Projection engineering (Jan 1932-Mar 1933)

Page 18 PROJECTION ENGINEERING Fig. 3. sity level of the test tone which is available. The method just described was developed by the writer to avoid the possibility of error which is apt to enter when other methods are employed which involve the operation of marginal relays. Furthermore, the exact manner in which the decay takes place is at all times known when the oscillographic trace is made. This facility is very important for the research worker who must be on the look-out for new and unexpected phenomena. The effect of reverberation on speech is shown in an interesting way in Fig. 2. In this case, two microphones were set up in a particular theatre. One was placed about four feet in front of the loudspeaker behind the picture screen giving the lower trace, while the other was positioned out in the auditorium, giving the upper trace. In this way a direct comparison between the sound as it comes out of the horn can be made with the sound as it is received by the listener out in the audience. The last three syllables are "VI-TA-PHONE." Although the distortion is abundant because of reverberation it was worse before this house was acoustically treated, as will be seen in Fig. 3. The reverberation is so great that the individual syllables are scarcely discernible. When the reverberation is measured in the auditorium the investigator at once discovers that there is a considerable variation, depending on the frequency. Some typical curves are given in Fig. 4. The curve marked T-l has an excessive period of 4.0 seconds at 128 c.p.s., while at 4096 c.p.s. the period is only 1.5 seconds, which is a ratio of almost 2.7. The curve T-3 from a theatre which has been treated acoustically has a period of 3.5 at 128 c.p.s. and 1.0 at 4,096 c.p.s. Here the ratio is even greater — 3.5. Curve T-2 has the smallest ratio — 3.2 to 1.8 seconds, which is approximately 1.8. At once, one is led to the suggestion that perhaps the relative slope of the reverberation curve may be of considerable importance. In other words, the fact that low-frequency sound is absorbed less efficiently than the high frequency may be most valuable in determining the degree of intelligibility with which speech is understood. In Fig. 5 (lower right portion), is given the reverberation curve of a particular average theatre with a seating capacity of 1,800. It will be noted that the reverberation at the higher frequency depends upon the relative humidity of the air. Recent investigation by Dr. Knudsen and others have established the absorption of the air at different values of relative humidity. Their findings have been quite startling Alii UNITS pj tct-SSUtt 1-Q 6-IVg FLtfr Cl>«V £ 125 ZSO 600 1090 7-990 *-099 gCOO -,;o° -.,o%. CHAHGB in AvktA&E T» ai~>t CtjH C_o_E_£j IZS *S» A"«0 foo xooo — +MO 8«w I3,S 2SO 5*0 1099 X099 4-0OO *?CCO iiy zso 5QO loop xaoc gage Soo" ^ y/I.SCHi.hHKtR. but entirely in accord with the experimental as collected by competent observers. Professor Sabine, himself, recognized the fact that the moisture content of the air is a factor in the dissipation of acoustic energy but he concluded that (to quote from p. 171 of his Collected Papers), "this form of dissipation instead of being an important factor, is an entirely negligible factor in any actual auditorium." If the bounding surfaces of this auditorium were perfect reflectors the reverberation at 4,000 c.p.s. would be 4.4 seconds due to the absorption of the air at a relative humidity of 20 per cent and room temperature of 70° F. On the other hand, if the humidity is 70 per cent the period is almost twice as great — the absorption at 4,000 c.p.s. o ?2 6 > 1 THREE TYPICAL THEATERS 1,000 to 1,500 Seats — i — _ Without balcony treatea1 with material with coefficient similar to A 178 256 512 1,024 2,048 Frequency, C.P.S 4,096 Fig. 5. Reverberation curves. Fig. 4. Reverberation curves of three typical theatres with different acoustical treatment. being almost negligible for all practical purposes. In fact, this no doubt accounts for Sabine's failure to detect the effect of humidity, since he may have done practically all his work in humid air. The effect of dry air must be taken into consideration since most of the theatres have very dry air in the winter time. I have been informed by the Carrier Engineering Corporation that the humidity will run as low as 25 per cent in the winter where no air conditioning is provided. The practical ideal for winter is 35 to 40 per cent as is maintained by the air conditioning equipment. It can be concluded that at frequencies above 2,000 c.p.s. the relative humidity must be considered in determining the acoustical absorption and its effect on reverberation. In the upper right-hand portion of Fig. 5 is given the absorption as calculated from the reverberation given in the curve immediately below. It will be noted that the curve has an alarming slope at a relative humidity of 70 per cent. At an average humidity of 45 per cent the total absorption is 12,000 units at 4,000 c.p.s. and only 6,000 units at 125 c.p.s — a ratio of 2.0. For purposes of later consideration the units of