Download Aided threshold interpretation - online at vanasch

AIDED THRESHOLD INTERPRETATION
Paul Peryman , Senior Audiologist, Van Asch Deaf Education Centre, November 2013.
Aided thresholds may be provided on an audiogram as a means of confirming the audibility of
test sounds at different frequencies. This testing may be done as part of guidance counseling,
verifying the programmed amplification of a bone conduction aid or a cochlear implant,
verifying that frequency lowering is effective in a hearing aid, and assessing functional gain
when real ear microphone measures can’t be taken.
If you receive an audiogram and are unsure if it has aided thresholds entered, check that there
is a key to the symbols used on the audiogram, or ask the child’s audiologist.
Aided hearing thresholds must be interpreted with caution.
Recall that the definition of a threshold for hearing is the level in decibels at which a test signal
is just faintly audible 50% of the time.
This applies to any threshold measurement, unaided or aided, and when tones, noises or
speech signals are used as stimuli.
Sounds that are heard above the hearing threshold are described as being at supra-threshold
levels, or at a certain sensation level above threshold, in decibel units.
The definition of suprathreshold level is a signal of sufficient strength to produce a reliably
perceptible physiological effect.
The definition of sensation level is the amount in decibels that a stimulus is above the hearing
threshold. It is expressed as dB SL.
An aided threshold only allows us to infer that a signal is just audible, but the signal may not
necessarily be discriminable or identifiable as distinct from a similar or even quite different
signal. This will especially be the case for speech phonemes. To be discriminable or identifiable,
the signal needs to be at a certain sensation level or suprathreshold level so that it’s distinct
characteristics are clear enough to the listener.
When we speak to each other and to hearing impaired children, we use a level of voice effort
that we know or at least assume is loud enough for speech to be clearly perceived and
understood. The fact that the listener understands and responds appropriately to what is said
confirms that our speech is loud enough as well as intelligible.
For a normally hearing person, the sensation level of conversational speech at the ear is about
20-30 decibels above the normal threshold range. Certain speech phonemes may vary in
intensity around this overall level. The overall level of conversational speech at 1 to 2 metres
from the talker may range from 30 to 50 dBHL (50 to 70 dBSPL).
For a hearing impaired child wearing hearing instruments, the sensation level of this amplified
conversational speech will vary depending on the degree of hearing loss (unaided thresholds),
the amount of amplification programmed into the instrument, the distance between the child
and the talker, and the level of the talker’s voice.
Aided understanding of speech and other important signals can only be assessed using suprathreshold speech stimuli to confirm what the child can actually discriminate and identify via
their hearing instruments; and not simply predicted by looking at the aided thresholds.
The actual ideal listening range for understanding speech can be measured by those
communicating with the child, as it may vary between talkers. Each teacher working with a
child needs to find this ideal listening range for their own voice, and in the acoustic
environment in which they typically work with the child.
While hearing aids are amplifying the speech of others to an optimal volume level at normal
conversational distances (1-2 metres), any surrounding noise present will also be amplified to
some extent. Speech understanding and a fluent conversation is more likely to occur with
talkers being as close as is practical to the hearing instrument wearing child when talking.
For further reading, please see Appendix G of the Auditory Skills Programme, 2008.