Our research staff and clinical audiologists often discuss the challenges involved in fitting infants and children with hearing aids. These conversations keep the researchers up to date on clinical trends. Recently, the topic has revolved around the speech intelligibility index (SII) and the optimal SII value for a child who wears hearing aids.
The speech intelligibility index describes how much of a long-term average speech signal is audible for a listener (American National Standard Methods for Calculation of the Speech Intelligibility Index, American National Standards Institute [ANSI] S3.5-1997 [R2007]). It is usually expressed as a number between 0 and 1, where 0 means that none of the acoustic energy in speech is audible to the listener, and 1 means that the entire speech signal is audible. The SII is occasionally expressed as a whole number between 0 and 100, but the concept is the same, with a higher number reflecting more audibility of speech.
The question posed by one of our audiologists was an excellent one: “If the purpose of fitting hearing aids on children is to make speech audible and support speech and language development, should we try to give them the highest SII that we possibly can when we fit their hearing aid?” While the idea of providing the child as much amplification as possible seems logical, the answer is not quite that straightforward.
CHILDREN NEED MORE AUDIBILITY
We know from past research that children with hearing loss need more audibility than adults do to reach maximum levels of speech recognition (J Acoust Soc Am 2001;110:2183-2190) and to support vocabulary development (J Speech Lang Hear Res 2012;55:764-778). Because children are still developing the speech, language, and cognitive abilities needed to understand speech, they have a greater reliance on the acoustic information in the speech signal, while adults are more efficient at decoding and understanding speech that has been degraded by noise, reverberation, or hearing loss.
A child with poorly fit amplification that leads to limited audibility is likely to experience delays in speech and language, which are the very skills needed to support speech understanding when audibility is low. The goal of hearing aid fitting in children should be to maximize the audibility of speech and prevent loudness discomfort across the range of potential listening situations a child could encounter over the course of a day.
WHAT'S THE MAGIC NUMBER?
Clinicians often want to know what SII value should be their goal when fitting infants and young children with hearing aids. Unfortunately, the degree to which audibility can be restored heavily depends on how much hearing loss a child has. For example, a child with a mild hearing loss may often achieve audibility of 0.85 or higher with amplification fit to appropriate prescriptive targets, whereas a child with severe hearing loss may have audibility of less than 0.40 for the same average speech signal and prescriptive targets. Much of the difference in achieved audibility is related to the fact that a listener's dynamic range decreases as the degree of hearing loss increases.
The degree of audibility that can be achieved is also contingent upon the configuration of a child's hearing loss. Because the speech intelligibility index weights the importance of each frequency band's contribution to speech understanding, it can be dramatically different between two ears with the same pure-tone average but different hearing loss configurations. Unfortunately, all of this means that there is no single magic SII number to use as a guideline for all children.
However, there is a reasonable solution. Researchers at the University of Western Ontario—who developed the Desired Sensation Level (DSL) prescriptive approach for hearing aid fitting, which will be discussed in The Hearing Journal's May Journal Club—have published a normative range for SII as part of the Pediatric Audiological Monitoring Protocol (PedAMP). The normative range for SII is plotted as a function of pure-tone average for both average (65 dB SPL) and soft (55 dB SPL) speech input levels so clinicians can estimate how much SII to expect if the hearing aid fitting is matched to DSL targets.
Research by Derek Stiles and colleagues revealed that children with an aided SII of 0.65 or higher had better receptive vocabulary outcomes than did children with less audibility (J Speech Lang Hear Res 2012;55:764-778). Considering the PedAMP normative range for SII in the context of these results, the good news is that, for children with less than an 80-dB pure-tone average, audibility of at least 0.65 can be achieved if DSL targets are matched. In other words, matching DSL targets should provide reasonable audibility in most cases.
LIMITATIONS OF THE SII
While the speech intelligibility index allows for an estimate of speech audibility, some important limitations should be considered. The SII represents the amount of speech that is audible in a typical listening situation where the speaker is approximately one meter in front of the listener. Audibility in the real world will vary depending on the location of the listener and the distance from the talker. Because of the variability in the acoustics of speech across different listening situations, the SII may overestimate or underestimate audibility. Another recent study demonstrated that similar SII values can result in very different levels of speech understanding depending on which frequency bands are audible to the listener (Int J Audiol 2011;50:34-40).
While these limitations are important to consider when comparing speech audibility across different listeners or listening situations, the overall utility of the speech intelligibility index for estimating audibility is strong in spite of them. Children with hearing loss need to hear the acoustic information that makes up the speech signal in order to develop speech, language, and cognitive skills. Clinicians should use the SII for their fittings as a reference to determine whether or not amplification is adequate to support auditory access to the environment and the development of communication.