Enormous megaphone from silent film era - photo

[Jon]

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.

[Frank S]

Is that where the term he has a loud mouth came from

[mwmkravchenko]

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.

[12bolts]

.....A good horn.....

Is there a specified ratio of length to diameter? Would a longer but narrower horn project sound further?

[mwmkravchenko]

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.

[awright]

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

[MeJasonT]

Did anyone else get the pun - silent films?

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

[Jon]

Ingmar Bergman directs The Devil's Eye in 1960. The twirling circular tool on the left is used to cast shadows of foliage across the driver's face, to give the illusion of motion.

[Jon]

Buster Keaton stunt.

This is (one of) the great things about older films: you get to experience genuine curiosity/wonder/awe when considering how certain scenes were accomplished. For modern movies, the answer is usually the same: "computers".

[12bolts]

Perfectly choreographed. The positioning of the 2 pursuers and their subtle movements make the cut and splice indistinguishable. The tiny shift in the womans shoulders and dress hem are only noticeable after several views in stop start motion. For a single viewing at normal speed and not knowing what to expect the audience would be pretty impressed with that stunt