I'm sure this is probably a newbie question but I never really knew what they affect or do so I just wanted to ask the UG community.
I think its an extra speaker that sounds higher frequencies, it lets you turn it up with a bit of gain and still have some definition and have your sound cut through without farting... I'm not a bass amp expert though
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it lets old people slowying traffic up know that they should pull over or be destroyed

funny guy.

i'm not sure but i think it's for higher frequencies.
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I give up.

yeah, your bass amp has a horn (tweeter) so that it can get the higher frequencies out without it sounding bland and dull. and to protect the subwoofer in some cases.
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yeah, your bass amp has a horn (tweeter) so that it can get the higher frequencies out without it sounding bland and dull. and to protect the subwoofer in some cases.

Isn't a horn different than a tweeter?
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Without meaning to steal the thread, my new warwick amp has a "HF horn (I assume this means high frequency horn)". In case you're wondering which one here's the link:

I suppose the reason this particular amp has a horn is to boost the trebles and/or mids, as it's speaker is a 15 inch.
Anyway, if a horn is for middle and a tweeter is for highs, then how can you have a "high frequency horn"? In fact, my perception of what a "horn" is has just been completely muddled.
Some horns cover mid range, some cover high depending on the size of the driver, etc. One horn usually more or less covers both in a two way hi-fi or PA cab, whereas a lot of bass cabs I think it mostly handles highs.

I'm pretty sure that while a tweeter is just a naked speaker, a horn is a speaker with, well, a horn attached to further amplify and shape the sound. so think of a tweeter being housed in a box, and then that box opening into the horn housing and that's a horn.

Let's see... wikied.
Looks like that pretty much sums it up.
I posted a description in the FAQ in response to a similar question. Simply, a tweeter is a plain cone speaker; a compression horn is a small speaker driving into the narrow end of a 'trumpet', much like shouting into a megaphone. Very power efficient, but the horn can only be tuned for a narrow band of frequencies.
Check out Cerwin-Vega's site for a lowdown on their 18" folded horns. Better still, find one in a club and steal it - save it from a life of sh*t dance music, stick it under an SM900, and destroy your house...