size of the driver

"Hoffman's Iron Law". First formulated back in the early 1960's by Anthony Hoffman (the H in KLH), Hoffman's Iron Law is a mathematical formula that was later refined by Thiele and Small, whose work now forms the basis of all modern loudspeaker design.
Hoffman's Iron Law states that the efficiency of a woofer system is directly proportional to its cabinet volume and the cube of its cutoff frequency (the lowest frequency it can usefully reproduce). The obvious implication is that to reduce the cutoff frequency by a factor of two, e.g. from 40 Hz to 20 Hz, while still retaining the same system efficiency, you need to increase the enclosure volume by 23=8 times! In other words, to reproduce ever lower frequencies at the same output level you need an extremely large box! This is why we see so many subwoofers or low eff multidriver towers. Using 5-7 in drivers. Larger woofers are better woofers, larger cabinets are more efficient and produce deeper bass with less thermo compression and Superior transient response. If one compromises size efficiency or range one can make a smaller design.

Probably a topic for a different thread, but I would be really interested in better understanding how the better known Walsh style driver/designs differ from the more conventional approach in terms of these common parameters that help determine speaker performance.

How does a downward oriented, open-back Walsh driver manage to deliver seemingly flat and extended frequency response at very high SPLs horizontally in a largely omnidirectional manner compared to a say a single similar sized conventional driver generally firing more directly at the listener?

The answer seems to lie somewhere in the domain of "wave bending" in the Walsh theory, as opposed to pistonic motion which I believe accounts for most of the output associated with traditional dynamic designs?

I kind of understand the theory based on wave propogation through materials of different density "bending" or diffracting the wave, but would have no clue how to relate it in technical terms comparable to what Drew and others here have so eloquently related, nor how to apply it effectively in practice, other than via trail and error perhaps.

Here's what Wikipedia has to say about it FWIW:

"Bending wave loudspeakers
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Bending wave transducers use a diaphragm that is intentionally flexible. The rigidity of the material increases from the center to the outside. Short wavelengths radiate primarily from the inner area, while longer waves reach the edge of the speaker. To prevent reflections from the outside back into the center, long waves are absorbed by a surrounding damper. Such transducers can cover a wide frequency range (80 Hz to 35,000 Hz) and have been promoted as being close to an ideal point sound source.[49] This uncommon approach is being taken by only a very few manufacturers, in very different arrangements.

The Ohm Walsh loudspeakers use a unique driver designed by Lincoln Walsh, who had been a radar development engineer in WWII. He became interested in audio equipment design and his last project was a unique, one-way speaker using a single driver. The cone faced down into a sealed, airtight enclosure. Rather than move back-and-forth as conventional speakers do, the cone rippled and created sound in a manner known in RF electronics as a "transmission line". The new speaker created a cylindrical sound field. Lincoln Walsh died before his speaker was released to the public. The Ohm Acoustics firm has produced several loudspeaker models using the Walsh driver design since then.

The German firm, Manger, has designed and produced a bending wave driver that at first glance appears conventional. In fact, the round panel attached to the voice coil bends in a carefully controlled way to produce full range sound.[50] Josef W. Manger was awarded with the "Diesel Medal" for extraordinary developments and inventions by the German institute of inventions."

05-01-12: Drew_eckhardt
04-29-12: Johnnyb53
>04-29-12: Tamule1
>real bass comes from moving a large surface area gently -not a small surface violently . 6.5" is not a woofer size IMO

"Violent" isn't a problem until you reach the linear or physical limits which geometry dictates you do when using such small drivers.

>And you base this opinion on what?

I draw upon personal experience with speakers including transmission lines built with similarly small drivers...

Drew, it's pretty obvious I was responding to Tamule1 and not saying anything concerning your previous statements. I question his setting a size definition on woofers that essentially excludes 90% of them. A driver doesn't have to function well as a subwoofer in order to be a competent woofer.

Your enthusiasm for larger drivers is already well known. You play down the disadvantages of the flex and weight of a larger diaphragm, the cost and bulk of a larger magnet structure, the dispersion enhancement of a waveguide (I've yet to see a waveguide on a 12" driver to turn it into a midrange) while pointing out the disadvantages of longer excursion, small radiating area, etc. of smaller diameter bass drivers. It's really a matter of picking your poison.

If cost, size, and weight are not important factors, fine. Knock yourself out and put in 12" and 15" drivers. If WAF, floorspace, and bass speed at affordable prices is important (and it's intuitive that this package is an easier sell to both sexes), then the narrow column anchored by multiple 6.5" woofers rules. And that's why they dominate the sweet spot (not the highest end) in high performance loudspeakers.

I tend to look at it from the perspective of basi physics.

Size matters in order to pressurize air more effectively.

Larger rooms and lower frequencies at higher volumes require more air be pressurized to maintain a smooth frequency response. Larger drivers have the advantage.

The dilemma is that larger drivers will tend to have more mass. More mass means more inertia. Inertia is the enemy of transient response. Transients are an important part of real music. Therefore larger drivers are at a disadvantage when it comes to transient response.

A lot of what works best comes down to a matter of scale ie how much speaker is needed to deliver the desired scale of performance in the target roo m.

All you have to do is go to Parts Express or Madisound websites and look up the specs on various drivers of different sizes and types, including "pro". Some have detailed frequency response and impedance graphs. You can go further by plugging some of those T/S numbers into a speaker box calculator.

For example, plain and simple...
http://www.bcae1.com/spboxad2.htm

Some of you might be in for a shock. The evidence speaks for itself but interpretation varies.