Prosthetics

Hearing can fail in many ways, and to some extent, becoming hard of hearing is an unfortunate but inevitable consequence of normal ageing processes. Given how much our society relies on verbal communication, losing the ability to hear clearly can be devastating.
Thankfully, much progress has been made in recent decades in the area of auditory prosthetics: hearing aids and cochlear implants. The science behind these devices is very interesting, and is discussed in chapter 8 of "Auditory Neuroscience" . The following web pages provide supplementary material.

Early hearing aids and ear trumpets

Early hearing aids were often simple "ear trumpets" designed to funnel sound to the ear over a larger area to amplify the sound. But the users of hearing aids have always been concerned about the cosmetic side effects of their devices. The wonderful Becker Collection pages describe a variety of ingenious ways of trying to conceal hearing aids and ear trumpets, among them the chair of King John VI of Portugal shown here and mentioned in Chapter 8.1 of "Auditory Neuroscience." 

 

The Becker Collection also shows ear trumpets designed to be disguised as flower vases, to be concealed in a hat, or a walking cane, or under a bushy beard. 

Hear it Like Your Grandmother - Simulating Age Related Hearing Loss

As we age, our auditory sensitivity often declines, and the average decline is perhaps a lot more than you might think! Use the buttons below to start or stop a little stand-up comedy video clip, and to select to hear it the way it would sound to an average elderly person some 60, 70, or 80 years old, or someone completely deaf. You can also add "multi-speaker babble" background noise. Hearing the comedian is, unsurprisingly, more difficult in the presence of background noise, but it is even harder with old age hearing loss on top of the noise.

Background Noise

If you are fairly young and your hearing is good, you may be surprised, even appalled, at how much worse the simulated "elderly hearing" is. Is it really that bad? The answer to that is: it depends. First of all, people vary considerably in how much they are affected by old age hearing loss, but this simulation is based on data from a study of hearing in elderly Americans, and it reflects the average hearing loss seen in elderly ladies in South Carolina, and, for still unknown reasons the average hearing loss in men in this study was even worse! Also, this little demo is only a rather "rough and ready" approximation of what old age hearing loss is like. All it does is to use digital filters to reduce the sound in varying frequency bands according to the average loss of sensitivity. In this figure below, the average age related loss according to Schmiedt (2010) is shown with little circles, while the solid lines show the filter transfer functions of the digital filters I have constructed for this demo.

average hearing loss graph

There are additional problems that often arise with hearing in old age ("broadening of auditory filters" and "recruitment"), and which this demo does not capture. (There are also more sophisticated and accurate hearing loss simulators but they would be difficult to implement in a web browser, and the loss of sensitivity is by far the largest and most important effect, so as a simulation of what your hearing is likely to be like when you are old, it is not far off).

The upshot of all this is: chances are 50/50 that your hearing in old age is likely to be if anything worse than this demo would suggest, and that, by the time you are 70, chances are that you will need hearing aids in order to take part in social activities. (Sorry to be the bearer of bad news!)

Cochlear Implants


Cochlear Implants aim to restore hearing by direct electrical stimulation of the spiral ganglion, the part of the cochlea where the cell bodies of the auditory nerve fibres sit. This picture, kindly provided by MedEl, shows a typical modern cochlear implant. Contemporary implants consist of a receiver unit implanted under the scalp, from which a stimulating array electrode is run through the temporal bone and inserted into the scala tympani of the cochlea. Contemproary implants have in the order of 20+ contacts which deliver electrical stimulation along the tonotopic array.

Cochlear implants often represent speech formants quite well and enable implantees to have telephone conversations unaided. However, contemporary implants are unable to deliver sufficient temporal and spectral fine structure information to allow implantees to appreciate pitch and musical melody, or to localize sound sources accurately. These limitations make it hard for implantees to follow conversations in environments with high background noise. These shortcomings notwithstanding, cochlear implants are often remarkably effective in improving the quality of life of the severely hearing impaired.

A cochlear implant patient (youtube video)

 

This extract of a youtube video shows a 5 year old boy, Trever, who was born deaf but fitted with a cochlear implant early in life. What is remarkable about this video is how unremarkable it is. The things to note are that:

 

In Trever's case the implant was clearly successful, but the auditory experience of cochlear implant patients is nevertheless poor compared to that of individuals with healthy ears.

What do cochlear implants sound like? - 1: speech

Noise vocoded speech is sometimes used to simulate what speech would sound like through a cochlear implant. These sound files give an example of normal (well, fairly normal, British) English speech, and the same speech passed through an 8 channel vocoder. The speech is somewhat "rough", but comprehensible with some practice, at least if there is not too much other background noise.

As is explained in chapter 8 of "Auditory Neuroscience", the speech processors connected to cochlear implants may operate in a number of different modes, such as "compressed analog" (CA), "continuous interleaved sampling" (CIS) or any number of variants of these modes. What exactly they sound like will depend in the speech processor setting, and some settings are impossible to mimick by sound delivered to a healthy inner ear. The noise vocoded speech presented here is probably best thought of as a reasonable approximation of CA coding. CIS should have a rather different subjective sound quality which is impossible to mimic with real sounds, but it is not necessarily more informative of the sound source.

"Family viewing" - normal:



"Family viewing" - vocoded:


For more demos of what the world sounds like through cochlear implants, check out the next page, as well as this podcast.

What do cochlear implants sound like? - 2: music

Beethoven

For reasons explained in chapter 3 of "Auditory Neuroscience", cochlear implants do a very poor job at conveying musical pitch. Consequently, cochlear implant users can get the rhythm of a piece of music, but have tremendous difficulty recognizing or appreciating melodies. Their enjoyment of music is sadly much diminished as a consequence.

Here we again use noise vocoding to simulate what the world sounds like through a cochlear implant, but this time, instead of a speech sample, we run the noise vocoder over a short snippet of music.  

The piece we use for this illustration is a short extract of Ludwig van Beethoven's"Kreutzer Sonata", a very sophisticated piece for piano and solo violin. Beethoven himself famously became deaf, losing his hearing in adulthood. The cause of his deafness is uncertain. Nowadays he would be eligible for cochlear implantation, but while that would have restored his ability to understand speech in quiet, it would have done little to facilitate his work with music, as you may appreciate when you compare the normal extract of the Kreutzer Sonata with the vocoded ("cochlear implant filtered") one. In the vocoded example, the melody is almost completely lost, and it is similarly very hard to make out which instrument is playing when. 

Kreutzer Sonata - normal:


Kreutzer Sonata - vocoded:


A Cochlear Implant Success Story

In this informative and touching short video, Helen, who has been living with a cochlear implant for over a decade, tells her story. The implant enables her to take part in the hearing world at a very high level, studying at a top university for a demanding science degree, and even taking part in a dance sport team. But in the video Helen is also honest about the limitations of the technology, and reminds us that the implantation is a fairly major surgical intervention - nobody's idea of fun.

 

Music to Deaf Ears - podcast

This podcast by science journalist Dr Carinne Piekema explores how hearing loss affects people, in particular how it affects musicians, and what modern prosthetic devices such as hearing aids or cochlear implants can and cannot do for these patients. It contains insightful interviews with inspirational deaf musicians, some of the UKs leading hearing researchers, as well as simulations designed to show to normal listeners what it would be like to have to rely on a hearing aid or a cochlear implant.

You can listen to the podcast here,


or you can download it by right-clicking on the "Attachment" link below and choosing "Save link as...".

If you enjoyed the podcast, you may also wish to check out Carinne Piekema's blog, or follow her on twitter (CarinneP).

Sheldonian Theater Ceiling

AttachmentSize
MusicToDeafEars.mp320.53 MB