Speech/voice acoustics in relation to hearing loss
The following factors need to be taken into consideration when thinking about speech intelligibility.
Speech banana
The speech sounds (phonemes) of a language occur across a range of sound frequencies and over a range of sound intensities; from low frequency to high frequency (eg 50Hz to 5KHz) and from low intensities to higher intensities (28dBHL word endings to 65dBHL word initial sounds). This is shown as a banana shape on a Pure Tone Audiogram (PTA), based on speech delivered at 1 metre from the ear at a ‘normal’ intensity with no competing noise. This is referred to the speech banana.
For the brain to perceive a particular phoneme it must hear the key frequency/frequencies at about 12dB louder than the ear’s threshold of hearing. The PTA threshold is a reaction to a sound, the perception of a phoneme requires information to be analysed by the brain necessitating a 12dB louder input. This is why the Speech Banana shape showing individual phonemes - and the Dotty Audiogram (see below) - is ‘quieter’ than the one showing the effect of all sounds.
Adults with a fully developed speech and language system learned through normal hearing can follow speech (to a degree) even when reduced to hearing 70% of the phonemes. Deaf children are
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hearing a reduced quality of speech through hearing devices
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using a decoding system of delayed language development
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trying to learn new vocabulary and language about new subject areas every day.
Peter Keen, Consultant Educational Audiologist, argues that this requires hearing at least 90% of phonemes with minimal background noise interference. He has developed a more accurate speech banana for the sounds of English, giving a clearer picture of what deaf children are accessing with and without their hearing devices, predicting phonemes not heard /mis-heard, using Aided Levels or demonstrating actual phonemes mis-heard or not heard from Speech Discrimination testing (eg AB word lists). The Dotty Audiogram uses the Aided Levels to show the percentage of phonemes likely to be heard in perfect listening conditions.
Teachers of the Deaf and Educational Audiologists use both of these tests to help fine-tune hearing aid programming and predicting areas where accessing language might be difficult.
These results are also used to predict the need for Assistive Listening Devices and/or sound treatment to a learning area when considering how the acoustic information shows potential access to speech.
Ling sounds
The Ling 6 Sound Test, designed by Daniel Ling, is a way of checking that speech sounds across the whole speech spectrum are likely to be heard. It can be used as a check that the hearing device is amplifying and programmed appropriately. The three vowels and three consonants used - 'mm', 'oo', 'ah', 'sh', 'ee', 'ss' - have sound frequency information specific to areas of the Pure Tone Audiogram. For an analysis of the test and how to use and interpret the results, see The Ling 6 Sound Test, by Peter Keen, Consultant Educational Audiologist.
Assessing speech discrimination
Another way of assessing speech intelligibility is to do speech discrimination tests. This is an important part of the role of a Teacher of the Deaf (ToD), particularly working in mainstream settings. Room acoustics and the impact of factors such as reverberation time on the student’s ability to hear should be assessed. Using a consistent method to assess speech discrimination allows a measure to be made of the impact on listening of a long or a short reverberation time. An acoustically good classroom will enable students to hear more clearly and accurately so their scores, for example in the BKB sentence test will be better; this supports ToDs in their requests that their students are timetabled for rooms that are proved to be acoustically good. There is a series of articles available from the British Association of Teachers of the Deaf Magazines which provide more information on speech discrimination tests using BKB sentences.
Impact of noise and distance
Especially in relatively poor acoustic listening conditions, the positioning in the classroom is critical. Sound energy decreases the further away from the source one goes. The doubling of the distance will result in a decrease in 6dB. (The inverse square law)
