I have recommended your spreadsheets on several occasions, both for transmission lines and for bass-reflex cabinets. They have been especially helpful to people who wanted to build a loudspeaker from my plans, but with some cabinet modifications. If they intend to make a tower, I frequently suggest they check the impact of their modifications using your bass-reflex spreadsheet.

About insulation, I don't think that myth is limited to builders of bass-reflex speakers. I've seen it coming from several sources. We've all seen people talk about lining some part of a cabinet with felt, or something else thin like that. But the best place for an absorbent material is spaced away from the boundary, so a thin sheet would be best hung, suspended inside the cabinet. If attached to the wall, it doesn't do anything.

Wayne

Now, the point I was REALY making, and apparantly not very well, is that once the quarter-wave genii is let out of the bottle, it must be delt with. You CANNOT build a tall, thin BR without concern for quarter-wave modes.

This is the modeled output of a 53 liter cubic box with a 3"x4" port, the same as on the quarter-wave pipe to follow -- a pure helmholtz resonator.

The peak at 900 is the half-wave resonance between the walls of the box.

This is the modeled output of the 53 liter MLTL with the same port. The driver and port are at optimum positions.

Just in case you think that the model is blowing smoke, here is the actual nearfield output of the MLTL.

If you do not model your design, then the only way to know if quarter-wave modes exist is to do a nearfield measurement of the port. The answer will be obvious.

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Another point you raise is that you can mix helmholtz resonance and quarter-wave modes in the same design. Yes you can, a few years back, there were a number of very complex designs with multiple pipes and resonators, all designed to tame quarter-wave resonances in essentially conventional TL's. Unfortunately, these were complex solutions to a simple problem. Proper pipe geometry and driver/port placement does the job in a simple, straight pipe.

One more graph. This is the outdoor, 1 meter farfield, no splice output of my 53 liter MLTL.

The dip at 150 Hz is primarily floor bounce.

See, it works!

Bob

You wrote :

"If the Qts is in the low 0.20's or lower, the driver belongs in a back horn. Performance in a quarter-wave pipe will be disappointing."

My latest thinking on these "back-loaded horn" designs is that almost all of them are really TL's for the lowest frquencies. If you look at the mouth area, multiply it by between 2 and 8 depending on room placement, you will find that the horn cut-off frequency is probably much higher then the designs claimed low end performance. So the enlcosure exhibits TL responses for two or three modes and then transitions to a horn like response. Compare Nelson Pass's Kleinhorn with the Fostex designs. There is nothing wrong with this approach and I am using it in my latest design.

Martin

I also wanted to make a point, a clarification of a definition really. If a cabinet uses Helmholtz resonance as the primary feature and is designed in such a way to eliminate or suppress standing wave modes, then it is a bass-reflex enclosure. If it is designed to use the primary quarter-wave mode as its main feature, then it is a quarter-wave pipe. This is less determined by appearance and more determined by the actual mechanism at play. The only way to really know is by modeling or measurement.

There is evidence that shows each type of speaker can be optimized to perform well. I've seen good examples of both types. Transmission line enclosures must size the port and cabinet so that Helmholtz resonance is out of band. It must also suppress quarter-wave harmonics. Bass-reflex cabinets must be sized to move standing waves out of band, or they must be suppressed using the same techniques as transmissions lines employ, i.e. port/driver placement and stuffing. In my opinion, there are more similarities than differences.

I think that we are actually in complete agreement on the physics of the problem. We are arguing some nits in the impementation that boil down to personal preference and technique. Everyone comes from a different place and is walking a different path.

Bob

Doug

True, the trick is to blend the action. As far as the mouth size i disregard the normal final expansion and make the mouth open up in a more radical fashion and with the sloped final stage it allows the wave to expand more rapidly and gives the effect of a larger mouth as well as brings the wave centerline (maxium response axis) up closer to the driver which helps the mechanical crossover of horn mouth/driver.
However this requires a slight fudge on the wave form as at the lowest frequencies it is more of an upright oval than a true circular wave. This is all accomplished for a smaller footprint and slightly exceeds the old rule of a 1:1.6 aspect ratio. However i tried a 1:1 aspect ratio and while the lower Hz was a bit cleaner the size of the mouth required was not to my liking.
ron