JF Vented Horn

Unfortunatly i dont have any software. I did that graph myself in excel. I agree that 30Hz is the lowest point but if you look at the hornrespimpedance graph and bear in mind thatthe portis tuned to 37Hz on the simulation. I think that 34Hz on my graph is at the same point between the peaks as the 37Hz is on the predicted response, (if you get me???)

I think that this tuning is too low... but i have already glued the box. I have to make three more for an event im doing (in just over a week!) i think i will tune the a bit higher and just have one box with a lower tuning freq, i can replace it later and use the prototype as a bed room sub!

Cheers man
Jake

How easily can you get to the port from the driver access? No way of albeit untidily cutting the port down?

I suspect what has happened is that you've got some effective port extension from its position in the cab. Shelf ports always behave as if they're a bit longer than their physical length.

Long story but... I wanted to make the tuning slightly higher so i cut the port down a bit but when i added bracing and a "shelf" for the access panell to sit on (it was quite near the port) i hoped it wouldne make much difference, but it seems to have stayed the same tuning as before i sawed a section of port off. I believe with a shelf port you need to add half its width to its length.

I dont think i could hack the port away, no room. do you think there would be a problem with using 3 cab tuned to 38Hz and one tuned to 32Hz (for example). I know its not great but it'll have to do for this event, then i'll make another one.

Cheers

I have been thinking....
WhenI take these cabs outto do an event, say they are playing some 35Hz sine waves with 700w going into them.... (this will be at port tuning frequency).
Then... what if some jebend comes along and covers the port? it turns into a pure horn and over excurses and rips my driver! what could i do to stop this, put some razor wire around the port?

Hi jake, could you post a paint for the shelf and other bracing around the port? I'll do one for an X1 extension [img]smileys/smiley4.gif[/img].

Also, for port protection, I think a fence and a grille go a long way.

On later models, recess the whole port and horn, essentially extend the side panels a bit (In the style of a full grille type cab), I can say that works.

Edited by: Deadbeat

a pic:

Thanks. It doesn't look like it could do anything much, I think it has to do with the end correction as Chris specified.

The port end correction factor, stated as 0.732, is really only valid for round ports in certain situations. I hope you find this useful:

Quick explanation of end correction (curiously in woodwind instruments) and mouth reflection.
www.phys.unsw.edu.au/jw/musFAQ.html#reflection

I linked the ARTA suite earlier as it had LIMP in as well, a very very useful tool for this.
Here's a very useful thread on end correction factors in different shapes, some rough math for figuring out your real port length (and theoretical tuning), and some more recommendations as you already have the box...you could find the real tuning and therefore your real port length.
www.diyaudio.com/forums/showthread/t-97204.html
Pay attention to the remark about extreme height/width ratios, I have found this to be true in the field.

Jake

A few points:

1. You can always try adjusting your original model to match your current measurements. I think Chris suggested this earlier. It's a good idea because if you don't object to the frequency response, phase response and excursion behaviour you actually have, then maybe you don't have a problem at all

2. It may beeasier to *extend* a port in a built cabinet by just gluing more wood on... Maybe the wood could be clamped from the front by putting one of those nifty "long G-clamp" things though the port. If so, you're erring on the side of caution if you make the ports shorter.

3. Have you considered the effects of stacking multiple cabinets? Both Rog and Chris have claimed to have created ported horns that don't change their response, but it's not clear that this will be true of all designs. The effect might be to gain the bottom end relative to the rest of the response, in which case that might compensate for under-porting.

I'm sorry that my three observations all seem to lead in different directions - that's the nature of the game I'm afraid. It's the choices you make at this stage that define your personal style as a speaker designer and hence make you unique and not just another sheep.
Edited by: jsg

Did I say the response didn't change with stacking?

Something I definitely posted at some point was an observation with the simulations that unlike a normal reflex cab, the port tuning of the horns actually drops with multiple cabs. A quick check of the simulation with my horn shows that for a single cab in 2Pi Rad, the tuning is about 44Hz but for 1 cab in 0.5Pi Rad (or four cabs) it drops to just below 40Hz.

This unfortunately means that it would be better to port the cab too high as a single because it will drop in multiples. I suspect the extra air loading on the front of the cone is dragging down the resonance in the rear chamber.

If your first design and build turns out to be useable then you are very lucky. My first two horn loaded prototypes were built, tested briefly and scrapped due to being entirely wrong!

chaudio wrote: Did I say the response didn't change with stacking?

Oh sorry. I thought you did. My bad.