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 efficiency between the speaker driver (typically made of paper or, more recently, more exotic materials such as titanium or mylar) and the air (which has a very low density). Since air is very light compared to speaker cones there is an impedance mismatch which leads to inefficient coupling of the speaker driver to the air, much as using a tractor to pull a bicycle trailer would not efficiently make use of the tractor's power. Horns couple the speaker driver to the air so that the driver appears to the air as though it were very large in surface area and very light (more like air), while the air appears to the speaker driver as though it were smaller in area and heavier. 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 and vice versa. It does this through the gradual exponential increase of the cross sectional area of the horn.

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.

Read more at Wikipedia.org