XNUMX. Work ear theory
Hearing aid is a micro-amplification system for personal use, which has four main parts: microphone, amplifier, earphone, and power supply.Any hearing aid is inseparable from these four basic structures (Figure 2-43)
How hearing aids work
The input transducer is composed of a microphone (microphone), a magnetic induction coil and other parts.Its function is to convert the input sound energy into electrical energy and transmit it to the amplifier. .The amplifier amplifies the input electrical signal and then transmits it to the output transducer.The output transducer is composed of earphones or bone conduction vibrators, and its function is to convert the amplified signal from electrical energy into sound energy or kinetic energy for output.The power supply is an indispensable part of supplying the working energy of hearing aids. In addition, it is equipped with peak clipping (PC) or white motion gain control (AGC) to suit the needs of patients with different degrees of deafness.
Second, the main technical indicators
To understand the acoustic effects of hearing aids, we must first analyze the technical indicators of the hearing characteristics of hearing aids.The main technical indicators include gain, frequency response, maximum sound output, distortion, level input voice and dynamic range, etc., these technical indicators can be side-by-side through the hearing aid analyzer.
1. Acoustic gain The amplification rate of a hearing aid is expressed by gain, which is the difference between the sound pressure level of the earphone output of the hearing aid and the input sound pressure level of the microphone.
2. Frequency response The relationship curve of the change of the output gain of the hearing aid with the change of the input signal frequency is called the frequency response curve.
3. The loudest output
When the external signal gradually increases from 6dB to 90dB input, the output value number also gradually increases, but when the input signal is greater than or equal to 90dB, the output signal does not increase correspondingly. At this time, the output is the maximum sound output.Acoustic output = input + gain.
For example, using the 2cC coupling cavity test, the sound output measured when the 90dB SPL sound is input is generally the maximum sound output (Figure 2-46).
4. Dynamic range
Dynamic range refers to the difference between the maximum sound output of the hearing aid and the gain.The dynamic range can be changed with the adjustment of the volume control.
After the external sound is amplified by the hearing aid, except for the amplification of the wave radiation, the cross-conversion ratio of any other sound waves is distorted.Such as harmonic distortion, intermodulation distortion, overlap distortion, transient distortion, etc., harmonic distortion is the most common.