Repairing the L-pipe on your Subwoofer (Page 1/1)
Toddster JUL 15, 11:47 AM
For whatever reason people tend to lose or throw away the L-pipe on the subwoofer. I can't recall how many of these I have found in the wrecking yard with the L-pipe missing. It is a frustrating experience since it makes no sense!
The L-pipe is there for a reason. It serves to divert compressed air away from the passenger side dash speaker. Without the L-pipe, compressed air from your subwoofer will distort the sound coming from your dash speaker which is directly above it.
As such, the L-pipe re-directs that compressed air into the dashboard in a harmless direction.

Since they are not available there is a solution; Obtain

(1) a length of 1" PVC from your hardware store.
(2) a 90 degree elbow
(3) a 1" to 3/4" threaded reducer
(4) a can of PVC cement



First, using a rasp, open the top hole in the subwoofer just enough to fit the reducer.


Using the PVC cement, insert the reducer from the underside of the subwoofer and let it set-up.


Now, cut the bottom of the elbow so that the bend is low enough to fit inside the dash.


Next, press the elbow onto the reducer (do NOT use PVC cement here) this will fit well enough without cement and will allow for adjustment and positioning



finally, cut off about an 11" length of pipe and press fit into the elbow. you may use PVC cement here or not, as you prefer.


Problem solved!

[This message has been edited by Toddster (edited 09-29-2019).]

pmbrunelle JUL 15, 12:54 PM
To elaborate a bit more on the L-pipe, I don't think a simple elbow is sufficient. While the elbow alone will direct the air blast away from the dashboard speaker, the length of the pipe matters too.

This is a bass-reflex subwoofer; in addition to the sound emanating from the vibrating cone, the sound output is augmented by the slug of air in the pipe that oscillates back-and-forth.
Assuming one wants to retain this feature, the full length of pipe must remain!

If the inside diameter of the replacement pipe is not the same as that of the stock pipe, formulas are available (from speaker cabinet how-to guides) to compensate for the different inside diameters by adjusting the pipe length to suit, if one wanted to retain the stock port tuning.
Toddster JUL 15, 04:31 PM

quote
Originally posted by pmbrunelle:

the length of the pipe matters too.




Exactly. 11" is the length of the factory pipe. But the PVC pipe is smaller in diameter so a longer piece might help. But one should never use anything shorter. I find that this solution works quite well, especially if you have upgraded to Rodney Dickman's new speaker!
fierofool JUL 15, 05:24 PM
In order to maintain the same compression on a bass note, would not a smaller diameter relief increase the compression if it were the same length as a larger relief pipe? Seems that the smaller diameter pipe would need to be shorter in order to allow release and intake at the original ratio. Longer pipes of any diameter have more resistance than larger pipes of the same length, whether it be pneumatic or hydraulic.
Toddster JUL 15, 07:49 PM
You have to keep in mind the "bottleneck" concept. Since the smallest part of the opening at the top of the box is relatively unchanged, the minor reduction in the pipe diameter makes virtually no difference. I suppose someone with better sound perception than I could tell the difference, but I sure can't. But I can DEFINITELY tell the difference from having this, and having nothing.
pmbrunelle JUL 16, 12:56 PM

quote
Originally posted by fierofool:

In order to maintain the same compression on a bass note, would not a smaller diameter relief increase the compression if it were the same length as a larger relief pipe? Seems that the smaller diameter pipe would need to be shorter in order to allow release and intake at the original ratio. Longer pipes of any diameter have more resistance than larger pipes of the same length, whether it be pneumatic or hydraulic.



You're thinking about the port like someone blowing a continuous flow of air through a straw, or other restriction.

A speaker port works more like this:

Imagine that inside the port, a solid piston of metal oscillates back-and-forth with the cone movement. At mid and high frequencies, the piston has too much inertia to move much at all. At a certain low frequency, the piston's mass resonates (meaning, it vibrates in-+and-+out) in conjunction with the springiness of the air inside the box. The piston pushes the outside air in and out; this is the extended bass response we hear from a ported enclosure.

Like a piston in a car engine, the piston doesn't travel very far; it doesn't slide out of the block. In the ported speaker enclosure, rather than having a physical metallic piston, the piston is created by the mass of the air within the port.

Anyway, I just provide this information for someone who wants to take this already good post, and then take it to "concours-level restoration", at least functionally speaking. Also, just find it good to have the background information to know why we do things.

If I wanted to do port calculations, I think I would consider the pipe inside diameter, and ignore the bottleneck.
Toddster SEP 29, 12:18 AM
Great Timing, this is going into the New Formula!

http://www.fiero.nl/forum/Forum1/HTML/098963.html