Sunday, March 30, 2014

3 Inch HiFi PVC Pipe - The Alphorn (MLTQWP)

Finally I found some time to tell/show a little bit more about the speaker enclosures I was bragging about so much lately.

Most importantly, why was this so interesting to be posted on a blog concerned with RF. Well, to my very own understanding, the principles behind emitting sound waves it somewhat similar to the principles of emitting radio waves.
Here is why:
Analogy between a quarter-wave vertical and a quarter-wave speaker enclosure
On the left hand side, the above sketch shows the good old vertical quarter-wave antenna driven by a gamma-match. Indicated in blue, the current distribution along the radiator. Of course, this current will emit a magnetic fields (green), which was the purpose of the antenna in the first place.

On the right hand side, you see a loudspeaker cabinet called quarter wave pipe (QWP). In such a pipe design, similar to the gamma-match of the vertical, a driver (in acoustics loudspeaker chassis are called drivers) creates pressure waves (green) somewhere in the middle of the "conductor". Similar to a quarter-wave antenna, the conductor is excited at a resonance frequency. The blue line indicates the air speed, i.e. current, through the pipe. The red arrows indicate sound emissions.
  • The driver emits sound to the front side of the cabinet.
  • The standing wave emits sound at the open end of the pipe.
While the driver emits what ever is present in the AF signal, the pipe predominantly emits sound at its resonant frequency and harmonics thereof. The latter, of course, is a problem! In musical terms, this could lead to a "One Bass Note Samba", something nobody would enjoy, contrary to the "One Note Samba" having quite some more notes than one only.
Consequently, the bandwidth of the pipe needs some severe broadening!

Back to aerials, there are 2 ways to make an antenna broadband:
  1. add more resonators (e.g. log-per, dipole fan)
  2. add Ohmic resistance (e.g. T2FD, Beverage antenna)
In acoustics, both can be done too. As I indicated before, antennas and speaker cabinets have a lot in common!
In acoustics, one can add more resonators by tapering a restrictive volume and add resistance by stuffing said volume with dampening material.

For a HiFi speaker cabinets one needs a very homogenous emission over several octaves, i.e. close to 0Hz up to 22kHz (those are, of course, extremes). A mixture of multiple resonances and some severe resistance is used. Actually, there is an added bonus on the resistive part of things... not only does stuffing material add resistance, it also lowers the velocity of sound within the medium. A stuffed enclosure looks larger to sound-waves than the same enclosure not being stuffed.

PVC piping, at last we leave the theory part of things, is a very convenient stuff to work with. Now we talk about pipe in the sense of water pipe. Have I forgotten to mention that the word pipe could have several interpretations, sewage pipe, water pipe, organ pipe, pipes of bagpipes, etc. OK, pipes, i.e. PVC pipes, fittings and stuff thereof. Here is the B.o.M:
PVC parts

The parts serve the following functions:
  • tubing form the pipe's body, obviously
  • the elbow is the resonator's "mouth", bending pressure waves towards the listener
  • the T-piece is to the driver
  • the reduction pieces will taper the resonator
  • and the end cap will be the end cap, i.e. terminating the resonator.
The elbow, T-connector and fat tube form the lower part of the pipe. This is all 3" piping. The length of the tube it 1m.
The upper part of the pipe is tapered down to 2" and 1.5" PVC tubing. The respective tubes have a length of 50cm each. Obviously, fittings add length to the final product.
Adding 1 additional diameter to the taper, will just add 1 more resonance (and its harmonics). Hmmm, "Two Not Bass Samba", really?!  No, that does not help!
The tubes therefore receive insets. The process of making those is pretty easy. My preferred method is using heating pipe insulation foam tubing. Cut in half, the stuff can be easily be cut diagonally. A result of this is tapering for pressure waves.
Tapering inserts

That's the tapering part dealt with... Those speakers are called TQWP.

However, before putting everything together, we need to address the resistive part of broadening the bandwidth of the pipe and lowering its resonance. There is no photo of this step. The material involved is polyester from an IKEA pillow, the cheapest acoustic stuffing on the market!
I complemented the inserts with the pillow stuffing and shoved it down the respective 50cm PVC tube.

We are nearly finished... there is just one other addition, before the "cabinet" can be assembled. In order to prevent the creations of unwanted harmonics, dampening material (cleaning cloth!) has to be added behind the driver (within the T-piece).

Here is a photo of the finished speaker:
The Alphorn speaker

Yeah, this is a very tall speaker. For obvious reasons, I call this speaker "Alphorn". The image above shows the thing from floor to ceiling, as installed in my attic.
The sound of those speakers is amazing! From classical to hiphop, via kizomba, funk and rock... the sound of those speakers blows me away!
Mind you, those are very cheap 3" drivers...

The drivers came in a stereo set priced less than €40.
Why am I mentioning this? The price for the PVC plumbing parts, used for this project, is actually > €42.The passive parts of the project exceed the price of the active ones... not sure what that means in the context of modern electronics.
Speaking money, the Alphorn speakers sound like speakers in excess of at least 1k€.

Back to theory... the total length of the pipe is about 225cm. 2,25m in quarter-wave reflects 9m of wavelength, which in itself equates to a sound frequency of about 33Hz. This would be the resonance of the empty (non-tapered) pipe.
As said before, the upper part of the pipe is stuffed with IKEA pillow material, hence the real resonance frequency will be even lower. Stuffing enclosures is called "Mass Loading".

There are 2 sharp steps in the taper, one at 120cm and one at 175cm. Those steps add resonances at 62.5Hz and 43Hz, which of course will also have harmonics too.

The resulting speaker enclosures qualify for the MLTQWP... and... they sound amazing!

UPDATE on alternative design thoughts. Thoughts only. On the internet a couple of designs float showing curved structures, involving a plurality of bends and elbows. Similarly to stepped tapers, those bents create reflections and disturb the air-column of the standing wave pretty good. Bending the design to limit height of the speakers seems however a very attractive thing to do.
My gut feeling tells me that 45º bents at carefully selected lengths could work. Maybe two of those just above the feeding T, followed by a reducer and two more 45º bents. Such a design would keep the tallness of the speaker at about 140cm. However, I still believe that such a measure will reduce the amplitudes of the lowest frequencies.

Monday, March 17, 2014

More Pipiness in the Audio Department

More improvement to report on the PVC pipe speaker enclosures (Voigt pipes).

Today I decided to stuff the upper pipe resonator. Stuffing reduces the velocity of sound due to increased density of the resonance volume. Such an increase of density lets the resonant volume appear larger to the sound-waves.
Quick excursion to a previous post: those pipe-enclosures are just broadband quart-wave resonators, having a pressure node at the closed end of the pipe and a velocity node at the open end.
Reducing the velocity of sound inside the pipe will therefore lower the quarter-wave resonance, resulting in more bass.

Poor man's stuffing could be cotton wool (cotton batting). This stuff introduces a severe problem! The pipes will be home of moths!

There is help, however, a real poor man's stuffing: polyester fiber from real cheap cushions. At a modern Swedish furniture store, such cushions are as cheap as €2/pc containing 400g of fibers, which is a lot more than needed.

I loosened up the cushion fiber, in order to create a very light and fluffy stuffing. Mind you, with 2" drivers, you don't want to dampen too much. It took just a few grams to lightly stuff the tapered resonators.
The result was quite pleasing. However, running the converters w/o the upper resonator revealed some adverse effects of the horn (lower part of the pipe/resonator).
To remedy this effect, I dropped a quarter cleaning cloth, cut the long side, into the tube, covering the back of the driver and following down as deep as possible.

Finally, my PVC-plumbing Voigt pipes are pretty close to Voigt pipes made from wood, as found on the world-wide-waiting-network, i.e. stuffing above the driver, damping behind the drivers and a horn like bass mouth.

As announced before, the next step has to be the homebrew low noise amp.

Thursday, March 13, 2014

PVC Pipe Nightmares

Uhh?! What is going, first he told us how cool those pipe are, and now nightmares?! What is going on?!
Very simple, when pushing one inferior variable to the best possible, another one will take the place of being the disturbing one... the one that spoils all the fun and excitement.
Well, this time, it was the stereo that was driving the fun, as I thought.
Although I still believe this is a very nice device, I have to report that this device creates a hizzzzzz under "normal"circumstances. As soon as any of the buttons is tampered with, the hizzz is absent, just to return after a few seconds... VERY ANNOYING!

This "feature" was audible only with my PVC Voigt pipe-type speakers. What do we learn from this?!
  • When using a cheap stereo, use the speakers supplied, so you don't hear the deficiencies!
  • When hooking up a cheap stereo to some decent sound converters, you may be able to hear the defects the respective sound converter has.
  • Dimensions given by your sound-Guru are relative... does a 1.5m Voigt pipe sound better than a 2+m Voigt pipe?! Probably not... shorter pipe, less bass..
Am I happy with my more than 2m long Voigt pipes driven by 2" drivers? Yes I am! I think those are the most amazing cheap speakers I ever listened to!

Remedy for the hizzzzzz problem: build a dedicated AMP (opamps/LM368) in a classical way...

Wednesday, March 12, 2014

Pipe Dreams X(X)L

Guess what, I am still experimenting with sound reproduction, HiFi audio that this. And of course, I am still on the cheap, sort of.
In my previous pipe dreams post, I discussed improvements of a stuffed transmission line made from PVC pipe. I still like the results, however, there is some bass missing... Depending on the genre to listen to, this is fine, however, lets assume the bass is really needed in a certain presence.

There was a certain Mr Paul G.A.H.Voigt, who is a simple speaker design named after. The Voigt pipe. And indeed, this design in a pipe, the like pipe of an organ. Actually a bass pipe, a broadband bass pipe.

The interested thing about such pipes is that they work similarly to what we know as "gamma match" in radio. At a certain fraction of the length of the pipe, a standing wave is created. While in a aerial, the current at 0 Ohms determines the radiation. In an audio speaker, the open end (zero resistance) aka mouth, does the same thing. As in aerial design, speakers have near and a far field responses.

Here I am, running a "gamma match" speaker, i.e. a speaker that functions as a pipe of an organ.
An organ pipe, of course, resonates at exactly one frequency. This is not ideal, unless playin' the "One Note Samba" by A.C. Jobim at the resonance frequency.
When using RF dipoles having (slightly) different resonance frequencies, the bandwidth of the antenna system can be widened easily (cf. log-pers and dipole stacks). 
Also, added Ohmic, i.e. lossy, resistance does a trick of broadening the resonance (cf. T2FD).
Pressure waves in sound follow the exact same principles. Instead of a dipole array, a resonating body is tapered. Instead of adding Ohmic resistance, damping material is added.

Here is a difference:
  • In radio, we want to transmit on one particular frequency. Any other frequency, we want to suppress as much as possible (unless running wide spread spectrum, that is).
  • In HiFi audio, we want to emit the entire spectrum as evenly as possible, the flatter the better!

However, there is a problem in HiFi audio with deep frequencies (long wavelengths). Pressure of such wave seems to collect in upper corners of rooms having a finite size, creating unpleasant rumble/mumble. So, the question is, how low do we need/want to go?! In a workshop about studio acoustics a once learned that studio monitors smaller than 5 inches are better suited, since those don't create said deep bass frequencies being so disturbing.

So, let's see what we have... 2 inch scrap speakers (salvaged from a dead small flat screen TV) as full range drivers.
In the precious posts, you see how those did in a damped 1m TL PVC pipe setup.

Same divers, similar back horn setup (removed all damping material from the back horn). Well, actually, there is no real back horn any longer. What used to be the back horn is now part of a pipe.
PVC Voigt pipe
The elbow containing the driver is replace to a T-piece. The upper part of the T-piece is connected to a reducer piece, which reduces the pipe diameter of 32mm.
The length of the closed 32mm pipe is 1m.
A Voigt pipe is not only closed, in order to create a standing wave (to the mouth), but also tapered, in order to widen the bandwidth. In my setup, I decided to taper the pipe by adding a angled cut of a cult in half insulation foam pipe, which I was writing about before.
Here a photo of the such a cut in half foam pipe.
imagine this diagonally cut in half, creating 2 similarly tapered pieces
Such foam is inserted in the upper (closed) end of the tubing, creating a tapered resonant body (stacked dipoles).
The lower half of the upper pipe's tubing is stuffed with 1/8-ths of a cleaning cloth, in order to reduce further reduce resonances (Ohmic part of the game).

To further improve sound response, some damping material was added behind driver. Care should be taken not to reduce the volume the driver breathe from.

Up to now, I played with 2 inch drivers only, there are drivers of greater diameter in my scrap box.

=> Stay tuned for more speaker madness!

Sunday, March 9, 2014

PVC Pipe Dreams (audio)

You have seen speaker enclosures I made from PVC plumbing pipes. For stuffing the pipes, I initially used heat pipe insulation (the grey stuff in the image below). It worked OK, at least to an extend I was surprised, concerning those tiny drivers.

speaker stuffing material

I experimented a bit with household cleaning cloth, as shown in the image... the stuff you would clean for bathroom with. The stuff was on sale, €1,- for 2 sheets.
What the hack, I rolled one up and stuffed it in the speaker pipe. Clearly, the mouth of the pipe now only did "mmmm mm mmmmm" (referring to very deep bass notes). The all over volume of the speaker went way down. I guess, I restricted cone motion quite a bit with that much mass.

This stuffing:
  • 0.5m of the insulation stuff
  • 1/4 sheet of rolled up cleaning cloth
was the next experiment.
Now the driver has got nearly half a meter of pure air column to breathe, followed by the insulation foam and some loose stuffing of soft cloth.
In a way, this is like tapered increase of acoustic density.

Again, the result is really interesting, seen the small / cheap drivers. Of course, you would not expect a hammering deep bass.

Still more to experiment, I pushed the cloth stuffing about half way into the PVC tube. This way, I think, the driver has to some volume to breathe from and the longer wavelengths, that made it through the stuffing, can form more homogeneous pressure fronts, before exiting the enclosure.
Again, some improvement could be noticed.

Speaker enclosures, made from PVC piping, seem to offer endless opportunities to experiment.

Monday, March 3, 2014

PVC Pipe Speakers

Yep, I know, this is not RF, this is AF. However, this post my be of interest anyways.

Recently, my first ever LC-Television died on me. Of course, the device had to be source of parts. And it was! The sound of this TV was not bad, hence, I also took the 2 tiny 2" full range speakers out before I crapped the rest.

After collecting some dust, the chassis are finally put into operation again, wit the help of 2" (55mm) PVC pipes.

The speakers are house in elbows, taped in by electricians tape:

the driver

Followed by 1m of 55mm PVC pipe, providing the air column. The pipe is actually filled with a "pipe insulation jacket" as to reduce resonance effects and also increase the acoustic resistance.

the speaker box

Of course you would not expect the sound of a Klipschorn from this device, however, you will be surprised how much bass a 2 inch speaker can deliver, being placed in a decent enclosure.
I am sure that sound quality can be improved by stuffing the tube properly, however, the foam tube insulation was just so tempting to do, cheap and easy, and it works surprisingly well.