Drivers & Horns

A horn speaker is a speaker that uses a "horn" to get more sound (volume) from the driving loudspeaker. more...

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The horn itself does not amplify anything, but rather improves the coupling between the speaker driver (typically made of paper or, more recently, more exotic materials such as titanium) and the air (which has a very low density). Simply put, air is very light and speaker cones are relatively very heavy, so horns "trick" the speaker cone into believing it is very large in surface area, and very light (more like air). The small part of the horn next to the speaker cone "driver" is called the "throat" and the large part farthest away from the speaker cone is called the "mouth".

How it works

In this sense some people have described a horn as an "acoustic transformer". Stated another way, it converts large pressure variations in a small amount of air into a low pressure variation (the human ear is very sensitive indeed to pressure variations — even quite loud sounds are actually very small pressure variations!) in a large amount of air.

The most well known early horn speakers are those on mechanical phonographs, where the record moves a heavy metal needle that excites vibrations in a small metal diaphragm that acts as the driver for a horn. The horn improves the loading and thus gets a better "coupling" of energy from the diaphragm into the air, and the pressure variations then get smaller as the volume expands and the sound travels up the horn.

A modern electric horn speaker works the same way, replacing the mechanically excited diaphragm with a dynamic or piezo speaker.

Advancements on the basic principle

As usual, once a principle of operation has been defined, the technology can be adapted and improved almost without limit.

The horn should not just be a cone shape of fixed length, since this would resonate at the natural frequency of its length. Modern horn designs typically feature some form of exponential flare, such as the tractrix taper. Roughly speaking, the slower the flare rate, the deeper and lower frequencies the horn will reproduce. For example, a horn area growth rate of 30% per foot will allow reproduction down to about 30 Hz; 1000% per foot (10 times area) per foot provides midrange reproduction; 100 times area per foot is used in high frequency horns.

Modern high output horns also make the throat area of the horn smaller than the cone diaphram area. This is called the "loading" or "compression" ratio of the horn. The compression ratio is the cone area divided by the throat area. Typically for bass and midrange frequency the compression ratio is from (1.5 to 1) low compression to normal compression (2 to 1) to high compression (3.5 to 1). High frequency tweeters sometimes have compression ratio's as high as 10 to 1! The higher the compression the greater the horns ability to amplify, until the compression ratio is so high that it actually begins to limit and impede the ability of the cone to move. At this point the sound output power from the horn will begin to lower. As a proof, place a cone woofer face down on a concrete floor, the compression ratio is very high, however sound output is very low.

Read more at Wikipedia.org