15” Driver in an 18” cabinet design

Hi everyone,
I’m looking for technical feedback on a horn-loaded cabinet project shown in the attached image.

First of all, many thanks to Mr Flexy for sharing this design and making it available to the DIY sound system community.
The cabinet shown is Mr Flexy’s original 18” horn design, and my goal is to adapt it for use with a 15” driver, while keeping the original concept and geometry as intact as possible.

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Reference design (from the image)

• Original design by Mr Flexy, intended for an 18” driver

• Front-loaded / scoop-style folded horn

• Overall external height: approx. 1016 mm

• Cabinet width: 724 mm

• Internal folded horn with angled reflector panel in front of the cone

• Original cone-to-first-front-wall distance: 120 mm (as shown in the drawing)

• Multiple internal dimensions and path segments visible in the image (e.g. 241 mm, 287 mm, 314 mm, 402 mm, etc.)

The horn path, folding scheme and mouth size are kept exactly as in the original drawing.

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Modification for 15” driver

Although the cabinet was designed for an 18”, I am planning to load it with a 15” driver.
To do this, I applied only one targeted modification, without changing the horn path or external dimensions:

• Original cone-to-first-wall distance: 120 mm

• Modified distance: 90 mm

• This is achieved by adding a 30 mm solid panel in front of the original first wall

The idea is to:

• Reduce excessive throat volume for a smaller cone 

• Improve coupling between the 15” driver and the horn

• Keep the original folding, expansion and mouth geometry untouched

Everything else remains exactly as in Mr Flexy’s original design.

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Driver

• 15” sound system–oriented woofer

• Low Qts, strong motor

• Intended operating range roughly 35–90 Hz

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Construction notes

• Plywood thickness: 18 mm

• Additional 30 mm panel used only to reduce the cone-to-wall distance

• Glued and braced construction

• No damping material planned inside the horn path

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What I’d like feedback on

• Is reducing the cone-to-first-wall distance from 120 mm to 90 mm a reasonable and effective adaptation for a 15” driver in this horn?

• Would this change significantly affect throat loading, compression, or response smoothness?

• Do you see any potential issues running a 15” in this geometry without further changes?

• Would you personally adjust anything else (rear chamber volume, throat area), or is this a sensible “minimal intervention” approach?

The goal is not to redesign the cabinet, but to make a practical, respectful adaptation of Mr Flexy’s original 18” design for a 15” driver.

Thanks again to Mr Flexy for the original project, and thanks in advance to anyone willing to share advice or experience.

You can see the original project in the video down below:

https://www.youtube.com/watch?v=018thiAJZFY

this is my HornResp simulation, with the external box size of the 18" but with the mods to accomodate my 15.

Edited by johnnyXXx - 04 February 2026 at 12:45pm

right! changed the config to tapped horn and it dosent seem bad.. what could be the downside of building this?

I think the answer that would really help here is why? If you let us know why you're planning on putting a driver that is completely different to the one the box is designed for it would really help and people could help you achieve your end goals. If your end goals are sonic, as in you want a certain frequency range covered with a specific tonality the this is definitely not the way to go about it. Tell us what drivers you have access to and then people will recommend options. If you have the driver and like the look and size of the box, are happy with a box that is almost certainly bigger than the driver needs and in all likelihood not get the best out of the driver then people can suggest how to get the best out of it.

ok i get that going with an existing deisgn is much is easier, but from horn resp simulation with my B&C 15Ps768 this dosent seem so bad, what am i missing?