scribes a new noiseproof subway car designed by engineers of the Interborough Rapid Transit Company, New York City. The car amounts to what is really a noiseproof box. Windows are kept closed and all cracks through which noise could enter are sealed. Forced ventilation is provided by power blowers, the air ducts being acoustically treated to prevent passage of noise through them, on somewhat the same principles as the noiseproof ventilators now used for windows.
Just to what degree the noise in a subway car is reduced by correct soundproof insulation was not left to argument which undoubtedly would arise were the question to be settled by the subjective recordings of the human ear. Rather, noise meters were put on the job so that little uncertainty existed regarding the magnitudes of sound intensities registered by these novel instruments. Noise meter measurements made in ordinary subway cars with the windows open, as is necessary in summer, indicate an average noise level of about 95 decibels, which is about the noise of Niagara Falls as heard from the closest point which a visitor can reach. For subway riders to talk to each other at this noise level they must shout loud enough to be heard, in quiet country at night, for almost two miles. Independent measurements indicate the noise level inside the new cars has been reduced to 75 decibels or one-hundredth of the former value. This is about as quiet as the average Pullman car with its windows closed. Ordinary conversation often reaches this 75 decibel level. Home radio music and the noise of city streets frequently are louder.
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