Design Phase - 2 Monster Front Loaded BR Subs

[Brian Steele]

[snip]
It was used in this configuration for a while and tested with plenty of actual music.
Not bad then.
This is "Test 1"

The impedance plot for test 1 looked a bit lossy around Fb. Was the box lined or stuffed for that test?

 

[Brian Steele]

Port tuning is very roughly independent of its dimensions for a given volume - it is the mass of air in the port which matters.

In a ported speaker the air in the enclosure is a spring and the air in the port is a mass. Imagine a mass hanging on long soft spring, gently bouncing up and down - the frequency of oscillation is not affected by the shape of the mass! (In reality we have friction, air resistance, and other losses so these generalisations are not accurate to ten decimal places...).

A few general rules about ports:

1. Begin the design with a port area equal to the speaker's effective area (Sd) and make it smaller until it fits in the box. (I have only ever once achieved port area = cone area!).
2. Use a single circular port.
3. Ensure that the port length is no longer than three times its diameter.
4. Radius the port at both ends to ensure equality of air movement at each end to avoid port rectification.
5. Make the port pretty rigid, and support large ports if necessary.
6. Don't fret about decimal places when port tuning - in the real world it will change several whole Hertz simply with temperature and output level.

Attention to good port design reduces distortion and increases output; 10dB differences can be measured between best and worst.

I usually have one rule for vented subwoofer alignments - make sure that vent is at least 1/3rd Sd and preferably larger. Typically that leads me to end up designing the subwoofer as a TL instead ...

 

[Databass]

Thanks guys.... Jeez, this forum is awesome !!


I will reply to the members who posted in some kind of chronological order, interspersed with comments and
information from myself.


@ MrKlinky - Thank you for your input....
My so called L'Acoustics "L - Vent" has been widened so much to raise the Fb, it is now
effectively just a regular ol' rectangle slot port with (very) mildly flared ends.
And even after all my reading, I had never known until now, that the port could also bee
shortened to raise Fb as well.
But the interesting curiousity is that the length and the width of the port is in some sort of
magical ratio with itself.
Apparently. as the port is widened to increase the tuning frequency, it must also bee shortened in
some kind of similar fashion.

Now my drawing from Post #80 on page 4 of this thread makes absolute sense.
https://www.diyaudio.com/community/...ster-front-loaded-br-subs.406026/post-7605404
I had not known, nor noticed this correlation before.

Bass reflex is getting less and less like science and more and more like voodoo.


Thank you also @ maxolini

maxolini's post #99 {page 5) is pure gold and should bee studied closely.
I found some of it almost counter-intuitive but that's how it is sometimes.

But my God, have I learned a fantastic amount since I kicked off this project and it just keeps giving and giving.

Must bee hard to keep up sometimes but I am very happy this thread contains a huge and very vast amount of
knowledge and information for anyone who wants to study the disipline of bass reflex.


Now I will answer @ Brian Steele's two questions above this post
"The impedance plot for test 1 looked a bit lossy around Fb. Was the box lined or stuffed for that test?"


Am, no... And what do you mean by "a bit lossy around Fb"
For Test 1 the box AND the port are smaller than Tests 2 and 3
For Test 4 I removed some box volume leaving the port quite wide.

There was no lining or stuffing used in any of the tests.

@ Brian - You mention a couple of times about simulating bass reflex enclosures as transmission lines when the port(s)
get too big.

I find this super interesting and have wondered many times before, at what point does a port become big
enough to bee a horn.... Because the L'Acoustics "bass reflex" L - vented enclosure is surely a back loaded horn of some description.



For now I think, I will stitch the KS18x back together and use it for listening to music for a while.
I think the engineering term is "good enough". I have just acquired a UMIK-1 so an SPL measurement is on the cards in the not too distant future.

The box has been rebuilt a few times by now and it is just too far gone to bee honest.
Shortening the port will not bee easy...

 

[Turbowatch2]

Read about "ground plane measuremens", now that you havea mike. Then do one.

If you have an unbraced, flappy cabinet, all rules may not fit and simulations are far off.

Remember that it is not the panel thickness that makes a good sub, but bracing that prevents them from resonating. The builds you pictured in this thread are an example of how not to build a subwoofer or any bass cabinet. If you build such a thing and call it a "test cabinet", the test will be worth nothing.
It doesn't help to buy good, expensive drivers and put them into some crappy enclosure.
Some do not get the idea that a cabinet with huge open ports has to be solid and air tight, but this is how these constructions work.

 

[maxolini]

I think the engineering term is "good enough". I have just acquired a UMIK-1 so an SPL measurement is on the cards in the not too distant future.

below are some tips for bracing your cabinet as Turbo commented, no bracing will lead to a flappy wobbly cabinet whose freq output measurements will be not valid.

Descriptions of Bracing with Visual Context:

1. Cross-Bracing:
   • Appearance: Imagine wooden struts (e.g., 1" x 2" or 2" x 2") running horizontally and vertically between opposing walls, forming an "X" or "H" shape. These are glued and screwed into the inner surfaces of the side panels.
   • Example: Think of the inside of a shipping crate—simple beams connecting the walls, but cut to avoid blocking the horn path.
2. Window Bracing:
   • Appearance: Picture a large panel (like the side of a TH sub) divided into a grid of smaller rectangles by intersecting braces. It looks like a windowpane with multiple sections (e.g., 4x4 or 3x3 grid).
   • Purpose: Each small section is too small to flex significantly, stiffening the whole panel.
3. Driver Baffle Reinforcement:
   • Appearance: The baffle where the driver mounts might have a second layer of plywood glued on, with four or more triangular or straight braces radiating from the driver cutout to the edges, like spokes on a wheel.
   • Visual Cue: Similar to how a car wheel hub connects to the rim, but in wood.
4. Horn Path Gussets:
   • Appearance: Small triangular pieces of wood (e.g., 6" x 6" right triangles) glued into the corners of the horn’s internal bends. Sometimes paired with a long brace running parallel to a straight horn section.
   • Look: Like the corner supports in a bookshelf, but inside a folded horn structure.
5. Mouth Reinforcement:
   • Appearance: A thick frame (e.g., doubled-up plywood) around the horn exit, often with additional diagonal braces at the corners. It looks like a sturdy picture frame reinforcing the cabinet’s front edge.
6. 
7. 

 

[Brian Steele]

Am, no... And what do you mean by "a bit lossy around Fb"

The impedance dip at Fb would be a bit lower (as in the other impedance measurements) if the box wasn't lossy. I've seen measurements like that in cases where the box is lined or stuffed.

 

[MrKlinky]

It is quite possible to design an enclosure which doesn't require massive bracing to work properly due to its inherently unsuitable shape!
The greatest unbraced width of any panel on my 21" 130l subs is 75mm.

Think out of the 'box' (non-cuboid)...

 

[maxolini]

So are you using fiber/resin panels ?
Plywood bend quite a lot.

 

[Turbowatch2]

Different shapes, like a sphere, may not need bracing.

 

[mark100]

When the impedance peaks in a bass reflex system are not the same height, it indicates an asymmetry in the system’s response that can reveal specific design characteristics or issues. Let’s break down what this means and why it happens.

Great post ! Maybe the best I've seen on assessing reflex impedance..thx.

 

[MrKlinky]

So are you using fiber/resin panels ?
Plywood bend quite a lot.

Flat run-of-the-mill Chinese 15mm hardwood ply (12mm for the 18" versions) and 6mm MDF bracing.

I tried kerf-cut cylinders but was never happy with them, although structurally very sound. I've made some curved laminated flexible ply 12" mid/tops too but it's a lot of work!

 

[MrKlinky]

Different shapes, like a sphere, may not need bracing.

They certainly wouldn't, but only really DIY-practical in GRP or perhaps fibre reinforced concrete. I've used IKEA stainless steel hemispherical fruit bowls for small speakers which are brilliant when properly damped - light and stiff; just what we want!