How easily does air need to flow within the volume of the speaker cabinet?

[Wolf]

Not sure it is relevant here, but Wolf recently made an aperiodic sealed cabinet where half the volume was restricted from the other half. It did make a difference in the impedance so it can theoretically matter.

Sounded different too!
Bar-Gain build thread will show the results.
Thanks for the notice and reference to my ice cream sandwich loading.

Later,
Wolf

[Steve Lee]

No, this is simply twice the surface area for a pressure wave to travel through in parallel. No different than having two or more ports in a ported system. There is no cancellation of any kind with two ports, either. They are both in phase and add due to superposition.



Recall that in a ported system you tune the system by selecting the port(s) diameter and length required to provide the necessary mass movement (inertia) to resonate at the desired Fp. It's the mass movement that tunes the port. The port air is not static in a compression wave. The port is a second order air mass/box compliance system.

You do the same thing for a passive radiator, only the surface area is much larger than that of a port, so the physical displacement is much small for the same volume displacement. That is air movement, in both port and PR. Air movement in the gap between pipe and box wall creates...a rectangular port. It has compliance on both sides, just as it is for a standard port. The compliance in this case is much lower in the small top chamber. They both contribute to the "tuning".

That's why I'm interested in seeing some impedance measurements, to quantify the affect.

dlr

After reading and absorbing your response more thoroughly - I really think we are saying very similar things and are probably in agreement yet my understanding of the terms differ from your more learn-ed uses.

However I'd like to quantify the test enclosure's internal volume and not just arbitrarily remove and then re-install the mid-range speaker tube.

The tube needs to be installed in all tests but restricted differently at the side-wall junctions such that one side closed can be compared to both sides open for a woofer impedance check. Additionally, the port frequency response should be recorded in both cases as well as the near field woofer response.

Cool?

[Liberator of Magic Smoke]

Sounds like a plan. I would probably make the test box a bit wider, say an inch, then "shim" the box to the same volume in different ways. That way you could see what would happen with a considerably wider space between the tube and the side of the box. Filling in the top and bottom with temporary pieces of MDF or whatever would get you back to the same volume, which should not affect the port or anything. Then you could take those pieces out and re-test with the sides filled in to get the configuration of the original design. Lots of possibilities.

Interesting question and interesting thread, by the way. I'm always learning things I didn't know I didn't know from threads like this.

[a4eaudio]

Yes, this is almost exactly what I'm going to do. And the shims yet to be used will just sit in the bottom of the cabinet until moved up to the "shimmed position" between the tunnel and wall, keeping the volume exactly the same for each variation.

[Steve Lee]

Excellent and thanks for taking this-on as an experiment for our mutual edification, mang!

[Steve Lee]

I suspect the delay time between perceived SONIC PRESSURE WAVE and AIR FLOW SMELL will be recorded in minutes to hours rather than milli-seconds.

[a4eaudio]

I created a test box similar to what my design will be but constrained to be made out of a single 2' x 4' sheet of 1/2" MDF. Woofer is a Tang Band W6-1139SIF. Test box is 0.9 cu. ft. tuned to 32Hz. Driver is not flush mounted, but I did chamfer behind it, used gasket tape, and put a small amount of foam on the interior walls.

• Interior depth is 13.5" which is close to the actual speaker.
• Because I needed to make the test box wider (10.5" compared to the actual 8") I moved the tunnel up so that approximately the same amount of volume ) about 0.20 cf above the tunnel in the test box as in the actual speaker.
• I added some blocks as "shelves" in the corners below the tunnel where I can drop in my shims (red rectangles in Pic 1) and a removable top for easy access.

Test 0: Original setup starts with 1 1/2" between the cabinet wall and the tunnel, thus a gap of 1 1/2" x 13 1/2" on each side of tunnel. (Actual gap was closer to 1 9/16 because I wanted to ensure my last shim would fit.)

Each shim was added one at a time, clamped with Titebond wood glue for 45 minutes and impedance sweep using DATS. Shims (Pic 2) were 1/2" (red), 1/2" (red), 1/4" (blue), 1/8" (yellow), partial shims of 1/8" (yellow), and then some gasket tape to almost fully fill in the gap (Pic 3, small yellow rectangle of about 1/8" x 2" was the gap).

[Note, while I intended to keep the shims in the cabinet when not used to keep the net volume constant I forgot to do this. Thus, Test 1 lowered the box volume 0.014 cf; Test 2 lowered the box volume another 0.014 cf; Test 3 lowered the box volume 0.007 cf; Test 4 lowered the box volume 0.0035 cf. Thus, not only was there not much difference due to the gap volume, the impedance sweeps are overstating this because there were small changes in box volume and thus tuning.]

[a4eaudio]

Test 1: Insert a 1/2" MDF shim; gap is 1" x 13 1/2" on each side of tunnel.
Test 2: Add a 1/2" shim; gap is 1/2" x 13 1/2"
Test 3: Add a 1/4" shim; gap is 1/4" x 13 1/2"
Test 4: Add a 1/8" shim; gap is 1/8" x 13 1/2" (This is the closest to the actual speaker cabinet)
Test 5: Add two partial 1/8" shims to each side
Test 6: Add a strip of speaker gasket tape in the entire gap and two partial strips of tape leaving only about a 1/8" x 2" gap on each side.

Figure 1 shows impedance sweeps for Tests 0 to 5 which don't show much variation at all.
Figure 2 shows impedance sweeps for Tests 0, 5, and 6 to illustrate that the much greater restriction in Test 6 make a little more of a difference, but still not a lot.

[dlr]

One thing I'm curious about. What does the impedance look like if you totally block the gaps? I'm curious because if there is so little change with the gap almost filled, is this showing that a tiny gap is sufficient or that even the larger gaps are having more influence than anticipated?

In this case, overlay of fully open gap and fully closed gap.

dlr

[a4eaudio]

According to my calculations my original box with no shims (Test 0) is approximately 0.9 cf tuned to 32Hz. The red impedance curve (Test 0) supports that. Even leaving just the small gaps did not seem to make a huge difference. Test 6 from before is the blue impedance curve.

I filled that gap with gasket tape. Then placed green painters tape across the whole gap to try to ensure it is completely sealed off. My rough estimate is that this leaves a net volume of about 0.67 cf. Since my port is fixed I played around in WinISD and keeping my vent at 7.75" says it would be tuned to 37Hz. The orange impedance curve (Test 7) is pretty close, suggesting 39Hz.

The minimum impedance at the tuning frequency has increased from 6 ohms to 8 ohms and the 2nd peak has dropped about 6 ohms from about 27 to 21. Does that make since if it is basically reducing the box volume 0.2 cf and increasing the tuning frequency?

[wogg]

Sweet! Even the tiny gap seems to open up the box volume effectively. With the impedance spikes, I'd expect them to equalize in amplitude as you raise tuning until the lower spike passes it, maybe somewhere in the mid 50s. That means very little acoustically though, as that would probably be pretty boomy in the 60Hz+ range.

Really cool experiment!

I would stuff the top in your speaker though, for no reason really. Give it a cool trademark audiophile technique name for fun.

[Liberator of Magic Smoke]

Wow, that's interesting. My guess was that the narrow gap would make more difference than that, but it was just a guess.

[Steve Lee]

Thanks for doing this man!

I know it was a lot of work/effort but is an eye opener for the mind.

Best!


Steve.

[a4eaudio]

Wow, that's interesting. My guess was that the narrow gap would make more difference than that, but it was just a guess.

Yes, that was my guess too. I just did the test with the really small gaps it because I thought since I wasn't getting much difference I would go extreme and see what it did expecting more.

I would stuff the top in your speaker though, for no reason really...

That is exactly what I was thinking, including the "no reason really" part!

[a4eaudio]

I will redo all my tests filling the box with molasses this weekend!

[Steve Lee]

Good thing the weather is getting warmer!

[Wolf]

Can only do molasses testing in January with slow sweeps. It's Feb, so it's too late to work accurately.
Wolf

[dlr]

Wow, that's interesting. My guess was that the narrow gap would make more difference than that, but it was just a guess.

My thoughts as well.

dlr

[billfitzmaurice]