The EarLens system: new sound transduction methods

Abstract

The hypothesis is tested that an open-canal hearing device, with a microphone in the ear canal, can be designed to provide amplification over a wide bandwidth and without acoustic feedback. In the design under consideration, a transducer consisting of a thin silicone platform with an embedded magnet is placed directly on the tympanic membrane. Sound picked up by a microphone in the ear canal, including sound-localization cues thought to be useful for speech perception in noisy environments, is processed and amplified, and then used to drive a coil near the tympanic-membrane transducer. The perception of sound results from the vibration of the transducer in response the electromagnetic field produced by the coil. Sixteen subjects (ranging from normal-hearing to moderately hearing-impaired) wore this transducer for up to a 10-month period, and were monitored for any adverse reactions. Three key functional characteristics were measured: (1) the maximum equivalent pressure output (MEPO) of the transducer; (2) the feedback gain margin (GM), which describes the maximum allowable gain before feedback occurs; and (3) the tympanic-membrane damping effect (D(TM)), which describes the change in hearing level due to placement of the transducer on the eardrum. Results indicate that the tympanic-membrane transducer remains in place and is well tolerated. The system can produce sufficient output to reach threshold for those with as much as 60 dBHL of hearing impairment for up to 8 kHz in 86% of the study population, and up to 11.2 kHz in 50% of the population. The feedback gain margin is on average 30 dB except at the ear-canal resonance frequencies of 3 and 9 kHz, where the average was reduced to 12 dB and 23 dB, respectively. The average value of D(TM) is close to 0 dB everywhere except in the 2-4 kHz range, where it peaks at 8dB. A new alternative system that uses photonic energy to transmit both the signal and power to a photodiode and micro-actuator on an EarLens platform is also described.

Figure 1

Illustration of the EarLens™Electromagnetic System, with the tympanic-membrane transducer, ear-canal transceiver, and sound-processing unit subcomponents shown.

Figure 2

Close-up view of the electromagnetic tympanic-membrane transducer in situ on the tympanic membrane. The green marking dot is used to align the transducer with the malleus.

Figure 3

Placement of the electromagnetic EarLens system in the ear. The sound-processing unit coupled to the ear-canal transceiver can be removed by the user on a daily basis while the tympanic-membrane transducer remains in place on a long-term basis.

Figure 4

System signal-flow diagram from the ambient sound input at the microphone to the eardrum motion due to the tympanic-membrane transducer, leading to the amplified perception of sound.

Figure 5

Mean and mean ± 1 standard error (STE) of the initial audiograms for the sixteen subjects participating in the present study, at standard audiometric frequencies. To qualify for the study, each subject’s audiogram was generally expected to lie above the “inclusion guideline”, although this was not strictly enforced above 6 kHz.

Figure 6

Mean and mean ± 1 standard deviation (STD) curves of the maximum equivalent pressure output (MEPO) for the electromagnetic EarLens system, across sixteen subjects. Also shown is a dB-SPL version of the “inclusion guideline” from Figure 5, which was calculated by adding that curve to the Killion (1979) minimum audible pressure curve, which is also shown. Note that the MEPO varies across subjects due to the way that anatomical differences affect the coupling between the coil and the magnet, and that it does not depend on a subject’s hearing level. If a subject’s anatomy were such that their MEPO curve lies above the inclusion guideline, then the EarLens system would be capable of generating audible sounds for that subject as long as their threshold of hearing were at least as good as the inclusion guideline. For the population used, 86% of the subjects have a MEPO that lies above the lower curve labeled ‘mean-STD’. Because the inclusion guideline is below the ‘mean-STD’ curve for frequencies at and below 8 kHz, amplification can be given at those frequencies. Similarly, for 50% and 14% of the population, amplification can be given for frequencies up to 11.2 kHz and 12.5 kHz, respectively.

Figure 7

The feedback gain margin (GM) of the electromagnetic EarLens system, which indicates the maximum amount of gain that can be applied to the signal between the microphone and the coil before positive feedback begins to occur. The mean and mean ± 1 STD are shown.

Figure 8

The tympanic-membrane damping effect (D_TM), which represents the extent by which a subject’s threshold of hearing is made worse by the placement of the tympanic-membrane transducer on the eardrum. The mean and mean ± 1 STD are shown.

Figure 9

Long-term wear results for tympanic-membrane transducers. Photographic documentation is shown from three representative subjects (rows), for the both the left ear (left three columns) and the right ear (right three columns). For each ear, a photograph was taken at the start of the study (day 0), on the day the EarLens transducer was removed (up to 127 days later for the three subjects shown), and within one week after removal.