimentation as to the acoustic conditions for recording which will produce a record that most nearly simulates music and speech as heard by an audience from an actual stage, yet the principles governing the behavior of sound within closed spaces are well known, and quantitative data for producing the desired conditions can be had, once these conditions are determined. Having determined the acoustical conditions that are desirable it is not necessary to guess at or experiment with means of obtaining these conditions. Let me illustrate by an example. In listening to a stage production, the audience hears the voices of the actors as they are modified by the acoustic conditions of an actual stage. What is the best means of securing this illusion in the case of the talking moving picture ? Is it to make a record that is entirely free from « room effects », and then put in the room effects by reproduction upon a stage that will introduce them, or will it be better practice to record under conditions that will include the « room effects » in the sound record? Only trial can answer this question, but once answered, I think it is safe to say that our knowledge of acoustics is at a point where the desired conditions for recording, whatever they may be, can be secured without further experimentation.
At the risk of indulging in the thankless task of carryng coals to Newcastle, by speaking of matters upon which many of you are already informed, may I very briefly outline the subject of the acoustics of rooms in general as an introduction to the consideration of the specific problems of the sound recording room.
Sound consists of the rapid to and fro motion of the particles of the air or other elastic medium.. This motion is propagated from particle to particle with a velocity of about 1120 ft. per second at ordinary temperatures. This vibrational motion of the individual particles results in a cyclic variation of the pressure, above and below the normal atmospheric pressure which in turn is accompanied by a corresponding fluctuation in temperature — a rise in the compression phase, and a fall in the rarefaction phase.
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