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frequencies in controlling intimacy. This has led to our standardizing a pulse length of 10 to 20 msec.
Other investigators have chosen to use short pulses generally produced by a spark discharge, but in our experience this presents so much information covering such a wide frequency band that it defies analysis.
As the pulse length is increased above 20 msec, the pulse pictures tend to lose their simple character due to interference between pulse components that arrive at the microphone position by paths of differing length. Figure 8 is an example of the changes that occur as the pulse length is increased from 10 to 46 msec.
Experience tends to indicate that our subjective assessment of sound quality is in good agreement with the results using a 10 to 20-msec pulse.
While we are certain that pulse methods are a powerful new weapon in exploring an auditorium, we feel that there is still much to be done. At present there is insufficient mathematical background and while we know that strong reflections have deleterious effects upon sound quality, we also know that a complete absence of reflection can lead to unsatisfactory results. The basic question "How much reflection do we want?" can be answered only in broad terms.
Change in our viewpoint and requirements during the investigation led to considerable modification in the test equipment. More recently, the equipment has
been carefully rebuilt by Messrs. Owen and Webb as illustrated by Fig. 9, the complete equipment packing into two cases approximately 23 X 16 X 12 in. The basic principles remain as illustrated by Fig. 3.
The pulse technique described was developed during 1937-1940 in close association with C. A. Mason and is more fully described in Reference 2, but a deeper realization of the significance of the pulse technique developed in the immediate postwar years.
We are indebted to H. L. Webb for most of the experimental work and our thanks are due to the Directors of the British Thomson-Houston Co., Rugby, England, for permission to describe the results of the investigation.
References
1. See, for instance, Architectural Acoustics, V. O. Knudsen, Wiley, New York, 1932.
2. C. A. Mason and J. Moir, "Acoustics of cinema auditoria," /. Inst. Elec. Engrs. (London), vol. 88, part III, pp. 175-190, Sept. 1941.
3. J. Moir, "Reverberation time as an index of room performance," Report of the Physical Society (British) Acoustic Group Meeting, 1947, Physical Society, Prince Consort Gardens, Kensington, London.
4. C. A. Mason, "Interpretation of pulse measurements," Report of the Physical Society (British) Acoustic Group Meeting, 1947, Physical Society, Prince Consort Gardens, Kensington, London.
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