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London Science Museum - Exponential Horn

Between May and July 2014 the London Science Museum had a rather special exhibition for those of us into audio (and horn loudspeakers). I took the opportunity of a visit on the final day to take some pictures, which I thought would be of interest to others.

But first, a bit of background. In the 1930s the Science Museums curator of Electrical Communication designed a massive horn speaker, which was rightly intended to be used as a benchmark, offering some of the highest sound quality available at the time. This proved to be a very popular exhibit for almost a decade, and was in use until the outbreak of World War 2.

Sadly in 1949 a demolition accident destroyed all but the first 9 feet of the horn, but what is shown here is a faithful recreation that has been built over a period of 8 months, using that original first section. Instead of being made from lead and tin like the original,  the new sections were made from fibreglass, but their dimensions remained the same.

The shear scale of this true exponential horn is quite breathtaking, pictures really don’t do it justice, it’s 27 feet (8.23m) long, and has a mouth that is over 7 feet square (2.16m sq.), and starts at the throat from a diameter of just 1 1/16 inches (27mm).


Above: flash photographs of the insides of the horn, click to enlarge

The driver used is the legendary, period correct, Western Electric 555. This compression drive unit is considered amongst the finest drive units ever made (and to this day one of the most valuable). It is renowned for having an incredibly flat response over a wide frequency range. It just predates strong permanent magnet drive units, so uses coils of wire (called a field coil) to generate the magnetic field, powered from a separate low voltage supply.  

It is claimed that this horn and driver combination will reproduce frequencies from 32 to 6,000 cycles per second (using Hertz for something designed in the thirties feels wrong), which is a remarkable range from a compression driver (although it wasn’t clear what -dB points this was measured at). Naturally it is also enormously efficient, which means the limited power handling of such a driver is somewhat irrelevant (except at very low frequencies, where presumably some filtering was used to protect the driver).

When I visited they were playing period recordings making it difficult to judge absolute fidelity by modern standards, but that idea at the time seemed laughable. Here was a sound so effortless, spacious, real, enveloping and simply special that it was in a different league from almost anything else you could imagine (even if you’re familiar with more conventionally sized horn loudspeakers).

What really makes it so special is the single driver covering such a wide frequency range, with no crossover or multiple sound sources needed to reproduce the majority of the audio spectrum. The treble was less rolled off than I’d have expected, and the figures suggested. As with all horns the dispersion at higher frequencies was a little narrow, but due to the shear size of the horn the sweet spot was still several feet wide at the listening position. I’ve heard large multicell horns (where there are internal divisions inside the horn to guide higher frequencies over a wider area) do a worse job.

I think the Science Museum should be congratulated for going to the lengths they did to recreate this very special speaker, and I hope it will get regular outings in the future - to hide it away in a store room would be a crime.

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