Many manufacturers of hearing instruments use their own trade names to label circuits, but the generic terms that apply to hearing aid circuits are these:
- Analog (can be Linear, Class A, B, D, or H, compression, or ASP)
- Programmable (also Digitally Programmable)
- Digital
We'll go into the differences between linear and compression circuits a later. Many other names are used, but they can all be fit into one of the above categories. If you understand some of the basics, you will be able to follow along easier when discussing hearing aids with the technically inclined.
Let’s start with the difference between analog, programmable, digital and digitally programmable hearing aids.
Analog hearing instruments amplify the sound wave by simply making it larger. They use transistors in a circuit to amplify and modify the incoming sound. Any changes to the sound of the hearing aid are made with the volume control, small trimmers to adjust the response, or by sending the aid back to the manufacturer to have components changed.
- Long time hearing aid users sometimes find the sound more acceptable because they are used to it.
- Cost is much less than digital.
- Batteries usually last longer.
- Are sometimes more powerful than digital.
Programmable hearing aids can have their response (sound) changed in the dispenser's office rather than sending it back to the factory. If you need more power, a change in frequencies, or other changes, the hearing aid fitter can plug a cord into your hearing aid and modify it on the spot. Some programmable hearing aids are analog, not digital.
- Sound and results can be changed in dispenser’s office without shipping back to manufacturer.
- Less expensive than DSP (Digital Signal Processor) digital.
- Sound can be adjusted to better fit your own likes and dislikes.
Please note: a digitally programmable hearing aid may still be analog, not digital. The first use of digital technology in hearing aids was to use a digital function to change the sound of the hearing aid. These hearing aids are often still analog hearing aids; they just use digital programming to make changes to the sound. A hearing aid that is truly a digital hearing aid is referred to as “DSP”, for digital signal processing.
A true digital hearing aid (DSP, or digital signal processor) takes the incoming signal from the microphone, converts it into a digital format, and then processes the signal using digital technology before converting it back into an analog sound to be delivered to your ear. Digital hearing aids are normally much more flexible and can be used to process sound more selectively than an analog.
- Certain “noise reduction algorithms” can be programmed into the circuit to help reduce background noise.
- Digital feedback reduction can make more gain available without whistling occurring.
- Some instruments are able to have multiple memories for different listening situations.
- Some instruments select and follow the sound of voices with a self-focusing directional microphone.
- The greatest degree of flexibility in adjustments is available with a good instrument.
Some low priced digital hearing aids have reduced or eliminated most of the benefits above to cut the cost. Just because it is advertised as “digital” does not mean it is a quality product. Sometimes you may experience better sound quality with an analog aid than with a budget digital. A high-end, high quality digital instrument can sound terrible if programmed incorrectly by the fitter. The complexity and flexibility create a steep learning curve to the dispenser. Ask the dispenser how long he has been fitting the aid he is promoting. Don’t be a guinea pig. Just because it is expensive does not mean it is a good hearing aid. Read about different brands on the Internet or in a library. Talk to hearing aid users to find out their experiences.
Hearing aids amplify and process sounds. Two basic concepts regarding amplification are linear and compression amplification.
Sound pressure (thought of as loudness) is measured in decibels. Sound enters the microphone of a hearing aid and comes out of the receiver (speaker). The sound that comes out of a linear amplifier is in direct proportion to the sound coming in. If a linear amplifier is set to increase the level of the incoming sound by 10 dB (decibel), then it will increase any incoming sound by that same 10 dB. If 50 dB is delivered to the microphone, then 60 dB (50 dB plus the 10 dB of gain) comes out the speaker. If 30 dB is delivered to the microphone, the 40 dB (30 dB plus the 10 dB of gain) comes out the speaker.
Compression amplifiers are used to reduce dynamic range--the span between the softest and loudest sounds. There are four basic parameters of compression: compression ratio, threshold or knee-point level, attack time and release time.
Ratio is a way to express the degree to which the compressor is reducing dynamic range. Ratio indicates the difference between the signal increase coming into the compressor and the increase at the output level. A ratio of 10:1 would mean that it would take an increase of 10 dB coming into the compressor to cause the output to only increase 1 dB. Ratio is a constant value, as it doesn't matter how much compression is taking place; the ratio of the input change to output change is always the same.
Threshold is the level of the incoming signal at which the compressor amplifier changes from linear gain amplifier into a compression amplifier. The compressor has no effect on the signal below the threshold level setting. Once threshold is reached, the compressor starts reducing gain according to compression ratio. The exact moment the compressor starts gain reduction is called the knee-point.
Attack time refers to the time it takes the compressor to start compressing after threshold has been reached. Typical attack times range from less than 1 millisecond at the fastest to more than 100 milliseconds at the slowest. Attack time settings affect the sound quality in terms of overall perceived brightness or high-frequency content. If you use very fast attack time settings, the compressor will activate very quickly, reducing gain instantly at the waveform level of the sound. Since transient information at the front portion of sound conveys brightness immediately reducing it with the compressor will dull the sound. Selecting a slower attack time will allow the transient portion of the sound to pass through before the compressor starts clamping. However, if the attack time is too slow, loud sounds will not be softened quickly enough.
Release time is the time the compressor uses to return to unity gain after the input signal has fallen below threshold. The compressor is said to "release" from gain reduction. Long release time settings are a form of distortion, since gain reduction is clamping the sound down for an unnaturally long time. "Pumping" and "breathing" are engineer jargon words for obvious compressor side effects with maximum compression. Sudden and usually unwanted deep gain reduction is called pumping, while a slower return (release) to operating level with a noticeable rise of the noise floor is called breathing. Newer compressor designs have predictive and adaptive formulas that reduce these side effects, making the compressor's action nearly undetectable or at least tolerable in most intense gain reduction situations.
