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peniku8

Shaped Ports

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I'm in the process of designing a small vented PA cab for B&C's 12BG100, tuned to around 40Hz. The current design utilizes two straight rectangular ports with a combined port area of 120cm².

Problem with the port is that the handle placement is so awkward, that I'd have to make the cab wider than I'd like to, which is why I'm planning to go with a shaped port. The fact that doing so would also lower port compression a little is a nice side-effect, but it's not my main concern here. I basically want the port to "bend around" the handle. Pics below.

I'm simulating a rear loaded horn in hornresp to be able to be more flexible with the port input data, but I'm not sure if this is the correct way to approach this, or if there are better alternatives. I had to increase the chamber volume by 10% to match the simulated response of the vented cab.

I sliced the port into 4 segments to model it as accurately as possible. The cab in the middle is far from done and needs a few modifications still, since I just quickly threw this idea together in Inventor to grab the measurements for hornresp. The sims are very similar between the straight port and the shaped one.

Am I on the right track?

ZU4spV8.jpg

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Nobody knows? Alright, I'm just gonna build the cabs and see how it goes. It's just 70$ of wood for two cabs so w/e

And even if the response is not ideal, these will be used with dsp amps exclusively so I can iron out some flaws at least.

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Interesting question, but I do not know what will happen sim v reality for this.  Will you have shaped and non-shaped cabs built to compare?

JSS

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47 minutes ago, maxmercy said:

Interesting question, but I do not know what will happen sim v reality for this.  Will you have shaped and non-shaped cabs built to compare?

JSS

No, too much work. The straight port version is not practical for me anyways. I have built an MDF prototype with a straight port, but that one was tuned to 30Hz instead. I figured that even the 12BG100 doesn't generate enough output to justify a tuning so low for my application. In addition to that, the port area on that one was a lot less and way too small.

I will build two to be used for a live setup for weddings and smaller gigs. Either way, most 12" subs on the market are tuned to 50Hz or higher, which just doesn't cut it for me and most of the remaining subs which would fit my SPL and extension needs are bigger/heavier than just 40lbs.

That said, the 12BG100 surprised me with its weight. 19lbs is quite a lot for a 12" pro woofer with a neo magnet imo.

I've finished the CAD files and am currently working on the CNC code. I hope to have one measured in the next few weeks.

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Modelling it as a rear loaded horn does simulate exactly the same, as I exemplified here: 

 

 

Interested in the results! 

 

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1 hour ago, Droogne said:

Modelling it as a rear loaded horn does simulate exactly the same, as I exemplified here: 

 

 

Interested in the results! 

 

Yes, my models of the pure vented cab and the RLH were exactly the same (except that the tuning frequency of the RLH somehow ended up higher, even when using the same straight ports, so I changed the back chamber volume to match the original vented sim). Particle velocity at the mouth was reduced by 50%, which might give me a db or two more output at maximum output levels.

Making those ports was probably 10x the work of simply doing a straight port, it better pays off. Look, it's a potty!

EedNjfA.jpg

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Hey man. Nice looking build. I'm way behind and catching up around here. 

I like the method of shaping the ports and combating turbulence. 

The very first thing that I see though is wasted volume inside of the "toilet seat" (LOL...Can't unsee that now!). That volume could be used for the chamber or even the ports. You could knock a big hole or 2 through the bottom panel of the chamber into the toilet bowl area and get that volume back for the main chamber. Every little bit helps. Doesn't have to be pretty. No one will ever see that. 

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Hey Josh!

I‘ve made a hole into the port wall which basically traces the port waveguide‘s dados. Aint wasting no volume ever! :D
You can see it better in this pic:

 

2776C338-8E7E-43AE-9874-2D6A298EF449.jpeg

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20 hours ago, peniku8 said:

Hey Josh!

I‘ve made a hole into the port wall which basically traces the port waveguide‘s dados. Aint wasting no volume ever! :D
You can see it better in this pic:

 

 

Awesome! Already ahead of the game. 

Is that your shop?  I'm jealous of the tools and space. I have nothing like that.  

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38 minutes ago, Ricci said:

Is that your shop?  I'm jealous of the tools and space. I have nothing like that.  

I know.  Such a small table saw. ;)  I have a CNC but that table saw is so nice!

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31 minutes ago, Ricci said:

Awesome! Already ahead of the game. 

Is that your shop?  I'm jealous of the tools and space. I have nothing like that.  

It's my dad's and my shop, yes. The biggest benefit is that my recording studio and my equipment storage area are in the same building. Plus everything is well isolated (acoustically) so that I can work way past midnight without anybody caring. The CNC was the latest addition to the shop last summer and it has easily been the most used machine in the room ever since 😊

It may look spacious, but it really isn't. There are machines everywhere!

 

Unbenannt-4.jpg

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On 3/11/2020 at 1:20 AM, klipsch said:

How well has the extruded aluminum worked for you?  I've heard good things...

What exactly are you referring to? The T-Slot table?

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I saw quite a few extruded aluminum shapes pieces. Perhaps the extruded aluminum comes with those machines from the manufacturer. Many here in the States add the extruded aluminum for guides and fences as aftermarket enhancements. A few companies in the States use extruded aluminum as hobbyist / small business CNC bases. 

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Gotcha. No I haven't done anything custom with extruded aluminum profiles yet. All aluminum components you see came with the machines and are not off the shelf profiles.
They've been sturdy enough for me to never break anything. All moving parts on the table saw are aluminum and everything else is steel. The machine is over 2000lbs, but using it feels pretty easy.

I thought about doing the CNC stand with Bosch/Item profiles but my dad offered to make one from wood, which is what you see there. I wish I had welded a steel frame. The wooden construction moves too much for my taste.

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Colour me impressed, whatever I throw at it, I can't get the ports to make excessive noise.

Tuning came in at around 38Hz and even 35Hz sines at Xmax produce very tolerable wind noise. The results are very impressive and they also sound great without a dsp dialed in yet.

I designed them to have increased sensitivity where excursion is low (slightly rising response towards the top end plus a bump around tuning), which will be corrected via dsp, since those subs were never intended to be used on non-dsp amps. I will get a free field measurement tomorrow. Unless it rains.

They weigh just 19kg (42lbs or so) and have an external volume of 85L. I designed them to realize smaller shows alone (weddings etc.).

They're also surprisingly stable when I pole-mount my EV ZLX12 on top. This is a great compact setup. I was going to do like 5 weddings with em next month, but the current health crisis delayed those by a few months. No actual gigs in the near future 😞

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Observations:

Actual Frequency response vs Simulated HR+Edge Output:

R5zQFdi.png

I didn't expect the native frequency response to be +- 1db from 35 to 100Hz, that is very nice. Average sensitivity is 89db at 2.83V @ 2m groundplane outside from 35-100Hz.

The HR response here has been scaled down by 4.5db, the measurement was 1m, normalized to 2m (-6db). I don't know how the sensitivity ended up 1.5db higher than the sim, but I ain't complaining.

Feeding the cab a 28.3V sweep showed zero compression between 40 and 80Hz. I didn't get a louder sweep yet, but the cab started to produce wind noise with 70V signals between 30 and 50Hz.

It was hard for me to make out a tuning point as there wasn't a clear excursion minimum, but it seems to be 37-38Hz. I will get an impedance sweep tomorrow.

 

Conclusion:

I'm not entirely sure if this is actually the case, but it seems like the shaped port made the peak from the port loading a bit broader, which is why the peak is much smaller compared to the sim and the sensitivity around the peak is a bit higher. Maybe this is also the reason why I had trouble finding the excursion minimum.
The tuning is quite low for a 12" 85L (3cuft) PA cab. The back chamber is about 50L (2cuft). I might go for a slightly higher tuning in v2, making the port entrance flare a little bigger and maybe adding some port area. The average port area is 120cm². It wouldn't hurt (ergonomically) if the cab was like 1cm wider. F6 is 31Hz, which is a good 10-15Hz lower than most PA subs this size.

And most importantly: I like the sound!

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Performance looks great, more output than my subs in half the size.  You also don't have any deep cancellation notches from the port pipe mode.

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