This article was submitted to HR by Thomas Behrens, MSc, a research engineer for Oticon A/S at the Eriksholm Research Centre, Eriksholm, Denmark. Correspondence can be addressed to [email protected] or .
Being able to understand speech in a complex, dynamic, and often noisy environment (eg, a dinner with friends), is taken for granted by normal-hearing people. A normal-hearing person is able to find all sound sources in the environment, focus on what is judged to be the most important sound source, and ignore competing sounds. Their mental resources can be focused on the information conveyed by a foreground speaker, while spending limited resources on surveying speakers and noises that are perceptually placed in the background.
However, for the hearing-impaired listener, these tasks are much more challenging. The organization of a complex sound environment takes a lot of mental capacity, detracting from one’s focus on the information coming from the foreground speaker.
|FIGURE 1. Spatial separation. Epoq is designed to make it easier to perceive talkers as distinct, and thus enable the user to focus attention on the most important message while suppressing the rest.|
Hearing aids have been shown to be quite effective in helping hearing-impaired individuals improve in complex environments.1 Still, the hearing aid user is left with a number of challenges associated with speech perception and organization of sounds in dynamic and complex daily life situations.
Enabling people with hearing loss to be able to better organize the spatial complexity of typical daily life environments has been a key goal in the development of the Oticon Epoq hearing aid. Allowing two hearing instruments to wirelessly exchange information, then using this information for coordinating compression binaurally, is a key feature of the device.
This article presents evidence that suggests vital cues for spatial hearing in complex sound environments are rendered more accessible for the user when wireless processing is combined with an extended frequency bandwidth and better output acoustics via the BTE receiver-in-the-ear (RITE, also known as receiver-in-canal, or RIC) hearing aid style. With Epoq, sound sources can be perceived more distinctly, resulting in a number of improvements in performance in challenging listening situations, including better speech intelligibility, improved ability to suppress competing voices and interfering sounds, and increased ability to locate moving objects. Thus, improvements observed with Epoq are found in complex and changing sound environments, where the use of spatial hearing abilities is crucial to be able to perform well.
Spatial Complexity and Audiological Solutions
|FIGURE 2. Interaural Level Differences (ILD) and Interaural Time Differences (ITD): blue and black lines represent sound at the listener’s right and left ears, respectively. ITDs, or the time differences of sounds reaching the right ear versus left ear, are the key for locating sources in the horizontal plane. ILDs, or the sound level differences between the two ears, appear to play an important role when there are uncertainties in the environment (ie, less-predictable listening situations).|
Environments with spatial complexity are common in the world of today. Imagine sitting at home watching TV. When only the TV is heard, the situation is simple. But, when a car passes by outside, our attention is attracted to it, and we can deduce a lot about the car even though we cannot see it. What direction is it driving in? Is it headed for our driveway? As we start noticing other things than the TV, the environment becomes complex. From the sounds of the car, we are able to deduce the location of the car every instant, and from that we determine if it is something we need to pay attention to.
Interaural time and level differences. When we perceive sound in a spatially complex environment, the key components of the sound that enable our brain to decode the spatial complexity are the differences in time and the level of the sounds reaching our two ears. These are known as interaural time differences (ITD) and interaural level differences (ILD).
It has long been widely accepted that the interaural time differences, or ITDs, are the key for locating sources in the horizontal plane. This also holds true for familiar situations in which the listeners’ expectations completely match the actual course of events.
However, when there is uncertainty in the environment, which occurs in many daily life situations, the interaural level differences, or ILDs, seem to play an enhanced role. Singh et al2 have shown that, in a situation where only ITDs were present and the location of the target talker was uncertain (ie, the subject had to listen for a specific cue to find the correct response), average speech recognition was around 20%. When ITDs and ILDs were both present, the performance rose to 50%.
That the ILDs may play such a large role is an important finding for the design of compression systems in hearing aids. In Epoq, for example, compression is coordinated between the two hearing aids in a way that better preserves the ILDs, while still allowing dynamic range reduction.
Bandwidth and spatial hearing. Bandwidth is another important aspect of spatial hearing. The ITDs are predominately a low-frequency phenomenon, being effective only below 1500 Hz, and thus quite robust relative to most types of hearing loss. However, ILDs first start reaching a magnitude of 5 dB at frequencies higher than about 1000 Hz; above about 6000 Hz, they can reach 20 dB or more (Figure 3).3
|FIGURE 3. Magnitude of the interaural level difference, or the level differences between a speech (blue line, right ear) andnoise (black line, left ear) signal. ILDs first start reaching a magnitude of 5 dB at frequencies higher than about 1000 Hz; above about 6000 Hz, they can reach 20 dB or more. Graph based on data from Sivonen and Ellermeier (2006).3|
Consider a situation where a noise source is placed on a listener’s left side and a speech source is placed on the right. The speech reaching the right eardrum is amplified by the blue curve on Figure 3, whereas the noise reaching the right eardrum is amplified by the black curve, creating the so-called “better-ear” effect on the right ear. Therefore, the more audibility is provided at higher frequencies, the more access is provided to locally favorable signal-to-noise ratios (SNR) at high frequencies. To take advantage of this, Epoq has an extended bandwidth to allow access to such high frequency information.
Frequency response of the RITE. Spatial cues are mediated by the sound sources that are active at any moment in time. Each sound source has its own unique characteristics, including how much energy is present at different frequencies. The frequency response of the hearing aid contributes to determining how we perceive the source.
Hearing aid acoustics change the way sound is perceived. One aspect of this is the output stages, which, in the case of a thin tube BTE solution, impose peaks and notches on the frequency response of the hearing instrument. These peaks and notches can be mistaken for, or can partly conceal, monaural spatial cues that are used to determine whether a sound source is located directly in front of or directly behind the listener. However, a RITE-type solution will not create peaks and notches of a nature that will distort monaural spatial cues.
Assessing Performance in Complex Environments
A relatively new but well-documented tool that can be used for assessing user performance in complex environments is the Speech, Spatial and Qualities questionnaire, also known as the SSQ.4
The SSQ has recently been used to assess the effects of bilateral versus unilateral hearing aid fitting.1 In this study, it was demonstrated that the extra benefit of two hearing aids relative to one hearing aid is especially apparent in complex environments where divided attention or rapid switching of a person’s mental focus is necessary, as well as in situations involving discrimination of distance and movement.
Combining the Results of Two Clinical Studies
As part of evaluating Epoq, a clinical study at Oticon in Denmark used an abbreviated version of the SSQ for assessing performance in typical daily life situations with spatial complexity. The 58 adults taking part in the study were experienced and satisfied hearing instrument users, with relatively active lifestyles and moderate hearing impairment. None of them were trained listeners. All subjects wore the Epoq hearing instrument (no name on shell) for a test period of at least 4 weeks (details of this study can be found in Hansen5). Interview-administered ratings from the abbreviated SSQ were obtained for both the subjects’ own hearing aid and Epoq.
|FIGURE 4. Frequency response of RITE (blue) versus a typical thin-tube solution (grey). The peaks and notches of the thin-tube BTE output have the potential to conceal monaural spatial cues that can help determine the location of a sound source.|
The results of the clinical studies are presented below, along with results from the Noble and Gatehouse study (2006).1 In order to be able to compare the results of these two studies, some adjustment of the ratings was necessary. Epoq was compared to the subjects’ own aid, which was typically the Oticon Syncro. The bilateral and unilateral data from Noble and Gatehouse1 were based on other older technology, so it was necessary to adjust the ratings to account for differences in technology.
Adjusting data for fittings between two studies. The only comparable data was bilateral data obtained on both the devices used in the Noble and Gatehouse study1 and in the Epoq clinical studies. First, a technology-level difference was computed as the difference between the bilateral ratings obtained on the Syncro hearing aids in the Oticon clinical study and the bilateral ratings obtained in the Noble and Gatehouse study. This technology-level difference was then added to the unilateral ratings of the Noble and Gatehouse study to obtain a unilateral rating corresponding to what one would likely have obtained from hearing aid users wearing one Syncro device. The net result of this is that the unilateral ratings shown in Figure 5 are higher than those presented in Noble and Gatehouse.
In summary, the unaided ratings are directly reproduced from Noble and Gatehouse,1 the unilateral ratings have been adjusted as described above, and the bilateral and Epoq ratings have been reproduced directly from the Epoq clinical study.
Results. Figure 5 shows how hearing aid users rate their ability to hear in 7 different listening situations. From the unaided ratings, it can be observed that, in quiet, performance is judged to be much better than in the group and outdoor situations. A unilateral hearing aid fitting increases average ratings by about 2 points (on the 1-10 scale). Bilateral ratings are about 1 scale point higher than the unilateral ratings in the group and outdoor situations. Finally, it can be seen that the ratings based on Epoq fittings are on average almost 1 scale point higher than the bilateral ratings. Thus the improvements observed with Epoq relative to bilateral ratings are approaching the magnitude of the improvements found with bilateral ratings over unilateral ratings.
The improved performance observed in group settings is indicative of Epoq users being better able to spatially separate different talkers. This, in turn, could allow them to focus on one particular talker and thereby ignore the other interfering talkers—a process known as auditory selective attention.
Auditory selective attention, or the ability to ignore interfering sounds and voices at will, is used whenever communication is taking place in a competing background. A recent study by Neher et al6 indicates that elderly hearing aid users can have significant residual abilities in this domain, provided that the different talkers differ clearly in terms of their spatial cues.
|FIGURE 5. Results from ratings on selected items from the Speech, Spatial and Qualities (SSQ) of Hearing Scale of unaided subjects, as well as subjects fitted unilaterally, bilaterally, and with two Epoq hearing aids. This data is based on Hansen5 and the adjusted data from Noble and Gatehouse1 (see text for adjustment method).|
Binaural compression, increase in bandwidth, and improved hearing aid acoustics with the RITE can help improve access to the acoustical cues used for navigating spatial complexity. The improvements observed with Epoq are obtained in a number of dimensions relating to listening in complex environments. These range from indoor social settings, where there is little or no change in the physical layout of the situation, to orientating in a rapidly changing outdoor environment.
The author acknowledges the valuable input for this manuscript received from colleagues at Oticon.
- Noble W, Gatehouse S. Effects of bilateral versus unilateral hearing aid fitting on abilities measured by the Speech, Spatial, and Qualities of Hearing scale (SSQ). Intl J Audiol. 2006;45(3):172-181.
- Singh G, Pichora-Fuller MK, Schneider BA. The effect of interaural intensity cues and expectations of target location on word identification in multi-talker scenes for younger and older adults. In: Proceedings of the International Symposium on Auditory and Audiological Research; Elsinore, Denmark, 2007. In press.
- Sivonen VP, Ellermeier W. Directional loudness in an anechoic sound field, head-related transfer functions, and binaural summation. J Acoust Soc Am. 2006;119(5):2965-2980.
- Gatehouse S, Noble W. The Speech, Spatial and Qualities of Hearing Scale (SSQ). Intl J Audiol. 2004;43(2):85-99.
- Hansen LB. Success in the pursuit of meeting users’ expectations. Hear Jour. In press.
- Neher T, Behrens T, Kragelund L, Petersen AS. Spatial unmasking in aided hearing impaired listeners and the need for training. In: Proceedings of the International Symposium on Auditory and Audiological Research; Elsinore, Denmark, 2007. In press.