Sealed test enclosure

[collo]

Inside SPL vs Outside SPL for a spacing of 500mm - (unsmoothed graphs)



Red is the original measurement with the SPL meter through the baffle measuring the resonances inside the sub.

Blue is measured outside the sub, 100mm (4 inches) from the driver - couldn't get the meter any closer due to the way the driver was mounted.

Gold is measured outside the sub, 1 metre from driver.

The 1m measurement is basically the same as the close miked result apart from picking up some extra environmental reflections.


To get a clearer view, here's just the first two, with 1/3 octave smoothing applied.....



- No sign of a quarter-wave resonance around 170hz.
The first peak is the 1/2 wave resonance around 340hz
The second peak is the ( 1/2 * 2 ) wave resonance around 680hz
The third peak is the ( 1/2 * 3 ) wave resonance around 1020hz

- The "dip followed by peak" is presumably due to phase differences as the sweep moves through each of the resonances. The resonance initially interferes with the driver, and as the frequency is raised, comes into reinforcement.

- The intensity has dropped from a 20dB peak inside the box, to a +/- 7dB ripple for the fundamental, and each successive harmonic diminishing by half.
Naturally the details would depend on the implimentation, but the principle is there....

[avaserfi]

Interesting results. I still wonder about something to do with perhaps pipe harmonics preventing any quarter-wave anomaly from showing. This anomaly can clearly be seen on Ricci's test box as well as some other measurements I have seen on undamped systems.

Sadly, my understanding of these physics is not strong enough. Due to this I have invited an expert on the subject to the discussion, Martin J. King. Hopefully he joins us!

[Ricci]

Interesting. I'm not very strong on physics and wave propagation either, but I am surprised that there is no evidence of a 1/4 wave effect at all. It would make sense that the 170hz anomoly in my box was a half wave resonance from the longest dimension from baffle to back wall but it should be slightly higher in frequency. To correspond with the 880mm distance. I suspect that I may have something higher up around 300-320hz like was suggested also due to the side to side resonance which would have 4 walls instead of 2 interacting, but I don't remember seeing anything. I'll have to look again.

[collo]

avaserfi wrote:

...I still wonder about something to do with perhaps pipe harmonics preventing any quarter-wave anomaly from showing. This anomaly can clearly be seen on Ricci's test box ....

.....I have invited an expert on the subject to the discussion, Martin J. King...

The 170hz on Ricci's box is certainly something. I'm just not convinced that it's quarter-wave. The front-to-back half-wave is the most likely culprit, but the round trip path length is too short by around 160mm.

With the magnet occupying most of the central hole in the brace, I wonder if there's a path that goes through one of the smaller holes in the brace and returns via another.

I'm just fishing here. Truly, an expert pair of eyes on this would be great!

[thxgoon]

avaserfi wrote:

Interesting results. I still wonder about something to do with perhaps pipe harmonics preventing any quarter-wave anomaly from showing. This anomaly can clearly be seen on Ricci's test box as well as some other measurements I have seen on undamped systems.

I find this interesting as well. I wonder if the measurement technique only picks up harmonics as if it were in a closed-open pipe. But I'm not sure why this would happen. Have a look at this article I found.

[avaserfi]

A big thanks to Martin King for his help and analysis. He just got back to me .

The only time a quarter-wave cancellation will occur is in a system that is closed on one end and open on another. In a closed system such as the one Collo tested, there should only be half-wave resonances as found.

In explanation of Ricci's resonant dip/peak he said that it is likely a half-wave resonance on the 36.75" axis. To explain for the discrepancy between the found anomaly and actual half-wave node King said that it was likely a small air leak and/or cone movement causing this movement.

Glad we got this cleared up. Looks like your results were dead on Collo. Thanks!

Time to go brush up on my physics .

[collo]

That's good news - I can go ahead and fix boxnotes and sonosub.exe with confidence.

Thanks to all involved. I just love the internet.. if you're not scared of a bit of criticism, you can get to the truth of just about anything fairly quickly.

Thxgoon, thanks for that link. I've been looking for some decent drawings and animations of standing waves for ages. The ones on that site are perfect!

[Ricci]

Cool. I feel like I learned a bit here..

Andrew,

To clarify...Mr. King was saying that the reason the 170hz blip doesn't quite correspond with the internal box length (it should be slightly higher in frequency) could be due to an air leak or the cone movement? This could cause it to be slightly lower in freq? That's a new one for me...

[avaserfi]

Ricci wrote:

Cool. I feel like I learned a bit here..

Jordan,
To clarify...Mr. King was saying that the reason the 170hz blip doesn't quite correspond with the internal box length (it should be slightly higher in frequency) could be due to an air leak or the cone movement? This could cause it to be slightly lower in freq? That's a new one for me...

Pretty sure you are asking me this.

That is what he speculated as the potential cause for the lowered resonance. Always learning, a great part of this hobby!

[Ricci]

Yes. Apparently it was too late for me to be posting as I couldn't even keep my names straight!

On a sad note I'd like to have a moment of silence for my poor SRM sound card. It has gone to the big electronics guru in the sky. I fried the input on it. I made a stupid mistake trying to measure the FR of a CE4000. A new one is on the way.

[Ricci]

I finally started on the first pair of sealed boxes awhile ago. It's slow going. I only have 14 clamps and very little time to devote. The basic dims are 24Lx24Wx36D. About 9 cu ft internal after displacements. I'm doing a few things with these that are going to be out of the norm. They'll have removable baffles like I've done on my last 2 boxes, removable back panels and probably some removable bracing. I was only going to do that on one of the 4 enclosures but it's almost easier to build all of them the exact same. I'm sort of modifying things as I go. I'll be able to configure the enclosures to use different drivers, ports, PR's, dual opposed drivers, etc for personal enlightenment. I don't want to have to build a new box, or chop one up when I want to try something out. Finish will be truck bed liner. Rugged and simple since these will be abused I'm sure. The basic idea is I make a bunch of baffles and just swap those out for whatever I want to do. I should be able to churn out a new baffle for a new driver or whatever in less than 2 hrs and cheaply.

[Ricci]

More progress...Sloooooow...

[collo]

They certainly look solid!

[Ricci]

It's getting there. I have a lot more bracing left to add, which I'm sort of modifying as I go and can only clamp so much at one time. I'm working on the driver baffles currently. Still a lot left to do.

[collo]

I think I have clamp envy ...

[Ricci]

More progress is being made. I'll post some pics up tonight. I've got the baffles and backs done and mounted up. I'll be putting a 1/2" roundover on the cabs tonight. From there I have to seal up the inside, install the weatherstripping, install lining, put the handles on, install the speakons and figure out what I'm going to do for bracing the back panel. It must be removable to match the back panel. I've got an idea but until I actually try it... After all of that gets done it'll be time to start on the second pair of cabs. I should get done with those about this time next year. At this point I've got about 5years worth of projects I want to do.

[Ricci]

Updates...

[Mike P.]

Looking good! What subs are you planning on trying out?

[Ricci]

These are going to end up housing XXX 18's long term. One sealed in each enclosure. A total of 4 of em powered off of 2 QSC PL9.0's. It should be about 3250w per driver. My 220v 50A line for those is being run on Friday. I've had all of the drivers and the 2 amps for almost a year now. It'll probably take me another year to get the whole thing up and running.

However I plan on using these to do some port, stuffing, lining, bracing, dual opposed and driver tests as well. I should be able to clear up to a 21" driver past the baffle mounting bolts. A 22" may take some ingenuity if I ever wanted to do that. 9 cu ft net should be enough space to test most 15-21" drivers sealed, or most 12-15's ported.

[spanish68]

Good job! What size are those wood braces? 2" x 2"?

[Ricci]

Yep standard 2x2 which is actually more like 1.5 x 1.5.

[Ricci]

Further progress. The first 2 enclosures are done and I've started on the second set. They should go a lot quicker because I know what I'm doing this time. I ended up using roll on Duplicolor truck bed liner on them. I tried to use the spray on stuff but it just wasn't working and would've cost a fortune. I've also got the 220v lines for the amplifiers installed.

[Mike P.]

The cabinets are looking good and those XXX's always look impressive. Teaching the little guy from a young age, eh?

[sub_junkie]

What's so special about the XXX 18 besides the 54mm Xmax, high power handling, and the 15.60hz resonance frequency? Does it have really low distortion like the LMS 18's? I just don't really see why it's worth $1500+ at some places. Is it really that good?

By the way, nice looking boxes. Looking foward to some results

[Ricci]

You sort of answered your own question Dylan. Ruggedness, power handling, low end extension without EQ, low distortion, uncompressed deep bass dynamics, etc. I got mine for $700-$850.