15" lowmid horn

Well, there's only a few compression driver horns that I would trust anywhere near 1khz, especially for high levels.

Then I was correct, hornresp after all is a horn modeling program, and shouldn't take what I suggested in. Think of it as the horn throat being so large the driver's direct radiation comes into play. There is a reason why the Altec VOTT simulations look so horrible (but in real life...), I guess. You'll have to google for it...or check diyaudio.

Hornresp is deadly accurate displaying what it does, after all, it's a big graphing calculator, but you have to realise what it does and what it takes in to look at the graph and take what you will.

_djk_:

Hornresp is accurate in prediction power response.
That's one of the best explanations I've found. Just so you're satisfies, here's another from diyaudio:

Hornresp models the power response of the horn. This is different than the on-axis response which you might measure with a microphone. The power response is what you would measure at a point if sound radiated evenly in all directions away from the horn, within the solid angle specified in the ANG input. So the modeled results should be fairly accurate up to the frequency where the horn starts to have directivity - where the polar pattern starts to narrow. This is typically at the frequency where the wavelength falls below the diameter of the horn mouth. Above this frequency, Hornresp will predict lower SPL levels than what you would measure on-axis. Hornresp now includes tools to investigate this effect. Once you calculate the model, go to the SPL Response chart. Under Tools, select Directivity. If you enter a blank input, you will see the power response. If you enter 0, you will see a prediction of the on-axis response. You can also enter other angles. Also under tools, you can look at the Pattern tool. This will predict the polar pattern at the frequency you input and show you the DI, directivity index at that frequency. Directivity index is a number in dB giving the gain over what the level of the power response is.

I'm sure David McBean will be in here any minute telling us what's actually going on...


Edited by Deadbeat - 28 August 2008 at 10:20am

huh, it looks much better on axis with constant directivity.

throat size is 800cm2. It seems like the horn reaches 1kHz easily...
But it is quite directive, 30° off axis response begin to fall at 800Hz.

Like this? 

Remember, when you click 'calculate' on hornresp, it only describes the power response of the horn! Not taking into account driver response, which on certain designs increases the HF ability of any given horn. Plug in an Altec A7 and you will see that....hey, the frequency reposne is crap! It don't go high as it's rated! What were they thinking! Well, measure one and you will see why they rated them to go with the compression drivers straight up. I'm not sure whether 'combined reponse' can do this...

On modeling plugs - a simple thought exercise is to design your horn with the high compression ratio, then split the throat apart keeping the same area in the cross section, if you know what I mean.

Well, if you look at the issue of making a front loaded horn cab go high enough, there are two ways. The first way is to compression load the driver, with a phase plug. Nowadays, with the cone strength of today's speakers, this is a viable option, though as you may well have noticed, it is not so in older designs:
Figure A, a Funktion 1 Res 4, using one of the later phase plug designs, even the folded horn is plugged:


There's also the other well accepted way, which is to make a very large throat that combines the driver response WITH the speaker response. You may have noted my earlier statement about this, and realise that even if you make the throat much bigger in hornresp, it still looks crap. I'll repeat it again - that button calculates horn response.

Now, the reason why these older cabs don't use the fancy new method of plugging a horn is simple - the drivers just couldn't stand up to it. Not at all. Build a high compression horn with that driver and it will rip from it's surround with 250W.

So. you are really looking for something like this:

Don't mind the HF (apart from the direct xover to the comp), and concentrate on the woofer section. It is very large. Now simulate it in hornresp, there are plans on the net. What? It's a piece of crap! Not so...

I suggest you try a throat size of...128 square inches. This is calculated from the above plans:
http://www.lansingheritage.org/html/altec/plans/1968-plans.htm

Did you block off the port?

What frequency range did you run it in then?


Edited by Deadbeat - 29 August 2008 at 9:22am

If you calculated the chamber like I thought you might, it would load the other side of the driver as effectively as the line on the altecs. Halving the throat would alter it more, I'm not sure exactly but unless it was enough to fully compression load the driver, I feel that the upper cutoff would droop slightly. Did you run measurements before and after?

Try it without the throat restrictor and the horn still in the port, and see, I can't say exactly how but that's my opinion. Someone with more knowledge will set us both straight, I hope.