Calculation of cabinets and ports
Copyright 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:
http://www.mh-audio.nl/sql7/ViewDownloadFiles.asp
Both files in this zip file:
Download.

Tutorials

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.

This file 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 perfect match.
Dealing with high-Qt drivers (e.g. Qt > 0.4) we use cabs too small and for low-Qt drivers (e.g. Qt < 0.3) we tend to use too large cabinets.
Remember, there's no "right" here. You can use whatever you want, but be aware of the consequences of what you're doing.
Too small cabs may mean no bass and too large cabs may produce a boom-box with un-precise bass performance (e.g. 2.5 clone).

Closed system:
Try a cabinet volume making Qtc = 0.7 and see what happens. In this case we reach 24 liters and this is not too far off. Try inserting 30 liters (change cell B11)
and you find the response (purple curve) reaching a bit lower. What you also see from the graphs is that the closed box will make much less bass at 40 Hz com-
pared to the vented system. If you're not a bass freak, you may prefer the short and dry bass coming from the closed box compared to the more voluminous bass
from the ported system. I often recommend making the ported system and you can always stuff the vent with a sock if the bass becomes too boomy. If you ad a
minor amount for e.g. dacron/acoustilus/MDM3 to the port you're having an aperiodic system and this may not be bad at all. Try it out!

Ported system:
The 18W drivers are mostly seen in 28-33 liter ported cabs and we have a nice (almost) flat response down to 40 Hz. Only a minor increase of ~1 dB in
the 60-100 Hz range. No problem. We're ~5 dB down at 30 Hz and this is not a common view I can tell. This driver goes deep from a 33 liter vented cab.
The price to pay for this is efficiency on the low side.

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.


SpeakerBoxDesigner:

Download Speaker-Box-Designer for 18W/8531-G00 here, and you will see this:


I don't have a password for this file, but we can view the formulas used for calculation. The port length is not corrected to fit reality and if you use this file, multiply port length by 0.7.

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.

So, I 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 low-end.
QB3 uses a smaller cab and a high tuning resulting in a flatter response and a steeper roll-off profile.
SC4 uses a sligtly larger cabinet compared to the QB3 with a fairly low tuning, not too different from the SBB4 tuning.

Here's the LspCAD results based on SS data:

QB3, SBB4 and SC4 all prescribes boxes of 15-20 liter volume!

Here's what we usually do: 33 liter and a port tuning of 30-33 Hz. Confused? No wonder!
Considering the impact of damping material on virtual volume as shown below, I'd make a 28 liter cab for the 18W reaching a virtual volume of 32-34 liter
when properly damped.


The impact of damping materials on box volume

Adding 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 volume.
So, time for a small study:

Materials: test cab, 33.4 liter net volume, internal dimensions 460 x 280 x 260 mm (volume of two fillets for mounting front panels subtracted)
Driver: ScanSpeak 21W/8555-01, Fs = 17 Hz, Vas = 169.5 liter
Green felt, approx. 10 mm thickness.
Egg crate foam, 30 mm
Monacor MDM3, 33% polyester foam and 67% sheep's wool.

To cover internal walls with 0.5 m^2 of damping materials were used.

Results:

- As can been seen the green felt did not reduce volume, rather increased virtual volume by 6%.
- For egg crate foam and MDM3 we see an increase in virtual box volume of around 15%.

As always: Take into account the impact of damping matrials on your box volume.


Red = unit in free air, empty box (blue), green felt (green), egg crate foam (brown) and MDM3 (yellow).