Tuning reflex boxes

Evening all..

Last timeI designed a reflex bass cab was before the days of winisd etc, and I had to rely on alignment charts and calculations. With a suitable driver (ie qts ~ 0.35, sensible vas and fs) this always worked well for me.

My latest design is a for a 15" bass cab to be used for disco/wedding parties etc, where they wouldn't appreciate a big pile of horns. I've got several fane 15xb's lying around gathering dust, so I'll use these. Due to the rather low qts of this driver the conventional alignment charts suggest a box of around 30litres tuned to 37 hz. Unsuprisingly this models as giving very little output below 100hz. I therefore need to be a bit more adventurous with this design.

A box of around 100-120litres would be suitable in terms of size, and seems realistic for a 15" driver. I don't need it to play really low, a 40-45hz rolloff would be fine.

The problem I have is that I read somewhere that if a reflex box is correctly tuned the two impedance spikes will be of equal size. The only way this happens is when I tune the box to 37hz, the drivers fs. If I tune higher the lower spike increases, which I believe is a disadvantage, but port length and air velocity decrease, clearly an advantage.

Does anyone know what the effects of unequal peaks might be, and how different they can be before causing audiable problems?

Any pointers will be much appreciated, I don't have long until I need to use these cabs, so it needs to be good enough to use first time, I won't be doing prototypes.

thanks

Irrespective of the tuning conclusion. The chuffing that occurs in ports may only be heard on test. It is quite common to hear this when going low with a single tone. With pink noise you can't hear it and certainly when in proper use, it can't be heard, not by my ears anyway. I thought I would say this to put some perspective on the subject. In other words, if it does happen it's not that serious.Edited by: Tony.A.S.S.

I normally tune 15" reflex boxes to 40-45Hz depending on the driver and application. 100-120L is perfect volume.

In WinISD tune it to give a flat response or slightly rolling off at the low end. Avoid peaks at the tuning frequency.

Sometimes with Low QTS driver they have to be tuned fairly high just to get fairly level response.
A friend of mine loaded some PD154's into old Peavey boxes of about 110L and complained about the sound.
I eventually measured them and found that the port tuning (not revised from original drivers) was at a too low 40hz causing a great big suck-out in the mid bass region.
I retuned them to about 48 hz which gave a pretty level response and he was as happy as Larry.
If possible much easier to use drivers with a Qts 0f around 0.35

Tony

I presume you're aware of this?

www.fane-acoustics.com/pdfs/Cabinets15inch125L.pdf

If not it might save you some brain exercising/prototyping, unless thats part of the fun for you.

Thanks chaps.

Tony- I'm not too bothered about chuffing (less so now I've seen your reply), as I doubt this application will involve high power levels, and I imagine they'll only be playing average disco music, nothing with sweep tones.

Chaudio- I do get a peak at tuning when tuned to driver fs (37hz), well more of a dip above actually, but it seems tricky to remove this completely without sacrificing a lot of low output. Is a small peak acceptable? (a couple of dB, not talking 6dB) I take it a peak will make them rather one note or muddy?

AJW- I would prefer to use a driver with higher qts, however I've got xb's spare, and don't have time or funds to get something more suitable.

Imageoven- The first box I modelled was the fane design, and I'm confident I can do better- this seems to be a generic design as they list 3 drivers as suitable, all using the same tuning. This shows a lower peak of around 100ohm at 30hz and an upperof 55ohm at 75hz, clearly nowhere near equal. It reaches xmax with 600w at 50hz, cutoff is around 55hz (tuning is at 57 hz), group delay peaks at 21ms at 57hz.

Thanks for your input folks, I'll do some more modelling and show you the results.

spl plot.
white is 110 litres, 37Hz tuning
yellow 100, 37
blue 80, 37
green 100, 40
Plot with huge peak is Fane's suggestion, 125 litres, 56.5Hz
Impedance plot below is a bit crowded. The upper peak on the green line is around 80 ohms



Any thoughts?

And if you heard them it would probably be the Fane box that sounded initially the most impressive, however I would imagine that it would show some signs of one note bass.

I personally would choose an alignment that is shown by the green plot.

Most commercial boxes would probably be more like the Fane alignment!!!

Thanks AJW. I'm liking the look of the green plot. It has noticably less output than the fane one, but I'd rather have a smooth response as I don't need huge spl (I've got a pile of horns for that!)

Yup, green one or maybe try taking it slightly towards 45Hz and see how it looks.