Projection engineering (Jan 1932-Mar 1933)

Record Details:

Something wrong or inaccurate about this page? Let us Know!

Thanks for helping us continually improve the quality of the Lantern search engine for all of our users! We have millions of scanned pages, so user reports are incredibly helpful for us to identify places where we can improve and update the metadata.

Please describe the issue below, and click "Submit" to send your comments to our team! If you'd prefer, you can also send us an email to mhdl@commarts.wisc.edu with your comments.




We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.

Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.

Page 20 PROJECTION ENGINEERING also determined by its frequency characteristics. Fig. 8 shows an interesting comparison of two traces which were taken In a studio which was heavily treated acoustically. The upper trace shows the room noise which runs as low as 30 c.p.s. up to 200 c.p.s. The lower trace shows the same room noise with high frequency noise of a motion picture camera superposed. These traces were taken separately but within 15 minutes of each other. Once more the room noise is found to be made up of low frequency components. It has been noted how multiple reflections build up the sound intensity when a single tone is sustained long enough for a steady state condition to be established. In dealing with speech, however, the duration of the individual component is very short. The time of the average vowel is 0.3 second while the average consonant can be taken as 0.05 second. Inasmuch as the vowels and consonants are entirely different in frequency, duration, and intensity, it seems reasonable that each should be considered separately. For purposes of diagnosis, the writer has developed an electrical syllable which can be used to simulate a component of speech. If its frequency is set at, say, 250 c.p.s. with a length of 0.30 second, it will represent a vowel. If its frequency is set at, say, 4,000 c.p.s., with a length of 0.05 second, it will represent a consonant. In this way an auditorium may be tested by acoustically projecting the vowel and the consonant separately and recording the sound as it is picked up in various parts of the house by a microphone. Fig. 10. Intensity level of vowel above intensity level of consonant: tv = .30 sec. tc = .05 sec. 'J 3.0 SEC A« -.10-.20 2 "- ^ [^i^. rao '- „ •= \ "* ..so INITIA LEVEL ^— _X0 -10 k^ "=T r— ab 'too _i Z < z o (/) z o 0 Id > o CO 3 § u. o -J VOLUME OF RO0M4(0.000 CU FT. > Id Av 4.0 SEC iff )> ..•29" 40 ^ VOLUME 9000 OF ROOMCU. FT. 0 (0 .20 .30 .40 .50 .60 .70 .BO .90 1.00 O 10 ..20 .10 .40 .50 .60 .TO .80 .90 (.00 Ac COEFFICIENT OF ABSORPTION FOR CONSONANT Ac Four different oscillograms are reproduced in Fig. 9. These were taken as suggested in the last two paragraphs. In fact, they were all taken in the same theatre but in different positions. The traces on the left were taken in the center of the orchestra while the ones on the right were taken on one side of the orchestra. Space will not permit more than a glance, but it is obvious that a great wealth of information may be gained from a careful study of such soundings in an auditorium. The particular orientation of the horns will also control to some degree whether the high frequency projection will "clean cut" like the one at "M-2" or ragged like the one at "M-5." After a careful consideration of the properties of speech and its interpretation by the ear, the writer has finally concluded that the true relation between reverberation and articulation can be traced to the short length of the funda moo-pulse m-z ceni 3300~FVLS£ MS SIP£ y^^m^? > ~ 8~PUL$£ M-2 CENTS f* sas PULse m-s siob '.':' Fig. 9. Traces made in an acoustically treated theatre. mental speech sounds. If the vowels are assigned a duration of 0.3 second and the consonants 0.05 second with a difference of 15 decibels in power we will have a good average representation. The vowel can have a frequency of 250 c.p.s. while the consonant can be assigned 4,000 c.p.s. The intensity to which each will build up can then be calculated where the average coefficient of absorption of the room is known for each frequency. It is at once apparent that the consonant will build up to only a small fraction of the steady state value because it is so short in duration. On the other hand, the vowel which is six times longer will attain a much greater fraction of its steady state value. As has been seen, the vowels are about 15 db. higher than the consonants. One can then proceed to calculate the difference in level as determined by the duration of the sound, the absorption at the particular frequency of the vowel or consonant, the volume, and total surface of the room. Fig. 10 shows the results when all the possible coefficients of absorption are assigned to the vowels and consonants, respectively. The true vertical distance represents the intensity level of the vowel above that of the consonant for any particular value of the consonant coefficient. For example, take the point marked 0.8 second on the chart for the room with a volume of 9,000 cubic feet. This represents a consonant with a coefficient of 0.20 and a vowel with -a coefficient of 0.20 also ; the level of the vowel being 17 db. above the consonant. The 0.8 second is the reverberation for this particular coefficient. The dotted line passing through this point is a locus of all conditions in which the vowels and consonants have the same coefficient. In other words, this line represents a balanced absorption with respect to frequency. The upper dotted line represents the condition in which coefficient of absorption for the vowels (Av) is only one