Subject: Re: [AUDITORY] Ear Probe Mics and detecting the heartbeat? From: "M.A. Stone" <mas19@xxxxxxxx> Date: Sun, 30 Jun 2013 21:27:00 +0100 List-Archive:<http://lists.mcgill.ca/scripts/wa.exe?LIST=AUDITORY>On Jun 29 2013, Jelfs, Sam wrote: >Dear List, > > Just a question for those of you with experience using ear probe > microphones, do you ever hear the heartbeat? I've built a couple of > in-ear microphones, and can hear my own heartbeat, but the quality is > relatively poor, and only works in very low-noise conditions. I was > looking in to using dedicated probe mics, such as Etymotic ER-7C > (http://www.etymotic.com/pro/er7c-ts.aspx) and whether that might give me > a better signal-to-noise ratio, but just wondered if anyone has any > experience of this before I buy some. Sam, The er7 has too high a noise floor for this. I would suggest the er10c, the dpoae system from etymotic. http://www.etymotic.com/pro/er10c.aspx However, the main problem is that the low-frequency response of these systems is not adequate for these measures, since the heartbeat energy is centred below 100 Hz, and even lower (1-40 Hz). The er10c can be modified quite easily to do this (but lists at $2000- each). Alternatively, you could build your own microphone system, We used flat-response microphones from the Knowles FG series electrets, with a suitable in-house preamplifier, and could show flat response down to below 10 Hz. Additionally/alternatively, depending on how much you occlude the meatus, you can let the occlusion effect bring you up above the noise floor. We observed circa 40(average) - 50 (max) dB of occlusion below 40 Hz with realistic plugging (performed by a competent audiologist and/or ENT surgeon). Our recorded SPLs turned out to have a total power between 80-90 dB SPL (4-30 Hz, obviously unweighted). The work is in press to appear in a future edition of Ear & Hearing by Stone, Paul, Axon & Moore. sincerely, Michael Stone. -- Michael Stone Senior Research Associate Auditory Perception Group Dept. of Psychology University of Cambridge Downing Street Cambridge CB23EB Website: hearing.psychol.cam.ac.uk