One of my biggest concerns as an audiologist for the open source project is the need to ensure SAFE LISTENING LEVELS. How this concern is addressed will likely be requested by most Investigational Review Boards. Clinically, I have seen more than one amateur audio engineer report an incident years before coming to the clinic of intense noise exposure while experimenting with sound. Sure enough, these individuals presented with a classic ‘noise notch’ in their audiogram.
It is always good practice when playing with sounds of unknown levels to slowly bring the earphones to your ears to be sure that whatever you are listening to will be at a comfortable/safe listening level. For those of us with normal hearing, this is one concern. When programming a device for someone with hearing loss, this is another concern altogether since there is often a need to make the sound output louder than normal-hearing listeners would tolerate. Therefore, a ‘listening check’ will not suffice. If this issue is ignored, there may be SUBSTANTIAL RISKS for the research participants. Furthermore, as the sound output is coupled deeper in the ear canal or if the ear canal is un-occluded (vs. supra-aural or circumaural headphones), it becomes more difficult to predict how individual ears will shape the sound across frequency. The only way to know this is to obtain probe microphone measurements near the tympanic membrane. These measurements will need to be compared to the measured or estimated (via scientifically-accepted prescriptive methods) loudness discomfort levels.
The Tympan unit comes with a potentiometer (blue volume knob) that controls gain. At first, I was concerned that this would introduce error in setting gain due to lack of precision in its placement. However, I learned that the ‘Serial Monitor’ in the Arduino software allows one to see the gain as the volume knob is turned (see figure). I assume that the adjustments are made in discrete steps of 5 dB and that it NOT the case that adjustments are continuous while the reporting of the gain is discrete. Another concern is that an experimenter or research participant will accidentally or intentionally turn the volume knob after it is set for the experiment. Therefore, it would be nice to be able to deactivate the volume knob after the gain is set or to conceal it (perhaps, something as simple as black electrical tape would suffice). Upon review of the example Tympan Library codes, it appears as if the former is easily accomplished with programming.