Would a transmission line behave correctly if its cross section was real oblong?
For example, how about a 44" untapered rectangular pipe with a cross section of 3"X11" or something like that?
I'm trying to come up with a design for a cabinet that's very shallow, to go right against a wall. It can be pretty wide or tall.
For example, how about a 44" untapered rectangular pipe with a cross section of 3"X11" or something like that?
I'm trying to come up with a design for a cabinet that's very shallow, to go right against a wall. It can be pretty wide or tall.
I've built a Daline type system using rectangular steel pipe. I believe it worked fine (history section of our web site). The main difference would seem to be the shape of your cross-section.Dave Jones said:
I'd say that it's worth a try.
Re: Re: Oblong pipe for TL?
That's interesting. What is patent on the "stand line" for? It doesn't make any practical difference to me, because I won't be selling them, but I'm kind of curious. I thought about putting the pipe in a stand, BTW, but not folded around because I don't need the length. I've seen pictures of other speakers that used the stand as a port.
7V said:
That's interesting. What is patent on the "stand line" for? It doesn't make any practical difference to me, because I won't be selling them, but I'm kind of curious. I thought about putting the pipe in a stand, BTW, but not folded around because I don't need the length. I've seen pictures of other speakers that used the stand as a port.
Re: Re: Re: Re: Oblong pipe for TL?
Ahhhh. C'mon. What is the patent for? You can tell me.
7V said:
Ahhhh. C'mon. What is the patent for? You can tell me.
Re: Re: Re: Re: Re: Oblong pipe for TL?
There were two patents. The first related to the use of a loudspeaker stand as a tuned pipe. The second related to the use of an egg-shaped sub-enclosure mounted inside a speaker cabinet.
Oh, I see what you're asking.Dave Jones said:
There were two patents. The first related to the use of a loudspeaker stand as a tuned pipe. The second related to the use of an egg-shaped sub-enclosure mounted inside a speaker cabinet.
Dave Jones said:
I think MJKs worksheets include one which will calculate the behavior of a TL section-by-section. I imagine you could massage the information you get as a "best-guess" analysis of an oblong TL.
Just a thought.
Dave
Hi Dave,
I don't see any reason why a cross-section shape like the one you describe would not work well. I would recommend placing the driver at the midpoint of the longer side to minimize the excitation of the first half wavelength standing wave across the this dimension.
I don't see any reason why a cross-section shape like the one you describe would not work well. I would recommend placing the driver at the midpoint of the longer side to minimize the excitation of the first half wavelength standing wave across the this dimension.
MJK said:
Thanks a lot, MISTER King. 🙂
Interesting... I was sort of imagining the speaker on the wide side at a golden section ratio or something - you know - to reduce defraction effects. Not necessary? Middle, eh?
The idea is to make a speaker that's designed to go right smack up against the wall (in my office). That's one reason I want them real shallow.
Can you say any words of wisdom about baffle step and defraction considerations for this kind of setup? Suppose it's up to 16" wide and 4" deep (outside dimensions).
[Added later.]
Oh wait, do you mean placing it in the center top-to-bottom? I'm afraid that would kind of defeat the purpose. I want to get the (full range) driver up a bit. Hmmm.
But I've got another idea for a once folded design that would allow me to put the speaker at 1/3 of the way down the tube while still being near the top of the cabinet.
OK, rereading my post I can see it was slightly confusing. So let me do a better job describing what I was trying to say and give some simple reasons for my ideas.
First, I think the golden ratio stuff is grossly over emphasized, I don't even think about the ratios of enclosure dimensions. I don't think the ratios are that important. (OK, collective gasp!)
So, your original idea was a TL that was 44" long, without a fold, and having a cross-section of 11" x 3". Here is what I would start from if this were my design.
1. Mount the driver 1/5 to 1/3 of the length from the closed end. This will help kill the ripple from the higher standing waves and not reduce the bass output significantly.
2. Stuff the top 2/3 of the enclosure with about 0.5 lb/ft^3 of polyester fiber fill.
3. Mount the driver in the middle of the 11" dimension. This will pin the maximum velocity of the first half wavelength standing wave across the long dimension of the enclosure. For a half wavelength standing wave, the maximum velocity is in the middle and the pressure is zero. So the acoustic impedance will have a null. Mounting the driver slightly off center will probably also work. This mode should now have a minimal impact on the driver. Hopefully the stuffing will kill the higher harmonics. This assumes that the mode can be supported which is a function of the location of the open end of the TL.
4. The very shallow depth will push the half wavelength standing wave in this short direction higher in the frequency range and hopefully the stuffing will do the job on these modes. I seem to remember that Boston Acoustics had wide and shallow cabinets in the late 70's or early 80's for this reason.
5. If the enclosure is to be pushed against the wall I would neglect any baffle step correction circuit. My guess is that the room placement will take care of the baffle step problem on its own.
This is my off the cuff assessment of a design like the one originally described. Simulations and more detailed thinking will probably improve on the ideas stated above.
First, I think the golden ratio stuff is grossly over emphasized, I don't even think about the ratios of enclosure dimensions. I don't think the ratios are that important. (OK, collective gasp!)
So, your original idea was a TL that was 44" long, without a fold, and having a cross-section of 11" x 3". Here is what I would start from if this were my design.
1. Mount the driver 1/5 to 1/3 of the length from the closed end. This will help kill the ripple from the higher standing waves and not reduce the bass output significantly.
2. Stuff the top 2/3 of the enclosure with about 0.5 lb/ft^3 of polyester fiber fill.
3. Mount the driver in the middle of the 11" dimension. This will pin the maximum velocity of the first half wavelength standing wave across the long dimension of the enclosure. For a half wavelength standing wave, the maximum velocity is in the middle and the pressure is zero. So the acoustic impedance will have a null. Mounting the driver slightly off center will probably also work. This mode should now have a minimal impact on the driver. Hopefully the stuffing will kill the higher harmonics. This assumes that the mode can be supported which is a function of the location of the open end of the TL.
4. The very shallow depth will push the half wavelength standing wave in this short direction higher in the frequency range and hopefully the stuffing will do the job on these modes. I seem to remember that Boston Acoustics had wide and shallow cabinets in the late 70's or early 80's for this reason.
5. If the enclosure is to be pushed against the wall I would neglect any baffle step correction circuit. My guess is that the room placement will take care of the baffle step problem on its own.
This is my off the cuff assessment of a design like the one originally described. Simulations and more detailed thinking will probably improve on the ideas stated above.
Martin, thanks again! I'll read what you said very carefully. I've got a lot of work to do for my day gig in the next couple of weeks. When that's done, I will be ready to get this DIY project going in earnest.
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