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OmniMic future feature request (spatial averaging with multiple OmniMics)

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  • OmniMic future feature request (spatial averaging with multiple OmniMics)

    Hello Bill,

    I currently own two original (V1) OmniMics and one V2 OmniMic. I would love it if you were able to (potentially) allow them to operate with their own unique calibration files in a mic multiplexer mode, or if the USB standard doesn't allow such fast switching, simply have the OmniMic software make a measurement with one OmniMic than move on "automatically" to the next mic for perhaps 30 to 60 seconds (per OmniMic) and load the unique OminMic cal file for the 2nd OmniMic, repeat and store a measurement and continue in this fashion and then average all the OmniMic's measurements together to get a spatial average using up to 5 OminMics. There is another firm that makes the THX acoustic analyzer (AcoustX @ www.acoustx.us and their D2 Win RTA software / hardware). Their system does the mic multiplexing in "software" (switching between the mics used to measure) but admittedly it isn't hampered via the use of the USB interface to do this sort of thing, (but still doing it in "software" which is the only such mic multiplexer unit I'm aware of that does it in this fashion - - all the others are hardware based). Your current OmniMic software allows an end user to have / use up to 5 OmniMics unique calibration files loaded / stored to do polar measurements of a DUT but the end user has to manually plug and unplug each OmniMic with its associated unique (and stored) mic cal file and repeat a new measurement, etc. What I'm suggesting is a new feature either via an automatic way to switch using the OmniMic software to control and load each individual OmniMic mic cal file for such spatially averaged measurement using multiple OmniMics or if the USB doesn't support such fast switching, etc., than allowing each individual OmniMic to make a measurement for 30 to 60 seconds using pink noise and then store that, the OmniMic software automatically loads the next OmniMic mic cal data that's stored in it, repeats a new 30 to 60 second pink noise measurements, stores, it and so on up to 5 OmniMics. After all the various OmniMic measurements are made / stored then the OmniMic software post-processes and averages all the various OmniMic measurements together (but the end user can keep all the various OmniMics, up to a maximum of 5, all plugged into the USB ports using a USB dock to increase the # of USB inputs). Would this all be possible in some later version of your OmniMic software by chance? Thank you in advance for your consideration of this request.

    Warm regards,

    SM

  • #2
    It does have built in averaging. I'm not sure what you are asking that's extra.

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    • #3
      Hello Erik,

      Microphone multiplexing allows for far faster acquisition time of measurement data since the mics can be independently placed in different locations in a given room vs. taking one mic (OmniMic, whatever) and doing a steady state (pink noise, a.k.a. "time blind") measurement at one location, stopping, physically moving the mic to another location, repeating the measurement again in the new mic location, saving that 2nd data acquisition, going to a 3rd and 4th place and repeating again, etc. After this, we use the post-processing in the audio analyzer (again, OmniMic or whatever) to mathematically average all the various microphone positions together to derive a composite frequency response that is "spatially averaged"). Shown below are some URL links to examples of what I'm referring to:

      - http://www.linearx.com/files/pdf/cinemapp.pdf (please see page # 11 on microphone placement)

      - http://www.av-iq.com/avcat/images/do...mx399intro.pdf (please see the 2nd page under MX399 Testing Setup)

      - http://www.rsem.com/pdf_manuals/usl/mmp10mane.pdf (please see page # 8 on microphone placement)

      - http://www.film-tech.com/warehouse/m...OLBYMPX500.pdf (please see page # 4 on microphone placement)

      - http://www.gold-line.com/pdf/manual/pa_mx4.pdf (please see basic introduction as to the rational for use of a mic multiplexer)

      - http://www.bobgolds.com/Sencore/FlatResponsePart2.htm (see portion of article on microphone multiplexing)

      With regard to why should someone make a spatially averaged measurement since we one can't physically have their head in each of test different microphone positions, ergo how does such a spatially averaged measurement correlate with how we hear vs. that of a single spot (location) measurement technique? The answer is that for small rooms (those with less than a 50 foot dimensions, e.g., height, width, depth) the lower frequency sound (below about 300 Hz) become dominated by standing waves so if one chooses only one mic position they may be in one standing wave pattern or another (I won't go into this topic in too much depth - - entire books have been written about it ;-)

      Some say that standing waves take time to develop (true, i.e., sustained [time wise] low frequency spectrum sounds) and that both pink noise (pseudo random Gaussian noise) or warble tones (amplitude modulating a sine wave) tend to "break-up" such standing waves from developing, ergo their choice to use in steady state measurements. All of these things are true. However, combining the use of steady state pink noise measurements that are both temporally (time) averaged as well as spatially (over different spaces in a given room) averaged is better IMO for steady state measurements than one spot (mic location) steady state measurements. Sure, one can use a mic and make a steady state pink noise measurement that's temporally averaged, store it, physically move the measurement mic to a new location in the room / space and redo it all over again, repeat this process and average them all together, and then post-process them after the fact to get the same picture as a mic multiplexer would do. This takes an inordinate amount of time and a mic multiplexer approach speeds this process up considerably. That's why I would like Bill Walso to develop the type of mic multiplexer capability / function into a future software build of OmniMic (if possible). By the way, my personal preference is pink noise over warble tones for such measurements (I won't go into why in this post - - I've bored you enough I'm sure :-)

      Anyway, by using multiple microphone locations throughout the space of the room (ergo the term "spatially averaged") one tends to minimize the potential possibility of a false measurement result due to standing waves at a given spot vs. averaging together several mic positions throughout the room). Additionally, even the use of one mic location doesn't necessarily correlate perfectly well with how humans hear since one single mic does measure in the same means as how we hear with two ears, i.e., we hear binurally and above about 400 Hz we hear our sense of direction based on interaural intensity differences (i.e., the amplitude [loudness] of sound arriving at one ear vs. that of the opposite ear) since the human head serves as a good baffle to attenuate higher frequency sounds that arrive from off-axis locations and the higher the frequency the more baffling effects due to the smaller wavelengths of sound at higher frequencies beaming and not bending around ones head to the opposite ear. Below about 400 Hz our sense of directional hearing works by detecting phase differentials since the the lower frequencies are longer and can transverse around a person's head. Below about 80 Hz our ability to detect where a sound is originating from is really not at play IMO.

      Please note I haven't even touched on other aspects, e.g., pinna (our outer ear flaps ;-) transformations, etc. These also come into play insofar as our sense of directional hearing is concerned, but less so than the aforementioned, (we can even hear a bit [not too well] in terms of "height" information based on pinna transformations. However our primary localization capabilities are far better on the horizontal plane vs. that of the vertical plane.

      So why mention the above? Simply to illustrate that single mic measurements, regardless of how well done, be they quasi-anechoic or true anechoic or steady state, etc. aren't capable, when used alone, to tell an end user how a given loudspeaker or room (or combination of the two) will sound. We need multiple measurements to do that (and the education and experience in terms of how to tell the end user how to properly interpret such measurements).

      With regard to how to "mathematically integrate," such steady state measurements, please see:

      http://www.linearx.com/files/pdf/LMS-4_Manual.pdf (please turn to page # 154 in this PDF manual and see "Scalar RMS" term)

      I think a combination of multiple quasi-anechoic measurements combined with multiple in room measurements tend to reveal a plethora of valuable information to a well informed / read (in terms of acoustics and psychoacoustics) practitioner / end-user. When one combines such measurements with binaural measurements such as IACC (large rooms) or Diffuse Field Transfer Function (DFT) and Interaural Cross-Correlation Fluctuation Function (IACCFF) the latter two in small rooms (who's dimensions are less than 50 feet, e.g., height, width and depth) - - see: http://epubs.surrey.ac.uk/562/1/fulltext.pdf I won't even touch on other highly interesting ways of looking into the complex room acoustic interface issues such as depicted here: http://www.acoustics.org.nz/journal/..._D_NZA2011.pdf

      Finally, I think the above paragraph's measurement techniques should be combined with measurement a modulation transfer measurement that's designed for music vs. that of speech (not Speech Transmission Index [STI] which is also a modulation transfer type of measurement) but one like MATT (see http://www.acousticsciences.com/matt and http://www.acousticsciences.com/art-...-sound-quality) or better yet AQT (see http://pcfarina.eng.unipr.it/Public/...153-AES110.PDF) would be used.

      Well, enough of my rant for now. Hopefully the above gives you insights as to why I would like to have Bill Walso add microphone multiplexing functionality to multiple OmniMics in some future build update. Have a great day and Happy Thanksgiving to you!

      SM
      Last edited by Sound_Man; 11-23-2016, 11:43 PM.

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