SB17NBAC in a transmission line with active crossover

[Dave Bullet]

I've been hankering to try a transmission line for a while. I realise many people use these for subwoofers and 3 ways, but I thought why not try it with a 2 way, using a low Qts driver to really make my job hard?

To make it even harder, why not throw an active crossover into the mix? I also want to experiment with faceted baffles.

The active crossover will take the form of a Raspberry Pi 3b+ with HAT attached Suptronics DAC which takes HDMI to 7.1 channel unbalanced analogue output via RCA (sadly these are NLA - so I better treat it nicely). I have a media server built on the same (albeit IQAudio 2 channel DAC). Using either Ecasound or CamillaDSP - I can implement the IIR crossover. Modeling may either be done in VituixCAD or Charlie Laub's Excel based tools (Charlie built these to produce the required ecasound parameters to apply the filters - so plug and play).

The drivers - SB Acoustics - so no originality awards here:
SB17NBAC-4 and TW29DN-8

Onto the enclsoure...

I modeled this using David McBean's excellent Hornresp tool. It takes a little getting used to the UI, but once you have the basics down, quite powerful.

I came up with a 2.6:1 (reducing) taper transmission line ~ 43 litre volume (169cm line length). This is quite a bit larger than the many vented options but the TL gives a theoretical F3 of 35Hz.

Now I know this isn't going to win any SPL awards, so will be volume limited to about 25w with bass heavy music. They'll be used in a smaller listening space and are experimental, so I am happy with this compromise.


Here's the HornResp model with stuffing to smooth the TL ripple. I used a path offset of 64.2cm being the distance between SB17NBAC centre and terminus centre:

NB there is no baffle step rise in this modeled response - so expect the overall sensitivity to be ~ 87dB for this 4 ohm version of the SB17

[xandresen]

As an electrical engineer (engineers use electrical transmission lines quite a bit), let me point out a problem in many folded transmission line cabinet designs.

The folds have to look like a transmission line to sound waves. For example, if you shine a flashlight (sound) straight at a glossy smooth white wall (at a 90 degree angle), the reflection returns to where you stand. It does not go off in some other direction. Transmission lines are all about reflections.

When we make a 90 degree fold in the cabinet's internal shape, the sound waves will be (mostly) reflected straight back to where they came from. This design may sound OK to someone but it is not what transmission line modeling software models - the calculated results are for a transmission line.

To make a transmission line fold, think of the flashlight again. The light (sound) needs to bounce off mirrors at the corners so that it will exit through the cabinet's exhaust port.
In other words, a fold needs a 45 degree corner to reflect the light (sound) at 90 degrees.

Do THIS for a 180 degree fold
/ . \ <---use two 45 degree angled corners to get a 180 degree fold.
| | |
| | |
in out

Not this
___
| . |
| | |
| | |
in out


(note: sound is partially reflected at the line's exit port, the modeling software accounts for that.)

Hope this is clear.

[Dave Bullet]

Thanks. Good point. I will add those to the model

I'm going to measure both before and after adding the corner radii to see how much of a difference it makes

[Paul K.]

You are absolutely wrong. The stuffing in the line is an acoustical low-pass filter which removes, or greatly attenuates most of everything other than bass. The bass wavelengths are so much longer that those frequencies go around those right-angle corners quite well, not being reflected straight back. If you model correctly the right-angle corners and then build what you modeled, you will get the predicted results. Using corner reflectors is almost like some people who say only long-fiber wool should be used for stuffing.
Paul

As an electrical engineer (engineers use electrical transmission lines quite a bit), let me point out a problem in many folded transmission line cabinet designs.

The folds have to look like a transmission line to sound waves. For example, if you shine a flashlight (sound) straight at a glossy smooth white wall (at a 90 degree angle), the reflection returns to where you stand. It does not go off in some other direction. Transmission lines are all about reflections.

When we make a 90 degree fold in the cabinet's internal shape, the sound waves will be (mostly) reflected straight back to where they came from. This design may sound OK to someone but it is not what transmission line modeling software models - the calculated results are for a transmission line.

To make a transmission line fold, think of the flashlight again. The light (sound) needs to bounce off mirrors at the corners so that it will exit through the cabinet's exhaust port.
In other words, a fold needs a 45 degree corner to reflect the light (sound) at 90 degrees.

Do THIS for a 180 degree fold
/ . \ <---use two 45 degree angled corners to get a 180 degree fold.
| | |
| | |
in out

Not this
___
| . |
| | |
| | |
in out


(note: sound is partially reflected at the line's exit port, the modeling software accounts for that.)

Hope this is clear.

[Dave Bullet]

You are absolutely wrong. The stuffing in the line is an acoustical low-pass filter which removes, or greatly attenuates most of everything other than bass. The bass wavelengths are so much longer that those frequencies go around those right-angle corners quite well, not being reflected straight back. If you model correctly the right-angle corners and then build what you modeled, you will get the predicted results. Using corner reflectors is almost like some people who say only long-fiber wool should be used for stuffing.
Paul

Thanks Paul - I have also read this.

I suppose the real issue with direction transition, is maintaining the taper ratio. Without some sort of radius / angle, the line would go wider then narrow varying taper along the way.

[Paul K.]

Dave, what you said is true, but if you correctly model how the area changes going around the corners, you will know exactly what to expect, including whether the tapering not being continuous is causing a problem. Now, in all honesty, if you want to build with corner reflectors as shown in your cabinet drawing in Post #1, there's nothing wrong with that, only that not using them would not cause reflections as Xandresen stated.
Paul

Thanks Paul - I have also read this.

I suppose the real issue with direction transition, is maintaining the taper ratio. Without some sort of radius / angle, the line would go wider then narrow varying taper along the way.

[xandresen]

Thanks. Good point. I will add those to the model

I'm going to measure both before and after adding the corner radii to see how much of a difference it makes

That would be great - please post your results.

[Paul K.]

Using Martin King's software, I modeled two versions of an SS18W/8531 in a 10:1 tapered TL 72" long. The first version was modeled with a continuous taper to simulate a single-fold line that has angled corner reflectors. The second version was modeled as a single-fold line having right-angle corners instead of angled reflectors at the line's U-turn. The attachment shows the predicted system bass responses of the two lines, which are virtually identical. Dave, I'm not saying you should not use angled reflectors, just that their effects, are at best, minuscule. I used the SS8531 because I've done several builds with it. My results may not be completely indicative of what yours will be with the SB17 but should be useful and informative.
Paul
SS18W comparisons.doc

[Dave Bullet]

Using Martin King's software, I modeled two versions of an SS18W/8531 in a 10:1 tapered TL 72" long. The first version was modeled with a continuous taper to simulate a single-fold line that has angled corner reflectors. The second version was modeled as a single-fold line having right-angle corners instead of angled reflectors at the line's U-turn. The attachment shows the predicted system bass responses of the two lines, which are virtually identical. Dave, I'm not saying you should not use angled reflectors, just that their effects, are at best, minuscule. I used the SS8531 because I've done several builds with it. My results may not be completely indicative of what yours will be with the SB17 but should be useful and informative.
Paul
[ATTACH]32403[/ATTACH]

Thanks for mocking this up Paul - very interesting!

I intend to hold the angle pieces in place with blobs of blutack. This is for easy removability but also to not reduce the volume (ie. the gaps behind the angles will still contribute to the enclosure volume). I want to minimise the differences as much as possible (so the only volume reduction will come from the MDF of the angles themselves).

I may not get to this for a month or so. Being sick, then home improvement list takes priority But I will get to it.

[rj45]

Dave,
I have designed and built a couple of MLTL projects, and Paul K has been a huge help to me.
If you build a MLTL, you can measure the FR at the port, and see that the output is mostly bass, with greatly attenuated mid range output, just as the Hornresp model shows.
Another interested experiment I did was to build a ported bass reflex box that I could convert to a MLTL by adding one internal divider. Although the boxes measured the same, I thought the MLTL sounded much better.