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Highly Directional Close up Microphone

Discussion in 'Microphones' started by ctilley83, Jun 7, 2013.

  1. ctilley83

    ctilley83 Active Member

    would like to record specific audio for the following scenario. I have a piece of equipment that rotates a drum via an electric motor. The drum contains food product that is heated by air entering the drum. The food inside the drum cracks very lightly when it is fully cooked. The problem is that the cracks are audible, but barely. The whine of the motor and inflow and outflow of air is drowning out the audible cracks.I figured a solution would be a certain microphone that would concentrate it's sensitivity close to the drum, therefore avoiding all the background noise. Unfortunately I am NOT an audio expert in the least. What microphone would work for this setup? Any other tips with how to filter the microphone and process the audio in real time.I am a computer programmer so designing software would not be an issue for this. I mainly need help picking out hardware and fully understanding the problem domain so I can design a solution.
     
  2. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

    I would bet some sort of contact pickup would be the best solution. A piezoelectric transducer placed under the food product might do it.
     
  3. DonnyThompson

    DonnyThompson Distinguished Member

    Boy, I dunno, guys. That sounds like a real tough one. Regardless of the directional narrowness, with a motor going like that, any isolation on particular freq's is gonna be very tough. I guess I'd have to second Boulder's suggestion of a contact pickup of some kind, but I'd be surprised if it was as successful as it sounds like you want it to be.

    You might.. and I mean might be able to use corrective or forensic EQ on it, but it's going to be a long shot with all that other noise happening at the same time.

    Maybe a better solution, if this is for a video or something? ...would be to overdub a sampled sound similar to the "cracking" you described. it's not as if you're lying about the food doing that, you can hear it in real time, you just need to bring it out, and a way to do that would be to consider overdubbing a sampled sound similar in scope/tone.

    One last question... does the food stop "cracking" immediately when you shut off the motor? Because if it still continues to make that sound, even for a few seconds after the motor is off, and you're ready to go with your mic and recording rig, you could grab the sound quickly before it stops, and if the motor is off, you wouldn't have the added noise. You could then save the "cracking" sound as an individual sample and use that to overdub, and simply copy-paste throughout the recording...at which point you would have discreet control over the sound you desire - in both volume and tone...

    Just tossin' that out there...

    fwiw
    -d.
     
  4. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

    I'd experiment on the cheap with a piezo tweeter from a cheap boom box. Once you get it out of the plastic speaker box it's just a flat disc about 3/4" in diameter. You'll need a preamp with a very high (10 megohm) impedance.
     
  5. ctilley83

    ctilley83 Active Member

    What about a shotgun mic?
     
  6. dvdhawk

    dvdhawk Well-Known Member

    Did you know that if you watched the broadcast of the olympic rowing competition in London, you were hearing oars that were pre-recorded and foleyed in? The helicopters necessary to provide good TV coverage were drowning out the natural rowing sounds as they chased the boats down the river. Hollywood magic to the rescue.

    So I guess what I'm saying is, like Donny I would also try to find a way to get the food product to make it's cracking sound minus the noise of the motors and blowers. Even if it's in some other form of heater. If the manufacturing noise is also part of the presentation, then mix the two together.

    If the motor & blower noises are a pretty constant waveform you may be able to record the noise (without the food product) out-of-phase , and see if carefully combining it with the normal in-phase recording of food product (plus noise) results in any significant noise cancellation. Even if you could place (destroy) a wireless mic placed directly inside the heated drum all you're going to get is wind noise.

    I think this is going to be very difficult to do without some cheating.
     
  7. Boswell

    Boswell Moderator Distinguished Member

    I think we need to know the purpose of the recording. If it's for promotion, advertising or whatever, then adding the snap-crackle-pop after recording the video is what everyone else would do. It would be very difficult to use conventional microphones inside the drum, not least because of the temperature and airflow conditions.

    If the purpose is scientific, you either need one or more highly-directional external microphones plus some sophisticated signal processing, or you need to re-design the experiment to include audio capture as a major factor in the list of design requirements. You might, for example, be able to use pulsed air flow so the popping was more audible during the off periods of air flow.

    The other thing I will draw your attention to is the work that is being done in the field of processing data from arrays of microphones to achieve very high directionality coupled with suppression of ambient noise. This technique is being used in areas as diverse as hearing aids to military target identification. One of the leading exponents in the hearing area is Prof Brian C. J. Moore at Cambridge UK, and it may be worth your contacting him to see if he could provide you with any pointers.
     
  8. ctilley83

    ctilley83 Active Member

    The purpose is to monitor the roast and tell whether it's done or not. I would like to do the processing in real time so I can listen with a pair of headphones. The problem with roasting coffee is that there are two stages when you know the roast is complete. First crack, and second crack. Unfortunately my machine is rather loud, and noise from the motor drowns out the popcorn sound that coffee makes when it nears completion. This is not for advertising, bogus special effects, etc. It's to develop a precise instrument to measure a process.
     
  9. bouldersound

    bouldersound Real guitars are for old people. Well-Known Member

    This is the type of piezoelectric transducer I'm talking about. It's about diameter of a coin but thinner.

    APS4812B-LW100-R PUI Audio | Mouser
     
  10. Boswell

    Boswell Moderator Distinguished Member

    My feeling is that the internal conditions of the cooking drum proscribe the use of conventional microphones (or piezo transducers connected as microphones) sited inside the drum.

    The only way I can see to get a useful result for these conditions is to use a multi-microphone array outside the drum coupled with processing of the resulting audio channels to achieve the necessary sonic localization. MEMS microphones make this a realistic proposition.

    There are many papers on this type of processing, and also commercial products based on multiple microphones. Here's an app note on beamforming by processing the output of a microphone array.
     

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