Spatial Hearing and Hearing Aids

Abstract

Spatial hearing is a central part of everyday listening. The ability to determine the direction of a sound source is a natural and effortless skill that is only remarked on in rare or challenging circumstances, such as when a driver cannot determine the direction of the siren of a passing ambulance. But spatial hearing performance by hearing-impaired listeners is considerably degraded, and generally hearing aids do not offer any benefit.In most circumstances -excepting only if a sound source is exactly directly ahead, above, or behind -the fine details of a sound's waveform at the left ear will differ in some way from those at the right ear. The auditory system is sensitive to these interaural ("between the ears") differences. Imagine that the source is to the right of someone's head. The left ear is further from the source than the right ear, by about 18-20 cm, so the sound will arrive there a fraction of a millisecond after it arrives at the right ear. The left ear is also in the acoustic shadow cast by the head, so the sound's level will be less there than at the right ear. Both interaural cues physically occur at all frequencies, but their importance varies with frequency. The interaural level differences (ILDs) can reach 20 or 30 dB at frequencies around 5-10 kHz, but are less than 5 dB below 500 Hz. ILDs are therefore generally regarded as being most useful at high frequencies. Conversely, the interaural time differences (ITDs) -at most, about 600-800 millionths of a second -are primarily a lowfrequency cue, useful up to around 1500 Hz Many laboratory experiments have demonstrated that normal-hearing listeners are remarkably sensitive to these two cues: at best, people can discriminate changes of about 0.5 dB of ILD or 10 millionths of a second of ITD. But in everyday listening people are never called onto discriminate pure changes in ILD or ITD in isolation: when the direction of a source changes, both cues vary. At best, the discrimination for changes in direction is about 1 degree of spatial angle. This is the "minimal audible angle": one can reliably distinguish the left vs. right direction of two sounds, presented in quiet, 1 degree apart. These measurements can be made directly using very closely-spaced loudspeakers or with robot arms to accurately move loudspeakers by very small amounts. An alternative approach is to use a fixed loudspeaker array with relatively large spacing between loudspeakers, such as 15 degrees, and then measure how inaccurate listeners are at reporting the direction of sounds presented from them.One degree of angle is really not very much: it corresponds to the apparent width of the index fingernail held at arm's length. Comparative measurements in mammals have demonstrated that humans have amongst the best performance: of all species measured so far; only dolphins and elephants do as well or better (Heffner, 2004). It is curious why we should be so good at distinguishing sound direction in quiet. Perhaps it is a by-product -an evolutionary spandrel (Gould and Lewontin, 1979) ...

Dynamic sound field audiometry: Static and dynamic spatial hearing tests in the full horizontal plane

“…within 40 minutes, with 5 minutes to reliably assess the MAA at a given azimuth. Especially with multiple real sound sources at multiple azimuths, it is a difficult test to perform (Akeroyd & Whitmer 2016). Our results confirmed that the MAA increases with increasing displacement from the forward direction to the sides (Mills 1958).…”

Dynamic sound field audiometry: static and dynamic spatial hearing tests in the full horizontal plane

“…For the time-consuming MAA, we used a robot-supported, automated test procedure that acquired free-field MAAs in the full azimuthal plane at every 45 • position (i.e., 8 tested directions in total) within 40 minutes, with 5 minutes to reliably assess the MAA at a given azimuth. Especially with multiple real sound sources at multiple azimuths, it is a difficult test to perform (Akeroyd & Whitmer 2016). Our results confirmed that the MAA increases with increasing displacement from the forward direction to the sides (Mills 1958).…”

Head Movements Allow Listeners Bilaterally Implanted With Cochlear Implants to Resolve Front-Back Confusions

“…It has been shown that listeners with hearing loss are far more prone to front-back confusions than are normal hearing listeners (Akeroyd & Whitmer, 2016; Vaillancourt, Laroche, Giguère, Beaulieu, & Legault, 2011; Van den Bogaert, Carette, & Wouters, 2011). While there is substantial inter-subject variability, the overwhelming trend is that wearing hearing aids offers little-to-no improvement in distinguishing front from back and can even make matters worse with hearing aids placed behind the ear (Best et al, 2010; Van den Bogaert et al, 2011).…”