Hearing

Hearing is the ability to interpret sound vibrations. People can identify simple sounds such as beeps and tones and complex sounds such as speech and music against differing background noise. Hearing also allows for speech perception which forms the basis of our ability to communicate with other.

On this page:

The human ear is divided into three parts: the outer ear, the middle ear and the inner ear. The outer ear collects sound energy and channels it to the middle ear via the ear drum. The sound energy is converted into mechanical vibrations of small bones in the middle ear. These vibrations are transmitted to cochlea in the inner ear.

A membrane in the cochlea picks up these vibrations and converts them into neural impulses. These impulses are then transmitted to the brain along the auditory nerve.

When blockages exist anywhere in the passage from the outer ear to the middle ear, they result in conductive hearing loss. This results in the loss of ability to hear faint sounds, and particularly those of high frequencies.

When the cochlea is affected by ageing or disease, it results in sensory neural hearing loss. This affects the quality of the sound detected and results in loss of ability to understand speech and discriminate various sounds, especially in the presence of noise. The ability to locate sounds also decreases with age, particularly for sounds with relatively low volume and short duration, such as the tones generated by watches.

Three major hearing functions are considered in design applications.

• Sound detection is the ability to detect beeps, tones and other sound output from various products.
• Speech discrimination is the ability to detect and understand speech in quiet and noisy environments.
• Sound localisation is the ability to tell which direction a sound is coming from.

Sound vibration can range from very low intensity to high intensity. This is perceived as loudness or volume. The frequency of these vibrations can also vary from low to high, and this is perceived as pitch.

Complex sounds such as speech and music contain a range of frequencies at various levels of intensity. In order to detect a sound, it must be of sufficient loudness and within a frequency range that is audible to the listener.

Detecting sounds in the presence of noise is more difficult than detecting sounds in isolation. For example, hearing a phone conversation in a crowded restaurant or hearing a phone ring while the television is on require the ability to distinguish sounds from background noise. Most real world tasks take place with some level of ambient noise.

Human ability to discriminate speech is an important consideration when designing products that facilitate verbal communication or use speech output.

Examples include telephones, cash machines, car navigation systems, and accessible technology that aids people with low vision through alternative speech output.

Speech sounds can be detected when they are of sufficient loudness, but discrimination of speech can be difficult for some people, especially in the presence of noise.

Male or female voices can be used for speech. Generally male voices are of lower pitch and are within the range of hearing. Various accents and intonation can also affect how well speech is understood.

Speech output can be prerecorded and replayed or speech can be synthesised. Synthesised speech is more difficult for older people to understand.

Speech can become difficult to understand if it is recorded, played back or synthesised at high speed.

Sound localisation is the ability to tell the direction of a sound. A sound reaches one ear with a slightly higher intensity, and fractionally before it reaches the other ear, and the brain uses this information to work out where the sound is coming from. The ability to localise a sound therefore requires two functional ears, and this ability diminishes with performance reductions in either or both ears.

When sound localisation ability is low, it affects a person’s safe interaction in an environment. For example, if a person cannot tell the direction of an approaching bus or a car, he or she could be in danger of being struck by the vehicle.

Sound localisation is important when interacting with products that use sound to warn the user, or indicate where they are. The localisation can be assisted by other outputs such as lights, motion or vibration.

The sound of an ambulance siren is difficult to localise, because it contains only two specific frequencies. White noise is the easiest to localise, as this has a broad frequency range.

The most important factor that affects hearing is the presence of noise. Noise is essentially the ambient sound environment that interferes with the perception of the sound of interest. Noise introduces hearing demands in that the user has to discriminate the sound of interest from a mixture of other sounds.

Spaces that introduce large amounts of reflection and reverberation of sound can cause problems with hearing. The sound becomes distorted and more difficult to discriminate from the background noise. This occurs in public spaces where announcements are important, such as train and underground stations, sports arenas and music halls. Increased reverberation affects people of all abilities, but affects those with reduced ability to a greater extent.

Hearing aids are least effective in noisy environments, as they amplify the background noise indiscriminately. Induction or T-coil loops transmit sound directly to a hearing aid, which can be set to only receive this signal, thereby eliminating the background noise entirely.

• Provide adjustable volume levels where possible, failing that, ensure sufficient loudness for the ambient noise level
• Ensure that the frequencies of beeps and tones are within the range 800 to 1000 Hz in order to maximise the number of people able to detect them
• Use natural recorded speech in preference to synthesised speech, and avoid high pitch speech
• Use intonation, an appropriate word rate and clear pronunciation to help speech recognition
• Think about assisting those with hearing impairments by supplementing information through visual or tactile means, with due consideration for information overload
• Think about enabling the user to customise the tone and volume of auditory outputs
• Consider using sounds that contain multiple frequencies to help people determine where the sound is coming from
• Think about providing inductive couplings to assist communication with hearing aids
• Contemplate sound reflection and reverberation when designing environments and spaces
• Ensure that systems that transmit and reproduce speech do so with sufficient clarity

Hearing ability level (in increasing order)

Ability level is measured with any desired hearing aids.

No demand

• The user is not required to perceive anything by hearing

Low demand: The user is required to have sufficient ability to do things like:

• Understand someone talking in a loud voice in a quiet room without difficulty.
• Follow a TV programme with the volume turned up

Moderate demand: The user is required to have sufficient ability to do things like:

• Understand someone talking over a normal telephone in a quiet room
• Notice a doorbell or alarm clock

High demand: The user is required to have sufficient ability to do things like:

• Follow a conversation against background noise without great difficulty
• Follow a TV programme at a volume that others find acceptable

Demand levels assume that any desired hearing aids will be used.