closed end high frequency transmission lines

[thadman]

I'm interested in the optimal alignment for absorbing the rear radiation of a high frequency loudspeaker (ie ribbon).

A stuffed transmission line is intuitively the alignment that comes to mind as it theoretically dissipates waves completely above a certain frequency that is relative to the length of the line. What happens to waves that are larger than 1/4WL of the longest dimension?

For example, lets assume that a ribbon loudspeaker is mounted at one end of a closed 12" transmission line that is stuffed with polyurethane open-cell foam (similar to what Dr. Geddes uses in the Summa for the foam plugs). What happens to waves >48" (1/4WL, 279hz).

[bear]

They get reflected back minus the attenuation from the absorption of the foam.

I would not stuff a transmission line with the foam that Dr. Geddes uses for his plugs. The intent is very different, that foam has to essentially transmit HF energy without significant attenuation.

We could debate what Dr. Geddes' application in the throat of a horn is doing, but that is another topic.

The best thing to do is to fully absorb the rear wave of a ribbon, unless you are doing some sort of tuning using the 1/4 wave frequency. Imho.

You also have to be aware of a cavity resonance behind a ribbon.

What ribbon, what size? Sounds like something like a large BG or the like you are thinking about??

_-_-bear

PS. your dimensions seem at odds... you mention 12" and then 48"... maybe you mean 12 ft and 48 inches? dunno

And at the 1/4 wave, the thing doesn't look like an open window...

[thadman]

Originally posted by bear
They get reflected back minus the attenuation from the absorption of the foam.

I would not stuff a transmission line with the foam that Dr. Geddes uses for his plugs. The intent is very different, that foam has to essentially transmit HF energy without significant attenuation.

We could debate what Dr. Geddes' application in the throat of a horn is doing, but that is another topic.

The best thing to do is to fully absorb the rear wave of a ribbon, unless you are doing some sort of tuning using the 1/4 wave frequency. Imho.

You also have to be aware of a cavity resonance behind a ribbon.

What ribbon, what size? Sounds like something like a large BG or the like you are thinking about??

_-_-bear

PS. your dimensions seem at odds... you mention 12" and then 48"... maybe you mean 12 ft and 48 inches? dunno

And at the 1/4 wave, the thing doesn't look like an open window...

How would you go about fully absorbing the rear wave? The transmission line is the only solution I'm aware of that fully absorbs it, below the operating range of the transmission line the wave is not fully absorbed and is reflected back to the origin reradiating through the ribbon element. With the ribbon element being of negligible mass, reflections could possibly be detrimental to performance.

I was interested in using open-cell foam because it would be easier to model and optimize compared to fiberglass since its fairly consistent throughout its volume.

1/4WL resonance of a 12" tube would be 48" or 279hz if I'm not mistaken.

[Henkjan]

this is the same direction as B&W went with the nautilus... also the tweet has the ball&horn...

[FrankWW]

I was interested in using open-cell foam because it would be easier to model and optimize compared to fiberglass since its fairly consistent throughout its volume.

Geddes uses reticulated foam - all that remains of the cellular structure is the skeleton, the walls are completely blown out. So it doesn't absorb much with pore size he uses (@ 30 ppi) but you could probably get greater attenuation with a smaller pore size. This stuff is expensive.



1. What is often referred to as open-celled foam is the kind which has holes blown in the cell walls, like a sponge. (Often used in shape conforming mattresses). This might be useable. But,

2. The foam that's sold as acoustic absorption which is sort of in between - is sort of reticulated but not completely. You can see little "shiners" which are cell walls that have not all been blown out. It's about the same as that used for lining instrument cases. It doesn't absorb low frequencies worth bupkus but OK for highs. This is probably the easy one to experiment with. I stuffed a horn with this foam and got about 10 dB attenuation in the overall SPL.

You can get either of these easily from foam shops. Much less expensive than reticulated foam.

[bear]

I think that the main issue is if the attenuation through the path at frequencies other than the 1/4 wave is sufficient to not send a strongish reflected wave delayed by the time it takes to travel down the "pipe". And iirc, at the 1/4 wave frequency the pipe looks rather stiff, not soft and spongy... of course that could be at the 1/2 wave freq, I am easilly confused anyhow...

There are means to absorb LF, they do it in anechoic chambers for example...

The usual TL has an opening on one end. I think this is a technically an acoustic labrynth not a TL.

And you have a 12" ribbon that goes down to 279Hz.??

It would be easier if you fully described the project/parameters.

_-_-bear

[planet10]

A closed transmission line is a 1/2 wave resonator. Your line needs to be 1/2 wl long of the lowest frequency of interest (note that tapering the line allows for 1/2 wl support in a shorter box.

dave