Change proportions of enclosure for front port location? (Helix Dome MT)

[Dougment]

Hi -

This is my first post to the forum and I appreciate any help you may be able to offer. I am considering building a pair of Jeff Bagby's "Helix Dome MT" speakers but the space in which they would be used requires that they be on a shelf with only 1" to 2" clearance behind them. I'm considering changing the box design so the port is on the front. I've read a number of posts on this forum and others suggesting that as long as the internal volume of a speaker enclosure remains the same, the proportions of the speaker can change with little or not affect on the resulting sound.

First - is this an accurate statement? And second - If so, I'm okay with using the provided cylindrical port on the front of the speaker. Could I simply make the speaker the same width as designed but increase the height and decrease the depth (front to back)? Does the location of the port relative to the woofer and tweeter matter?

See the attached diagrams showing what I'm thinking about doing.. I am a woodworker and am confident in my skills to modify the box design but don't want to do any modification of the crossover design or driver selection. : )

Any advice is welcomed. I searched this forum for similar questions with no luck. If there's a thread that addresses this question, please direct me to it.

THANKS!

Doug

[PWR RYD]

I am not familiar with that particular design from Jeff, but if he designed it with the port on the back panel then he certainly designed it with the intent of it being stand mounted and at least a couple of feet away from the rear wall. That means that Jeff most likely designed his crossover with some degree of baffle step compensation (BSC).

Yes, your statement about keeping the volume and port dimensions the same holds true for box tuning... it doesn't really matter if the port is on the front, top, bottom, or the back for that matter. But...

If you place this speaker design on a shelf, an inch or two from the rear wall, it will have a totally different frequency response than what Jeff designed it for. It will have bloated bass and midbass. While I personally love lots of bass (frequencies below 80 Hz) I don't like bloated midbass (frequencies in the 100 - 400 Hz range). That just makes the music muddy. I hope this is helpful. Moving a speaker closer to or farther from the rear wall makes a big difference in tonal balance.

[Dougment]

I am not familiar with that particular design from Jeff, but if he designed it with the port on the back panel then he certainly designed it with the intent of it being stand mounted and at least a couple of feet away from the rear wall. That means that Jeff most likely designed his crossover with some degree of baffle step compensation (BSC).

Yes, your statement about keeping the volume and port dimensions the same holds true for box tuning... it doesn't really matter if the port is on the front, top, bottom, or the back for that matter. But...

If you place this speaker design on a shelf, an inch or two from the rear wall, it will have a totally different frequency response than what Jeff designed it for. It will have bloated bass and midbass. While I personally love lots of bass (frequencies below 80 Hz) I don't like bloated midbass (frequencies in the 100 - 400 Hz range). That just makes the music muddy. I hope this is helpful. Moving a speaker closer to or farther from the rear wall makes a big difference in tonal balance.

Thanks, Craig -

In your experience, would moving the port to the front of the Helix Dome MT (as shown in my 3D model) help diminish 'bloated bass and midbass'? Or would it be foolish to modify the Helix Dome design under the circumstances?

Perhaps I should be considering another design altogether... Do you (or anyone) have suggestions for a predesigned kit speaker that will be on a 13" deep shelf in the +/- $400 - $500 range? I want the best sound possible for that price. No need for BIG bass - clarity is more important to me.

Best -

Doug

[billfitzmaurice]

Bloated bass and midbass doesn't come from the port location. IME the only reason for putting a port on the back is when the baffle is too small to fit it on the front. The concern with mains placement close to a wall is the resulting response notch where the baffle to wall distance is 1/4 wavelength. To get around that subs are best placed close to the wall, where the 1/4 wavelength distance is above their pass band, while mains are best place far from the wall, where the 1/4 wavelength distance is below their pass band.

[buggers]

"Could I simply make the speaker the same width as designed but increase the height and decrease the depth (front to back)?"
Changing the baffle dimensions can have a negative effect on the sound. I would suggest a different speaker for your situation.

[djg]

Look at another Bagby design, the Continuums. Now Continuum IIs with a better tweeter. These were designed with flexible placement in mind and are sealed, no ports. The original was Mr. Bagby's favorite speaker. You will need a sub.

I built a pair of the original design, they are wonderful.

https://techtalk.parts-express.com/forum/speaker-project-gallery/1444247-continuum-build

[billfitzmaurice]

Placement of sealed versus ported is the same, as it doesn't affect boundary sourced cancellations. The usual reason for using sealed mains is that when used with subs they don't need to go below 80Hz, so the added complexity of porting isn't justified, nor is the higher cost that's often associated with woofers/midbasses optimized for ported enclosures and response below 80Hz.

[djg]

I can't find where Jeff Bagby said anything about "flexible placement", but they're still great speakers.