Well… yeah. The question pretty much speaks for itself. I’m under the impression that it’s louder than normal, but I don’t really know the extent of it. Can anybody tell me?
It’s loud. But a quick Google search turns up even louder ones at 130 dB. Typically they run around 90 dB or so.
Pretty damn loud – this link has some example levels
http://www.glenbrook.k12.il.us/gbssci/phys/Class/sound/u11l2b.html
At first I though that you wouldn’t get a horn that loud, but some consideration and some checking turn out that you can and thinking of the other things people do to their cars I’m surprised I doubted it.
SD
Car nut and car-alarm installer here. I highly doubt they’re rating those horns by any strict standard. How far away is the sound meter----six inches? one foot? one millimeter? And where is the horn mounted in the car’s engine compartment----are other components in a place where they can impede the sound?
I was in Pep Boys yesterday looking at replacement horns for the dead ones in my car. They were selling original-equipment horns rated at 125 dB, and for $1 more, an extra-loud model at 132 dB. But I smell tomfoolery-----when I looked through the clear packaging, both horns sported the
exact same part number! So I put in some used Cadillac horns instead; they have a nice sound.
Also, for what it’s worth, many car alarm sirens have stickers on them claiming 120 dB.
You could ask your friend to beep it for you. 
Since the dB scale is logarithmic, given Q.E.D.'s figure of 90dB for a normal horn, it would be 1000 times louder than your average horn.
But personally, I’m with antechinus. Get your friend to beep it for you.
Hehe…
Is that 120 dB of “meep!” or 120 dB of “WHONNNK!”
I think Chris has the right idea - those mid-70s Caddy horns (four tones, rather than the usual one or two) had a good strong tone that cut through just about anything and said “MOVE IT! Something BIG is coming at you!”
My pocket ref book says that a car horn at five meters is 100 dB; 120 dB is described as “Thunder, diesel engine room (Incidentally , my lovely wife arranged for me to tour the engine room of one of the Alaska state ferries, big ocean-going boats; it was god-damned LOUD), nearby riveter.”
Also, car horns sound much more authoritative when you have two horns in unison, tuned maybe a third apart.
My pocket ref book says that a car horn at five meters is 100 dB; 120 dB is described as “Thunder, diesel engine room (Incidentally , my lovely wife arranged for me to tour the engine room of one of the Alaska state ferries, big ocean-going boats; it was god-damned LOUD), nearby riveter.”
Also, car horns sound much more authoritative when you have two horns in unison, tuned maybe a third apart.
Yeah, but rule of thumb is that when you halve the distance, the SPL increases by 6 dB, pretty close. So the 100 dB car horn at 5 meters is a ~113 dB horn at 1 meter. Obviously, a point source like a horn sounds louder the closer you are to it, so tucked into any noise measurement is some distance measurement*. This place says a typical car horn is 115 dB at 3 ft (darn close to Rocketeer’s cite); other places seen to peg it anywhere between 110 dB and 120 dB (without the distance).
One meter is kind of a standard measuring distance, so often a “120 dB” figure given with no distance implies one meter. However, given that marketing forces are at work here, I would be hesitant to assume that’s the way the horn’s being measured. But hey, 120 dB ain’t that much louder than 115 dB.
Additionally, there’s a couple perception issues involved. First, the human ear is more sensitive to certain frequencies than to others, so a 120 dB noise at one frequency can seem much louder than a 120 dB noise at another. Often, noise will be cited using “A-weighted” dB, which increases or reduces the “actual” dB based on the perceived loudness of the noise frequency. Second, there’s something of a noise quality issue, where certain noises are more jarring (and likely to be noticed) than others.
*Anecdote: I occasionally use a dB meter at work. This meter is calibrated with a 140 dB noise source that fits directly over the tip of the microphone. Thus, the source is 140 dB…at 1/2 inch (or whatever) distance. If you hold the source in your hand and turn it on, it makes about the same noise as someone whispering a couple feet away.