8 – Hearing Aid Features

How Hearing Aids Work

A ‘hearing aid’ or ‘hearing instrument’ (in these modules the term ‘hearing aid’ is used throughout) refers to any device that sits in, on top of, or behind the ear that takes in sound, amplifies that sound in a specific way to match an individual’s hearing loss, then sends the amplified sound into the ear canal/auditory system.

An ‘analog’ hearing aid is one in which the sound wave

A ‘digital’ hearing aid is one in which the sound wave

As of this writing (2012) most hearing aids manufactured today are digital, although a few companies still make analog aids. The reason digital hearing aids have supplanted analog ones is the complex processing that can be applied to the sound. Based on decades of research about how the ear works and the characteristics of the speech signal in noise, this complex processing can be implemented to help the wearer understand speech better.

See more on how hearing aids work.

Hearing Aid Features

Directional microphone requires that at least two microphones be installed on the hearing aid. In the most basic configuration, one of these mics faces forward and the other faces backward. This microphone configuration is used to partially ‘cancel out’ noise that goes into the hearing aid and help emphasize speech that is in front of the listener with a goal of making speech louder than the noise. The rationale is that in most listening situations the listener is facing the speaker and much of the noise may be behind the listener. In digital technology directional microphones can now ‘track’ the noise and speech sources and set the directionality of the microphones to the best advantage for a particular listening situation. A top-of-the-line digital hearing aid may have this function automated; mid-level hearing aids may require the user to turn the feature on manually; basic, entry-level digital aids may not have this feature but have a directional mic facing forward only.

Telecoil technology, the development of an electromagnetic field that conveys sound, is the same in all hearing aids. The sensitivity of a telecoil depends on its actual size and orientation within the hearing aid casing. Telecoils are needed to work with landline telephones as well as ‘wired’ or ‘looped’ large spaces such as churches and auditoriums. However, many large spaces may have  FM technology  in addition to, or instead of, telecoils/looping.

Direct audio input (DAI) requires a wire to be connected to hearing aids from some other type of device, such as a Sony Walkman, iPod, MP3 player, etc.

FM capability is available for both analog and digital behind-the-ear hearing aids. Many aids require a separate piece, called a “boot”, that snaps  into the bottom of a behind the ear hearing aid, to receive the FM signal. In some digital hearing aids the FM receiver has been incorporated inside the hearing aid and does not require a separate piece. An FM receiver is not available for any hearing aid that is cast to fit in a particular ear.

In digital hearing aids, the use of microchips has allowed more complex processing and many additional features to be added as compared to analog  hearing aids.

Multiple Programs

Although users  can change the volume setting, there  are few additional changes they can make to the hearing aid, no matter what type of listening situation they  find themselves in. Digital hearing aids, by contrast, allow for two, three or four programs depending on the level or expense of the hearing aid. Often, the primary program is set with an omni-directional microphone, which means the microphone will pick up sounds in a circular pattern from all around the listener. Program two is often designated as the ‘noisy situation program,’ because the directional microphone is engaged when the hearing aid is set  to that program. Program three may be the ‘telephone program’ and will use the internal telecoil with a landline phone. Most  cell phones do not operate on the electromagnetic principle, and so cell phones can be used directly with a hearing aid or through a Bluetooth connection.


Analog hearing aids are made up of a single filter across the entire frequency range. That is, most analog hearing aids have  a bass knob and a treble knob but no more than that. Digital hearing aids have many filters across the frequency range, which allow the audiologist to more precisely fit the amplification needed in each frequency region to a particular person’s hearing needs.

Feedback Cancellation

If you have ever been physically close to a person wearing a hearing aid you know that sometimes the hearing aid will start to squeal. This happens because the amplified sound coming out of the hearing aid is fed back into the microphone and is re-amplified. If this occurs several times the circuit is overloaded and the hearing aid emits a high-pitched tone. Feedback happens a lot with analog hearing aids and often the user is unaware of the problem.

Digital hearing aids have sophisticated programming to  identify feedback as soon as it happens and  end it. Manufacturers  use different strategies to get rid of feedback, all which are much better than the strategies for handling feedback in analog hearing aids.

Speech Enhancement

Digital hearing aids are tiny computers in a person’s ears. The term “digital” means that the sound wave is converted to a series of zeros (0s) and ones (1s). The features and programs within the hearing aid are in essence mathematical equations that operate on the zeros and ones. This conversion to zeros and ones is the reason digital technology is superior to analog hearing aids. Once the programs have operated on this signal and changed it, the ones and zeros are converted back into sound to be delivered to the user’s ear.

Because such sophisticated programming is possible, many digital hearing aids help the listener focus on a speech signal and pay less attention to background noise. The hearing aid is programmed to recognize both a speech signal and a noise signal, and to decrease amplification of the noise while increasing amplification of the speech. This process, however, is far from perfect; current hearing aid technology or any computer application is simply not as good as human  hearing and our brains at distinguishing between speech and background noise. Therefore, while this feature may be helpful, it cannot replace the auditory system and should only be thought of as an aid to speech understanding.


Bluetooth technology has been adopted by most hearing aid manufacturers in order to couple digital hearing aids with other communication devices, such as cell phones and televisions.

New Realities About Hearing Aids

It is probably clear after reading through the features of today’s hearing aids that they have changed substantially even in the last 5 to 10 years. In the past hearing aid costs were driven by the size of the hearing aid; the smaller the hearing aid the higher the price. That is no longer true; the cost now is driven by the level of technology included in the hearing aid.