Physical Measurements of Audition and Their Bearing on the Theory of Hearing*

Abstract

Synopsis: The author states his purpose to be the presentation of certain facts of audition which have been determined recently with considerable accuracy and the discussion of the theory which best explains these facts. Making use of data of Knudsen's as well as his own measurements of the auditory sensation area, the author estimates that the normal ear can perceive approximately 300,000 different pure tones. This is taking account of all possible variations in both pitch and intensity. Knudsen's data show that for considerable ranges the minimum perceptible difference in intensity bears a constant ratio to the intensity and the minimum perceptible difference in frequency bears a constant ratio to the frequency. These relations have been termed by psychologists “The Law of Weber and Fechner.” A loudness scale is proposed such that the difference in loudness between two tones is equal to ten times the common logarithm of their intensity ratio. A pitch scale is proposed such that the difference in pitch is equal to one hundred times the logarithm to the base two of the frequency ratio. A method for measuring the loudness of complex sounds is mentioned but is to be discussed in a later paper. A method is proposed for expressing quantitatively different degrees of deafness. Reference is made to data obtained by the author on the masking of one pure tone by another. The minimum audible intensity of a pure tone depends upon the presence of another tone of different frequency. A low pitched note will, in general, exert a surprisingly large masking effect upon notes of higher frequency. The masking of a low note by a higher is not nearly as pronounced. From his observations, the author draws certain interesting conclusions. For example, given a complex tone consisting of three frequencies 400, 300 and 200 cycles with relative loudness values of 50, 10 and 10, respectively, the ear would hear only the 400 cycle tone and the 200 cycle tone. If the sound is now increased 30 loudness units, without distortion, the 400 cycle tone and the 300 cycle tone only, will be heard. Binaural masking in which each ear receives one of the two sounds is considered and the conclusion reached that the masking effect noted results from conduction of the masking tone through the bones of the head to the ear receiving the masked tone. It is stated on the basis of data obtained by Wegel and Lane that the oscillatory system of the ear, comprised by the membranes and little bones of the middle and inner ears, does not obey Hooke's Law regarding the proportionality of stress and strain. Consequently, the ear, when stimulated by a pure tone, introduces harmonics and the workers cited have observed harmonics as high as the 4th order. The non‐linear transmission characteristic of the vibratory system of the ear is held to account for the greater masking of a high frequency by a lower. A theory of hearing is advanced which pictures the basilar membrane as being caused to vibrate by incident sound waves. In the case of a pure tone, the membrane is su...

Newborn Incubators Do Not Protect from High Noise Levels in the Neonatal Intensive Care Unit and Are Relevant Noise Sources by Themselves

“…Although the audible range lies between 20 Hz and 20 kHz in humans, speech can be regularly identified within the more narrow band width between 300 Hz and 3.4 kHz, which therefore has been used as the "telephone bandwidth" according to the standards of the International Telecommunication Union [31,32] Additionally, frequency-dependent attenuation changes the amplitude spectrum of speech as the corresponding sounds enter the incubator. According to Fletcher et al (1923), the intelligibility of speech depends on the sum of different frequency bands, speech intensity, and temporal properties of the stimulus [33,34]. Consequently, the reduction of speech loudness by 15 dB(A) at frequencies above 250 Hz will impair the transfer of speech into the incubator.…”

“…However, the acoustic environment of preterm infants is rather characterized by unstructured, sudden and unpredictable noise sources. Knowing that at 1 kHz and above 40 dB, the perceived loudness doubles if the sound pressure level is increased by 9 dB [33], it visualizes that the loudness perceived by preterm infants increases substantially by minor manipulations such as accessing the baby, by alarms, or cabinet use. This is because perceived loudness roughly doubles with each 10 dB increase in SPL (more accurately, it doubles with each 9 dB increase in SPL at 1 kHz and above 40 dB).…”

Towards the Use of Sound Exposure in Environmental Noise Standards

“…The wish to have a unit independent of frequency led to the de nition of a "transmission unit" [7] a dimensionless unit when the proportionality constant is exactly one tenth. At this time also, Harvey Fletcher [8] was Working in the Bell System Laboratories on the direct ratio scaling of loudness, using the voltage induced into a telephonecircuit as the measure of sound pressure. From his research he introduced a "loudness unit" for an incremental change of one tenth in the logarithm of the mean square pressure.…”

Recent research in the field of audition.

“…Acoustic absorption is likewise dealt with. Fletcher (20) gives detailed figures on the masking effect of sound and on the sensitivity of the human ear. Foley (21) notes the velocity of sound for different media.…”