Adults with a moderate sensorineural hearing loss have a need for soft sounds to be amplified to help with clarity of speech without going over a level which the person finds too loud. Moderate sensorineural hearing loss is caused by damage to outer hair cells, which can lead to a reduced dynamic range and ultimately, recruitment. The dynamic range is the range between the threshold of hearing and the uncomfortable loudness levels (ULL). Venema (1998) refers to this as the floor (threshold) being raised and the ceiling (ULL) remaining the same. When the ULL’s are unchanged, as thresholds worsen, an irregular increase in loudness is perceived typically referred to as recruitment. In order to distinguish between different types of hearing aids and find the most suitable for this type of hearing loss we have to look to see if the hearing aids can encompass the person’s dynamic range without going over their uncomfortable loudness levels. It has been suggested that output limiting compression (CL) and wide dynamic range compression (WDRC) hearing aids are more beneficial for this type of hearing loss compared to linear hearing aids with peak clipping. Ultimately, for a moderate sensorineural hearing loss it is believed that WDRC is the most beneficial type of amplification at this time.
The outer hair cells in the organ of Corti have been referred to as the amplifiers of the cochlea (Brownell, et al., 1985). In the absence of outer hair cell function, a moderate sensorineural hearing loss of around 40-50 dB is present (Ryan and Dallos, 1975). The most prevalent type of hearing loss in adults is presbyacusis or age-related hearing loss (Valente, et. al. 2008). Presbyacusis begins as a bilateral, symmetrical, high frequency sensorineural hearing loss affecting the outer hair cells in the basal end of the cochlea. People with this type of hearing loss tend to complain about background noises such as speech babble in a noisy pub. This can account for, what is commonly referred to as the upward spread of masking, which is caused by lower frequencies masking higher frequencies (Valente et. al., 2008). This results in softer, higher frequency sounds from speech such as consonants being masked by lower frequency speech sounds such as vowels. Presbyacusis causes a subtle decrease in hearing over time (Valente et. al., 2008) and as a result, patients do not usually attend clinics until their families notice that the television is too loud or the patient themselves realize that they cannot hear as well in noisy situations as they used to.
Hearing aids can include different types of compression circuits, which can benefit different types of hearing loss. Let’s first look at input and output compression circuits. They differ to each other depending on where the volume control is located in the circuit. Output compression circuits have the volume control before the compression takes place. This type of compression affects the compression kneepoint and the gain but not the maximum power output. It is also the type of circuit used with CL amplification strategy and is associated with high compression ratios and kneepoints. Input compression has the volume control located after the compression circuit; therefore the sound is compressed before the volume control affects the sound. This means that the kneepoint is unaffected while the gain and maximum power output are. This type of compression circuit is what tends to be used with wide dynamic range compression (WDRC) strategy and is associated with low compression ratios and kneepoints (Venema, 1998; Dillon, 2001).
The first type of compression is output limiting compression amplification. The input is linear until it reaches a high kneepoint and then it compresses the sound with a high compression ratio (Venema, 1998; Valente, et. al., 2008). This type of compression is very similar to peak clipping (PC), which is found in linear hearing aids, however it is more pleasant for the listener than PC because there is less distortion. People with normal hearing or mild to moderate hearing loss will notice that the quality of speech is more distorted with limiting when compared to people with severe to profound hearing loss who will not notice this effect as much (Dillon, 2000). In a study of 12 adults with mild to moderate sensorineural hearing loss, sound quality and clarity were improved with output limiting compression when compared to peak clipping (Hawkins and Naidoo, 1993). It is generally accepted that linear hearing aids with peak clipping no longer have a place in audiology clinics and hearing aid companies have stopped manufacturing them.
Wide dynamic range compression (WDRC) is a compression strategy that aims to amplify soft sounds by a lot, medium sounds by a moderate amount and loud sounds by a small amount (Souza and Turner, 1998). WDRC tends to give more gain to soft sounds and has fairly short attack and release times (Marriage, et al., 2005). WDRC is a nonlinear compression strategy, which tries to mimic the non-linearity of the cochlea and attempts to account for loudness recruitment with sensorineural hearing loss (Moore, et al., 1992). The threshold kneepoint is usually low at around 50 dB in order to amplify quiet sounds, compressions ratios are usually lower than 4:1 and attack and release times are short so that consonant sounds are not masked by vowel sounds (Valente, et. al., 2008). WDRC is a relatively new compression strategy that is used commonly in modern digital technology hearing aids.
There are mixed views as to whether WDRC is of more benefit than linear amplification. It has been noted in some literature that measurable benefits of WDRC include improved hearing for soft speech sounds (Souza and Turner, 1998), speech in quiet, speech in noise, more comfortable listening situations for loud speech (Moore, et. al., 1992; Davies-Venn, 2009) and improved acclimatisation (Yund et. al., 2006). In contrast it has also been reviewed that WDRC may improve audibility but not necessarily intelligibility when compared to linear amplification (Marriage, et. al., 2005; Souza and Turner, 1998). WDRC may be of more benefit for people with mild to moderate sensorineural hearing loss compared to people with severe to profound sensorineural hearing loss. This may be due to the suggestion that as hearing gets worse i.e. in severe to profound sensorineural hearing loss that temporal cues are relied on more heavily to understand speech. Since fast WDRC can change temporal cues it may be that this population of hearing aid wearers benefits more from compression limiting (Jenstad and Souza, 2005; Davies-Venn et. al. 2009).
In 1992, Brian Moore, et. al. tested twenty subjects with moderate sensorineural hearing loss, measuring speech discrimination ability in quiet and speech reception thresholds (SRTs) in noise. The subjects were fitted with two types of hearing aids: Linear amplifiers and two-band WDRC compressors. They were tested with their new hearing aids and also in an unaided condition and with their own original hearing aids. With the compression hearing aids the subjects had good speech discrimination scores at all intensity levels in the quiet and the other three conditions showed decreasing speech intelligibility as the intensity level got quieter. The WDRC aids proved to help subjects achieve lower SRTs in noise compared to the other conditions. Patients with reduced dynamic ranges also benefited from the compression hearing aids more than the linear aids in that they found the loud sounds more comfortable. When surveyed the subjects also preferred the sound of the WDRC hearing aids (Moore, et al., 1992).
Another benefit of WDRC over liner amplification is improved acclimatisation. Acclimatisation is the time it takes for the brain to get accustomed to sound from a particular type of amplification and to have increased speech recognition. Yund et. al. (2006) did an acclimatisation study with 39 subjects with mild to moderate sloping sensorineural hearing loss, who had never worn hearing aids. They showed that subjects who wore the WDRC hearing aids experienced acclimatisation, whereas the patients who wore linear hearing aids did not show any increased speech discrimination scores. They believed this was because the WDRC hearing aid was able to process the normal hearing dynamic range into the dynamic range of subjects with mild to moderate sensorineural hearing loss. After a period of wearing linear amplification, subjects were then fitted with WDRC hearing aids. These subjects still struggled with acclimatisation after a period with their WDRC hearing aids and needed extra help in the form of auditory training to get rid of the effects of the linear amplification on the brain. Overall, it was concluded that hearing aids with more sophisticated technology may be the best aids for acclimatisation (Yund, et. al., 2006).
One study compared the benefits of linear and nonlinear hearing aids with speech tests and Glasgow Hearing Aid Benefit Profile (GHABP) questionnaires. The majority of subjects preferred the WDRC nonlinear hearing aids compared to the linear hearing aids. They showed better scores on speech tests, had better speech recognition, and preferred the overall listening experience with the WDRC hearing aids. WDRC hearing aids can be programmed with fast or slow attack and release times or a combination as this can be adjusted for different channels. In this study the researchers found that there was more of a preference for slow attack and release times for the most comfort and satisfaction compared to fast WDRC (Gatehouse, et. al., 2006). In comparison, Shi and Doherty (2008) found better speech recognition scores for both slow and fast, attack and release times compared to linear hearing aids, however found no difference between scores for slow and fast times in WDRC. When attack and release times are shorter the soft speech sounds are amplified more than the louder ones. If the release time is long then the soft and loud speech sounds are amplified at the same level, which may result in the softer phonemes being masked by the louder ones (Valente, et. al., 2008). Where to set attack and release times may be different for each patient depending on their preference; however in these studies it has been shown that having attack and release times using WDRC improves speech recognition scores compared to linear hearing aids.
WDRC multi-channel hearing aids have a distinct advantage over single channel hearing aids because they have the ability to use BILL and TILL (features of WDRC) at the same time (Sandlin, 2000). BILL is the “bass increase at low levels” and TILL is “the treble increase at low levels” (Dillon, 2001, pp 169). BILL will tend to go into compression a lot more with low frequency sounds and not as much with high frequency sounds. The strategy of BILL is to allow the hearing aid wearer to hear better in background noise. TILL will go into compression more often with high frequency sounds and not as much with low frequency sounds. The strategy of TILL is to increase audibility of high frequency sounds. Both BILL and TILL used in conjunction can create a good fitting strategy for a flat moderate high frequency sensorineural hearing loss (Venema, 1998).
Dillon (2000) described two problems that can arise with WDRC hearing aids. The first problem is that while WDRC hearing aids amplify very soft speech well, they also amplify very soft background noises such as the clock ticking or the sound of clothes moving (Dillon, 2000). Fortunately with newer digital technology, hearing aids are able to separate speech from background noise more intuitively than with analogue technology. A way to deal with these very low level background noises is to use expansion. Expansion is the opposite of compression and aims to make the weakest sounds in the quietest environments unnoticeable as it is below the listener’s aided threshold (Valente, et. al., 2008). The second disadvantage is the problem of feedback being introduced when the hearing aid wearer is in a quiet environment and the gain is increased (Dillon, 2000; Valente, et. al., 2008). In the past few years digital feedback suppression/cancellation has become more sophisticated and this does not seem to be a problem with WDRC in hearing aid wearers as long as a suitable earmould is fitted.
Wide dynamic range compression has been shown to have advantages over linear amplification using compression limiting and peak clipping circuits. In some researchers opinions it has still not been unequivocally proven that WDRC is the best fitting strategy for all types of hearing loss. As levels gets worse than moderate sensorineural hearing loss, the loss of outer and inner hair cell function causes temporal cues to worsen. It is unclear whether fast WDRC may be causing distortion in speech signals due to this. What is clear is that for mild to moderate sensorineural hearing loss, most commonly observed with presbyacusis, WDRC seems to improve speech recognition in quiet, in noise, overall comfort and it is easier to acclimatise to wearing hearing aids. There is not a great amount of recent literature on the subject of the benefits of WDRC in the moderate sensorineural hearing loss category. It would be interesting to see new research conducted to determine whether there are more benefits in multichannel WDRC with newer, more intuitive, digital technology hearing aids.