How to Spot a Psychopath

A reader writes:

Why do horns make things louder?

I mean, I accept that they do, on gramophones and megaphones and PA speakers at the train station and brass instruments and so on, but what's actually going on there? Why does the sound of your voice get louder just because you're holding a conical thing in front of your mouth? Is it just making it more... directional?

Dennis

The great problem of audio production, and audio reproduction too, is coupling the sound-producing thing to the sound-transmitting medium, which is usually air.

Air is very light. Most things that make sound are, in comparison, very heavy. The moving parts of loudspeaker drivers, the strings of a violin or piano, the lips of a trumpet-player who's blowing a sort of highly controlled raspberry into the mouthpiece of the instrument; all very very heavy, compared with air. All not good at moving lots of air, which is what you want your sound-making thing to do. Wave a brick around in the air and you'll invest a lot more energy in accelerating and decelerating the brick than you manage to impart to the air.

One way of solving this problem is to make your speaker driver very light too. Electrostatic speakers use a big flat sheet of super-thin plastic as a driver; the sound-producing element in a plasma speaker is made out of ionised air (or other gases, if you're a big wuss who doesn't want ozone poisoning).

Horns are a simpler way of solving, or at least reducing, the coupling problem. When you put a heavy-compared-with-air vibrating object at the small end of a horn, the only air it can move is the air right in front of it at the small end. Moving this air is still pretty easy, but the restricted air's mechanical "impedance" is nonetheless quite a bit higher than it'd be if it were unconfined.

As sound pressure waves move down the horn, the gradually widening shape of the horn (for loudest results, an exponential curve) allows the small amount of higher-pressure air next to the driver to transfer its energy to a large amount of lower-pressure air. The end result is that more of the energy of the driver ends up as sound waves.

A sealed-box loudspeaker has an acoustic efficiency - the amount of the input electrical energy that comes out as sound energy - of about one per cent, at best. Horn speakers can manage thirty per cent without much trouble, and quite a bit more if you design them for loudness rather than fidelity. Take the horn off a phonograph and you'll have to put your ear right next to the diaphragm to hear much of anything, but with a big horn on it, a wind-up phonograph making sound by scraping a needle over a disc of shellac can legitimately be described as quite loud.

(Some phonographs let you remove the horn, or never had a horn in the first place, and allowed you to listen through one or more rubber tubes that went to a headset of some sort - essentially, primordial headphones. This allowed you to listen to your records in privacy, albeit with weird stethoscope-y sound colouration on top of the lousy fidelity of the phonograph system in the first place.)

Outside of Physics Experiment Land, acoustic horn design and implementation has many engineering tricks. For instance, modern horn loudspeakers usually have a horn throat that starts out much smaller than the diaphragm of the actual driver, which may be in its own actual rectangular speaker box stuck on the small end of the horn. There are also horn loudspeakers, like the legendary Klipschorn, that use various workarounds to fold something that acts somewhat like a horn into a speaker that can be mainly built out of flat wooden panels.

Also, the lowest bass frequency a horn can reproduce is determined by the size of the mouth of the horn; that's why public-address and hand-held megaphone speakers always sound tinny. Speakers like the Klipschorn have their horn mouth on the back of the enclosure, and are meant to be shoved into the corner of a room, so the walls behind them can provide a bit more effective horn size. Horn loudspeakers are also deliberately designed to be further away from an ideal horn shape than is strictly necessary, to balance the efficiency of the horn with the hard bass cut-off that a "pure" horn, with a mouth small enough to fit in a room, has at low frequencies.

The old phonograph horns have been reborn, too, as "amplifiers" for MP3 players and cellphones. The phone, MP3 player or ear-bud headphones plug into the small end of a horn, and suddenly the tsss-tsss-tsss of someone else listening to their iPod on the bus turns into actual music.

Some of these devices are very fancy and very expensive, but if you search eBay for "amplifiers" for MP3 players you'll find lots of cheerful-coloured horn doodads among the actual electrical amplifiers. The going rate for a combination iPhone stand and horn "amplifier" now seems to be about two bucks delivered.

Psycho Science is a regular feature here. Ask me your science questions, and I'll answer them. Probably.

And then commenters will, I hope, correct at least the most obvious flaws in my answer.