# What Happens When You Put a Big Driver on a Small Horn?



## Patrick Bateman (Sep 11, 2006)

In another thread, a forum member was talking about putting a big driver on a small horn. In his case, a BMS midrange compression driver, about the size of a small melon.

Matter cannot be created or destroyed; all that a horn does is take the existing output of a driver and focus it in one direction. What happens when the mouth is too small is that the wavelengths exceed the dimensions of the horn, and you start to get ripples in the response. The 'fix' for the ripples is to use a larger mouth.









A picture is worth a thousand words, so here's a pic showing what happens when you use a mouth that's 2" x 12", versus a mouth that's 4" x 12". Basically doubling the mouth size 'brings up' the output at the low end, and smooths the output across the entire bandwidth.

Horns are tricky devices; you can't just slap any driver onto a horn, you have to balance a ton of factors. (Mouth size, throat size, depth of the horn, beamwidth of the horn. There's also some uber esoteric stuff, like using the horns resonance to nullify the resonance of the driver.)

You can see in the measurement above that there's still a dip at the low end, even with a mouth that's 4" x 12". Going to a full 12" x 12" would be optimum.

One really cool thing about car audio horns is that we can use the dash to extend the curve. This is one of the reasons that some people have better results with mini horns versus full size horns. Basically the dash itself continues the curve. (Both horn types benefit from coupling to the dash, but a shallow horn is a better match for most dashes than a deep horn IMHO. Take a look at your dash and picture it extending the horn.)


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## oabeieo (Feb 22, 2015)

So pat, that put a lot of what I think some of us (me) try to put into words but just can't explain it like you do. 

Which in turn make me go into a deep pondering about horn termination (and chicks in bikinis  ) 

So around the lowest usable frequency of let's say a car horn (which usually is crossover for most horn users) the air around the mouth of the horn starts to break and snap if you will. 
We add round overs ( or knee bolsters-lol) or bigger horns to better terminate the mouth to make the high pressure and low pressure equalize , so we don't hear the air snap , it sounds better and there's none of this ripple from the air change. 

Obviously a smooth transition also doesn't disrupt critical intermodulation that is pattern forming. Does it? 

So let's ask, 

does the lower dash that extends the horn (on only one vertical side) the other side of the horn is dispersing a mirrored pattern that fires into the footwell (off axis) and on axis into the seat:

so the lower side of horn exit that doesn't have a roundover or a dash to extend the horn would it still have high pressure at the lip of the exit? 

Or

On the side that does have a dash against it, would the smooth transition equalize the entire horn mouth? Or so would the wave front be re-directed upwards towards the listener by reason that the path of least resistance has shifted to one side of horn? Because the resistance is the low pressure air that now has to be shoved out of the way.

Thanks


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## Patrick Bateman (Sep 11, 2006)

The easiest way to see what happens is to simulate it in the wavefront simulator in Hornresp.

It's easy to do - sometimes I think I'm the only person that uses the wavefront simulator. (I rarely see anyone discuss that part of the product online)

In a nutshell, if you remove one wall from a four sided horn, the sound radiates towards that side.

IE, if you look at a HLCD that's properly mounted in a car, the door extends the left wall of the horn, and the dash extend the top wall. This means there are two walls that are missing: the bottom wall and the right wall.

The net effect is that the sound radiates DOWN and to the RIGHT.

Having the sound radiates DOWN probably sounds bad obu it's not a huge deal, because the effect is frequency dependent. Basically each segment of a horn controls a segment of the bandwidth:

The first half inch controls the very highest frequencies 
The next inch controls the upper treble
The next two inches controls the lower treble

Etc..


That's why we can get away with shortening the horn; if you remove a few inches at the mouth it doesn't have a huge effect. But remove an inch at the throat and that's a different story.

Basically a well mounted HLCD uses the dash to extend the mouth. But there's a catch - the right and the bottom wall are absent. (Only at the mouth, of course. For the first six inches, all four walls are there.)

If you really want to make your head spin, check out the BEM sims I did three months ago, you can see that the sound doesn't even touch the walls of a horn in some cases! Basically it can't "see" all four walls, depending on the geometry of the horn.


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## oabeieo (Feb 22, 2015)

I love picking your brain. 
Yeah your right , it would do exactly that. And you make it make sense everyone can understand.


So to sum it up, would a big driver on a small horn get down to the 400s of the dash was just right, or is the side that has no dash or door make it impossible and make it the only usable range to the size of horn ?


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## thehatedguy (May 4, 2007)

Yeah, only the wavelengths at or below the Fc of the horn just fire straight through. So while the driver will physically play that low, it will not be loaded by the horn.


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## oabeieo (Feb 22, 2015)

One time I was using a vape pen ( didn't like it ) 
But the cloud of stuff was all over the dash and for an instant I think I got to see the midbass blow a smoke ring . Was kinda cool , tried and tried to do it again and couldn't do it .

But anyway smoke cloud that looked like a big smoke ring started about 4 inches outside the circumference of the driver. Was weird because it found its way around all the shapes up in the dash corner. It happened super fast too. Like was there and gone quick. Got to see it expand and bounce and dissipate so so fast 
I realized the radiation that's bigger than cone size vs. frequency pushes air way out side its physical size. Which would translate to the size of a horn mouth if it was a horn. 

Dumb story , but it was pretty cool to see but it made me understand comb filtering , reflection and loading in so much better. It's like it supported all the things I read .


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## Patrick Bateman (Sep 11, 2006)

oabeieo said:


> I love picking your brain.
> Yeah your right , it would do exactly that. And you make it make sense everyone can understand.
> 
> 
> So to sum it up, would a big driver on a small horn get down to the 400s of the dash was just right, or is the side that has no dash or door make it impossible and make it the only usable range to the size of horn ?


There's too many variables to make a blanket statement.

This is one of those things where you really have to fire up HornResp, and even then it takes years of trial and error to come up with a good solution.

Throat compression starts to become a huge issue here too. For instance, I generally try and avoid using a compression ratio higher than four to one, and two to one is even better. If you're using a horn with a 1" throat, *that limits you to a 2" diaphragm.* (Because a 2" diaphragm has an area of 3.14159", and a 1" throat has an area of 0.7854")


Synergy horns solve a LOT of these problems. You can have your cake and eat it too, by putting the tweeter at the throat, where the area is very small, and your midranges further down the horn, where the area is much much larger.


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