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Growing a Horn

Louder and more penetrating

By Julian Edgar

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This story isn’t about making your car go faster. But it is about improving one area of performance, and potentially increasing your safety – and that of other road users.

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One aspect of a recent car purchase really disappointed me. Like many European cars, the Skoda Roomster has an absolutely dismal horn. Living in a country of wide open spaces, with often those spaces inhabited by kangaroos, emus, wombats and birds, you get used to clearing a path with an effective horn.

So when I first sounded the horn in real need, and was greeted with just a dismal ‘peeep’, I knew an upgrade was a requirement.


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There are some excellent aftermarket horns now available. Electric horns, where a diaphragm is vibrated, are used universally as standard equipment. And you can also buy more powerful aftermarket horns that use this design.

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An alternative approach is to use air horns, where a compressor is matched to one or more trumpets. These are loud – in fact very loud. They are also penetrating (not the same thing as loud) and new design units that integrate the compressor with the horn assembly are more compact than the designs of yesteryear.

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In my case, I figured that a pair of electric horns from a wrecked car would be fine, preferably an Australian-built car – Australian car horns are almost always excellent. After watching eBay for a few days I decided to settle instead on a pair of second-hand horns from a Nissan Patrol. For the same reasons already mentioned (clearing roads of wildlife) horns from large 4WDs are also usually good. And at 99 cents plus $5 postage, you couldn’t really complain about the price!


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The first step in upgrading the Roomster’s horn was to find the factory one. It proved to be behind a pull-out grille in the bumper. Removing the grille revealed a single horn with a 400Hz output frequency.

Unplugging the horn and operating the horn switch showed, by the presence of a clicking sound, that the horn was operated by a relay. It’s important that you check whether a relay is used – if it is (as will be the case in nearly all modern cars) then you can upgrade the horns without having to fit a new relay. If there is no relay fitted, you’d better add one. (See Using Relays.)

The two Patrol horns were each marked as drawing 3.5A – so 7 amps total. That’s actually pretty low for horns so I was confident the factory relay would be fine driving that load.

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The Patrol horn design uses a chassis connection (ground) for one of its two connections. On the other hand, the Roomster horn is plastic-bodied and has two connections – a ground and a 12V supply. So on the Roomster horn, which wire was which?

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By connecting a multimeter and probing the plug it could be found which wire was 12V and which was ground. A check was then made between the ground wire and the negative terminal of the battery to ensure they had continuity (that is, they were connected together).

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The plug was then cut off and a spade terminal attached to the ground wire, which would be in turn attached to the bolt that would hold the new horns in place. The other wire was attached to a spade terminal that also ran to a second spade terminal, allowing both new horns to be powered.

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The new horns were bolted into place, with care being taken to ensure they could still vibrate freely on their laminated mounting strips. (Never sold-mount a horn – it will cause a large loss in a sound output.)


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Testing with a sound pressure level meter showed that that at a distance of about 150mm from the bumper, the standard horn had a maximum output of 112 dB(A). The new horns? Well they output 120 db(A) – with the logarithmic dB scale, that’s massively louder. Furthermore, the two discordant frequencies give much improved penetration.

For AUD$6 and an hour of my time, the upgrade was well worth it!

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