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    • Thread Starter

    I've been reading up papers about mapping sounds to their auditory representation, and I'm a bit confused about one bit.

    They've given some diagrams of sound waveforms spectral with the y-axis being spectral frequencies, and the x-axis being temporal modulations.

    What are temporal modulation frequencies in this case? I've been trying to find a simple, down to earth explanation on the net, but there seems to be a few different answers, and I'm not quite sure which one it is, if any.

    Here is an image of what I'm on about.


    If someone could give me an answer which an everyman can understand, I would be most grateful!

    "Spectral frequencies between 2500 and 5500 Hz, and temporal modulations in the range 1–16 Hz" - if that helps.

    Thank you!
    • Study Helper

    Study Helper
    Temporal Frequency Modulation?

    In engineering parlance, Frequency Modulation (FM) is an analogue term where a constant amplitude carrier wave of high frequency is modulated by a lower frequency signal. i.e. the lower frequency signal is imprinted on the carrier. In that sense the low frequency data signal is shifted to a much higher frequency range ready for radio or data transmission.

    The terminology Temporal Frequency Modulation is more like someone trying to differentiate between the engineering use of FM and experimenting with different ways of displaying information since a full frequency/amplitude plot would need to be log scale and therefore not easily discernible in a visual sense.

    Looking at the plots they seem to be the observations of infra-sound frequency modulation and its footprint on the audible sound swept frequency. In the analogy with FM, the audible frequencies are the carrier and the infra-sound the modulation spectrum.

    So the author is attempting to find a correlation between the spectral energy composition of familiar sounds: discordant vs pleasant:

    With pleasant sounds, the most energy appears to register lower in the swept frequency range below the most sensitive part of the human audible spectrum and also modulated by a higher average infra-sound spectrum.

    Discordant sounds appear to have most energy in the most sensitive part of the human audible spectrum and also infra-sound energy is in a lower average range than for the pleasant sounds.

    Of course none of these plots show the absolute amplitudes and hence energy of the signals concerned.

    Perhaps human evolution has meant that the human ear has developed a frequency spectrum sensitive to warning frequencies but also for the most efficient method of communication between humans at the atmospheric temperatures and pressure where humans most commonly evolved?

    Which would make the hypothesis that discordant sounds trigger a protective response heightening danger?

    Just me hypothesising. I'm not a neuroscientist!

    Hope this helps.

    And crucially, the plots are observations only.

    I get "access denied" for that link, unfortunately.

    I think you might need to refer to the specialist neuroscience/acoustics literature really.

    Fwiw My guess is that it's modulation of an audio frequency by a low frequency envelope - perhaps something like the sound emitted by the type of referees whistle that has a pea.
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