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Hearing Aid Circuits

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Our goal is to help you understand more about how hearing instruments function, the different circuits available and the advantages and disadvantages of these different circuits. Hopefully, this article will be in terms which will help clarify some of the circuitry in the hearing aid.

Hearing aids are comprised of three main components: microphones, amplifiers and receivers. Microphones take the incoming signal and filter it to provide the respective frequency response that we desire. Amplifiers take the resulting signal and make it louder. The receiver converts the signal back into the acoustical form of the signal that the ear can hear. There are several choices which can be ordered from manufacturers for specific hearing losses. The goal is to provide some degree of amplification of the in-coming signal, as accurately as possible.

Amplifier Options

Amplifiers maintain the frequency response of the microphone. All amplifiers generate some form of distortion. A good amplifier produces a large, completely accurate, undistorted copy of the input signal from the microphone. There are five main classifications of amplifiers currently available within the hearing aid circuitry: Classes A, B, D, Sliding Class A or Class H, and Digital Amplifiers.

The Class A amplifier is the amplifier first introduced in hearing aids. Therefore it is the least expensive and best understood amplifier. The Class A amplifier has low harmonic distortion at low input levels, but tends to have very high levels of distortion as the input levels rise. This amplifier is always on, that means the amplifier has current drain regardless of whether the individual is listening to something or sitting in a quiet room. This amplifier tends to have poor battery life.

The Class B amplifier works much like two Class A amplifiers put together. The Class B amplifier splits the incoming signal into two parts, with each part sent to one of each of the transistor pairs in the hearing aid. While one-half cycle is being supplied to one transistor pair, the other side is off; when the current is switched to the second pair or side, the first transistor pair is turned off. The Class B amplifier is often called a "push-pull" circuit because one side is always turning on while the other is off. There was a "crossover distortion" that often occurred as one transistor turned off and the other turned on. This type of distortion has virtually been eliminated so Class B amplifiers typically function without this type of distortion. The advantage of this amplifier class is that it allows us to generate a great deal of volume or gain without having significant levels of distortion or saturation. As the levels of the incoming signal get louder, the amount of battery drain increases. Consequently, the Class B circuit usually has lesser battery life than other forms of amplifier classes. The advantage of this amplifier is that it provides significant power for severe-to-profoundly hearing impaired patients.

The Class D amplifiers are more complicated, but essentially they have an oscillator which oscillates at 220 kHz, this oscillation is divided to create a square wave of 110 kHz. The square wave is further converted to a triangular wave by the integrator, and the amplified audio signal input that we wish to hear is added to the triangular wave via the adder circuit. The circuit contains a "duty cycle modulator" which generates a positive voltage for everything above threshold and a negative voltage for signals below the threshold level. A series of pulses remains. Loud sounds are coded via wide pulses while soft sounds have narrow pulses. The advantage of such a system is that the system is on only when there is an incoming signal present. Consequently, battery life is improved in the Class D amplifier. Also the amplifier is much smaller than either the Class A or B. This amplifier reduces harmonic distortion under higher input levels. The average hearing aid user is able to hear louder sounds with much less distortion when this amplifier is compared to the Class A or B amplifiers. The quality of sound is much improved. Most hearing aid fitters are choosing the Class D amplifier because the hearing aid user will gain much clearer, non-distorted sound, even in noisily situations.

The Sliding Class A amplifier or "Class H" amplifier works much like a combination of an A and D amplifier. It has the low cost of the Class A amplifier, has a similar frequency response, has lower distortion levels and better battery life. The current drain depends on the level of the incoming signal.

The Digital Amplifier samples the incoming analog signal from the microphone and converts the signal to a numerical representation using an analog to digital converter. The gain, any filtering and compression characteristics are all computed while in a digital mode. This resulting digital stream of numbers is converted back into an analog signal that the patient then hears in analog form. The advantages of the digital amplifier are many: less distortion, improved speech in noise, increased processing capabilities, elimination of feedback, allowance for modification which is software driven and elimination of volume controls.

Compression Circuits

There are many reasons why we might order compression for a patient: tolerance problems, small dynamic ranges, to reduce or control distortion, recruitment problems, to unify conversational levels of speech, to amplify only soft sounds, to improve the signal-to-noise ratio, to improve speech intelligibility by reducing peck clipping, to protect patients such as infants who cannot use a volume control, to protect patients from having to use a volume control when presented with loud sounds or to eliminate the volume control, etc.

Regardless of the goal or type of compression employed, an out-of-phase portion of the signal is fed back into the system to result in a compressing of the signal. To help you understand the science of compression, you will need some terms defined.

  • Kneepoint - The level of the incoming signal at which compression occurs
  • Compression Ratio - The ratio by which the sound level is reduced
  • Attach Time - The amount of time delay between the onset of a signal loud enough to trigger compression and the reduction of gain.
  • Release Time - The amount of time necessary for the gain to return to the pre-compression level.
  • Variable Attach/Release Times - Some circuits do not have fixed attack and release time buy vary the onset and offset time depending on the type of incoming signal.

Compression circuits are classified in many ways. They are called by the circuit type such as input or output compression; by the type of frequency response of the compression - BILL, and TILL; by the specific manufacturer circuit name; or by the dynamic characteristic such as output limiting, dynamic range or multi-channel compression.

  • Output Limiting Compression - usually has a high compression kneepoint. They also have high compression ratios, meaning that once the high level of input is reached they compress very aggressively. The goal for output limiting is to insure that loud sounds are not exceptionally loud, while still creating as large a dynamic range as possible.
  • Dynamic Range Compression - Compress the input levels into a narrow dynamic range. They use a low kneepoint. Small compression ratios are used, meaning that compression is very mild. The goal is to unify conversational speech and amplify only soft sounds while insuring that louder sounds are not too loud.
  • Multi-Channel Compression - Different compression ratios and kneepoints are provided for the frequents between 500 - 2000 HZ. The high kneepoints and ratios of output limiting are applied for all frequencies above 2000 Hz.
  • BILL - Low frequencies increase at quiet intensity levels and reduce at high intensity levels. When input levels reach certain intensity at a given frequency (usually around 70 dB SPL at 500 Hz), this circuit automatically reduces the low frequency amplification provided to the patient. The circuit is designed to assist patients in functioning better in background noise.
  • TILL - This is the exact opposite of the BILL. The high frequencies increase at low levels and reduce at high levels. When input levels reach a certain intensity at a given frequency (usually 2000 Hz), this circuit automatically reduces the high frequency amplification provided to the patient. This is designed to allow patients not to be bothered by high frequency consonant sounds and prevents the hearing aid from sounding tinny or sharp.
  • PILL - Programmable instruments can reduce either lows, or high, or both lows and highs, a combination of both BILL and TILL.

Digital Processing Circuits

The digital processing circuits are using many of the principles of sound management which are described in the compression circuits listed above. They all are doing some type of sound compression to control how loud the sound get. They usually have adjustments for the kneepoint and the output limiting. They also are able to enhance soft sounds. When the instrument senses speech sounds below your threshold they will increase the sound until you should be able to understand the speech. How long has it been since you have been able to understand when someone whispers?

The digital instrument actually changes the sound from the analog signal into a digital format of zeros and ones. This is where all the sound processing is done. This helps eliminate some of the distortion problems found in analog sound processing. They are also able to sample the incoming sounds. When it finds a "steady state" noise, the instrument will not increase the volume of that sound and will only enhance what it recognizes as speech. Therefore, you usually do much better in noisy situations then with a analog hearing aid circuit.

Summary

As you can see there are many choices when selecting circuits for hearing aids. You must consider cost, cosmetic preference, previous problems or preferences, the audiogram, unique listening situations, middle ear disorders, or a patient's specific request for a circuit or a manufacturer.

Regardless of the process by which a circuit is selected, choices will involve making determinations regarding amplification class, and if compression is selected, the compression variables. The choices are many and often confusing, but at least we are now able to give the patient a choice. You need to have the help of a hearing instrument professional to make this decision. This article is only meant to help you understand the complicated processes in hearing aid circuitry.

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