Tests on supraaural-and insert-type earphones for telephone applications, designed from measurements with real ears and the American National Standards Institute (ANSI) 2-and 6-cm a calibration couplers, are reported. Disagreements between real-ear and coupler data are rationalized by reducing the basic 2-cm a volume to 1.3 cm a and employing an approximate transfer characteristic to relate the supraaural coupler data to a common plane of reference corresponding to the tympanum position. Several experimental couplers for obtaining data within the telephone speech band, which permit measurements on the two types of earphone at the common plane, are described.
A carbon microphone is shown to have 5 dB less low-frequency sinusoidal output than a magnetic microphone, both judged to have approximately the same voice output level and tonal quality. Evidence is offered to show that the sinusoidal characterization of a carbon microphone does not properly describe its response to a complex, wide-band signal having a high peak factor, such as speech. This results from a greater-than-unity slope for the input-output curve of the carbon instrument. An improved characterization for estimating the subjective performance with speech input was obtained with an excitation signal consisting of a sinusoid plus white noise, the latter being filtered from the output. With this technique, the response curves for the two microphones became comparable and reasonably similar. This information is particularly important for testing carbon microphones designed to have a steep input-output characteristic to exclude farfield sounds.
This paper covers the use of the 6- and 2-cm3 ASA (Standard Z 24.9-1949) couplers employed to measure the response of earcap and ear-insert-type earphones, respectively. The investigation revealed that in order to compare the subjective loudness of the two types of earphones at any given frequency, a correlating adjustment had to be applied because the couplers measure the sound pressure developed at opposite ends of the auditory canal. In addition, to avoid a 3-dB error, the basic 2-cm3 volume had to be reduced to 1.3 cm3, coinciding with the unoccupied coupler volume. An experimental 6-cm3 coupler is described which measures the pressure developed at the tympanum end of the canal, and direct comparisons with data taken on the 1.3-cm3 coupler are presented. The necessary correlating information was obtained with ear probes projecting into the canal.
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