Hello Fons, Thanks for reading my post and for commenting on it. Years back, I picked up a book titled Acoustical Factors Affecting Hearing Aid Performance (Studebaker & Hockberg, Editors). Within the book was a chapter titled Transducers and Couplers (or something like this) authored by Mead Killion. Though I don't have this in front of me now, the chapter suggested that designers of miniature hearing aid mics have mostly overcome the inherent problems of mic self-noise, uniform response, etc. But I view certain claims with a dubious eye. The size of a tiny electret mic does impose physical restrictions and a good reason to raise an eyebrow. I'd guess that the "directional" characteristics, as purported by the manufacturers, do not include head shadow or the effects of head baffling (more z-effects than shadow-related attenuation), though both effects must be considered when determining the device's net response. As you know, most electret mics are intrinsically omnidirection, though Panasonic provides cadiod patterns in their line of electret mics. For the most part, mic -- or a hearing aid's -- directionality is obtained by using forward and rearward facing mics combined with basic signal cancellation. I can't imagine that there could be venting behind the diaphragm of a tiny mic that would then change its response (particularly at low- to mid-frequencies). Separate mic elements provide opportunities for directional manipulation, but I suspect the directional characteristics are highly frequency dependent -- just as the superposition of waves can be. I do know from experience that minute changes to the protective plastic components surrounding a mic makes noticeable changes for HA and CI users. For example, one manufacturer added what was intended to be a windscreen (really a hard plastic "windshield" -- not foam to break up turbulence). The size of the shield was miniscule compared to the wavelengths of speech-frequency sounds, but its size was comparable (or larger than) the mic diaphragm diameter. Users of the device complained of the change / addition, and measurable differences in speech discrimination scores supported their perception (even indoors and away from wind noise). The shield more-or-less formed a cavity with openings on the sides. One side was proximal to and parallel with the wearer's head. The end message I took from all of this was that small changes can make big differences, even to those with poor hearing. I'm not a mic designer (obviously), but I am interested in the combined effects of processing, bandwidth, and polar patterns in real-world listening. Even today, I recorded the ambient sounds of another popular cafe using my TetraMic. Once I have converted my raw wav files (A-format) to B-format (again, I use a TetraMic, hence no direct recording in B-format), I can do a number of manipulations. All of this is very cool. If all the channels were originally received via a left-pointing cardiod mic (as an example), then all the channels would be identical -- minus the phase anomalies that result from the four mic elements being located in slightly different places. From what I see in the equations, four identical A-formatted channels converted to B-format would simply yield a W channel with all information, and the X, Y, and Z outputs would be null or zero. Applying a HRIR to this would give a L and R output with both ears being the same in phase, response, etc. (albeit shaped by the head). Any attempt to rotate the "head" would just be aligning the head with the mic, not rotating the head and mic in space. I'm not a mathematician, so I could be way off here.
I can certainly create a HRIR using monaural material and then apply the IR of a cardiod mic to simulate the polar response, but this would simply give the equivalent of the mic (with head) pointed to a single, fixed source. If I wanted to study the effects of a stationary talker in a reverberant room, this would actually be useful. But trying to simulate the effects of ongoing, moving or multi-timbral sounds in 3-D space is not easy to solve. Again, I truly appreciate your expertise and insights. I'll also try to dig up data on actual HA mics. Best, Eric -------------- next part -------------- An HTML attachment was scrubbed... URL: <https://mail.music.vt.edu/mailman/private/sursound/attachments/20121108/95fc18c7/attachment.html> _______________________________________________ Sursound mailing list Sursound@music.vt.edu https://mail.music.vt.edu/mailman/listinfo/sursound
