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Otoacoustic Emissions: A simple test awaits

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Follow World Hum Map and Database Project on WordPress.com

Several years ago I reported on the research from Frosch that suggests the Hum and Otoacoustic Emissions (OAEs, or SOAEs) may arise from the same mechanism. One piece of evidence for this is that both the Hum and OAE are usually disrupted for several days by air travel.

I don’t know why I didn’t pursue this earlier, but OAEs can be recorded, even in newborns, using in-ear microphones. Here are some examples: http://www.otodynamics.com/supporthome.asp?CatID=2

Therefore, if the Hum is a type of auto-generated  emission, we should attempt to record it. The microphones used in OAE testing start around $500.00 and go up from there. I haven’t got the funding to conduct this, but if a motivated reader does, s/he could secure a place in history by being the first person to record the worldwide Hum. If the experiment fails to record the Hum, that would be an important finding as well.

Let me know if you intend to conduct this experiment.


  1. Charlie says:

    I have also wondered if it wouldn’t be possible to do this with the Hum. A while back I read a little about detecting OAE with mics with the view of actually trying this experiment,

    The $500 mic’s a bit of a worry. Neverthe less I’m going to look into this one – maybe there’s a cheaper option,

  2. Charlie says:

    One more thing, I was just looking at the Otodynamics page and I get the impression that these mics are used a bit in audiology. If that’s the case, I wonder if an audiologist, doing these types of tests, hasn’t noticed a strange Hum-like OAE in a few of their patients.

  3. jimvandamme says:

    Passive speakers and microphones are very similar, and one can be used as the other to some degree. These probes look like garden variety earbuds with good shielding, calibration, and a medical pedigree. If you just want to detect a signal without precisely quantifying its amplitude or making a medical diagnosis, why not stick an earbud in your ear and amplify it carefully?

    “The OAE Story” is well worth the download from Otodynamics. I had no idea how ears work, and the cochlear amplifier system is just amazing. As an RF engineer, I have struggled with amplifiers that exhibited oscillations. It seems logical that ears can go haywire sometimes too.

    • Charlie says:

      That is a good point. I have heard that speakers and some mics work on the same principle, ie movement of a coil within a magnetic field. My father was an electrical engineer and I remember him showing me how a paper cone speaker could also function as a microphone.

      I’ll check out the OAE story.

      Ok I have earbuds here, I’ll give it a go. I’ll test them to see if they function as mics, then stick one in my ear.

      • jimvandamme says:

        So, you’re way ahead of me, Bernie. What did you use for an amplifier, and indicator at the output? Did you try any high pass filtering?

      • Hi Jim –

        The amplifier was just two op-amp stages. The indicator was a sampling scope. (A non-storage scope could also be used with the advantage that the eye/brain will sort out a lot of traces fast; but no record, or easy means of checking frequencies). I used notch filtering to take out 60 Hz and the 2nd harmonic at 120 Hz.

        The summary note is at:


        which also leads to Webnotes 31, 37, 38, 39, 40. The full circuit (amps and filters) is in Webnote 38. The Webnote 31 tells about the elephant infra sound.


    • Jim – not quite that easy I’m afraid, but worth a try.

      I have never tried ear-buds but have tried a quality pair of stereo earphones (ear-protector size, with foam rings, very quiet) and they were perfectly good microphones when sitting on the bench. It’s trivial to pick up a low frequency audio test signal (say 50 Hz). In my work reported here, I used a full-sized stereo speaker as a microphone.

      With the earphones ON your head, it’s an entirely different story. There’s probably five times the signal of the bench, looking rather random. It’s generated (triggered) by your motion. So – why not just hold still! Right! Not easy to do. It’s hard enough to display anything with the phones on the bench. Wearing them is frustrating. Lots of rail-to-rail banging, etc. I saw nothing useful.

      I kind of suspected this would happen. My belt-worn elephant infra sound detectors were always going off even here in Ithaca despite a lack of elephants. People just can’t hold still. For example, you have to breathe. OAE or NOT, the motion noise is going to be a problem I suspect.

      So I suspect a challenge at best. Ear-bud might well work better. Perhaps you can go to sleep. I would suggest a team effort – one person to just lie as still as possible, the other to observe the scope.

      Even just one reasonably clean display of even just three or four cycles observed AT your pitch matched Hum frequency would be fantastic.


      • George G. says:


        I am using a slightly different approach which I believe has potential for enhanced OAE monitoring.

        I have removed the diaphragm from a stethoscope and attached the transmission tube end onto a electret mic capsule. The capsule drives a single stage pre-amp.

        The stethoscope earpieces are driven quite deep into my ears by a rather powerful clamp spring. Very uncomfortable, slightly painful, but spectacular results.

        Observing the output on my CRO is difficult but not impossible, and a waveform which I struggle to measure is present. Random environmental noise and light movement (as you mentioned) present a challenge, but the worst offender to a stable waveform is the periodic pulse from my heartbeat. It belts the trace periodically, most annoying.

        Nevertheless, there IS a waveform, which is NOT present when I remove the stethoscope from my ears.

        My best estimate is a period of 37mS to 43mS

        This gives us a frequency range 23Hz to 27Hz.

        I then switched the CRO to X-Y mode and applied a signal from a function gen. into CH2. (Stethoscope = X Genny = Y)

        The best circle I can produce (heartbeat kicking trace horizontally about once a second) is with 21Hz to 26Hz.


        I perceived The Hum as intermittent during observation, not constant. The waveform, however was constant.

        Mains hum here has a period of 20mS


        Audio Recording.

        The recording playback was all about me. Not a single hint of The Hum, even though I could clearly “hear” it, still at moderate level during the recording.

        Playback revealed a very deep, continuous rumble. This was present from start to finish, no variation in amplitude or frequency.
        The level of this rumble dominated the recording.

        My breath sounded like something being dragged along the ground in a cave, with echo.

        I even heard noises made by my stomach (gurgling etc.)
        Through my ears???

        But no Hum.


        During recording I tried my ear popping test every 30 seconds. This has the effect of making The Hum grow louder. At playback, there was no Hum, but during ear popping time slots a harsh grating could be heard. I am guessing this is friction induced vibration from movement inside my ears.


        I would like others to have a go at this. It’s not expensive. Just a cheap stethoscope, an electret capsule and a single transistor amplifier. Any high school kid interested in electronics could put one together.

        I do not know how to place pictures on this sight, I am emailing you pictures of the stethoscope/mic assembly and will also CC to Glen who will decide whether they should be posted.

        I have video of my CRO display but when I tried to email it a message said it was too large (259MB) so I will try and video a very short version.

        If you can offer any hints on how to trigger my ageing Hitachi CRO better than I can I would be most grateful.

        I would also like your thoughts on any trap I may have fallen into, such as spurious noise from equipment etc.



      • jimvandamme says:

        George, I like the stethoscope mod.

        Here’s a different setup somebody might want to try. I’m an analog scope lover as much as the next guy, but as a recent Nobel Laureate one wrote, “the times they are a’changin'”. The average cheap computer mic plugs right into a sound card or laptop, and has a preamp built in. Once you get it into the computer you can filter and record the signal easily. You can also crank up the gain quite a bit (10 dB boost). Computer inputs should be able to handle the low tones you mentioned, and might even help with the infrasonics. Cheap electrets will have a lot of hiss, but you can filter that. You don’t need broadcast quality.

        One thing that concerns me is that the sound at the end of a long tube can filter ambient wideband noise. Like hearing the “ocean” in a seashell. What happens when you record a non-hearer under the same conditions? It could be outside noise filtered through your body. You report that it’s constant, yet the Hum is intermittent. So which is cause and effect, or …..?

        Can you post a still from your movie that shows anything? Or a smaller snippet on YouTube, Vimeo or some such?

      • Good report – details are essential.

        My only guess about the approx 25 Hz signal would be the possibility of room resonances which I discussed in my ENWN-28 where I saw something around 17 Hz (my laboratory room being a very irregular basement perhaps 3 standard bedrooms in size).

        It’s hard to see. Try stomping your foot and see if a similar (even rapidly decaying) sinusoid jumps up. Stomping is better than clapping hands to excite lower frequencies.

  4. janetmenage says:

    This may be relevant to some Hum sufferers, however, it would not explain the partial amelioration of the Hum by bluetac earplugs, nor the vibrations felt in ears, eyeballs, skull and chest due to the Hum. US Navy experiments on LFN years ago clearly showed that these areas of the body are resonant with frequencies in the region of 50Hz. I doubt that anything self-generated would be powerful enough to set up physical resonances elsewhere in the body. The fact that air-travel may disrupt experience of the Hum could simply be that eustachian pressure changes have temporarily reduced tympanic membrane responsiveness to external LFN.

  5. Charlie says:

    Hi Bernie – you mentioned that breathing, bodily sounds etc interfered with your attempts to detect OAE. The thing is that audiologists/Otodynamics must have figured a way around that problem. I’ve been looking at the Otodynamics site and it seems that theres more to it than just the probes. At the very least it looks as though some special software is needed as well.

    FWIW I downloaded Audacity and I have just been trying out some earbuds as mics. They do seem to work, but not very well. At the moment the earbuds seem pretty insensitive. If I speak loudly they’ll pick it up, but sofar that’s about all. However I’m new to Audacity and there’s probably various input/ output levels that I can play around with. And I need to get hold of a regular mic for comparison purposes.

    But to be honest, after looking at the Otodynamics site i sort of doubt that earbuds and Audacity are going to be up to the job. No harm in trying though I suppose.

  6. George G. says:

    To Jim,

    Yes, it appears you have read me accurately. I’m the guy who still listens to SW with valve-driven radios (tubes to you guys)

    Much as I love my old CRO, it simply is not up to the job. I shall start shifting toward the digital era immediately, because I really want a “still” of this waveform.

    I have pictures taken with an iPod but they only show small portions of the trace. I should have expected that.

    As soon as I can get a trace with a computer I will attempt to post it.

    Your idea of recording a non hearer did not occur to me, but it is essential, and I shall start looking for a victim.

    To Bernie,

    The signal in question only appears when I’m wearing the device.

    Having said that, it just occurred to me that when I am wearing it the tubes are sealed, whereas when I remove it the tubes are open.

    This may play a role, which is what Jim warned of above. I shall repeat the test with the ends blocked.

    I will try to initiate some lower frequency resonance, just blinking my eyes produces signal, such is the sensitivity.

    One last detail, the audio recording was carried out in a separate building.

    • Charlie says:

      George – do you reckon it’d be possible to bypass the electronics and just have someone (a non hearer) to use a (perhaps modified) stethoscope in the regular way to listen to what’s happening inside the ear?

      With my crude arrangement of Audacity and dodgy earbuds I have succeeded in recording the sound made when I pop my ears, but nothing remotely humlike.

      Again I have to wonder why it is that audiologists using all the right equipment to detect OAEs haven’t noticed a strange noise in some (2%?) of their patient’s ears.

      • George G. says:

        Yes, I see what you are thinking Charlie.

        Anyone with a little mechanical ability should be able to put together a diaphragm small enough to fit into the ear.

        I am not familiar with Audacity but intend to learn about it very soon.

        A good point regarding audiologists, a good question. Perhaps they are aware of the deep rumbling I heard during playback but pass it off as noise. Or perhaps their equipment has sharp cut-off filtering below the bottom end of our audio range.

        We really need to talk to an audiologist for answers to such questions.

        Back to your earbuds, is there any way you could amplify them before the computer, it could make all the difference.

      • Charlie says:

        My computer says that it can amplify the input from a mic. I’ve tried using that feature and it has helped with the sound levels. I’m not sure if an amplified signal from the mic to the pc wouldn’t be better though.

        I’m new to Audacity and still messing around with the levels.

        Also with the rumbling sound you picked up on your audio playback – how loud was it compared to the other sounds?

  7. George –

    First, if you block the ends, in theory no sound gets in. In practice, I don’t know (through the walls?). In the case of having the couplers in your ear, we are speculating that acoustic energy would be supplied internally by a signal from the coupled living ear canal (now part of a closed chamber – closed at the eardrum).

    The tubes are from a physicist’s point of view “pipes” and the modes for “closed-open” (ends not in ears) are all odd numbers. I suspect that for closed/closed (with signal input!) all the mode are present.

    However, resonances of significance may or may not be present. The tubes seem to me to be relatively long relative to their diameter. In such a case this “pipe scale” may mean that friction of the air/walls dampens resonances. For example, for a trumpet the first two “open” (meaning here, no valves down) modes are C and G, the 2nd and 3rd modes. The first mode is damped (in some brass, available as a “pedal tone”)

    But do try it.


    • George G. says:


      I blocked the tubes and observed the CRO while the mic lay on the bench. The unidentified signal was not present, just ambient noise.
      The sensitivity astounds me.

      Back in my ears and the signal in question is back. I made made an all out effort to measure the period. The best I could do approximates 38mS.

      I had another brainwave. My old CRO has an output for CH1. I plugged the output into a frequency counter but cannot get enough level to activate the counter.

      This is now driving me nuts—–I MUST find the source of that signal, and the exact frequency.

      An obsession has begun.

  8. George G. says:


    The rumble is constant and approx. level is that of my heartbeat.

    Ear popping movement, swallowing, twitching etc. exceed the rumble in level.

    I wonder if I’m listening to blood flow. I really wish a medico would join us here.

    I know what you mean about audacity. I had a look and just can’t be bothered with it.

    I am going to look around for a computer-friendly grumpy-old-man-friendly oscilloscope program.

  9. George G. says:

    Bernie: CC Glen:

    The signal that has been bothering me is not present in the audio recording.

    It is not present in the house.

    It is not coming out of my ears.

    It only appears in my workshop, WHEN THE INVERTER IS SUPPLYING POWER for the CRO.

    Very embarrassing, my apologies to all.

  10. Charlie says:

    So the rumble was the inverter. At least you found out what it was.

    One thing with an oscilloscope program would be that you could run it on a battery powered PC.

    Audacity isn’t as bad as it might first appear. I’m very much a layman when it comes to acoustics but after a couple of days I managed to get my head around the basics of operating it. Anyway I don’t have a CRO, so I don’t have a lot of choice!

    • George G. says:


      The rumble is audio and still present.

      The strange waveform is RF, and only present in the workshop, displayed on a CRO.

      When I took the mic assembly away from my body I thought the strange signal had vanished, but it only diminished in amplitude.

      My body was acting as an antenna and rebroadcasting the RF through the mic assembly and into the CRO.

      The audio was recorded in the house (clean mains). I checked the mic assembly inside the house with the CRO this morning and it was clean.

      I would love a DC oscilloscope, digital of course, I have fallen into this trap before and should have known better.

      Again, I apologise for any confusion I may have caused.

      • Charlie says:

        The confusion could well be caused by me not reading things properly. The mystery audio rumble remains.

        Blood rushing does sound like a likely candidate, but I would have thought that some pulsing would have been evident. But like you say we really could use some informed medical input.

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