• WANTED: Happy members who like to discuss audio and other topics related to our interest. Desire to learn and share knowledge of science required. There are many reviews of audio hardware and expert members to help answer your questions. Click here to have your audio equipment measured for free!

desk tower 3way WIP.. (BlieSMa t25b + ETON 3-212 + Seas l26roy) + Hypex + PE boxes

OP
H

headshake

Active Member
Joined
Apr 20, 2019
Messages
297
Likes
238
Seems like wider than baffle the noise floor is higher with felt.
Odd, isn't it? My next plan is to take distortion measurements at diff. positions w/ and w/o felt.

If I smooth things and zoom in a bit, the w/ felt SPL's wide and narrow seem to track a bit better. This could be more about the felt shape than the felt.
 

Attachments

  • mmm_close1ft.png
    mmm_close1ft.png
    90.5 KB · Views: 111

bigjacko

Addicted to Fun and Learning
Joined
Sep 18, 2019
Messages
721
Likes
359
Odd, isn't it? My next plan is to take distortion measurements at diff. positions w/ and w/o felt.

If I smooth things and zoom in a bit, the w/ felt SPL's wide and narrow seem to track a bit better. This could be more about the felt shape than the felt.
How wide is the wide? I think the directivity should still be big enough from 500 to 2000Hz, so what makes the SPL lower for wide?
 
OP
H

headshake

Active Member
Joined
Apr 20, 2019
Messages
297
Likes
238
How wide is the wide? I think the directivity should still be big enough from 500 to 2000Hz, so what makes the SPL lower for wide?
Wider was 18" or so. About 5" on either side of the speaker. I think we are seeing 70-85 degree off-axis performance in the wide MMM measurements.

The spl's are lower because it is off-axis and I am very close to the speaker...well, that is my guess. The "outside of the baffle" MMM measurement shows a big drop (the first image on post 19, measurement 3). This is a MMM that never crossed the front of the speaker.

More measurements. I still think I am mostly changing the shape of the baffle adding felt to the top. I attached a comparison of an eq made to be used with 2 layers of felt and what happens as I remove each sheet. SPL's were not calibrated.

I decided I would hate the idea of never exploring a flush-mounted felt baffle so I will order one last round of shapes and then I hope to STFU about felt.




 

Attachments

  • 3msgate_30in_3layer_to_none_50db.png
    3msgate_30in_3layer_to_none_50db.png
    52 KB · Views: 192
  • 2layer_felt.jpg
    2layer_felt.jpg
    466.6 KB · Views: 322

bigjacko

Addicted to Fun and Learning
Joined
Sep 18, 2019
Messages
721
Likes
359
I think we are seeing 70-85 degree off-axis performance in the wide MMM measurements.

The spl's are lower because it is off-axis and I am very close to the speaker
I think the distance difference should be the cause, but why does wide with felt has lower spl than wide without felt in 500 to 2000Hz range?
 
OP
H

headshake

Active Member
Joined
Apr 20, 2019
Messages
297
Likes
238
I think the distance difference should be the cause, but why does wide with felt has lower spl than wide without felt in 500 to 2000Hz range?
I'll re-measure. Maybe a MMM at different distances and widths?

...

I did some mini-woodworking today. The first glued baffle is coming together. I did an "ok" job of gluing it. My rig to keep things flush let me down. All the warts will be hidden by the edge radius so no biggie.

A "chamfering of the driver hole" is something I wanted to pull off without complex tools. I made the driver holes slightly smaller than each other and sanded off the extra edge. The burnt laser edges make for a nice guide to see when you have sanded enough. A small flat file did the last stage of the job to get a nice edge.

http://www.troelsgravesen.dk/chamfer.htm
 

Attachments

  • baffle_startofchamfer.jpg
    baffle_startofchamfer.jpg
    599.5 KB · Views: 159
  • baffle_chamfer.jpg
    baffle_chamfer.jpg
    595.9 KB · Views: 164
Last edited:

bigjacko

Addicted to Fun and Learning
Joined
Sep 18, 2019
Messages
721
Likes
359
Wow is that bamboo? How did you get it? I think bamboo boards are expensive, some people think it has better damping than MDF.
 
OP
H

headshake

Active Member
Joined
Apr 20, 2019
Messages
297
Likes
238
Wow is that bamboo? How did you get it? I think bamboo boards are expensive, some people think it has better damping than MDF.

Yeah, just one layer is bamboo. The big woop is supposed to be bamboo mdf but I cannot find it. The stuff I am using is bamboo plywood; It is made from 3 layers of bamboo veneer with the middle going in the opposite direction.

I had a blurb about it in post 1 of this thread. Check out the first two descriptions of the photos and photo 2 from that. There is a link to a paper on bamboo vs ply for damping.

I almost went with aluminum but decided on the bamboo after reading the paper. The cost of one layer in bamboo was about $11.50USD vs $9.50USD for the MDF. It looks like they are out of the size I used- 6.7mm thick.

This is the service I used: https://www.ponoko.com/materials
 
Last edited:
OP
H

headshake

Active Member
Joined
Apr 20, 2019
Messages
297
Likes
238
Taking my measurement setup to the next level (I hope).

For more accurate off-axis measurements, I hacked the shit out of a part express speaker stand mount ($4.88USD). This will sit on the cheapest speaker stand ($30usd). I like this stand because it can rotate with the safety pin installed.

https://www.parts-express.com/Pyle-PSTND2-Tripod-Speaker-Stand-6-ft.-248-4652?
https://www.parts-express.com/Penn-Elcom-M1538-1-3-8-Speaker-Mounting-Top-Hat-Plastic-245-016?

I will measure to make sure the part that sticks out is not doing much.

Next up is making a label with lines to help me measure in 10-degree increments. This will sit on the pole below the mount.
 

Attachments

  • 245-016_HR_0.default.jpg
    245-016_HR_0.default.jpg
    82.6 KB · Views: 127
  • spkr_mnt.jpg
    spkr_mnt.jpg
    566.7 KB · Views: 127
OP
H

headshake

Active Member
Joined
Apr 20, 2019
Messages
297
Likes
238
https://journals.sagepub.com/doi/full/10.1177/1461348417744307

Found a paper on combining felt with micro-perforated panels (MPP). The math is beyond me, but I did copy the hole size and spacing for another little experiment. I made a new baffle layer with a support layer and a 0.9mm layer of bamboo for the top MPP layer. The felt would sit on top of this. The MPP baffle will have tiny air chambers (the white areas in the "support" image) that I might fill with fiber or melamine. Or maybe just the air gap is enough? It will be neat to see what shows up in the measurements.

Between the MPP and different felt layers, I should have plenty of combinations to test out. I have a 3mm and 6.5 mm air gap for the MPP baffle combinations.

The graph below is from the paper. It is showing what felt alone vs a sandwich of an air gap + MPP + felt.
 

Attachments

  • mpp.jpg
    mpp.jpg
    82.9 KB · Views: 109
  • support.png
    support.png
    25.7 KB · Views: 113
  • baffle_micro.png
    baffle_micro.png
    16.9 KB · Views: 107
OP
H

headshake

Active Member
Joined
Apr 20, 2019
Messages
297
Likes
238
Since I goofed on a new part by having overlapping shapes, I used the delay as a chance to make two more parts. When I do the MPP baffle test run, I will try 2 different hole sizes and spacings- 0.5mm hole/4mm spacing VS 0.75mm hole/3mm spacing. There is also a new layer that builds the airspace under the drivers to get as much space as possible.

Now the waiting.
 

Attachments

  • larger_MPP.png
    larger_MPP.png
    24.5 KB · Views: 87
  • mpp_base.png
    mpp_base.png
    20.9 KB · Views: 84

bigjacko

Addicted to Fun and Learning
Joined
Sep 18, 2019
Messages
721
Likes
359
Nice finding on MPP, it does look interesting, but how are you going to get it? One thing they did was making air gap, I think it should also be effective for felt only too. Felt works best at velocity maximum, right on baffle is pressure maximum, so leaving a gap between baffle and felt will make those felt work more effectively.
 
OP
H

headshake

Active Member
Joined
Apr 20, 2019
Messages
297
Likes
238
Nice finding on MPP, it does look interesting, but how are you going to get it? One thing they did was making air gap, I think it should also be effective for felt only too. Felt works best at velocity maximum, right on baffle is pressure maximum, so leaving a gap between baffle and felt will make those felt work more effectively.
The airgap will only be around 10mm- about 1/2 of what they did in the paper. Since the baffle is made from a bunch of layers, this was just a matter of adding more and removing my current top walnut veneer layer. The white areas of the MPP/support images are the air gaps that will be under the MPP layer.

The "mpp_base" is the first layer that is sitting on the baffle, then the "support layer" to make the airgap larger and surround the drivers, and finally, the MPP layer with the felt on top of that.

image:
white = airgap
black = driver cutout
red = 6.5 mm thick layer of mdf

I imagine the parts will be a bit delicate. The walls are going to be about 3.5mm thick/10mm tall and have a 0.9mm thick MPP top layer.

edit:
I forgot to add this sweet quote from the paper:

The sound absorption mechanism of porous material and MPP is different. Porous material absorbs sound by viscous and thermal losses whereas the mechanism of sound absorption of MPP is of resonant type.

Since the little MPP airgap will be resonant, I almost made it out of layers of cork. Cork should dampen better than mdf. Maybe I'll try that for some other speaker when I can make all of these things larger.
 

Attachments

  • support2.png
    support2.png
    21.3 KB · Views: 77
Last edited:
OP
H

headshake

Active Member
Joined
Apr 20, 2019
Messages
297
Likes
238
Did a little test with some laser scrap to approximate the new MPP parts. The spacing between the parts left a 2mm wide piece that is smaller than what I will use on the MPP baffle. I broke one out and glued it to the edge with some "titebond II". Once that was dry I added some "titebond quick & thick" to seal in the edge. I like that the "quick&thick" stays where you put it and dries clear. I also used the "quick&thick" to seal all of the seams of the PE boxes.

Once all the glues were dry I tried to break off the piece and it would only break in small chunks at a time. I think the new part will be stronger than I hoped. Success!

Ok, back to waiting for the real parts to be made.
 

Attachments

  • mpp_test_dryglue.jpg
    mpp_test_dryglue.jpg
    525.9 KB · Views: 120
  • mpp_test_wetglue.jpg
    mpp_test_wetglue.jpg
    523.4 KB · Views: 108

bigjacko

Addicted to Fun and Learning
Joined
Sep 18, 2019
Messages
721
Likes
359
I think MPP from the paper is a sheet metal with some 0.2 mm holes with 4 mm space in between. It looks hard to make unless you can laser cut sheet metal. Do you have any idea how to get it? Is your laser cutter strong enough for metal?
 
OP
H

headshake

Active Member
Joined
Apr 20, 2019
Messages
297
Likes
238
I think MPP from the paper is a sheet metal with some 0.2 mm holes with 4 mm space in between. It looks hard to make unless you can laser cut sheet metal. Do you have any idea how to get it? Is your laser cutter strong enough for metal?

The hole diameter is 0.5 mm and spacing between the hole centers is kept as 4 mm. The perforation rate is 1.23%. The thickness of MPP is 1 mm.

One of my plates will match the paper, the other test plate will be 0.75mm diameter with 3mm spacing.

Metal sheets are an option. I did 0.9mm bamboo because it costs 1/2 as much as the metal. I figured the thickness and hole size/spacing were more important to match. If one hole size shows better performance than the other maybe I will order the final one in aluminum. This is all just a test so I did not want to dump too much coin if it is a complete waste.

Edit:

I did a real quote rather than guess the cost difference and it was only $6 + resetting my order and delaying things another week. I ordered 1mm aluminum for the two test MPP layers. It will be one less thing to wonder about.

Edit again... for a few bucks more I did stainless steel. I am sure carbon steel is better, but I don't want rust.
 
Last edited:
OP
H

headshake

Active Member
Joined
Apr 20, 2019
Messages
297
Likes
238

Attachments

  • nomogram.jpg
    nomogram.jpg
    218.5 KB · Views: 101
OP
H

headshake

Active Member
Joined
Apr 20, 2019
Messages
297
Likes
238
Moving to metal I have a new limitation: the hole size has to be 1.5X the thickness. So 1.2mm is as small as I can go. Ugh... this was supposed to be easy way back when I was just assuming a bunch of things!

This is what I used to calculate the free space area of a mesh:
https://www.perforated-sheet.com/calculation/how-to-calculate-open-area.html

I confirmed that the numbers match the MPP paper when it comes to the free space calc. I now see with the nomogram how the MPP paper's panel was targeted (grey lines in the attached image).

We have seen that the bandwidth of high absorption is
proportional to the airspace depth, h. If only a restricted depth is
available, and we wish to achieve reasonably high absorption over a
wider band, we must design a number of tuned resonant absorbers
having different resonance frequencies to cover the whole required
frequency range:

Nevertheless, it is common practice to mount sound absorptive
layers directly against a wall, because it is very convenient to do so.
We must, however, realize that, in such cases, only the outer one-third of the thickness of the layer is effective in absorbing sound.
The rest of the material is simply acting as a convenient support!
Therefore, the values of R/rc for the 1/2" layer of material
given in Table 4, and the corresponding values of amax, assume that
this 1/2" layer is mounted near the perforated metal screen with, say,
an inch of empty airspace behind it. so that the entite 1/2" layer is
effective.
If the layer were mounted directly against a hard wall, the
tabulated values of R/rc would have to be multiplied by 1/3, and the
corresponding values of maximum absorption recalculated.

By contrast, with the partitioned back structure, not only does
the resonance frequency remain the same as the angle of incidence
increases, but the bandwidth for high sound absorption actually
becomes broader toward grazing incidence.
Finally, there is the effect of the density of the fibrous material
used to fill the airspace.
If it is too loose, the sound passes right through the material
without being absorbed. But if it is too dense, the sound is reflected
and cannot penetrate the material to be absorbed.
More detailed guidance concerning the trade-offs between
perforation patterns and depth of airspaces, as well as on the choice
of sound absorptive cavity filling, will be presented in PART TWO,
below.

All resonant devices have a preferred frequency of operation. For example, a ball suspended on a rubber band oscillates at only one frequency, when disturbed: that frequency is determined only by the mass of the ball and the springiness of the rubber band. In a resonant sound absorber, the oscillation involves the motion of air particles, in and out of the holes in the metal sheet, in response to an incident sound wave. The preferred frequency of this oscillation is determined by the mass of the air in the perforations and the springiness of the trapped air layer. At that resonance frequency, the air moves violently in and out of the holes, which pumps the air particles back and forth vigorously within the adjacent sound absorptive layer. There, the acoustic energy (carried by the back-and-forth motion of the air particles) is converted by friction into heat and is thereby removed from the acoustical scene. The practical advantage of the tuned resonant sound absorber is this: we have seen (page 11) that it requires a six-inch layer of sound absorptive blanket if we wish to attenuate sound effectively at low frequencies. Yet, as we have noted above, the treatment of a power transformer requires maximum absorption around 120 Hz. The one-inch layer of glass fiber (shown in the earlier figure on page 11) is only about 5% efficient at that frequency. But the use of perforated metal to make a resonant sound absorber especially tuned to 120 Hz can achieve efficient sound absorption at that frequency without requiring so much space and with only a thin layer of absorptive material.

from: http://www.iperf.org/files/1313/9265/8912/The_Acoustical_Uses_for_Perforated_Metals_Handbook.pdf

This has all of the goods. It answered a bunch of questions and saved me a ton of guessing.

I am now going to have patterns for different target frequencies and smaller compartments. Also 1/2 air and 1/2 damping in each compartment.

The idea from the MPP paper is to use the metal to target a frequency outside of the ideal range of the felt. I hope to do that but target a few different frequencies. Most of the lines are for the 1.2mm hole with different amounts of open area to change the target range. The green line uses a 3.4mm hole.
 

Attachments

  • nomogram_range.jpg
    nomogram_range.jpg
    263.7 KB · Views: 71
Last edited:
OP
H

headshake

Active Member
Joined
Apr 20, 2019
Messages
297
Likes
238
I got the new baffle done and it looks like something I could have just eyeballed. They are spaced according to the nomogram. Do I expect a little panel the size of a couple of grapes to do much? No. But then again, I did not think the felt would do much. I just want to hear it, measure it, and call it a road walked. I hope to re-apply what I learn to something larger.

The MPP + felt paper tuned things to 1000hz so I matched that with one baffle. The paper mentions how things will perform lower with the felt on top- their peak was at 800hz even though it was tuned for 1000hz. So my mixed range starts at 2000hz and shifts down.

It will be 0.8mm stainless steel for the top and MDF for the support.
 

Attachments

  • MPP_1_2mm__0_78percent_1000hz.png
    MPP_1_2mm__0_78percent_1000hz.png
    26.3 KB · Views: 141
  • MPP_multi_w.png
    MPP_multi_w.png
    27.6 KB · Views: 146
  • MPP_multi_1_2_and1_5_mm_nomogram.jpg
    MPP_multi_1_2_and1_5_mm_nomogram.jpg
    200.6 KB · Views: 127
  • baffle_mpp_support_fixed.png
    baffle_mpp_support_fixed.png
    29.1 KB · Views: 132
Last edited:

bigjacko

Addicted to Fun and Learning
Joined
Sep 18, 2019
Messages
721
Likes
359
I got the new baffle done and it looks like something I could have just eyeballed. They are spaced according to the nomogram. Do I expect a little panel the size of a couple of grapes to do much? No. But then again, I did not think the felt would do much. I just want to hear it, measure it, and call it a road walked. I hope to re-apply what I learn to something larger.

The MPP + felt paper tuned things to 1000hz so I matched that with one baffle. The paper mentions how things will perform lower with the felt on top- their peak was at 800hz even though it was tuned for 1000hz. So my mixed range starts at 2000hz and shifts down.

It will be 0.8mm stainless steel for the top and MDF for the support.
Do you have idea on what frequency can be attenuate by felt only and what frequency need to use MPP? I am thinking is whether 2000Hz a good number there or we can go lower and leave the higher frequency to felt.

Splitting the baffle can attenuate multiple frequencies, but does it make the effect lower because you have less area per frequency range?
 
Top Bottom