Are there sirens/horns that are like a car horn? I am going to start commuting on my bike, but I often use busy roads that are not cycle friendly and cyclists are not often noticed. I need a horn to be noticed. (and to vent my anger without losing my voice!! - I'm joking here. One of the only highway code rules I remember is "DO NOT sound your horn aggressively". If only car drivers knew that one!)

Is there any horn out there that is battery powered but does not sound like an alarm system, or has more than one sound? I just want to press it and have a sound like a car horn.

Are there any cheap ones out there that do this (I do not mind buying from China).
Here are a few loud horns:

The hornit:
A shop I used to work at sold these and they are extremely loud - around 140 decibels, but they sound like a loud beep rather than a horn. It takes 2 AAA batteries. Costs $45 US. The nice thing about this one is the button to press is remote, so you can have the horn on your fork or wherever. 

Airzound: http://deltacycle.com/airzound-horn
This one is slightly less loud (115 db), but it has more of a honk to it and requires no batteries - it takes air pressure through your standard bike pump, so there's virtually no cost beyond the initial investment. this one is slightly bulkier as you have the horn itself, the air cannister, which looks like a water bottle, and the tube that connects them. A friend of mine has one and he says it's pretty effective.  

There's also this kickstarter: https://www.kickstarter.com/projects/lansey/loud-bicycle-car-horns-for-cyclists
I've never seen this one in person, but the kickstarter can explain it for you. It has 112 db sound and sound the most like a car horn. it's $95 US though. 

GaryE also showed this one:


  I use a bike horm from another Kickstarter project. It has a friendly sound and a very loud 
  beep: ORP Smart Horn –  Gary E 


The loudness is 98 db, but this one also has a light included