Insert–type earphones are unique in that when tightly sealed in an ear canal, they have the potential to deliver sound down to 20 Hz and below. In practice however, obtaining an extended low frequency response is challenged by a difficulty for users to achieve an adequate seal between the ear canal and the insert earphone [1], [2], [3]. Achieving such a seal is not intuitive to users new to insert earphones. Measurements made on actual earphones show that a leak as small as .5 mm in diameter and 2.5 mm long can result in a reduction of bass at 50 Hz of approximately 15 dB. Such a loss results when the leak is present, since the actual volume seen by the transducer at low frequencies is considerably greater than for higher frequencies, where the impedance of the leak becomes much higher in value. The result is a very perceptible and often disappointing reduction in bass performance. This paper describes two methods by which a user can confirm the level of acoustic seal obtained for a given combination of transducer, ear tip / ear mold and ear canal by subjective and objective means. The subjective method is based on an experimental observation that even with a poor seal the output of insert earphones at 500 Hz, is quite independent of seal quality. By consequently subjectively comparing the output of a recorded tone at 500 Hz to one at 50 Hz adjusted to be equal in perceived level based on ISO 226:2003 equal-loudness contours, one can confirm a good seal when the levels tend to subjectively match. The objective method involves fitting an insert earphone with a miniature flat pressure microphone into the sound port, and observing the “in canal” frequency response by means of spectral analysis, while the earphone is in use. As the seal quality improves the measured low frequency response approaches that of the coupler response of the earphone as measured with a high quality seal.
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