by A.J. Bautista
The Chicago section of the AES spent its December meeting discussing the transducers and electronics in hearing aids. Tom Miller of Knowles Electronics told our group of 29 that persons with hearing loss can have up to 60 dB of attenuation, usually above 1 kHz, and especially around 2.7 kHz. Miller described hearing loss as sounding like non-encoded audio processed through a noise reduction decoder.
Hearing aid are designed to go behind the ear (BTE), in the ear (ITE), or completely in the ear canal (CIC). Miller explained that Knowles Electronics specializes in the electronics and transducer design of the hearing aid.
Physical requirements are demanding. Miller said that their customers require the assemblies to be 0.2" to 0.4"; have an efficiency of 115 dB in 1 microwatt; match the frequency response of the end-user; match the impedance of the circuit with the battery; provide for coupling with telephone receivers; design it to be friendly with the ear canal's environment; be able to withstand 10,000 Gs of force; amplify with less than 1% THD at nominal levels, and less than 10% of THD at 120 dB SPL; design DSP as necessary; and ensure low vibration.
Miller said that the heart of the transducer is the balanced armature, a moving magnet assembly that provides high efficiency with low distortion. An armature attached to the magnet is excited by a coil surrounding the armature. The armature is attached to a surface which serves as the speaker. Class D amplification is used because of it's high efficiency and small size. The armature is symmetrically balanced in the magnet, minimizing distortion. The materials required are miniscule, as are the cost for parts. Labor accounts for nearly all the costs of manufacturing. Miller then described the mechanical, acoustical, and electrical properties of the balanced armature. Shock protection is provided by snubbers, ribbed coils, and bumps on the armature which minimize deformation of the armature.
A demonstration followed. Miller explained that the apparent "brightness" of the sound is due to the outer ear. The hearing aid, which bypasses the outer ear's enhancement property, is designed to provide this equalization. However, by listening to the aid via a loudspeaker, the enhancement is provided again by the natural properties of the listeners' ears. Miller then applied a filter, allowing our group to audition the hearing aid more naturally.
Miller said computer modelling of the balanced armature is possible. PSpice modelling is used to predict linear behavior; non-linear modelling is created with nodal analysis in Mathematica.
. A question-and-answer session closed the meeting.
