I’ve been running through ideas for some cabinets for the Lorantz woofers.
The current fairly rough PA cabinets that came with them OK, but I’m not getting the full bass the speakers are capable of.
First step was some measurement with WooferTester3 to get the Thiele Small parameters. I have the values from the Lorantz Data sheet for this driver, but as I could borrow Col’s WT3 rig I made the measurements myself.
My measured values were close, but not quite the same as the ones in the Lorantz data sheet. For one thing the F(s) measured was 37.68 Hz instead of 40 (this is actually a bonus as I’d prefer lower), and the Vas was 221.2L instead of 170L. But, I used the added weight method to measure this (piling a known weight of coins onto the speaker cone), which may not be the most accurate way.
Regardless, the cabinet size I’m looking at work equally well with either “factory” or “measured” data so regardless of which numbers are more accurate, I should be OK.
comparing with running these same parameters in WinISD with the same scales chosen:
note: the strong line is with the measured T/S parameters, the weaker line with the ones from the data sheet. For the transfer function they do not vary more than 1dB.
simulation of current trap cabinet
I have guestimated using some trapezoid volume calculator web page that the rough volume of these current cabinets are about 90L, and they have two 8cm diameter bass reflex ports that are 6cm deep.
Playing around with WinISD it was pretty easy to come up with the transfer function of this cabinet, which shows it is tuned to about 54hz, with a very flat transfer function. So for what they are, these cabinets are quite OK for getting a lot of power out of the Lorantz speakers without making use of the lower bass frequencies they are capable of.
from the above I’m guessing I can drive these speakers with up to 340W each in these cabinets and they will still cope with it, at least if I stick to frequencies above 46hz?
idea for cabinet to build
Current idea is to have wide, not so deep cabinets something along the lines of the “Altec 19” older style speakers boxes. Right now 60cm wide, 75cm high and 39.6cm deep. roughly including bracing and the volume of the driver this gives me an internal volume of about 138.9 litres.
I’ve already got a sample of “BB Pine” plywood from Allboard distributors which is the current proposed material for the build, using 18mm ply for the sides and 25mm for the front baffle. The woofer will be mounted as high up the front of the baffle as possible to be as close to the waveguide as possible.
Using this volume, and tuning to 33Hz I get the following transfer function:
Not very linear, but then I’m intending to cross over at 500-700hz anyway, and the resulting “bump” at 300hz may already cancel out the measured dip at this frequency (baffle step??).
The other price for tuning the enclosure this low is loss of power handling, but according to the WinISP graph I still have plenty of headroom for the 130W amps I am using.. and thats in the “never gonna happen” case of me running them at full power:
My current setup is with a LR24 crossover low pass filter at 500hz for the crossover, handily I can add this filter in to WinISD to get an idea of the “crossover + cabinet” transfer function.. although I’ll very likely be runnig with quite a bit more miniDSP correction anyway:
I’m still a bit of a neophyte at WinISD, UNIBOX (I cranked the numbers into Unibox as well and got the same results), so any advice appreciated, it’s all just an idea on paper subject to refinement at this stage.
Other “design axioms” I am working from here, that on consideration I actually need to read up on rather then let them remain as rules of thumb clattering around in my mind are:
- make the drivers (compression driver and woofer) as close together as possible. insert reasoning here.
- roundover of sides of cabinet.. the bigger the roundover the better, but I’ve got 25mm of ply to work with.
- baffle dimensions – wider is better – justification.. not just because I like how it looks.
- plywood better than MDF (insert link to justify this)
..and a crappy sketch to round things off with: