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Two Important New Papers from Henrik

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Henrik, one of our “scientists in residence” has written a long-awaited paper on tracking down external environmental noise using very basic tools. I will feature prominently on this blog and on the Hum Map website for the near future. I hope that this paper will not only help create a purer dataset for our project but will aid people who are suffering from nuisance, low-frequency noise that arises from classic anthropogenic sources.

The paper is written in two parts.

Read Part One here.

Read Part Two here.


  1. Kurt says:

    I wonder if the iphone/ipad app: FFT Plot can be used for this purpose

  2. Henrik says:

    Maybe someone who has done this with iPhones can answer this specific question.

    What you should look for is a real-time audio analyzer with FFT (Fast Fourier Transform) functionality. We are searching for low frequencies, so we want a low sampling rate if possible, since we need a high frequency resolution, at least down to below 3 Hz. The frequency resolution (or Hz/bin) is the sampling rate divided by the FFT size. If the app has only 44.1 kHz or higher, we need an FFT size of 16384 to reach even 2.7 Hz resolution. This will drain the battery faster, since the processor load will be high. That’s why the Spectroid is good, since it has sampling rates down to 8000 Hz. I used 11025 Hz in the instruction to create a best match with the TASCAM recordings, but for just the visualization on the phone, any sampling rate is OK as long as we reach a good enough frequency resolution.

    It is also not ideal to use extreme resolution, since out of the man-made sounds, only the power line harmonics are absolutely stable. All machine sounds tend to “wobble” a little, so a resolution below 1Hz is no longer useful.

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