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4Pi - alternate compression driver mounting? [message #94478] Tue, 21 September 2021 13:16 Go to next message
areohbe is currently offline  areohbe
Messages: 6
Registered: May 2021
Esquire
Disclaimer: My first speaker build was a -- somewhat surprising -- massive success. I mainly followed the plans and didn't invest much time into actually understanding the physics/technical aspects of speaker design. I am planning to change with my upcoming 4Pi build (just placed crossover/waveguide order Very Happy ). But, sorry in advance if this is is a totally non-sensical question...

Would there be significant/any drawbacks to modifying the 4Pi plans such that the compression driver was mounted, exposed, on top of the cabinet? It's purely an aesthetic preference. I am willing to make some degree of sonic sacrifice, ideally it wouldn't be noticeable to the average listener.
Re: 4Pi - alternate compression driver mounting? [message #94479 is a reply to message #94478] Wed, 22 September 2021 08:57 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18167
Registered: January 2001
Illuminati (33rd Degree)

That's actually been a pretty popular mod around here. The main concerns are keeping the geometry the same between the midwoofer and the waveguide and keeping the geometry the same between the midwoofer and the cabinet walls and the port.

The easiest way to do this is to turn the cabinet upside-down, with the woofer high in the cabinet and rest the waveguide in a cradle sitting on top of the cabinet.

This ensures the polar response remains good because the woofer and tweeter positions are the same in relation to each other. And it ensures the internal standing wave alignment is the same because the woofer and port are each in the same place in the box.
Re: 4Pi - alternate compression driver mounting? [message #94480 is a reply to message #94478] Wed, 22 September 2021 13:11 Go to previous messageGo to next message
areohbe is currently offline  areohbe
Messages: 6
Registered: May 2021
Esquire
Thanks, Wayne! I tried digging around the forum for build threads with a similar approach, but didn't have any luck. Do you happen to know of one so I can spare you a bunch of follow up questions?
Re: 4Pi - alternate compression driver mounting? [message #94481 is a reply to message #94480] Wed, 22 September 2021 14:53 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18167
Registered: January 2001
Illuminati (33rd Degree)

I don't think there are any build threads from people that used that approach. But I've seen a lot of photos of loudspeakers made this way, so you might search again. Hard to search for photos, I guess, just gotta scan the internet for them.

Some people put the waveguide in a box and mount it traditionally, setting the tweeter box on top of the woofer cabinet. Others have the waveguide sitting on top, bare. Many people paint the waveguide.

When sitting the waveguide bare on top, the biggest challenge is what to do with the waveguide mounting holes. I would suggest using a filler material - like is commonly used in auto body repair shops - to fill the holes. Then lightly sand the front face so the filler blends and to remove any mold lines. Paint the waveguides with enamel and set 'em on a cradle. I use the cradle approach when I mount wood horns, and I think it looks really nice. See one of my tweeter cradles, below:

http://PiSpeakers.com/misc/Tweeter_Cradle.jpg
Tweeter Cradle

One last thing - I mentioned above that the woofer/tweeter geometry should be the same but you can center the tweeter. I assumed you knew that but wasn't sure, so I thought it best to clarify. It's just important that the waveguide face be flush with the midwoofer face and that the two are as close together as possible. That's why most people turn the cabinet upside-down - to make it closer to the tweeter.
Re: 4Pi - alternate compression driver mounting? [message #94485 is a reply to message #94481] Wed, 22 September 2021 22:53 Go to previous messageGo to next message
areohbe is currently offline  areohbe
Messages: 6
Registered: May 2021
Esquire
That looks lovely.

Turning the cabinet upside down makes sense. That's what I was planning on. Auto body filler is a great call!

I'd like to modify the port dimensions, also for aesthetic reasons. This is a bit over my head, but sounds like I'll need to consider standing wave issues. Think I will start with an MDF front baffle to take some acoustic measurements and go from there.
Re: 4Pi - alternate compression driver mounting? [message #94486 is a reply to message #94485] Thu, 23 September 2021 02:03 Go to previous messageGo to next message
areohbe is currently offline  areohbe
Messages: 6
Registered: May 2021
Esquire
Here's my working design:

  • Shortened overall height by 1", depth & width not changed
  • Port is now 8.75"x2.4" w/ a depth of 3"
This result is a cabinet volume of 2.79 cubic feet, equivalent to the original design. I'm sure there are potential drawbacks with this approach but doubt my ear could pick it up. Getting excited to begin the build!

/forum/index.php?t=getfile&id=2854&private=0
Re: 4Pi - alternate compression driver mounting? [message #94491 is a reply to message #94486] Thu, 23 September 2021 07:59 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18167
Registered: January 2001
Illuminati (33rd Degree)

That looks great, so my hat's off to you for the aesthetics. But you're right about the potential internal standing wave issues. The link you included in your post above describes them, so you know what to look for. Pay attention in particular to the region between 200Hz and 400Hz, and even up to 800Hz.
Re: 4Pi - alternate compression driver mounting? [message #94769 is a reply to message #94479] Wed, 10 November 2021 11:35 Go to previous messageGo to next message
alexg is currently offline  alexg
Messages: 10
Registered: June 2019
Chancellor
Wayne,

Top mounted wave guide looks interesting.

After flipping box upside down, does the woofer stay in place per plan or should the the woofer be moved higher up in the box to maintain the same spacing as called for in the plans?

Thanks
Re: 4Pi - alternate compression driver mounting? [message #94770 is a reply to message #94769] Wed, 10 November 2021 16:25 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18167
Registered: January 2001
Illuminati (33rd Degree)

There's the rub.

When flipping the box over, the waveguide and midwoofer are placed closer together than they would be if the woofer remained low in the box with the waveguide perched on top. So flipping the box over brings 'em closer together, pretty much the same distance apart as they are in the stock design.

I say "pretty much the same distance apart" but they're not exactly the same distance. Not that we need to split hairs about tenths of an inch or anything like that, but the difference is more than that - they're about an inch further apart.

That's not horrible, but it will draw the vertical nulls closer together making the forward lobe smaller. It's not what we want.

On the other hand, the forward lobe is pretty large and the nulls are spaced far enough apart we don't have to push it. That's why the stock design has a gap between the midwoofer and tweeter. If we needed, we could brings them closer together to get more distance between vertical nulls. But we really don't need that - the nulls are widely spaced.

Then we also have to consider the internal standing waves. If we move the midwoofer or the port, the standing waves will align differently inside the cabinet. So we cannot know for sure if modifications will create midrange anomalies without measuring the cabinet.

I moved stuff around for testing way back when I originally designed this cabinet, and I tried a lot of configurations both with mathematical models and then ultimately with measurements of physical loudspeakers. But that was a long time ago and I cannot remember what movements were OK and what weren't. I know this particular configuration measured well.

So for the person considering this mod, those are things to consider.
Re: 4Pi - alternate compression driver mounting? [message #94807 is a reply to message #94770] Sat, 13 November 2021 11:03 Go to previous messageGo to previous message
alexg is currently offline  alexg
Messages: 10
Registered: June 2019
Chancellor
Wayne,

Thanks for the info.

Top mounting sounds / looks cool but probably not worth messing with your design.
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