The reference test gain is a child of the ANSI S3.22 hearing aid standard (and the equivalent European IEC standard) and is that gain setting where many hearing aid tests are performed. This includes measures of internal noise, distortion, and the frequency response of the hearing aid.
In part one of this blog series about the reference test standard for musical instruments, the value of 77 dB was discussed. Reference test gain according to ANSI S3.22 is given by OSPL90 – 77 dB. And 77 dB is made up up 65 dB SPL + 12 dB where 65 dB SPL is average conversational speech at 1 meter and 12 dB is the crest factor or difference between the instantaneous broadband average or RMS of a signal, and its peaks. For speech the peaks are on the order of 12 dB whereas for many musical instruments, the peaks are roughly 18-20 dB (about 6-8 dB greater than for conversational speech). This relates to the overall less damping found in many musical instruments.
It was suggested that given the peakier peaks found in musical instruments, then the reference test gain for musical instruments through a hearing aid be set at a volume setting that was 6-8 dB quieter than for speech. A new definition of OSPL90- 83 or OSPL90 – 85 was proposed.
In this blog entry I would like to look at the other element of the reference test gain; namely the 65 dB SPL.
dB HL at 1000 Hz
|65 dB SPL||80 dB SPL||95 dB SPL|
The data in the above chart is adapted from FIG6 but it could equivalently been obtained from any other fitting algorithms such as DSL or one of the NAL approaches. Values of soft, medium, and loud for speech tend to be given for presentation levels of 50 dB SPL, 65 dB SPL, and 80 dB SPL. In contrast, values of soft, medium, and loud for musical instruments tend to be given for presentation levels of 65 dB SPL, 80 dB SPL, and 95 dB SPL respectively. That is, they are roughly at a sound level that is 15 dB higher.
From the chart we see that someone with a moderate sensori-neural hearing loss (e.g., 65 dB HL) may require 28 dB of gain for average speech, and 15 dB of gain for loud speech. However, they may require only 3 dB of gain for loud music. Instrumental music appears to be shifted up one loudness category than equivalently loud speech.
Returning to the reference test gain (OSPL90 – [average level + crest factor]), for typically loud musical instrument inputs to a hearing aid, the reference test gain should be OSPL90 –[ 80 dB + crest factor]).
The speech reference test gain is now roughly 20-22 dB higher than that for instrumental music. (80 + 18 = 98 versus 65 + 12 = 77). This 77 dB versus 98 dB value below the OSPL90 would be at an operating level for the hearing aids that would make the hearing aid appear to be quite different. Testing a hearing aid according to ANSI S3.22 one could obtain a reference test gain of 30 dB, for example; the very same hearing aid would have a musical instrument based reference test gain of only 10 dB.
I am not sure how this would change things- perhaps a little and perhaps a lot. Having a volume control setting near its minimum would yield different results than if the volume control was set to generate a level that was 20 dB higher, but with instrumental music, the difference would be more realistic.
It is true that ANSI S3.22 is “merely” a reporting standard and not a performance standard, but perhaps moving to a more realistic series of music settings such as a reference test gain that is 20 dB lower than for speech, may be the first step in developing a performance standard?