Modded a Loudspeaker a decade ago - I wonder what has theoretically changed

[Herbert]

Remember this one?

3A - Art & Acoustique Appliquée • Catalogue

Site consacré aux enceintes acoustiques du fabricant français 3A - Art & Acoustique Appliquée.

3a-art-et-acoustique-appliquee.fr

I bought them in 1990.
They used to be from France, their successors are still built in Canada:

REFERENCE 3A

reference3a.com

Anyway, because the cabinet used to scratch easily and also vibrated, I decided to glue the whole box
-with it's enclosure-into a new enclosure made of glued timber - hope this is the correct word.
This must have been about 15 years ago.
I remember "painting" alone took 6 weeks as it was not paint
but a special wood oil that hardens like varnish - but takes 2 days to do so for each layer.

I love the sound - warm, detailed - but ASR shows us that this does not mean fidelity at all.
And the sound must have changed because of the mod: The front face became bigger,
as the glued timber adds 40mm on each side. The front itself became 5mm thicker
as I used veneer and felt. The bass port became 40mm longer.
And there is an integrated stand.
Any ideas how that theoretically colorizes the sound?
Best, Herbert

 

[sergeauckland]

Many things will have changed, but whether any of them by enough to notice, is moot.

1) Making the cabinet thicker and heavier will change the panel resonances. It should reduce the amplitude and the frequency of any resonances, but increase the energy storage and hence reduce the Q of any resonances, so overall, I expect a benefit.
2) Making the front baffle a little (5mm) thicker but using felt I expect to have reduced diffraction along the front, so again overall beneficial. If the tweeter is now inset by 5mm, I don't expect that to have a significant effect.
3) lengthening the port by 40mm could be the one change that's not necessarily a Good Thing. This will change the bass tuning by reducing the tuning frequency. By how much depends on the diameter and original length, so can't say whether that's by enough to make a significant difference, but it's unlikely to be an improvement.

S

 

[Killingbeans]

The bass port became 40mm longer.

Out of the blue? Not based on any calculations?

 

[Katji]

^^Just the 40mm thicker panel. ...40mm is thick, like a scaffolding plank.

 

[Killingbeans]

Ahh yes, I see it now. I didn't notice that he had added 40mm to the panels. That's a thick boy!

 

[Katji]

I examined the pic then re-read post #1. ...So...I suppose it was not done with a router, and making a big wide flare would be too difficult with hand tools.

...Mass increased probably about 80%

 

[DVDdoug]

I agree the port change will be the only thing significant. There is software for designing/predicting woofer/speaker performance based on volume, port dimensions, and driver characteristics (Thiele-Small Parameters). WinISD is free.

But you probably won't find the Thiele-Small parameters for your woofer. They can be measured and there is a gizmo ($130 USD) that's supposed to make it easy. Otherwise, from what I remember it requires an oscilloscope. You'd probably have to remove the woofer from the box for testing.

I love the sound - warm, detailed - but ASR shows us that this does not mean fidelity at all.

These things can be "real" but the problem is, these words don't have an agreed-up scientific-engineering meanings and they can't be quantified. We can't say, "The speaker is 3dB warmer after the modifications."

"Audiophiles" often use flowery non-scientific terminology that invokes a feeling or impression without accurately describing the sound.

 

[Herbert]

3) lengthening the port by 40mm could be the one change that's not necessarily a Good Thing. This will change the bass tuning by reducing the tuning frequency. By how much depends on the diameter and original length, so can't say whether that's by enough to make a significant difference, but it's unlikely to be an improvement.

Thanks a lot for the replies! Well, I did "lengthen" the bass-port but tried to minimize the effect by sanding a fillet / flare around the hole.

2) Making the front baffle a little (5mm) thicker but using felt I expect to have reduced diffraction along the front, so again overall beneficial. If the tweeter is now inset by 5mm, I don't expect that to have a significant effect.

Well, there was a mishap. When you compare the mid/woofer of the original Midi-Master with mine, you will note a foam ring around the rubber corrugation/suspension. I assume it was for reducing diffraction. This foam-ring is missing in the mod. Not really missing - it sucked up the 2k-varnish I had to use on the frame of the woofer until it disappeared. ...

 

[alex-z]

Your port change was likely negative, unless the manufacturer dropped the ball when selecting the original port tune. I would strongly recommend doing an impedance sweep of the driver in free air with no crossover to get the T/S parameters. That will give you enough information to check what the appropriate air volume and port tune is for that woofer.

The thicker cabinet walls likely reduced overall resonance, which is good.

The grill detracts a bit from the sound. All grills introduce some degree of high frequency diffraction, even if you carefully round the edges.

Speaking of rounded edges, missed opportunity to make them bigger on the cabinet itself. Going for 1 inch or more makes a significant decrease in edge diffraction. The felt is borderline useless at preventing diffraction, because it is caused by sound waves detaching at the edges, not as they travel across the baffle itself.

Did you make any internal changes, like a crossover tweak to account for the increased baffle size?

 

[Herbert]

Did you make any internal changes, like a crossover tweak to account for the increased baffle size?

As I wrote, I simply glued the complete box into a new enclosure.
The bass port became longer,
but the extension consist of a fillet with a radius of about 20-25mm.
So I think the extension might be shorter than 40mm.
As the port had a sharp edge before I wonder if it had produced more noise.
Can't get around the grille. The mid/woofer was custom made so I need to
protect its suspension from UV light...

 

[puppet]

Quick and dirty method to find new port tuning would be to lay loudspeaker down and elevate it up so the port isn't blocked. Lay some uncooked rice on the woofer cone and run some low frequency tones. Rice will stop dancing on the cone around the port tuning frequency.

 

[Herbert]

This is either brilliant or a joke. There is just one problem: They soaked the cones with some kind of sticky stuff...

 

[sergeauckland]

This is either brilliant or a joke. There is just one problem: They soaked the cones with some kind of sticky stuff...

No joke, as at the port tuning frequency, virtually all the air movement comes from the port and very little from the cone. If the cones are sticky, then you may have a problem, but you may just be able to tell visually when the cone excursion is at a minimum.

S.

 

[dasdoing]

Quick and dirty method to find new port tuning would be to lay loudspeaker down and elevate it up so the port isn't blocked. Lay some uncooked rice on the woofer cone and run some low frequency tones. Rice will stop dancing on the cone around the port tuning frequency.

so why you need rice? you can feel it with your fingertips

 

[puppet]

It's just easier using something you can see.