Free 50 Best Homemade Tools eBook:  
Remove advertisements
Results 1 to 7 of 7

Thread: Enormous megaphone from silent film era - photo

  1. #1
    Jon
    Jon is offline Jon has agreed the Seller's Terms of Service
    Administrator Jon's Avatar
    Join Date
    Jan 2012
    Location
    Colorado, USA
    Posts
    12,460
    Thanks
    2,296
    Thanked 3,057 Times in 1,332 Posts

    Enormous megaphone from silent film era - photo

    Directing extras in the 1922 silent film Robin Hood. That's director Allen Dwan on the far right in the hat, and I think that's the lead Douglas Fairbanks standing at the middle of the megaphone. Microphone technology had been invented by 1922, but I believe it was not yet as effective as a simple giant cone. This makes me curious about the limits of using a megaphone to amplify sound.


  2. The Following 5 Users Say Thank You to Jon For This Useful Post:

    MeJasonT (05-26-2018), Moby Duck (05-26-2018), mwmkravchenko (05-26-2018), PJs (05-28-2018), Seedtick (05-26-2018)

  3. #2
    Frank S's Avatar
    Join Date
    Aug 2015
    Location
    Peacock TX
    Posts
    2,083
    Thanks
    363
    Thanked 2,019 Times in 974 Posts

    Frank S's Tools
    Is that where the term he has a loud mouth came from
    Never try to tell me it can't be done
    When I have to paint I use http://kbs.justoldtrucks.com/

  4. The Following User Says Thank You to Frank S For This Useful Post:

    MeJasonT (05-26-2018)

  5. #3
    mwmkravchenko's Avatar
    Join Date
    Nov 2017
    Posts
    13
    Thanks
    138
    Thanked 14 Times in 8 Posts
    I design loudspeakers amd horns in particular for a living. A good horn with proper coupling can multiply the sound level by 12 times.

    Not 100% on the quality of the coupling in the throat of the conical horn here. But the throat is large enough to push your face against so should be decent coupling.

  6. The Following 2 Users Say Thank You to mwmkravchenko For This Useful Post:

    12bolts (05-26-2018), PJs (05-28-2018)

  7. #4

    Join Date
    Sep 2012
    Location
    Hinchinbrook QLD
    Posts
    51
    Thanks
    8
    Thanked 23 Times in 11 Posts
    Quote Originally Posted by mwmkravchenko View Post
    .....A good horn.....
    Is there a specified ratio of length to diameter? Would a longer but narrower horn project sound further?

  8. #5
    mwmkravchenko's Avatar
    Join Date
    Nov 2017
    Posts
    13
    Thanks
    138
    Thanked 14 Times in 8 Posts
    Quote Originally Posted by 12bolts View Post
    Is there a specified ratio of length to diameter? Would a longer but narrower horn project sound further?
    There are balances. A horns length dictates how low it can support the sound. The size of the mouth ( large end) really dictates how ripple free the sound will be. Ripple as in not exactly like the sound input but rather a bit distorted.

    If you are interested there is a great program for designing horns that is free:

    Hornresp

    And a huge thread on how it works : Hornresp - diyAudio

    There is a pretty comprehensive manual built into the program.

    Sound projection is more a situation of the correct length to mouth diameter for the frequency range that you are interested in. Human voice is a fairly narrow bandwidth and not that hard to do right.

  9. The Following 4 Users Say Thank You to mwmkravchenko For This Useful Post:

    12bolts (05-27-2018), Jon (05-26-2018), Moby Duck (05-26-2018), PJs (05-28-2018)

  10. #6

    Join Date
    Mar 2015
    Posts
    11
    Thanks
    0
    Thanked 5 Times in 3 Posts
    A horn is actually an acoustical transformer. It increases the efficiency of coupling of the impedance of the small mouth (relative to the wavelength of vocal frequencies) to the surrounding atmosphere. It does not "amplify" the vocal power output which is in the microwatt range before and after passing through the horn. By enhancing the coupling efficiency it facilitates projection of the main part of the vocal frequency range. The main frequency range for vocal communication is from about 500 Hz to 3kHz. The wavelength of sound in air is roughly 1 foot at 1kHz.

    The other phenomenon exploited by the horn is increasing the directionality of the projected voice. As the outlet aperture of the horn increases, the directionality of the projected voice narrows into a sharper beam.

    Decades ago I fabricated a parabolic microphone reflector about 1 meter in diameter to identify sound sources in refineries from overlooking hills for our acoustical consultation practice. Testing the directionality by rotating the microphone and reflector in front of a sound source in an open field, we found that the directionality was very sharp in the multi-kHz range but essentially zero below 1 kHz. Directionality is directly related to "gain" relative to the bare microphone. (Our reflector was used for decades by Yosemite rescue personnel to communicate with climbers stranded on rock faces.)

    The conical horn shown in the poster's photo is very inefficient, acoustically. Far more efficient is an exponential flare with the rate of flare directly influencing the lowest frequency that can be propagated efficiently through the horn (and the diameter of the mouth influencing the efficiency of coupling of the low frequencies to the air). That is the reason for the familiar graceful curved appearance of horns on everything from truck and train horns to civil defense alarm horns. Most of the science of horn acoustics was developed subsequent to the use of the simple conical horn shown in the silent movie set.

    awright

  11. The Following 3 Users Say Thank You to awright For This Useful Post:

    12bolts (05-27-2018), Moby Duck (05-26-2018), PJs (05-28-2018)

  12. #7
    MeJasonT's Avatar
    Join Date
    Jun 2016
    Location
    Lake District in UK
    Posts
    54
    Thanks
    44
    Thanked 45 Times in 22 Posts

    MeJasonT's Tools
    Did anyone else get the pun - silent films?

    As i have a big mouth do i only need a small cone.


    Post your reply!
    Join 33,912 of us and get our 50 Must Read Homemade Tools eBook free.



    50 Must Read Homemade Tools
    Last edited by MeJasonT; 05-26-2018 at 11:33 PM.
    Smoke makes electronics work, if it escapes the equipment breaks.
    Got to keep the smoke in.

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •