The definition of sound quality may vary according to context. It is described differently when it comes to musicians whose understanding is determined by timbre and overtones, whereas hearing healthcare professionals, on the other hand, address sound quality in terms of loudness, background noise, internal instrument noise, bandwidth, etc. However, those who wear hearing aids may have a subjective definition based on their own listening experiences, sound perceptions, and preferences.

Hearing aid users can also measure sound quality as the way amplified sounds (through the use of a hearing aid) fit within their range of hearing. This can refer to the extent that sounds are distorted, and the degree to which undesired sounds (e.g., background noises, acoustic feedback, or signal processing artifacts) are heard.

Hearing aids that provide good sound quality usually ensure that the wearer experiences:

• An increased range of audible soft sounds
• Loud sounds to be comfortable
• A reduction in background noise
• Reduced feedback
• A feeling that sounds seem “natural” 
• A comfortable and natural sound of one’s voice.

Poor sound quality is usually characterized by feelings of discomfort caused by auditory input. Although a hearing aid may not restore hearing abilities to normal levels, an unnatural quality of sound that affects functionality in the hearing aid user may be a sign of poor sound quality.

Why is sound quality important?

The primary reason that hearing aids are fitted is to restore functionality in persons who are compromised due to the presence of a hearing loss. Although the focus is often on the understanding of speech, the quality of sound can be the deal breaker on deciding to continue wearing the hearing aid.

A  survey conducted in 2010  by Kochkin, indicated that sound quality was a primary factor concerning hearing aid satisfaction. Of the 10 most important factors found to influence overall hearing aid user satisfaction, 5 were related to sound quality (clarity of sound, natural sound, richness/fidelity of sound, comfort with loud sounds, the sound of own voice).

Good sound quality allows the user to not only be functional in understanding speech but enjoying auditory input such as sounds in nature, music warning signals, and sounds that can influence one’s work or hobbies.  It also impacts emotional wellbeing positively.

What affects sound quality?

This can only be achieved by hearing aids that provide sufficient amplification power, processing abilities, complements the user’s lifestyle requirements, and is comprehensively fitted by the provider.  The degree of hearing loss will impact the amount of amplification and processing that is required by the hearing aid. 

Naturally, persons with a greater hearing loss will experience poorer quality of sound. The better the hearing loss, the wider the frequency range for audibility. This means that persons with milder hearing losses are likely to experience an improvement compared to persons with more severe to profound hearing losses.  However, some hearing aids can move sounds that were previously inaudible into a frequency range that is audible to the user.  This feature is very helpful in improving the quality of the sound for users that have particular difficulty with hearing higher-pitched sounds.

How do manufacturers address sound quality issues?

Hearing aid manufacturers strive to maintain and improve hearing aids through advanced technology and innovation. They shape the sound quality of their devices by adjusting several parameters.  Manufacturers do continuous research and development to improve the experience for the users.  Some of this include:

Short Delay between the hearing aid and natural sound

Hearing aid users with normal hearing in certain parts of the frequency range are likely to hear both the sound which has been amplified through the hearing aid and the sound that enters the ear canal directly through the ear tip/mold. This can lead to a delay between the direct and processed sounds.  This results in the unnatural and poor quality of sound. Hearing aid manufacturers aim to reduce the delay in sound to less than 10ms to minimize the effects thereof.

Increased dynamic range at all sound levels

The human ear naturally distorts very loud sounds.  For example, sound levels that exceed 110 dB SPL will normally be perceived as distorted by a person with normal hearing (Killion, 2009).  To offer a natural sound reproduction, the upper limit of the hearing aid’s dynamic range should reach at least up to 110 to 115 dB SPL.

Wider frequency range

Traditionally hearing aid amplification focused on frequencies 0.25 kHz- 8kHz covering the frequencies required to understand speech. Some hearing aid manufacturers have now increased the frequency limit of 8 to 10 kHz which has shown to provide audibility of certain speech sounds.

Evaluation

Hearing aid manufacturers are involved in continuous research to improve sound quality and replicate the essence of “natural hearing”. The research involves extensive testing and feedback on the use of the hearing aid in varied environments. Questionnaires, interviews, and rating scales are used in controlled laboratory environments and in the real world to investigate sound quality. 

Promoting the use of bilateral amplification

Improved sound quality is achieved through the use of binaural (both ears) cues in bilateral hearing aid fittings. The advantage of listening with two instruments has been reported along several dimensions of sound quality, such as clarity, fullness, spaciousness, and overall quality. Providing the brain access to spatial cues by having the settings of the two instruments co-ordinated wirelessly allows for enhanced natural perception of spaciousness and externalization of sound.

In addition to the technical design parameters, another important contributor is the fitting rationale or the algorithm that automatically adjusts the hearing aid amplification (gain) to match the prescriptive targets for the individual hearing loss. Fitting rationales can be manipulated to achieve specific goals for the users. The goals are generally based on what the user would like to achieve most from the hearing aid. Examples of such goals can be:

• Maximum speech intelligibility
• Maximum comfort
• Maximum sound quality
• Immediate acceptance of hearing aids
• Maximum audibility across a broad range of frequencies
• Restoration of natural loudness perception

The majority of the large hearing aid manufacturing companies have either developed their proprietary fitting rationales for their hearing devices or make use of a generic rationale, such as the National Acoustic Laboratory (NAL) or the Desired Sensation Level (DSL) rationales. These rationales are designed to serve as a basis for the initial fitting ensuring optimal function of the hearing aid, adaptation for the user, and ultimately fine-tuning of the hearing aid settings once the user has been acclimatized to the use of the hearing aid.

It is also important to note that sound quality is likely to improve as the user adjusts to wearing a hearing aid. After the initial fitting of the hearing aid, it may be described as uncomfortable by the user as he/she is now exposed to a range of sounds that were barely or audible or inaudible and the brain has to now adjust to this new input. Modern technology has been developed to track user patterns and assist in the adjustment process. Furthermore, the hearing healthcare professional will provide fine-tuning of the hearing aid to achieve optimal user satisfaction.

Sound quality is crucial to the uptake of hearing aids. The increased satisfaction with the devices is meant to increase the usage and acceptance amongst users, and ultimately to functionally benefit from the hearing aids as intended.