size of the driver

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 (I've heard various examples and owned Definitive BP8s when I was young, naive, and less proficient with power tools ) and identify the underlying physics which cause such issues. They get strained at moderate listening levels when the source material contains bass, where that's low bass you get doubling where you hear tones at twice the fundamental frequency due to distortion which although inaccurate isn't too bad, and the IM distortion happening to midrange frequencies is offensive.

These are some of the same reasons "audiophile" speaker demos are done with female vocalists and not orchestral music.

>What then shall we call all those 5.25" and 6.5" drivers that provide real bass extension down to 25-30 Hz?

Having more measurable bass extension at low to moderate listening levels than stand mounted monitors with similar drivers, good marketing, or fiction depending on your perspective.

>You also left out the part about how the back wave is managed, which accounts for why the Atlantic Technology AT-1 extends usable bass to 29 Hz from a pair of 5-1/4" woofers.

Assuming the 88cm^2 Sd and 3.5mm xmax of the 5.25" drivers I used in my example with driver output attenuated per the Stereophile measurements they'd reach their linear limits with program material calling for 94dB 1 meter from the speaker at 60Hz (one driver will net 83dB, two 89db, and the Stereophile nearfield measurement has the drivers -5dB down with the remainder coming from the port/transmission line hybriddrivers and 89dB (one will net 71dB, two 77, and they're 12dB down) at 30Hz.

As stated I like my jazz at a moderate less-than-live 85dBC SPL average which can yield peaks 105-107dB 3 feet from a speaker.

Pulling _Take Five_ from _Time Out_ off the CD and feeding the two channels through second order IIR Butterworth low-pass filters at 60Hz using GNU Octave I find right channel peaks at -10dB below the full-range peaks; or 95-97dB SPL.

The arithmetic explains why such speakers don't work well - up to 9dB shy is off by a factor of 8.

This is also an optimistic simplification. Distortion product SPL is more a function of total driver displacement although the fundamental output is dropping for a given excursion at lower frequencies so you might find only half the total linear excursion is clean.

>Next thing we'll need a disclaimer:
"No violence was committed in the generation of these low frequencies."

I'd like something quantitative such as output levels and distortion numbers for given input frequency + level combinations like independent testers are starting to do with sub-woofers plus a practical frame of reference: In our small and large listening rooms, we could average xx and yy dBC SPL with Sir Solti conducting the Chicago Symphony playing Beethoven 9. Elevator music averages xxdB, the typical middle aged male gets to yydB when his spouse is home, and the average audiophile prefers zzdB in a darkened room with a tumbler of single malt.

My PMC Fb1i Signature speakers have a 6.5" driver and go down to 28 hz. I wouldn't have believed it before got them, but they have a transmission line design and really do go that low. IMHO, it's all about the design.

"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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The neutrality of this section is disputed. Please see the discussion on the talk page. Please do not remove this message until the dispute is resolved. (October 2010)

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."