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hearing aid

Hearing aids are miniature electronic devices that compensate for a congenital or later-onset hearing loss. They are acoustic aids to enable the hearing impaired to participate in daily life. Functionally, they are battery-powered audio amplifiers that receive speech and audio signals via a microphone, amplify and process it, and transmit the sound directly to the inner ear of the hearing impaired person via a MEMS speaker or other miniature loudspeaker. The purpose of hearing aids is to amplify sound in order to improve speech intelligibility

and directional sensitivity.

The electrical concept of hearing a

ids Hearing aidsmust functionally pick up sound waves, convert them into electrical signals, limit the level of these signals, amplify them depending on the frequency and emit them again as sound pressure. It is therefore an amplifier concept with frequency response boosts or cuts. Such a circuit concept can work analog or digital

Structure of an analogue hearing aid

Structure of an analogue hearing aid

The electronic structure of hearing aids consists of the basic components microphone, amplifier, loudspeaker and button cell. As far as the microphone is concerned, mainly condenser and electret micro phones are used, in recent years also MEMS microphones. All of these types of microphones can be manufactured extremely small, measuring only a few millimeters. The smallest microphones have dimensions of only 1 x 1 mm. Microphone preamplifiers connected downstream of the microphone amplify the microphone signals, which are only a few millivolts. In addition to signal amplification, gain control and frequency signal adjustments also take place here. Depending on the concept, the signal is boosted or attenuated in the high or low frequencies by means of Automatic Signal Processing (ASP) with Fixed Frequency Response (FFR) or Level Dependent Frequency Response (LDFR) and limited in level in the power amplifier by means of Peak Clipping (PC) or High Level Compressor (HLC) and fed to the electrostatic or piezo loudspeakers. To prevent hearing damage, the electronics use automatic gain control (AGC) and limit the maximum output pressure (MPO). With AGC control, the input sound pressure and the output sound pressure experience separate level limits. As for audio compression, three different compression methods can be used: Compression Limiting (CL), Automatic Volume Control (AVC) and Wide Dynamic Range

Control (WDRC).In digitally working hearing aids, the microphone signals are digitized in an AD converter, processed in a Digital Signal Processor (DSP), filtered with digital filters

and finally converted into analog signals ina DA converter

forthe loudspeakers.

Since hearing aids can be manufactured extremely compactly by modern manufacturing processes, there are versions with Bluetooth Low Energy (BLE). This allows signals from mobile phones, smartphones

and audio equipment to be transmitted directly to the hearing aid without being affected by ambient noise.

Hearing aid designs

In termsof the application concept, there are hearing aids that are placed behind the ear, Behind the Ear (BTE), those that are placed in the ear canal, Receiver

inCanal (RIC), and others that are placed in the ear, In the Ear (ITE), In The Ear (ITE) or Receiver in the Ear (RITE). Which hearing aid is best for the individual is ultimately determined by the indication.

Behind-the-ear

device (BTE), Photo: hoergeraete-otto.de

device (BTE), Photo: hoergeraete-otto.de

Behind-the-ear devices

are worn behind the pinna of the ear, the amplified sound is guided into the ear canal via a sound tube and an ear mould. Because the function of the pinna is no longer in effect with behind-the-ear hearing aids, a BTE device receives sound from the front and back with equal sound pressure. Directional localization, which is otherwise performed by the pinna, is thus impossibleIn-the-ear

(ITE) device in CIC design, Completely-In-Canal (CIC), Photo: zacho.de

(ITE) device in CIC design, Completely-In-Canal (CIC), Photo: zacho.de

It is different with in-the-ear devices. The hearing aid lies in the pinna and extends into the ear canal. The locating function of the pinna is thus preserved. ITE hearing aids are smaller and more compact and require less sound power

because of the direct sound input into the ear canal. Depending on how far the ITE hearing aid closes off in front of the ear canal or extends into the ear canal, a distinction is made between In-The-Ear (ITE), In-The-Canal (ITC) and Completely-In-Canal (CIC) designs.In the case of the In-The-Canal design (ITC), the hearing aid closes off with the front of the ear canal. To keep the hearing aid from standing out too much, the front of the hearing aid is tilted back. Even less noticeable is the Complete-In-Canal (CIC) design, as the housing lies inside the ear canal and is pulled out on a nylon thread. Since CIC hearing aids are extremely compact, they are powered by very small batteries, which limits their operating time

Characteristic values of

hearing aids As far as the characteristic values of hearing aids are concerned, we are essentially concerned with the parameters that directly influence speech intelligibility.

The internal modulation distortions caused by limiting circuits such as peak clipping or compression play a decisive role. The same applies to the frequency response and the boosting and cutting of individual frequency ranges. The transient response and decay are further parameters related to the control behavior of the amplifier, which influence speech intelligibility. The quality of the reproduction is also strongly affected by extraneous noise. It is reflected in the signal-to-noise ratio. To separate sound signals from background noise, the Acoustic Overload Point (AOP), which is decisive for the overload, is improved by technical measures. A further improvement in listening comfort is provided by Wide Dynamic Range Compression (WDRC), in which the compression depends on the sound pressure of the input signal.

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Englisch: hearing aid
Updated at: 30.09.2020
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