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Thoughts on AL's Hum

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  • kallio_mn
    AL s work was most interesting. I have though about this for a while now. That fact that the disk drive in his Skybox was the source of one Hum would imply
    Message 1 of 3 , Dec 5, 2006
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      AL's work was most interesting. I have though about this for a while now.

      That fact that the disk drive in his Skybox was the source of one Hum would imply that the room had a natural resonance at (or near) that frequency.  AL was in essence, inside the pipe on a pipe organ. 

      Now that the Skybox has been replaced, the source of excitation has been elimated.  But the room is still resonant at or near 90 Hz. (90 Hz * 60 min/sec = 5400 rpm = the rotational speed of the disk).  This is a function of the geometry.  Any other source of acoustic excitation could also cause the room to resonate as well. 

      There is a quality factor of resonance that reflects how close you can be to the exact frequency and still detect a response.  In electronics, this is referred to as the "Q" or quality factor.  High Q resonators have very sharp response curves.  At the exact frequency of resonance, the response is the greatest.  As you move away from that frequency, the response is less.  How quickly that response drops off determines the Q factor. 

      It would be interesting if AL would be willing to take a speaker, place it in the same place as his Skybox (maybe even inside the old box) and drive the room with different power levels and different frequencies and report on the response.

      Anything that has a sharp resonance curve can also be used to create oscillations as well.  If the excitation is reasonably steady, a steady note will emerge.   Blowing across the opening of a bottle is the classic example.  Sudden excitation (Impulses)  can also cause an oscillation.  How quickly a device stops oscillating after the excitation is removed (decay time) is also an indicator of  Q as well.   High Q resonantors will "ring" for a long time.  

      It may be that we live in environments that exhibit high Q at low frequencies, and are thus suspectible to noise (impluse) induced oscillations.  Our homes might be very good and taking low level environment noise and resonating at Hum frequencies.  If a disk drive in a Skybox can do it,  think what traffic, aircraft, and other low frequency noise sources might be able to do.

      Kallio 

       

    • dark_al1
      I completely agree and have been thinking along exactly the same line. This Is my reasoning behind the tone generator experiment which I have planned.
      Message 2 of 3 , Dec 6, 2006
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        I completely agree and have been thinking along exactly the same
        line. This Is my reasoning behind the tone generator experiment which
        I have planned. Basically, using the generator to create a 90hz tone
        and play it through a speaker in the room. I will plot volume vs
        response (resonance in fireplace) and also sweep the frequency from
        about 50hz to 100hz to locate the exact resonance frequency.

        With ref to your comment on excitation from traffic and other
        sources. I would also agree. It has been very windy here over the
        last week or so. While watching the TV the other day, I was getter
        very annoyed at the low frequency 'bottle tune' coming from the
        fireplace, Caused by the wind going over the chimney. This was very
        loud, even to the point of being painful. What suprised me most was
        that only when a very large sustained gust blew accross the chimney
        causing a very loud,low and constant note, which was very painful. My
        wife then said 'What was that?' She had been completely oblivious to
        the noise before then, while I had been sitting there grimacing. To
        me this highlights both your point of multiple exciters and also the
        hightened sensitivity that hearers seem to have. I have also noticed
        that when some traffic goes past with engine noise at a cetain
        frequency, it does seem to be amplified by the chimney. I would
        hazard a guess that the noise is also around 90hz.

        AL



        --- In humforum@yahoogroups.com, "kallio_mn" <kallio_mn@...> wrote:
        >
        >
        >
        > AL's work was most interesting. I have though about this for a while
        > now.
        >
        > That fact that the disk drive in his Skybox was the source of one
        Hum
        > would imply that the room had a natural resonance at (or near) that
        > frequency. AL was in essence, inside the pipe on a pipe organ.
        >
        > Now that the Skybox has been replaced, the source of excitation has
        been
        > elimated. But the room is still resonant at or near 90 Hz. (90 Hz
        * 60
        > min/sec = 5400 rpm = the rotational speed of the disk). This is a
        > function of the geometry. Any other source of acoustic excitation
        could
        > also cause the room to resonate as well.
        >
        > There is a quality factor of resonance that reflects how close you
        can
        > be to the exact frequency and still detect a response. In
        electronics,
        > this is referred to as the "Q" or quality factor. High Q resonators
        > have very sharp response curves. At the exact frequency of
        resonance,
        > the response is the greatest. As you move away from that
        frequency, the
        > response is less. How quickly that response drops off determines
        the Q
        > factor.
        >
        > It would be interesting if AL would be willing to take a speaker,
        place
        > it in the same place as his Skybox (maybe even inside the old box)
        and
        > drive the room with different power levels and different
        frequencies and
        > report on the response.
        >
        > Anything that has a sharp resonance curve can also be used to create
        > oscillations as well. If the excitation is reasonably steady, a
        steady
        > note will emerge. Blowing across the opening of a bottle is the
        > classic example. Sudden excitation (Impulses) can also cause an
        > oscillation. How quickly a device stops oscillating after the
        > excitation is removed (decay time) is also an indicator of Q as
        well.
        > High Q resonantors will "ring" for a long time.
        >
        > It may be that we live in environments that exhibit high Q at low
        > frequencies, and are thus suspectible to noise (impluse) induced
        > oscillations. Our homes might be very good and taking low level
        > environment noise and resonating at Hum frequencies. If a disk
        drive in
        > a Skybox can do it, think what traffic, aircraft, and other low
        > frequency noise sources might be able to do.
        >
        > Kallio
        >
      • dark_al1
        Just performed a quick tone generator experiment. Placed a PC speaker on the floor in front of the skybox and played a 90hz tone from the laptop.I stood in
        Message 3 of 3 , Dec 6, 2006
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          Just performed a quick tone generator experiment.

          Placed a PC speaker on the floor in front of the skybox and played a
          90hz tone from the laptop.I stood in front of the fireplace and I had
          my wife adjust the speaker volume, starting with it turned off and
          slowly turning it up until I could just hear the tone.
          When it became audible, it was percieved as non-directional as with
          the original fireplace hum caused by the skybox.
          Also as with the original hum, when I crouched to change my ear
          height the tone could not be heard and was loudest at the 5 to 6 foot
          level.
          It also has the same volume pattern around the fireplace and alcoves.
          stepping away from the fireplace directly toward the speaker, the hum
          could not be heard and was only just be heard with my ear about 2 1/2
          feet from the speaker. At this point the sound was clearly
          directional from the speaker.

          I tried an 80hz tone, which had the same dispersion and volume
          characteristics around the fireplace, but the speaker volume needed
          to be slightly louder.

          There is currently too much traffic going past and noise from my wife
          and kids. So I will do some more controlled experiments later when it
          is quiet. I will also try to get some Spectrum labs readings to see
          if there is a amplification peak at a specific frequency.


          AL
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