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!

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