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Discussion in 'Electronic Design' started by Korben Dallas, Apr 7, 2005.

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  1. Hello,

    I am requesting help to construct an ultrasonic device that emits sound at
    about 25Khz. Unlike most ultrasonic devices, I want the device to carry
    sound over 100 feet (or 30 meters) and still be over 140 decibels at 100
    feet (or 30 meters).

    Does anyone know how I can do this? Plans? Actual devices? Please reply.

    All I know of this is that the sirens on fire engines can do this for miles.
  2. It would help if you were to describe the medium,
    intended signal reaching the target(s), duty cycle,
    and the general purpose of your device.
    Your main challenge is going to be transducer design.
    This is probably not right forum for help with that.
    They do not.
  3. Mark Jones

    Mark Jones Guest

  4. Guy Macon

    Guy Macon Guest

    I will assume you mean dBA SPL, not dB. The Decibel is a ratio,
    not an absolute quantity. What you wrote above translates to
    "I want the device to ... be 30,000 times louder than at 100
    feet." 30,000 times louder than what?
    In that case, you have a big problem. The one thing that you think
    you know is completely false. A typical 100 watt fire engine siren
    puts out 130 dBA at 1 meter on axis. To get that up to 140 dBA at
    1 meter would take a 10,000 watt speaker (+10 dBa is x10 power and
    x2 perceived loudness).

    Sound pressure levels in a free field from a point source decrease
    6 dB for each distance doubling. So to increase that 10 kW 140
    dBA enough to reach that level at 30 meters instead of 1 meter,
    you would need these levels:

    1 meter 170 dBA
    2 meters 164 dBA
    4 meters 158 dBA
    8 meters 152 dBA
    16 meters 146 dBA
    32 meters 140 dBA

    ....and your 140 dBA 10,000 Watt speaker would have to be
    replaced with a 170dBa 10,000,000,000 Watt speaker.

    So, do you happen to have a 10 billion watt power amplifier
    laying about that you can use to drive this speaker?

    Now let's talk abhout the fence you will need to build.
    Federal maximum exposure standards are:

    90 dBA Sound Pressure Level for eight hours, or
    95 dBA Sound Pressure Level for four hours, or
    100 dBA Sound Pressure Level for two hours, or
    105 dBA Sound Pressure Level for one hour, or
    110 dBA Sound Pressure Level for 30 minutes, or
    115 dBA Sound Pressure Level for 15 minutes, or
    120 dBA Sound Pressure Level for 7 minutes

    You need to move the fence twice as far to get each
    6 dB of noise reduction, so the fence will be:

    32 meters away from the speaker for 140 dBa exposure.
    64 meters away from the speaker for 132 dBa exposure.
    128 meters away from the speaker for 128 dBa exposure.
    256 meters away from the speaker for 122 dBa exposure.
    512 meters away from the speaker for 116 dBa exposure.
    1 kilometer away from the speaker for 104 dBa exposure.
    2 kilometers from the speaker for 98 dBa exposure.
    4 kilometers from the speaker for 92 dBa exposure.
    8 kilometers from the speaker for 86 dBa exposure.
    That's a circle 16 km across.

    I have a suggestion. Why don't you tell us what you are
    trying to accomplish rather than asking us to help you to
    implement the "solution" that you came up with on your own?
  5. Guy Macon

    Guy Macon Guest

    This shows the power of a good design. What I calculated to
    require 10 billion watts using speakers and amplifiers only
    took 100,000 watts (180hp) using compressed air and a rotor.

    does anyone know why they are specing 10dB loss per distance
    doubling rather than 6dB?
  6. I read in that Guy Macon
    As you so presciently stated:

    (Note: every Usenet post that contains calculations is required
    by law to have at least one typo or bonehead math error...)
  7. mike

    mike Guest

    I suspect your main challenge is gonna be using the telephone after
    your first test experiment. The good news is that your life
    will be very quiet from then on...

    Return address is VALID but some sites block emails
    with links. Delete this sig when replying.
    Wanted, PCMCIA SCSI Card for HP m820 CDRW.
    FS 500MHz Tek DSOscilloscope TDS540 Make Offer
    Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
    Wanted 13" LCD for Mitac 6133 Samsung HT13X13-201
    Bunch of stuff For Sale and Wanted at the link below.
  8. (Followups set to sci.electronics.basics where this
    was inappropriately multi-posted by the OP.)

    Yep, close. Overlooking that the OP means dbA,
    all he needs to do is get that 3 ton device shipped
    to his place, drive it about 30 to 80 times faster
    with something better than its 180 HP engine, and
    stand back if he has any sense, (not to be assumed).
  9. Joel Kolstad

    Joel Kolstad Guest

    Absorbtion, reflection, etc. by the buildings, foiliage, etc. along the way?

    Back in antenna school it was discussed how, while power in free space
    should drop off as 1/r^2, for practical purposes a value between 1/r^2 and
    1/r^3 was used, with 1/r^2.5 not being uncommon. That gets
    you -8dB/doubling... I imagine with sound waves similar approaches are
  10. As Mr. Pefhany has helped me realize in sci.electronics.basics,
    the Decibel is also used for sound pressure levels, with the 0 dB
    reference point being 20 uPa. The dBA (and dBC) reference
    levels follow a function determined by perception among humans,
    and are not defined above the audio range. So the OP may well
    have meant "dB". (I agree that bare "dB" should be cast out of
    our lexicon for absolute measurents, but it's not up to us.)

    [Uncontested material cut.]
  11. I read in that Guy Macon
    When they measured it, it was facing into the wind. Maybe.
  12. qrk

    qrk Guest

    Don't forget to add in absorption. That will amount to about 15dB/30m,
    depending on humidity, temp, and other factors.

  13. Andrey

    Andrey Guest

    Looks like the guy in original posting is looking (dreaming) to build sonic
    Like American Technology Corp

    The premise is that at above 140 db air becomes unlinear media for acoustic.
    Hence, if Ultrasound is modulated by audial frequency it becomes demodulated
    on non-linearity.

    I yet to hear about success of this weapon. I heard trial units were sent in
    Iraq, ... and did not hear et how it helped American soldiers.

  14. hamilton

    hamilton Guest


  15. Ross Herbert

    Ross Herbert Guest

    From deep down memory I seem to recall it has to do with the fact that
    a perceived (audible) doubling (or halving) of sound level corresponds
    to a 10dB change. In real power terms the same change is only 6dB.
  16. Guy Macon

    Guy Macon Guest

    Thanks! I was not aware of that.
  17. Guy Macon

    Guy Macon Guest

    Hey, what's a x10 typo between friends? A billion watts,
    ten billion watts, 100 billion watts, who cares? It's just
    an added zero, and what is a zero, really? Nothing!

  18. Graham Orme

    Graham Orme Guest

    Irrespective of how many zeros there are, if you could generate that SPL
    and focus it using a parabolic reflector to produce a plane wave, then
    apart from acoustic damping in the air it should be the same at any

    I know reality never works quite the same as the theoretical model but
    140 dB relative to 20 uPa at 30 m might be achievable.

    We're going to have to get something done about reality. It messes up so
    many brilliant designs.
  19. Rich Grise

    Rich Grise Guest

    Probably ground clutter.

  20. Rich Grise

    Rich Grise Guest

    A billion here, a billion there, and pretty soon you're talking real
    I wonder what they guy's trying to do? Shoot down flying saucers?

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