36 Hearing Aids
It is estimated that there are 4 million people with hearing loss in the United Kingdom who could benefit from wearing hearing aids, but they don’t. There are many different types of hearing aids. Familiarity with these types allows a clinician to work as a team with audiologist to advise and help patients use the correct aid for their circumstance. Hearing aids augment hearing, but in patients with a sensorineural hearing loss will not return hearing to normal. Other forms of rehabilitation, such as hearing therapy, may further benefit such patients.
According to a survey by Action on Hearing Loss, the earlier hearing aids are used, the more the patient will benefit from them. Quality of life improves because these patients no longer feel isolated. In particular, their social interactions improve as they can understand and therefore contribute to conversations as well as benefiting from being able to hear better on the telephone, television and social media.
A hearing aid is any device that amplifies sound or assists the hearing-impaired individual, but in the present context it will be taken to mean an electroacoustic device used to amplify sounds. Cochlear implants are described in a separate chapter (Chapter 12).
It is important to be familiar with the basic design of a hearing aid as many patients attending the ENT department have them. You might be asked to describe one in an examination. The basic components of any hearing aid are a receiver (microphone and/or induction coil), an amplifier/processor, a sound transmitter (earphone, bone conductor) and a power source (primary cell). There are two types of signal processing systems: analogue and, the most currently used nowadays, digital.
1. Analogue systems
These systems are rarely used as hearing aids but there might be some long-term hearing-impaired patients who will not get on with digital aids or might need a body-worn type hearing aid. In analogue systems, an acoustic signal, which is a constantly varying sound pressure wave, is converted to its electrical analogue at the microphone stage of the hearing aid circuit.
2. Digital systems
Digital hearing aids are the current default hearing aids provided by the NHS and private sectors. They were introduced in 1996 and continue to be upgraded.
Digital systems work on frequency channels and bands. Frequency channels are a range of frequencies that are created by a digital filter or series of digital filters within a hearing aid. Most of the signal processing functions operate on a channel-by-channel basis. These functions are expansion (amplification), compression, digital noise reduction, feedback suppression and directionality (direction of sound). Multiple channels provide selective amplification and improve feedback suppression, digital noise reduction and speech discrimination in quiet and background noise. Multiple channels may equalise the loudness of several people who might be contributing to conversation. Frequency bands are the number of adjustment ‘switches’ provided in the programming software to obtain manipulation.
Digital hearing aids have directional microphones whose function is to enhance the signal-to-noise ratio by detection of clearer sound from the front of the listener and reduced sound from other directions. The microphone has two inlet ports: sound coming from behind hits the front port later than the rear port (external delay), and sound entering the rear port is delayed by a low-pass filter (internal delay). Directional microphones are more useful in the presence of ‘wordy’ sound and work best in those wearing an occlusive mould.
Environmental noise suppression is digitally managed by splitting the sound into various frequency channels. In each channel, the aid gives a predominant signal to either noise or speech, depending on the shape of the sound envelope. The aid analyses fluctuations in signal amplitude. Speech produces slow- and large-amplitude fluctuations, while noise produces fast- and small-amplitude fluctuations.
In some hearing aids, output sound can feed back into the microphone causing acoustic feedback. Digital systems can reduce this feedback by phase reversal (feedback frequencies are reversed in phase and played back when feedback is detected) and frequency clipping (gain cut). In addition, they reduce audibility of internally generated sounds or environmental soft sounds which will not benefit the patient. This is called expansion or microphone noise reduction.
Recent digital aid innovations include wireless facilities such as Bluetooth synchronising programme and volume changing. There is also the possibility of attaching the aid to a small ear-level receiver that attaches to a hearing aid. The teacher will wear a transmitter microphone that transmits via Bluetooth, sound to the ear-level receiver, and then into the hearing aid. Such a device can improve the ability to hear the teacher and so help students of all ages.
36.2 Types of Aid
1. Behind-the-ear (BTE) aids The body of the aid sits behind the wearer’s ear and is normally connected by a hollow plastic tube to an ear mould, which allows sound passage to the ear. There are three main groups of BTE aids available on the NHS, the 10, 30 and 50 series, with the power of the aids increasing correspondingly. Within each series are several models with differing patterns of frequency response. All contain an induction coil which can be used with telephones, televisions and in theatres and cinemas fitted with induction loops, to bypass much of the unwanted background noise. There are also very many similar models available in the private sector from the various commercial manufacturers.
2. In-the-ear (ITE), in-the-canal (ITC) or completely-in-the-canal (CIC) aids These commonly available commercial aids have an external shell, made usually of acrylic, and conforms to the shape of the wearer’s conchal bowl (ITE) or outer external ear canal (ITC) or wholly within the canal (CIC). These aids have their working parts in the ear mould.
They are less obtrusive than the standard BTE aids but are expensive and occasionally prone to feedback problems due to the proximity of microphone and ‘speaker’.
3. Body-worn (BW) aids These aids are usually worn with a strap around the neck and the body of the aid on the patient’s chest. By virtue of their size, they can be made very powerful, and the distance between microphone and earphone means that, even with high amplification, feedback is rarely a problem. They are, however, prone to picking up the sounds of rustling clothes and are rather cumbersome. There are two series available on the NHS: BW 60 and BW 80.
4. Bone conduction devices (Fig. 36.1) These are very similar to the standard body-worn aid but feed their output to a bone conductor rather than an earphone. They are indicated in the event of conductive or mixed hearing loss when use of a conventional air-conduction hearing aid is impossible, contraindicated or ineffective. The most common situations include chronic meatal discharge (chronic otitis externa and otitis media), ear canal stenosis or canal atresia. The ENT UK 2010 position paper on bone conduction devices states that because of the benefits of bone-anchored hearing aids over conventional bone conduction hearing aids, the latter have, on the whole, been replaced by the former.