A massively geeky question about cab tuning

[Bo Millward]

Right I need help tuning this old 2x15” bass cab I’m sorting out. It had two shagged H&H drivers in it when I got it, so I’ve replaced them with Celestion FTR15-4080FD’s. I know they're PA speakers but there’s reason in the madness. After looking for ages for a speaker that could reproduce a low F# fundamental I found these with a frequency response of 35-2500 Hz. (F#’s round 20 Hz, but for my £100 odd budget it’s as close as I’m guna get to it.) As well as getting the new speakers I’ve lagged out the inside of the cab to take out a bit of the “OOOOoooOOoooo” out and it sound sick. Plus I’m back in B now so it handles everything perfectly. The plan when I’ve got this cab sorted is to get hold of an old shagged 2x10 for a top cab sort that out then bang, sick rig.
Anyway, now I need to tune the f***er comes the massively geeky question bit: The cab is ported with two 3” ports, how long should I make the tubes in the ports to tune the cab to 31 Hz? Here’s the Thiele-Small Parametersand shiz to get you started;

Thiele-Small Parameters

Mmt (g) 129
Qms 7.2
Qes 0.24
Qts 0.23
Re (Ω) 5.4
D (m) 0.33
Vas (L) 203
BI (tm) 23.9
Cms (mm/N) 0.20
RMS (kg/s) 3.56
Xmax (mm) 6.0
f/p (mm) 10.0
coil (mm) 22.0

General specifications

Nominal diameter (") 15
Power Rating (AES, W rms) 1000
Nominal impedance (Ω) 8
Sensitivity (dB) 97
Chassis type Cast
Voice coil diameter (") 4
Surround material Cloth-sealed
Magnet type Ceramic
Magnet weight (kg) 3.1
Coil material Round copper
Cone material Glass loaded paper with
weather resistant impregnation.
Frequency range (Hz) 35-2500
Resonance frequency, Fs (Hz) 32

And here’s the cabs specks;

Height (cm) 80
Width (cm) 60
Depth (cm) 37
Ports 2 x 3”

Nice one!

Edited May 22, 2010 by Bo Millward

[LawrenceH]

Haven't got an answer for you (but could potentially plug it into WinISD and give one), but for 2000 watts into 2x15", twin 3" ports are nowhere near big enough to get the most out of your speakers regardless of tuning. What kind of power are you actually putting through the cab?

[Mr. Foxen]

Not geeky enough.

[url="http://www.talkbass.com/forum/showthread.php?t=474354"]Check this for info on the low F# fundamental.[/url] [Hint; you can't have it and you don't want it]

The 'frequency response' is fairly meaningless number for oversimplification purposes.

Download WinISD Pro [url="http://www.google.co.uk/url?q=http://www.linearteam.dk/default.aspx%3Fpageid%3Dwinisdpro&ei=hLz3S4fICdOM4gap4fFj&sa=X&oi=smap&resnum=1&ct=result&cd=1&ved=0CBgQqwMoADAA&usg=AFQjCNHgpQr-y6l7n8FDJF6hq4gb7dr7_Q"]here.[/url]

What is the cab made of? If it is good solid ply, might be worth messing about with. Firstly you'll need loads more port area. Might be good to start again with he ports and use some tube you can sleeve another one in and have telescopic ports so you can alter the tuning.

I'm not so good at eying specs and seeing what a driver is like from the specs, but I reckon that is the sort of driver to need a really big box to get really low.

[Dood]

Hi Bo' - I just tried to type a reply to your comment on FB, but the stupid heap of crap didn't save my somewhat long and wordy reply. That has really pissed me off!

Anyway - as it happens - most of what I typed has been said already - thus saving me a long retype! What I was going to add on to as well is pretty much what I'd said before about fundamentals with bass.

Ok, firstly the port. I can understand why you are tuning to 31hz - however - I think that forcing a port to a particular frequency as 'guess work' is or could be counter to what you actually want to achieve. WINSID can show you graphically what I am trying to explain. (You'll be able to see the frequency curve of the cabinet change as you force the size of the prot to new figures) You'd be better off working out the optimum tuning frequency for the cabinet and speaker combination. I have a good book on it somewhere - but I have forgotten the 'calculations' - thankfully - again WINISD is much easier on the brain and will give you those figures.

So, Speakers are not so efficient down on those really low frequency ranges and require a great deal of power to get those notes sounding. (If you can get the fundamentals to sound at all in such a cabinet) You may find that accentuating the overtone frequencies in your EQ and scooping some mids around the 300hz range will give the illusion of depth in the tone. Then, if you really need woofs of bottom end, rely on PA support. I learnt the hard way that I was throwing away lots of Watts of power trying to reproduce fundamentals through cabinets that were just not able to!

[Bo Millward]

Nice one for your help people, right i've got winISD so I'm guna has a mess and see what happens. I may have to quiz you guys a bit more if I get stuck but I'll keep you posted. Oh and as for LawrenceH's question; i'm driving it with a 300W Warwick ProFet 3.2

Thanks again!

Edited May 22, 2010 by Bo Millward

[Mr. Foxen]

You'll want a separate amp to run your 2x10 for top. might be better with a guitar combo/2x12 and head.

[Phil Starr]

your finished speakers are going to be overdamped which means that the bass, whilst going deep, will be somewhat squashed. With a Qts of 0.24 damping is very high and the speakers are designed to be used in a smaller box than the one you are going to put them in. Although the bass promises to be clean and have good transient response I suspect they wont be the bass monsters you are hoping for. In addition the lack of response above 2500Hz will mean that they will lack articulation and the ability to cut through the mix

[Bo Millward]

[quote name='Phil Starr' post='845252' date='May 22 2010, 06:11 PM']your finished speakers are going to be overdamped which means that the bass, whilst going deep, will be somewhat squashed. With a Qts of 0.24 damping is very high and the speakers are designed to be used in a smaller box than the one you are going to put them in. Although the bass promises to be clean and have good transient response I suspect they wont be the bass monsters you are hoping for. In addition the lack of response above 2500Hz will mean that they will lack articulation and the ability to cut through the mix[/quote]

Cheers for the advice man, if I partition the cab inside so each speaker is loaded in its own sealed enclosure would that help to prevent this? (Oh, and add ports to each section)

[Phil Starr]

[quote name='Bo Millward' post='845843' date='May 23 2010, 04:21 PM']Cheers for the advice man, if I partition the cab inside so each speaker is loaded in its own sealed enclosure would that help to prevent this? (Oh, and add ports to each section)[/quote]
I quite like partitioned cabs because they stiffen up the structure and reduce the interaction between two speakers however this won't solve the problem of the cabinet volume. I have done this by removing the back of a speaker and then putting a new back in place a few inches in front of the old back which I then put back to cover up what I had done. this makes the cab quite heavy but since the outermost 'back' is just cosmetic you could use a very thin panel.

Sorry I was a bit negative about your speakers but if you like the sound then they are good and if it is the sound you want then you should trust your ears and not some techie advice. Thinking about what you are trying to achieve though it would be quite interesting to use just one of the Celestions with a partition inside the cab and then another 15 with a better midrange/top end in the larger top half of the cab.

I don't know how you are getting on with WinIsd but if you are having problems I'll try and give you a hand with design.

[Mr. Foxen]

[quote name='Phil Starr' post='847063' date='May 24 2010, 11:52 PM']Thinking about what you are trying to achieve though it would be quite interesting to use just one of the Celestions with a partition inside the cab and then another 15 with a better midrange/top end in the larger top half of the cab.[/quote]

If that sorta mod is the plan, probably best to use a proper sub driver, a crossover and stick a smaller mid driver on a board sealing the top hole.