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Thread: Real World Frequency Forms

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    Senior Member Ducatista47's Avatar
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    Real World Frequency Forms

    OK. I admit this is idle curiosity. Forever, it seems, I have wondered what the waveform of real, complex music and sound look like when plotted. We are all familiar ad nauseum with the sine wave plots used to illustrate the workings of audio components. Music, even from a single instrument, has harmonics and such. Imagine the sound coming from a band or orchestra in all its simultaneous complexity.

    My gut feeling is that when this information is reproduced we get a single frequency at any given point in time. It is certainly not a smooth sine wave, but what would a plot of it actually look like over an interval of time? Perhaps it can not be plotted as a wave, but rather graphically represented with one of the other audio representations, like a waterfall plot or something. Good guess, I am not an audio engineer.

    I have never seen this represented or vividly described anywhere. I am sure it has been, but I have not seen it myself. Otherwise I would not be wondering about this.

    Clark
    Information is not Knowledge; Knowledge is not Wisdom

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    Senior Member 1audiohack's Avatar
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    I too think about that fairly often.

    I often watch music on an ociliscope while listening when I am testing something, the more complex it gets the more it looks unrecognizable. It's funny how you can look at a single instrument track and have no idea what you are seeing but a 10 for a dollar tiny speaker let's you know that it is a voice, a sax, or what ever.

    We have recorded musical instruments and other stuff and looked at the wave forms on a digital storage scope and I found some very interesting things, at least to me. One look at a kick drum wave sold me on polarity. One other thing, I find a saxophone sounds much more like a clarinette if the polarity is reversed anywhere.

    Closer to your original topic, the latest edition of EASE is capable of using the music as the sample stimuli for time and frequency response measurements. I plan on buying that this spring. Maybe one step closer to really seeing what we hear. How far can we go down this road I don't know, what does light sound like? If you could hear it, what could you tell about it? Does the language exist to do so?
    If we knew what the hell we were doing, we wouldn't call it research would we.

  3. #3
    Obviously... not a golfer grumpy's Avatar
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    looks like noise, with repetitive patterns ... sometimes more noise,
    sometimes more pattern

    The notion of a frequency at one point in time doesn't hold... there's only frequency over
    a time period. At any instant, it's only an amplitude value or pressure reading or ...

    Much like analog synthesis, you can add sine, square, triangle, noise (of various 'colors')
    and in varying frequencies together and get out something that sounds pleasant (or not).
    Interpreting it visually is ... difficult at best once you've mixed enough sources together.

    One can sample something {over a time interval} and analyze (filter) the result to get
    some idea of what's happening in frequency bands (think of an RTA display...). It can
    be pretty or informational, but I don't think anyone would say you can 'see' the music
    (a beat, perhaps with a fast sample-interval/repeat-rate).

    Not sure if that helps... pretty sure it doesn't completely answer your questions.

    (edit: 'hack brings up some good points)
    It's uh... uh... it's down there somewhere, let me take another look...

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    I watch it on spectrumlab first direct from source CD laserdisc DVD and bluray to pc then connect the SPL db meter up to pc for an in-room response, which is different due to a few room minor room nulls or some room gain. Some of the frequencies will mirror from direct to in-room response.

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    Senior Member WDJ's Avatar
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    An interesting plot might be to use a waterfall type display as in a sonobouy system.

    Use frequency as the horizontal axis (left is lowest, rising as you go to the right) and color to show intensity.

    A spectrum analyzer would give you a "picture" of what freqs are occuring "when" but the audio band is pretty narrow so it might not be very easy to see seperate freqs.

    (It's also "fun" to look at a few lines of video on a o'scope and try to figure out what would be on the screen....)
    Share what you know, learn what you don't...

  6. #6
    JBL 4645
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    Quote Originally Posted by WDJ View Post
    An interesting plot might be to use a waterfall type display as in a sonobouy system.

    Use frequency as the horizontal axis (left is lowest, rising as you go to the right) and color to show intensity.

    A spectrum analyzer would give you a "picture" of what freqs are occuring "when" but the audio band is pretty narrow so it might not be very easy to see seperate freqs.

    (It's also "fun" to look at a few lines of video on a o'scope and try to figure out what would be on the screen....)
    I agree I find the RTA a bit limited in precisely locating the frequency. Spectrumlab pinpoints it down to the Hz or KHz.

    If anyone wants to believe the pc RTA they spent $100.00 or so package they brought, be my guest. Spectrumlab is free and it’s good for what it is. Scroll down the page for download and enjoy it, I know I have. It might look like predators thermal vision it does take a while to get used to it. Play LP tape CD bluary even your own voice though.

    http://www.qsl.net/dl4yhf/spectra1.html

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    Senior Member Ducatista47's Avatar
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    I confess there was a corollary question driving my more fundamental inquiry. This is it: since a transducer/driver/microphone diaphram creates or detects sound by moving back and forth, and can be moving in either but not both directions at one time (all in the service of mimicking the pressure waves of sound)...

    ...what movement does it actually employ to reproduce or detect the complex signals/movements/pressures? It is going back or forth but manages to recreate or detect incredible complexity. Instead of a graphical representation of its movement (like an oscilloscope scope display), what does the dance of the transducer look like?

    This we probably do know, I think. It can be seen with the proper photographic techniques, but I have not seen it myself. It would be about amplitude over time. Each travel back and forth would be asymmetric and usually looking like a jittering object, I would think. The lower frequencies would dominate the travel and the higher frequencies would be riding on the low frequency's back. Perfectly demonstrating the relative power required to muscle each frequency into life.

    If this is at all the case in reality, that means the movement in one direction is indeed a summation of the numerous impulses present. Think about it. How else could a speaker function? Time cannot wait for the simultaneous waves to be generated one at a time. This was what gave me the gut feeling I have about all this.

    So it is a not so simple simple question. What does the movement of a transducer really look like? That would be the answer.
    Information is not Knowledge; Knowledge is not Wisdom

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    Senior Member 1audiohack's Avatar
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    To quote James Moir, "Anything obvious in acoustics is usually wrong." Maybe he was joking, maybe not.

    Physics teaches us that only rotational motion is absolute, all other is relative.

    I remember someone here saying of Zilch, "is exploring for the sake of exploring." I thought to myself, that's great! Is there any better reason?

    Well likewise I am incredibly curious. We have a precision high speed laser position sensor system here that is used to measure and display valve position in a non firing motor driven engine for valvetrain development and validation, it's called a SpinTron. You set up the laser with a view of on of the valves, motor the engine slowly to record the motion of the valve and set a data baseline and then increase the speed of the engine until the valve no longer follows the cam profile. There is a ociliscope on the console and you watch for distortion in the wave form so to speak. It's kind of like "The Rack" you just break stuff on it.

    Well it was no great leap to say hhmmm, linear motion, position, scope, hhmmm, signal generator, amp, speaker,,, a little signal conditioning and let's see what a speaker looks like.

    Back around to your thought about small high frequency ripples riding on the waves of low frequency, yup. As for not moving in two directions at once, I'm not so sure, at the glue line of the voice coil to cone juncture of a 2234, still visible before the dust cap is applied I could see motion to almost 8K, about 0.004" to 0.008" before the motion disappeared into the noise floor. At that point it's more than 60Db SPL below the 1K mark. That motion is not detectable by the laser any where on the cone, only the coil and former are moving at that point. It's funny if you do a constant voltage high power locked motor impedance sweep, ie with the voice coil former epoxied in place if youdrive it hard enough with a sweep you can hear the high frequency still! It must be just the wire moving at that point. I did that to a 2225 before I turned it into a 2234.

    So the next logical? step is to set up the scope to monitor the amp output on Ch1 and the laser on Ch2. Speakers (where minimum phase) do a fine job recreating single sinewaves, square waves? Not so good. We did this last year and I did not have a digital storage scope at the time, I want to revisit this now that I do! Then I could share with pictures.

    The real reason I did all that was to explore the "fast bass" thing, I "drag raced" every 15 I had.

    This is as close as I have been to what you are talking about, still it is exactly as Grumpy says, it just looks like noise.

    Richard Heyser rightly stated something like "Nature does not care how we look at it and if we don't or can't get the results we are after with the tools we have, no matter how comfortable we are with them, we must then reach for other tools."

    As for what does the sound of an orchestra look like, maybe Walt Disney got as close as we'll get in Pink Elephants on Parade.
    If we knew what the hell we were doing, we wouldn't call it research would we.

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    Quote Originally Posted by Ducatista47 View Post
    I confess there was a corollary question driving my more fundamental inquiry. This is it: since a transducer/driver/microphone diaphram creates or detects sound by moving back and forth, and can be moving in either but not both directions at one time (all in the service of mimicking the pressure waves of sound)...
    The eardrum is also just a diaphragm with about the same limitations as the mic/driver varieties, and it's able to reproduce all the sounds we hear regardless of source.

    As for the coils themselves reproducing sound, another tangent.

    In electric powered RC, some speed controllers emit a series of tones to indicate that they are armed, error codes, etc. But there is no transducer on the controller, the sound actually comes from signals sent through the motor windings.

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    Senior Member timc's Avatar
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    What you see when looking at the wave file on your computer, is the sum of all the preassures or velocities at the microphone diaphragm at that particular instant. The eardrum works in the same way. When sampled quickly enough, or recorded with analog equipment, we are able to play it back over a sound reproduction system.

    All sound can be mathematically decomposed into pure sinewaves. The real waveform is the sum of all the amplitudes and phases of the respective sinewaves.
    2213 + 2435HPL w/aquaplas + H9800 (Matsj edition)

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    Senior Member 1audiohack's Avatar
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    The exhaust fan motors in our chassis dyno cell 15HP each X2 and are inverter driven soft start. They emit from the motors about a 4K whine until about 60% speed the noise of the fans drowns it out. It sounds exactly like the beginning of a Pink Floyd song from Momentary Lapse of Reason, I forget which one but it's actually a very peaceful sound, right before something very violent and exciting happens. High HP is music!!!
    If we knew what the hell we were doing, we wouldn't call it research would we.

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    Obviously... not a golfer grumpy's Avatar
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    Hey Clark, it would have been illuminating for you to have seen the demo of the
    laser holography lab at JBL while it was still set up (who knows, now ).

    In the simple case, yes... you can think of the transducer moving as an infinitely
    stiff surface... moving with the air (mic) or moving the air (speaker) and that motion
    is the analog of air pressure changes ... and is turned to another analog: the electrical
    waveform you would see on an oscilloscope.

    In real life that infinitely stiff surface only mostly holds true over a specific range
    of factors and the whole thing can look like wave reflections in a bathtub... very
    orderly or chaotic. Both "problems" can and have been used to create workarounds
    or a specific sound (think guitar speakers).
    It's uh... uh... it's down there somewhere, let me take another look...

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    Senior Member Ducatista47's Avatar
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    Quote Originally Posted by timc View Post
    What you see when looking at the wave file on your computer, is the sum of all the preassures or velocities at the microphone diaphragm at that particular instant. The eardrum works in the same way. When sampled quickly enough, or recorded with analog equipment, we are able to play it back over a sound reproduction system.

    All sound can be mathematically decomposed into pure sinewaves. The real waveform is the sum of all the amplitudes and phases of the respective sinewaves.
    Thank you! That was my suspicion all along. I could not envision any other way the whole thing could ever work. I am sure, as everyone else has commented, that in reality it is more complex and a bit sticky. Even with pure pistonic response, if it were possible.

    This all invites the question: Should we pattern our high fidelity reproduction systems after the perfection of the original sources or the imperfect response of our ear diaphragm? Here I suspect that since our ears will do their thing with our hifi sound, the hifi should indeed mimic the source. Again, no other answer makes sense to me.
    Information is not Knowledge; Knowledge is not Wisdom

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    Quote Originally Posted by Ducatista47 View Post
    This all invites the question: Should we pattern our high fidelity reproduction systems after the perfection of the original sources or the imperfect response of our ear diaphragm?
    I was under the assumption that that was the entire idea behind building different/better/more accurate systems in the first place, to try to mimic the original source as closely as possible.

    But in the end, unless you are just comparing waveforms (and sound only travels as waves, after all), you'll need eardrums to hear it.

    The only other way is to take the reproduction out of the equation and just listen to the original source with your eardrums.

    Here I suspect that since our ears will do their thing with our hifi sound, the hifi should indeed mimic the source. Again, no other answer makes sense to me.
    Yes, not all ears are created equal.

    I'm reminded of the Ella Fitzgerald commercial, "Is it live, or is it Memorex?" Wasn't that the selling point, reproduction of a wave so precisely that ears couldn't distinguish between the original source and a recording?

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    Senior Member ratitifb's Avatar
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    Quote Originally Posted by Ducatista47 View Post
    Quote Originally Posted by timc View Post
    All sound can be mathematically decomposed into pure sinewaves. The real waveform is the sum of all the amplitudes and phases of the respective sinewaves.
    Thank you!
    please thank Mr. Fourier

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