I suppose this is my first scratch project. I've built kits
over the years (crossovers prewired) and modified cabinet/
speaker combinations. Had a successful mod of some old
Nakamichi enclosures that went with their mini receiver by
replacing the woofer with the Fostex FE87e and knocking a
big hole in the backs. This worked out well and I'm using those
as my computer monitors.

I got a couple RS 1197's when they were being closed out
for (I think) $3 ea. or so. And I got on to the PVC pipe
idea because it requires few tools and no power tools at all.

The PVC's I've seen have a small baffle made of MDF. To fit
the speak to a 90d elbow, I used a toilet adapter with a
short ring of 4" diameter pipe as a baffle.

The whole thing (I've completed one for testing) consists of
RS 1197 with foam insulation as a gasket screwed to the
toilet ring and inserted into the 90d elbow. I have a line
length of only about 50 cm total and that includes a PVC t-fitting
at the base which acts as a 4" diameter port.

I intend to make the final length 101 cm as the example
project shows. But to test I just used a piece of the PVC
to see how it sounded.

The rest of this test rig is as sparse and inexpensive
at the speaks: I have a Radio Shack SA155 mini amp and a
turntable from Pioneer that a friend gave me. I purchased
a Grado Black cartridge and that's my source for now.

Playing some mono LP's through the single speak
initially gave what I felt was a muddy end to string
bass and the low frequencies in general. (This is still
the 50 cm overall length.) I've been able to smoothe
out the sound of this little driver with some dacron stuffing
in the 90 elbow and nothing else down the length of the line.

My question is: will increasing the line length to
typical TL proportions make the lower end more crisp (string
bass sounding like a string bass and less muddy?)

There's a long way to go on this but it has been a big
learning experience.

• an example of a PVC pipe speaker

About transmission lines: Naturally, the longer the line, the lower the fundamental resonant frequency. If augmentation begins in the upper mid-bass where the driver is already strong, you might develop pretty strong peaking that would sound in a way that many would describe as muddy. For example, I would expect the driver is pretty strong above 100Hz. If the fundamental resonance is between say 80Hz and 160Hz, then your speaker will be maybe 10dB louder in that region. That's definitely going to sound "muddy."

A longer pipe that shifts the fundamental down to 50Hz would be better, and then you would need to damp the system so that the next successive higher frequency pipe harmonics (at 100Hz, 150Hz, and so on) were de-emphasized. They will be attenuated slightly from the fundamental anyway. But you will want to de-emphasize them as much as possible because your driver will probably be working strong by that point.

Ideally, you would want the cabinet to provide augmentation where the driver response begins to fall off. It might be good to set the fundamental frequency similarly to the way you would set the Helmholtz frequency on a bass-reflex system. I'd suggest checking with Martin King at www.quarter-wave.com about that. Another good reference is Daniel Russel's paper, "Acoustic Filters, Waveguides and Transmission Lines."

Thanks for the reply. I am using Rick Schultz's article on
Alpha transmission lines as general guide. According to that piece,
I could achieve 63 hz with some polyester stuffing (which is
what I have) and an overall length of 49 in. I suppose that is a good place to start. I can always get the length down, but I wanted to make sure not to trim too much for a first fitting.

On the other hand, I'd need 70 in of length to go down to 45 Hz
and that length would be awkward.

In any case, I am at last getting to hear what all the theory
is about with a $3 driver and some plumbing parts. At some point
I'm going to want to push some big band music through them. Anyone
know where I can get a good recording of Ted Heath's version of
"Love For Sale?"

You wrote:

>> According to that piece, I could achieve 63 hz with some polyester
>> stuffing (which is what I have) and an overall length of 49 in.

Sounds about right. One quarter wavelength of 70Hz is 49 inches, so the first peak should be somewhere in that area. Other factors than just pipe length come into play such as pipe area, baffle area, stuffing, etc.

>> I suppose that is a good place to start. I can always get the
>> length down, but I wanted to make sure not to trim too much for
>> a first fitting.

I think you're probably in the 60-70Hz ballpark with 49" length.

>> On the other hand, I'd need 70 in of length to go down to 45 Hz
>> and that length would be awkward.

Maybe so, but I'll bet your driver is pretty strong at 70Hz and so if your pipe is tuned to 70Hz, your system may be peaking quite a bit. Seems like 50Hz pipe tuning might be better, but then again, that's just a guess. I've not examined the facts, so I'm just thinking out loud here. If your pipe is tuned to 60Hz, that's splitting the difference and might be right where you want it to be.

The main things I would consider are the first and second standing wave peaks. If the first peak is rather high frequency - in the midbass where the driver is strong - then system output may be too high at that frequency. On the other hand, if the first peak is too low, then the second peak might be shifted down into this midbass region. Careful placement of these first couple of peaks must be the balancing act when designing and building speakers like these.

>> In any case, I am at last getting to hear what all the theory
>> is about with a $3 driver and some plumbing parts.

Well, there you go. That's where the fun is and you can't beat the price!

Wayne

Taking into consideration all your advice I got another part
for $1.06. I can try to straighten out the line and not use
the rear ported part. This would make it more like a traditional
line and I'd have to make a base with feet for the wave to exit
through the bottom. I'm going to keep as much length before
cutting as necessary to allow some wiggle room.

In a day or so I'll have a workable base. The base is more
odds and ends from my practice making cutouts.

This is a story in itself: To practice making cutouts without
access to a router or a spin saw (Rotozip) I got some rotozip
wood bits and chucked one up in my electric hand drill that is
in a Sears drill guide. I have my choice of three centers from the
available machinings on the guide. No trigger lock on the ol'
drill, so I got one of those new ratchet clamps and kept the
drill going that way.

Go ahead and laugh. I was surprised that it worked at all.
But I wear eye protection on this job. I snapped a couple bits
before I got the speed right. So my practice bases have
cutouts through a square of particle board stacked 2 high
and for feet... well the local Farm and Fleet had bags of
wood plugs so a couple of those glued up will be the feet.

Maybe someone in here can tell me if, with this combination
of driver and enclosure, I should attempt the other 99 cent
modification of the driver which is to damp it with rope caulk
around the basket (?)