Bose Bass Cannon VS Tapped Horn

[Patrick Bateman]

I've messed around with sonotube subs*, Bose Bass Cannons, and tapped horns. Never had much luck with anything but the latter.

Forum member DJK clued me in to how they're supposed to be built, and also clued me into the fact that the orientation of the cannon makes a difference. (It never occurred to me that turning the cannon 90 degrees or 180 degrees changes the output.)

With this knowledge in hand, I've come up with a method for optimizing the Bass Cannon** that seems to work well.

Here's how you do it:

Step 1) First, find a tapped horn that works well. I am going to use Art Welter's Keystone sub*** as an example of a good tapped horn.

Here's the specs, displacement, and response for a Keystone TH with a B&C 18SW115

Step 2) In step 2, we make a Bass Cannon that has a footprint that's about the same size as the tapped horn. This will require some trial and error, but it goes fast, because the cannon has no flare, it's just a big ol' tube. To get started, follow these steps:

a) Look at the tapped horn, and find the lowest resonance. In the Keystone sub, that's 34hz. Now set the length of the FRONT of the bass cannon to 1/4 of that. In this case, set the length to 250cm. (34000cm per second / 34hz / 4 )
b) Now set the REAR of the bass cannon to the same length as the front, divided by three.
c) Set the AREA of the segments to about half of the SD. In the case of the B&C 18SW115, that would be 605cm^2.​

Once you do that, you'll have three segments to your horn. A front horn, a driver, and a back horn. Hornresp doesn't support horn loading both sides of the cone, but you can do a bass cannon by making the rear chamber vented. This works because the bass cannon has no flare. If we'd used a flare on the rear chamber, hornresp wouldn't work. To make the rear chamber vented, go to "tools > chamber > rear vented"

I set a very very small rear chamber, because if you make "Vrc" empty you can't port the chamber. (Because it doesn't exist.) So I made "Vrc" just one half of a liter, which is plenty small enough. Realistically, the back chamber in a Bass Cannon will likely occupy a few liters, simply because of the size of the basket. (This statement assumes that the diameter of the cannon is smaller than the diameter of the driver.)

 

[Patrick Bateman]

It takes some juggling to line everything up. The most important thing by far is to get the excursion curve of the bass cannon to line up with the tapped horn.

If you look at my numbers, you'll notice that the rear segment of the bass cannon is 100cm long. Theoretically, it should be 81cm long. But in hornresp I found that it was necessary to lengthen that segment to get the excursion curve to look correct. This is really important with the bass cannon, do not skip this step. If you don't get the resonances to line up correctly the response will look terrible.

Perhaps this is the reason that people have had such bad things to say about the bass cannons? There's a lot of resonances here, and if you're off by a few inches, they're not going to be staggered properly.

Here's a look at the tapped horn and the bass cannon. The box size is within 1%. Note that the BC has the potential to be lighter and smaller if you use a sonotube, due to the extra rigidity that a tube gets you.

Here's the impedance curve. The dips are at the same frequencies, this is critical.

Here's the output of the FRONT of the bass cannon. See that dip at 74hz? That's important, because we're tuning the cannon so that the output from the rear of the cone 'fills in' the hole from the front of the cone. (Again, this is super critical. If you get these ratios wrong you'll get a peak and a dip.)

Here's the dialog in hornresp where we add the output from the rear of the cone.

And finally, here is the combined response. The grey line is the Keystone tapped horn; the black line is The Bass Cannon. The two subs are so close, it would probably be hard to tell them apart.

Of course, all of this is just Hoffman's Iron Law. But I hope that these pictures and instructions help illustrate how to make the Bass Cannon work. It's quite a bit trickier than a tapped horn, because moving the woofer by even five inches can change the response dramatically.

If I were to build one of these, I would want to use sonotube and a microphone, and then cut the tube down to size to get the curves right. (Use a woofertester to measure the impedance dips, which will correspond to the dips in the excursion curve.)

 

[weltersys]

Here's the dialog in hornresp where we add the output from the rear of the cone.

And finally, here is the combined response. The grey line is the Keystone tapped horn; the black line is The Bass Cannon. The two subs are so close, it would probably be hard to tell them apart.

The grey line is a Hornresp sim, the actual Keystone response does not look the same.
The Keystone goes from roughly omni at LF to showing some directivity at 100 Hz.

You did not add the path length difference from the port outlet to the listener minus the distance from the horn mouth in your Bass Cannon simulation, add it and see what the results are.
Without adding in a path length difference the cannon is firing left/right or up/down, needing a support.

The difference in output locations of the Bass Cannon will make the polar response quite odd (peaky) in the upper regions of the pass band.

 

[j.michael droke]

HR-horn w/mid position driver

Hi there Patrick: Have you tried HR,s "SC" wherein S1 thru S4 is one side of the driver and S5 to S6 is the other side. By juggling the input numbers you can balance volume and/or distances. Besides tube shapes, you can use cone shapes or other curves supported by HR. Fun and games, however, results may be hard to translate into buildable enclosures. ...regards, Michael

 

[Patrick Bateman]

The grey line is a Hornresp sim, the actual Keystone response does not look the same.
The Keystone goes from roughly omni at LF to showing some directivity at 100 Hz.

You did not add the path length difference from the port outlet to the listener minus the distance from the horn mouth in your Bass Cannon simulation, add it and see what the results are.
Without adding in a path length difference the cannon is firing left/right or up/down, needing a support.

The difference in output locations of the Bass Cannon will make the polar response quite odd (peaky) in the upper regions of the pass band.

According to Hornresp, "the path length difference is the distance from the port outlet to the listener minus the distance from the horn mouth to the listener."

Based on that, if the bass cannon is parallel with the listening seat, and directly in front of the listener, the path length difference is zero.

What's happening in The Bass Cannon is that the output from the REAR is flattening out the output from the FRONT.
That's why the ratio of the lengths is so important; if you're off by five or ten inches, the output from the rear will make the front peakier instead of flattening it out.

The pic above shows the output of the B&C 18" Bass Cannon when you're equidistant to the ends, and also when the pathlengths vary by one meter.

We see in the sims that the dip gets deeper, and that's because the front and the back aren't summing optimally.

Then again, the low bass gets louder, which is nice, and for the same reasons.

I imagine that in-room the bass cannon's output would be a lot more sensitive to orientation than a TH. That could be good *or* bad depending on one's needs.

 

[Patrick Bateman]

Michaels right, it looks like there's more than one way to model a bass cannon in hornresp.

I'm not sure what an "SC" is, but you can model a bass cannon by using the "compound horn" option, which basically puts a horn on each side of the driver

 

[weltersys]

Based on that, if the bass cannon is parallel with the listening seat, and directly in front of the listener, the path length difference is zero.
What's happening in The Bass Cannon is that the output from the REAR is flattening out the output from the FRONT.
That's why the ratio of the lengths is so important; if you're off by five or ten inches, the output from the rear will make the front peakier instead of flattening it out.

The pic above shows the output of the B&C 18" Bass Cannon when you're equidistant to the ends, and also when the pathlengths vary by one meter.

We see in the sims that the dip gets deeper, and that's because the front and the back aren't summing optimally.

Patrick,

Front and rear are left and right if the bass cannon is parallel with the listening area. Response will be different from the sim anywhere along the L/R listening position.
So, disregarding room modes (which is ridiculous, but that does not seem to matter to you 🙂) the response will only be the same at 35 Hz and 100 Hz, and within the sub pass band will vary from 5 to 15 dB across the (narrow) listening area, and provide +5 dB somewhere at 20 Hz, which sounds twice as loud.

That kind of response I can live without.

But like Rodney Dangerfield said about the horrible hat in Caddy Shack:

"Looks good on you "

Art

 

[salzburgsoundsystem]

If a bass canon is a disjointed tapped horn...

...how about making the two ends of the bass cannon meet?

Make a bass cannon clone but fold the longer section so that the "ends" of the canon are adjacent.

Isn't that nearly a tapped horn?

the "front wave" path is 3x the "rear wave" path.

Wouldn't this offer all the advantages of a bass cannon without having to sit in just the right spot?

Ben

 

[weltersys]

...how about making the two ends of the bass cannon meet?

Make a bass cannon clone but fold the longer section so that the "ends" of the canon are adjacent.

Isn't that nearly a tapped horn?

the "front wave" path is 3x the "rear wave" path.

Wouldn't this offer all the advantages of a bass cannon without having to sit in just the right spot?

Ben,

As far as I can tell, the only "advantage" of a "bass cannon" is ease of construction using Sono Tube, and the long thin tube shape potentially can be hung or stood in an unused area.

Sono tube is not really stiff enough for use at high power, and making a fold to connect two cardboard cylinders is actually more challenging than constructing a tapped horn with plywood.
If you were to make the fold, the form factor is no longer appealing, and much air volume is unused compared to a rectangular cabinet, the "U" will occupy more space.
Standard plywood construction also allows taper, which is a feature defining a tapped horn compared to other tuned pipe enclosures.

Using small drivers and PVC plumbing pipe and "U" fixtures would be an easy to assemble project that would not have as ragged response as a straight tube.

Art

 

[Don Hills]

... Front and rear are left and right if the bass cannon is parallel with the listening area. Response will be different from the sim anywhere along the L/R listening position. ...

You have to consider the original intended use. Typically, one or more BC would be mounted on the end wall of a picture theatre behind the screen. Room modes and path length differences are less of a problem in large spaces. As you pointed out, not suitable for a typical lounge...

 

[Soldermizer]

I'm still playing with my sonotube + 12" Peerless sub. I am quite impressed with what I could do with a cardboard tube, a used woof, epoxy and caulk. It cost, being generous, $160. And absolutely no tools required (ok, a jig saw to cut the tube to tune it.) + soldering iron, etc. Electronics were already on hand so not included either. So, tell me guys, how much did your work shop cost you? Put that in your router and smoke it 🙂

 

[just a guy]

So, tell me guys, how much did your work shop cost you? Put that in your router and smoke it 🙂

$40 for a good plunge router. $20 for a box of extra bits.
$120 (IIRC) for a decent table saw.
$40 ($5 each) for 8 big clamps.
$40 for a drill, $20 for a big box of bits.

That's $280 for all the tools needed to make just about anything I need (as far as speaker boxes go). I'd tell you to stick that in YOUR router and smoke it but I guess you don't have one. 🙂

Not only do I have those tools but I use them frequently for things not related to speakers and I have the experience gained from using them.

 

[Soldermizer]

I salute you sir!

Yours is a modest tool kit. I salute you more on your (no doubt) greater manual skills. For the rest of us (me!) we help keep local handymen employed 🙂