|Calculation of cabinets and ports
Copyright 2010-2014 © Troels Gravesen
|The files available here have
been found on the web and the first one, "box
simulation and room-gain", is an easy
quick-and-dirty file that tells us what's going on.
The second file, SpeakerBoxDesigner, is found at an excellent Dutch website:
Both files in this zip file: Download.
Go to: Damping adds to virtual volume
Download "box simulation and room-gain" excel file for ScanSpeak 18W/8531-G00 here, and you will find this:
From manufacturer's data sheet you get the TS data and you only need four values to get going, Fs, Re, Vas and Qt. Insert resistance of the coil(s) in series with the bass driver in cell B8, something like 0.2-0.4 ohms. It doesn't make a huge difference.
Now, TS data and their reliability: Basically we can get the data we want depending on the voltage applied during measurement. Here are some examples from an 8" polyprop bass driver:
Above the same driver from four different
measurements of impedance: Two are done from stepped sinusoidal
measurements and two with a sweep signal. Both are performed by added
mass (20 grams) and added volume (33.4 liter).
Next I tried added mass/sweep and 4 different input levels:
This looks more interesting as we get a huge
variation depending on voltage applied during measurements. So, what is
right? Good question! Actually none of these are correct as it was
possible here to measure the weight of the the moving mass. This is done
by taking the membrane with the voice coil and cutting half the rubber
surround and half the spider. At least this is as close as we can get
the moving mass. We can always argue if half the rubber surround and
half the spider is really the right amount of suspensions
adding to the moving mass. Anyway, Mms is closer to 25 grams - and this
is close to what we get from maximum CLIO output, added mass and sweep
measurements (column 4 in the first illustration). But Qt is really low
- and is this really true? Hard to tell. The problem is that
the suspension aren't linear. This shows up in Cms and Rms. Cms is the
compliance of the suspensions, where Rms is the damping of the
suspension and also from e.g. voice coil formers made from aluminum.
Using Kapton voice coil former the electrical damping is much less and
usually produce a much higher Qm.
The box simulation excel file linked above does not tell you optimum cab volume for a given
driver and we may question whether this is desirable at
all as only a few drivers will
fit in nicely when it comes to the math. For ported
systems, only drivers with a Qt close to 0.35 will make a
The port: Insert your port diameter in cell B22 and you can read the port length in cell B23 and you don't have to be nit-picking on this value. Anything from 13-14 cm length will do perfect. If you have a slightly conical port, use diameter at outlet (and not maximum diameter of a flared port).
Do not feel tempted to make a 50 liter ported system for this driver if you find you can get -3dB @ 30 Hz and think YES, this is what I want. There's a price to pay for this in terms of a boomy and un-precise bass performance. You may ask what the heck is the optimum size cab for this driver? Hmm.... read on.
Download Speaker-Box-Designer for 18W/8531-G00 here, and you will see this:
This is a more sophisticated program telling you optimum cabinets based on TS parametres. You need to add more data (cells with blue font) and we read a recommended closed box of 24 liters and a calc. ported box of 48 liters, making F3 = 31 Hz!
Now, we would never place the 18W driver in a 48 liter vented box. Or, it would be like making a modern 2.5-clone getting deep, deep bass although boomy, sloggish, one-note, etc. Not a good choice.
So, how do we really decide which volume we should use for a given driver? Only practice can tell. We need to try out the driver in various volumes, experiment with various Fb and we need to listen. We also need to listen to the midrange in case we making a 2-way with the 18W handling anything from the lower bass to 2.5-3 kHz. I've tried the 18W in 22 liter volume and I didn't like the midrange being flat with too much presence. The 18W as midrange in a 3-way system is another story, here 22 liters do really well.
What makes the whole thing even worse is that manufacturers' TS data may not always be right. Sometimes even very much wrong, so we have to measure TS data ourselves and see if this makes a more likely scenario. There's no international standard for measuring TS data, so the results will vary enormously depending on the voltage applied during measurements. Speakers are highly un-linear devices.
use the manufacturers' data and I use my own data. Next I
model the driver in LspCAD to see if this driver is
really possible! In LspCAD you can fiddle
with all driver parametres and make cross-calculation to
see it what we measure makes sense and not least if what
the manufacturer claim is really plausible.
Here you can also explore more ported tunings like
SBB4/BB4, QB3/SQB3 and SC4/S4.
An SBB4 tuning will typically use a large box with a low
tuning with a more shallow rool-off and extended at the
Here's the LspCAD results based on SS data:
QB3, SBB4 and SC4 all prescribes boxes of 15-20 liter volume!
what we usually do: 33 liter and a port tuning of 30-33
Hz. Confused? No wonder!
damping materials to a recent construction I started
wondering if the dense green felt I often use really
doesn't reduce box volume rather than increase box
test cab, 33.4 liter net volume,
internal dimensions 460 x 280 x 260 mm (volume of two
fillets for mounting front panels subtracted)
To cover internal walls with 0.5 m^2 of damping materials were used.
- As can
been seen the green felt did not reduce volume, rather
increased virtual volume by 6%.
As always: Take into account the impact of damping matrials on your box volume.