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Thanks Revv! Ya, it's a riot alright, especially with the Traynor driving it. More sound power than I'll ever need, except for bragging rights

So, I promised you guys some details on the post-jam test-&-measurement, and the surprise I received. I'll cover that first. If you just want to look at the pictures, scroll down - lots of construction pics and shots of the interior - like this one:



Test & Tune

All the experts and all the literature say that "ported cabinets are more susceptible to misalignment..." What they mean is, it's easy to get a ported box wrong, and have it sound like that $80 "Bass Tube" you can get from Canadian Tire, for your automatic Cavalier with the UFO lights on the bottom. So the important final step in building a ported speaker is to check and adjust the port tuning. So that's what I did after the jam.

If the ports are too long, it's not the end of the world; your bass response will just be a little weaker than you'd hoped. If the ports are too short, however, you'll have a big boomy peak in response somewhere around the mid-bass. If you're into SPL racing in your highschool parking lot, this is good. If you like music, this is bad.

The way you test port tuning is by playing a test tone, and measuring the impedence of the speaker while you vary the frequency. Remember, at some frequencies the cone is doing all the work, while at other frequencies almost all of the sound is coming from the ports. At the resonant frequency of the ported cab, the cone is moving the least, and there is a minimum in the impedence curve. This is the tuned frequency of your alignment. See graph, below.

Remember that my target box frequency was 34Hz, to match the free-air resonant frequency of the driver. I hooked up the test apparatus (this time using my home-theatre amp as a source, instead of my SS bass amp), and plotted the curve. My actual tuned frequency...

28 Hz !??



How did I get it so far wrong!? It's "only" 6Hz from the target frequency, but that's over a quarter of an octave. The ports must be too long. Let's see, check the design, they're supposed to be.... 7.5". Get the tape measure out...

Uhmmm...

Well, you look at the photo above and tell me if that looks like 7.5"!

Those bazookas are 10.75" long each!

Yeah. Turns out, I cut the straight tube sections to 7.5" long each. Then I attached the port flares at each end, adding over 3" to their overall length. Man, I'm glad I didn't use too much glue putting those babies together!

So besides the slip-up, we do have a new data-point here. With two ports of 3.91" inner-diameter and 10.75" long, the box frequency is 28Hz. Plugging that back into WinISD Pro and solving for interior volume, we get... 6.1 cubic feet (slightly bigger than the design target of 5.6 cu.ft.). This is not totally unexpected. The stuffing in the box serves to increase the effective accoustic volume, which is something I didn't bother to account for when drawing up the plans (because bigger is always better!). This is a volume increase of about 10%, which is consistent with a couple of inches of stuffing lining 5 walls of the box (if the box was 100% full, it's accoustic volume would be +40%, according to theory).

So, given a 6.1 cu.ft. box, WinISD Pro says the ports need to be 6.28" long for a 34Hz tuning. Time to start cutting.


I know I promised more photos, but it's my bedtime and I need my beauty sleep. To make a long story short, I spent last weekend in the garage. The cab is re-born with shorter ports, glued and caulked joints, some other improvements, and it's heavier than ever before!

-Bionic

_________________
Four strings is enough.

Okay it took me a week but i'm finally back with the rest of the photos...

First, a quick review of how I measured the impedence of the speaker:


The laptop's headphone output jack is connected to the auxilliary input jack of the home stereo amp (I kept the laptop volume low, because headphone jack output is not exactly line-level). The speaker output of the stereo amp is connected to the speaker, with a resistor Rm in series on one lead. For Rm (measurement resistor), I used a 10-Ohm, 1-Watt resistor.

The laptop is running WinISD Pro, which includes a signal generator utility. For each data point, I set the output frequency (tone) on the laptop, measured the stereo output voltage across points A-C, then measured the voltage drop across Rm, A-B. I entered these two voltages into two columns on a spreadsheet. The third column used Ohm's Law to calculate the impedence of the speaker at that frequency. I had to re-measure the stereo output voltage again for each data point, as the laptop-stereo combination was not linear throughout the frequency range of interest (not surprising at all).



Okay, so the result of all that work was to determine that my ports were way too long. Besides some tube-cutting, I also still needed to take the box apart and glue the joints, seal them with caulk, build a brace, and some other minor clean-up. Here are the pics...


By the way, here's the back panel and the bolts that hold it on. This way, I can re-open the cab when repairs or changes are necessary - which I'm anticipating will be a lot. I know, that joint doesn't look too tight; don't worry, the battens around the back panel have weather-stripping on them (see photo, previous post). All other joints will be glued and screwed. And caulked. You can also admire the recessed steel handles (there's weather stripping under them too) and the "Speakon" connector. No quarter-inch phone jack for me! But I had to build a 1/4"-to-Speakon adaptor cable, to join my '70s amp to my 21st-century cab.


Re-assembling, with glue this time. This is the bottom panel, with battens. Only the back batten has weather-stripping on it (remember, the back is removable, so I can't caulk that joint).


Speaking of caulk, here it is! You think these beads are bad, you should see my bathroom.


This is how you build a "shelf brace" (I'll explain what it's for in a minute). My neighbour was having an open house that day. Every time someone came to look at his house, I was on our shared driveway noisily making sawdust with some sort of power tool. Some even stopped to ask what I was building. "Giant bass speaker." Strangely, he didn't get any offers that day


Completed shelf brace.


Here's the shelf brace mounted inside the cabinet. The purpose of the shelf brace is to reduce resonant vibration of the panels. A brace like this is especially good at reducing BASS resonance, because it effectively divides each panel into two smaller panels - and these smaller panels resonate at higher frequencies. This increases response and reduces distortion in the bass region. Before I added this brace, the baffle (front) would resonate a LOT, because it is the weakest panel (with all those holes cut into it), and has the 17lb speaker mounted in it.


This is the bottom of the cabinet, and the sockets for the removable castors mounted therein. This cab is d@#% heavy, it needs wheels! Removable wheels, so it'll stay put while being played. I also have four rubber feet, which I'll mount after I carpet the cab.


Second-last pic, here you can see the castor sockets from the inside. Yes, they're airtight... mostly.

Then I cut three inches off each port (leaving them about 7.75" long each including flared ends), remounted the speaker and ports, stapled the stuffing back in and buttoned her up. After all that work, it's disappointing that the cab looks exactly the same from the outside!

But she's solider, air-tighter, heavier and can be wheeled about.

Now that we've seen our last warm weekend (hey, don't shoot the messenger!), it might not be til spring that I'll get around to carpetting 'er. *shrug*. I've always liked the look of bare wood. And splinters.

Oh, so I re-tested the impedence curve, and we're now at...


32Hz!

Pretty close to the target 34Hz. It might be a while before I get up the motivation to crack her open one more time to cut ports and go after those last two Hz.

So that's where we are for now. Thanks for reading this far Dad, and anyone else who stuck with me. Post any questions that you have. I'll record a clip next time I'm jamming with Steve. I'd better learn 'Pulling Teeth'...

_________________
Four strings is enough.

Awesome! Engineered to the nines. Must be satisfying to see the results so close to design. I know it sounded good so cant wait to hear it again in a session I will record.

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Hey, bionic.

Quality work, both in design and execution! An education for us all. Having learned all that, I'm glad I have open back cabs on all but MY Traynor!

Thanks, I truly enjoyed this thread.

JP