High efficiency Loudspeaker set for disco application
Applying exponential horn principles; the back loaded horn.
Download a printeble version: Hornspeaker.pdf...
Introduction
The current amplifier used in my son's drive-in disco is only rated 80 W RMS @ 8 Ohms driving two Karlson type enclosures with 12"loudspeakers which I built some 25 years ago.
Although these speakers also perform quite well, they are limited when using for audiences of over 150 people. The midrange exponential enclosures recently built increased the sound level only partially.
The need for extended sound pressure level would normally be solved by buying a more powerful amplifier and accordingly higher rated loudspeakers.
In this case the challenge was however to achieve this goal by using the same amplifier and applying high efficiency speaker enclosures.
This high efficiency can be obtained from speakers driving exponential horn
enclosures. Nowadays these enclosures are rarely used apart from large PA speakers that are used for open air concerts and can only be transported by truck. This looks not very suitable for a modest drive-in disco.
The disadvantage of the large volume of the horn enclosures can however be
limited with some tricks. The final product was still within reasonable proportions and can be transported by car.
The principle of exponential horns is very old. Trumpets and similar instrument apply this shape of enclosure for centuries to enhance the sound pressure level. In the early 20th century, the first turn-tables used a horn to amplify the vibrations of the needle.
This principle can also be used to amplify the sound level of loudspeakers. While HiFi speakers have a acoustical efficiency of 1 - 2%, applying a horn will increase this to a 30-50% efficiency.
The basic principle is to put the sound source (speaker) in a closed box, with only a small outlet. This outlet is connected to the rear end of the horn. The horn area is expanding according to an exponential curve.
Normally the frequency of these horns is limited to several hundreds Hertz.
In order to achieve a wider range, a back loaded horn is applied. Here the speaker is mounted in the front side of the enclosure and thus be allowed to transmit higher frequencies directly from the membrane. Still the small opening in the back can drive the horn for amplification of the lower frequencies (these bass frequencies would normally require the most electrical energy).
The size of the horn area is determined by the lower frequency.
The lower the required frequency is set, the larger the horn mouth.
The second challenge is to limit the size of the horn to reasonable proportions. Folding the horn into a rectangular enclosure requires some
experimental drawing, but results in a transportable enclosure with reasonable dimensions.
Calculated results
For determining the various parameters and dimensions, the following relationships can be found in literature:
Practice
Because
of the initial desired volume and power to be handled, 30 cm. diam. Disco loudspeakers were chosen.
The effective membrane diameter is 29 cm, resulting in:
The (inner) width of the enclosure is set at 40 cm for
practical (read: transport) reasons. Now we can calculate the hight of the horn at a distance x from the throat, while keeping a constant width of 40 cm.
x
