315 



TABLE 309A.— DISTRIBUTION OF LOSS OF HEARING ACUITY 108 



The following data are part of the results of the hearing tests conducted by the Bell 

 System at the New York and San Francisco World's Fairs in 1939. The first four columns 

 indicate the percentages of the population having hearing losses of 25 db or more at various 

 frequencies. A person having a loss of 25 db at all frequencies below 2000 cps may expe- 

 rience difficulty in understanding unamplified speech, as in an auditorium or church. The 

 second four columns indicate the corresponding percentages for losses of 45 db or more. A 

 person having such a loss experiences difficulty in understanding ordinary conversational 

 speech at distances greater than 2 or 3 feet. 



25-db loss 45-db loss 



Frequency in cps Frequency in cps 



Age group 440;880 1760 3520 7040 440;880 1760 3520 



10-19 men 17 1.6 4.5 8.0 .6 .6 1.8 



women 1.8 1.2 1.2 2.5 .6 .4 .3 



20-29 men 1.1 1.2 7.0 9.5 .1 .3 2.7 



women 1.8 1.6 2.2 3.5 .4 .3 .7 



30-39 men 1.8 3.5 15. 19. .3 .6 6.0 



women 3.5 3.5 5.5 10. 1.2 .8 1.6 



40^9 men 5.5 9.5 32. 39. 1.4 2.6 16. 



women 7.0 7.0 11. 24. 2.1 1.5 3. 



50-59 men 9.5 17. 48. 58. 2.6 6.0 27. 



women 13. 14. 22. 43. 4.0 3.0 7. 



109 Steinberg, Montgomery, and Gardner, Journ. Acoust. Soc. Amer., vol. 12, p. 291, 1940. 



TABLE 310.— ARCHITECTURAL ACOUSTICS 107 



Planning for good acoustics in a building requires careful consideration of noise control. 

 This includes consideration of the selection of a site, the arrangement of the rooms within 

 the building, the selection of the proper sound-insulation constructions, and the control of 

 noise sources within the building. The design of a room where people gather to listen to 

 speech or music should be such that its shape and size will ensure the most advantageous 

 flow of properly diffused sound to all auditors. Absorptive and reflective materials and 

 constructions should be selected and distributed to provide the optimum conditions for the 

 growth, decay, and steady-state distribution of sound in the room. The reverberation 

 characteristics of the room are controlled by the amount and placement of the absorptive 

 material. 



Reverberation time calculations. — Because of the importance of the proper control 

 of reverberation in rooms, a standard of measure called reverberation time has been estab- 

 lished. This is the time required for a specified sound to die away to one-thousandth of its 

 initial pressure, which corresponds to a drop in sound-pressure level of 60 db. The 

 reverberation time of a room is given by the following equation : 



0.049 V 6 



~~ -2.30 5- log 10 (1 — OC) -(- AmV 



where V is the volume of the room, 5" is the total surface area in square feet, and OC is the 

 average absorption coefficient for the room given by 



rc _ OCi St + OC^i -I-OC3 S* + _ £ 



Si + S* + St + 5" 



where OC! is the absorption coefficient of the area Si, etc. 



The second term in the denominator, 4mV, represents the effective absorption in the 

 room contributed by the air. The attenuation coefficient m at each frequency depends upon 

 the humidity and temperature of the air. Except in very large rooms the absorption in air 

 can be neglected below about 2000 cps. The values of m for a temperature 68° F are given 

 in figure 3 as a function of relative humidity for a number of frequencies. 



107 Taken from Acoustical designing in architecture, by V. O. Knudsen and C. M. Harris, John Wiley 

 & Sons, 1949. Used by permission of the publishers. 



(continued) 

 SMITHSONIAN PHYSICAL TABLES 



