Bill Fitzmaurice

[quote name='ShergoldSnickers' post='1317189' date='Jul 26 2011, 10:26 AM']Analogy? Take a torch and a sheet of glass. Clear glass gives a focused beam from the torch, as the beam is relatively unaffected. Frost the glass however and you'll get scattering, widening the beam. I'm guessing this is something like?[/quote] You can find a basic description of how diffraction works here, in section 1-5. [url="http://www.jblpro.com/pub/manuals/pssdm_1.pdf"]http://www.jblpro.com/pub/manuals/pssdm_1.pdf[/url] [quote]The acoustic foam certainly does attenuate at higher frequencies as illustrated by the NRC for good quality 1/2" to 3/4" foam, and a model based on simple absorption predicts well what is observed in practice. The hole is entirely necessary - covering the whole speaker in a uniform layer of foam would do nothing to alter directivity, it would merely attenuate the overall output.[/quote]Maybe. An alternative view is that the hole becomes the primary radiating plane for high frequencies, and as its diameter is small the dispersion is widened. The same mechanism is seen in slot loaded tweeters, which also make the slot narrow and high, for both wide horizontal dispersion and tight vertical pattern control. It's an easy enough theory to test, you just make a foam plate with a narrow high slot instead of a hole and measure it on both axis.

[quote name='ShergoldSnickers' post='1316598' date='Jul 26 2011, 03:32 AM']It's more to do with what other manufacturers don't do, than what BFM or Alex have 'invented' and put in. There is no new wheel, the theory and methods are already out there - they've just been ignored by most manufacturers, due to cost, laziness, manufacturing complexity etc.[/quote] +1. All Alex and I have done is to design our speakers using proper acoustical engineering principles that have been well known to the acoustical engineering community for decades. If there's a question to be posed it's why has the electric bass speaker manufacturing community in general ignored those principles from day one? Here's a hint: profits.

[quote name='LawrenceH' post='1316491' date='Jul 25 2011, 06:53 PM']Careful, or you'll reinvent the phase plug[/quote] I already have, they're used in the current versions of the OTop and Jack speakers and in the midrange horn of the Omni series. A phase plug is just another example of a diffraction device.

[quote name='LawrenceH' post='1316415' date='Jul 25 2011, 05:21 PM']Sigh...but the Geddes model is considerably more expensive to implement. For all intents and purposes in the frequencies where guitars output this model works by attenuation, and 3/4" of the right stuff is enough to make a worthwhile (ie audible) difference that can be confirmed by measurement directivity plots without creating a bloody great waveguide that adds volume to the cabinet. The main drawback is an overall loss of sensitivity in the HF but you gain a less uneven dispersion pattern. By contrast a 'beam blocker' creates a more complex pattern that when you measure it is all jagged and unven. Doesn't really go towards solving the problem so much as shift it about. You do like to be difficult sometimes Bill! [/quote]I think you'd find adding a beam blocker to the hole in that foam thingamajig would get a better result than either alone. The blocker probably works better than the foam if sized properly. I doubt that Weber ever did a thorough study of different sizes and shapes. I might someday if I have nothing else to do, it would only take an hour or so to map polars with different size blockers in different positions.

[quote]But in that design the hole in the middle IS what makes it work[/quote]Not if you do it right, which is with varying foam thickness across the cone. See the Geddes model. [quote]otherwise you just attenuate the sound without modifying directivity.[/quote]The amount of attenuation offered by 3/4 inch of foam is infinitesimal. The diffraction is fairly significant at the shortest wavelengths, enough to cause them to literally bounce off the walls of the foam's individual cells, redirecting their paths, while longer wavelengths pass though unimpeded.

[quote name='LawrenceH' post='1316303' date='Jul 25 2011, 04:23 PM']I think they like them not just for the look but the nearfield sound on stage (very full, bass-heavy and a PITA to mix). You just don't get that with a 1x12".[/quote]More than a few top acts don't have any amps on stage at all. They still have them, backstage. They hear what the audience hears, the PA feed, though both monitors and in-ears. 'Journey' adopted this arrangement at least 12 years ago. Geddy Lee is probably the most obvious proponent. Chicken, anyone?

[quote name='LawrenceH' post='1316247' date='Jul 25 2011, 03:56 PM']That has more to do with the fundamental limitations of 12" speakers than eg the small advantage gained by crossfiring. I'm a big fan of angling speakers back on stands to get the guitardist's ears closer to the 'beam of death' but a neater solution (IMO) is Jay Mitchell's 'foam donut' [url="http://www.stratopastor.org.uk/strato/amps/prii/speaker/foamdonut/foamdonut.html"]http://www.stratopastor.org.uk/strato/amps.../foamdonut.html[/url] Dead simple to implement and the beauty is you can tailor the foam type/thickness and size of the hole to give some flexibility to freq cutoff and amount of attenuation.[/quote]It works, but isn't at all a new or novel idea. Earl Geddes has been using foam for quite some time, and without a hole in the middle, as that's not what makes it work. The underlying principle is diffraction; a thirty year old example is the JBL 2301 perforated plate horn lens. [url="http://www.jblpro.com/pub/obsolete/acoustic_lens_family1.pdf"]http://www.jblpro.com/pub/obsolete/acoustic_lens_family1.pdf[/url] Beam blockers use the same principle.

[quote name='Marvin' post='1316190' date='Jul 25 2011, 03:16 PM']One may like to compare the whole issue to when James Dyson was getting nowhere with his 'cyclone' vacuum cleaner. Now everyone makes one.[/quote] I prefer the example of Ed Villchur. In 1954 he invented the acoustic suspension speaker. He showed it to every major speaker manufacturer, all of whom said, in effect "If something like this worked our engineers would have already invented it". As a result he started his own company, Acoustic Research. By 1963 AR held a 70% market share. The company tanked after he sold it to Teledyne in 1967 and the bean counters took over. [quote]I think it's more a matter of cost. To make a box like that of Bill's design is probably not cost effective for mass production.[/quote]Not at the price the average player wants to pay [i]if[/i] they're to maintain their profit margins. Alex manages it, but only because he doesn't have their overhead for marketing and distribution, which accounts for about 40% of what you pay for a store bought mass produced cab.

[quote name='ficelles' post='1316043' date='Jul 25 2011, 01:11 PM']Exactly... back in my sound engineering days I would regularly be deafened in 400+ seater venues by guitarists refusing to turn down their amps... "but it's only 50 watts and I can't hear myself properly"... worst offenders were those of the Hank Marvin school of tinnitus-inducing ice-pick guitar sounds! ficelles[/quote]The number one complaint of FOH engineers in 10,000 plus seat venues is stacks. To get eight twelves crunching hard the dB level from a stack can overpower even a 30kW PA at 100 feet, and FOH engineers go mad trying to get a balanced sound when said stacks are cutting a swath like a laser beam on-axis. With PA support no guitar'd player needs more than a 2x12. But Jimi and Pete and Richie etc., etc., etc., used stacks, so therefore...

[quote name='Mr. Foxen' post='1315930' date='Jul 25 2011, 11:38 AM']Moves the cone centers closer together, broadening with dispersion.[/quote] True, but that's only a small part of the equation. Here's the rest: [url="http://homepage.mac.com/randyhyde/webster.cs.ucr.edu/AudioStuff/audioStuff.html"]http://homepage.mac.com/randyhyde/webster....audioStuff.html[/url] [quote]it all depends whether one perceives a difference and - if so - whether one 'likes' the difference. Some may. Some may not.[/quote]If you're standing in front of the cab with it tilted so the axis is aimed at your ears you won't be able to tell much, if any, difference between a vertical and horizontal alignment. If you're in the audience, off-axis, and the main sound source is your rig rather than the PA the difference will be quite obvious.

[quote name='GarethFlatlands' post='1314854' date='Jul 24 2011, 10:52 AM']Make you wonder why most cab makers don't address the problem.[/quote] How many posts do you see about a new product where the poster says "[i]Wow! That looks great! I have to have one![/i] Manufacturers build what sells, what sells it what looks good, what looks good is what's familiar. Very few are willing to take a chance on introducing something that sounds better at the risk of it not selling for the lack of looking 'right'. [quote]Would separating, for example, a 4 by 10 internally into 2 sets of 2 by 10s make any difference?[/quote]Only if one vertical pair was low-pass filtered to prevent them from radiating mids above where the center to center distance of the horizontal spacing is over a wavelength, roughly 1kHz. Then you'd have the low frequency power of four drivers with the horizontal dispersion of a single ten. But that would add a few quid to the build cost, and so...

[quote name='GarethFlatlands' post='1314842' date='Jul 24 2011, 10:39 AM']So a 2 * 12 guitar cab would benefit then? I'll give it a try next practice. Thanks.[/quote]If Leo Fender had known anything about how speakers work he never would have put two drivers side by side. He didn't, so instead of making his amp chassis narrow to fit atop a vertical 2x12 he put the drivers side by side to visually match up with a wide chassis. The die was cast and remains to this day the intrinsically flawed standard.

[quote name='ficelles' post='1314807' date='Jul 24 2011, 10:01 AM']Maybe it's fallen out of fashion... it was all the rage in the 80s! A few years back I used to run a ... a 1x15 and a 4x8,[/quote] Flawed. One doesn't need four 8s to provide the mids to keep up with one 15; one would be sufficient. [quote]Worked pretty well, but not markedly better than my other rig which was a GK RB400IV into a single vertical 2x10 so in the end the simplicity of the single head/cab won out[/quote]No doubt, as you were lugging about three extra eights, and I assume the eights were in a 2x2 box, which results in the dispersion being halved. [quote]Does this apply to guitar cabs too? Is there any benefit of stacking them vertically?[/quote]Much more so than bass, as their frequency range runs an octave higher. One should not even use a twelve inch driver for best results with guitar, as it will beam above 1.5kHz. Put two side by side and beaming starts around 800Hz, total insanity for a bandwidth that runs to 4 to 5kHz.

[quote name='ficelles' post='1314761' date='Jul 24 2011, 09:13 AM']Really? As to my recollection crossovers - particularly in the context of bi-amping with different cabs & drivers for different frequency ranges - have been in use in bass amplification for decades. ficelles[/quote]AFAIK the only cabs on the market today designed for bi-amping are some G-K, and those are woofer/tweeter cabs, not woofer/midrange, where bi-amping is most useful.

[quote name='thodrik' post='1313849' date='Jul 23 2011, 09:35 AM']These threads really prove to me just how clueless I am to technical stuff, yet I still love reading them! I have two cabs bought when I was 22 based on the 'small speakers for highs, big speakers for lows' logic (2x10, 1x15).[/quote] The logic is sound. It was, and unfortunately remains, the implementation that was flawed. The hi-fi and PA (then cinema sound) guys knew back in the 1940s that you use big drivers for lows, smaller drivers for highs, along with a crossover so that they don't have overlapping coverage. The electric bass cab industry is still in the process of figuring that out.

[quote name='mart' post='1312953' date='Jul 22 2011, 11:33 AM']So you'd probably still want it vertical, but stood on top of a chair or beer crate or something, right? And, if I've understood this, then adjusting the height will, crudely, change which frequencies you're cutting - so raising or lowering it should help you find the ideal height to counteract the boominess.[/quote]+1. But it's easier to twist the frequency knob on a parametric EQ.

[quote name='4 Strings' post='1312797' date='Jul 22 2011, 09:56 AM']On a similar question, I usually use one of these 210s for my Motown band so its a warm, roundy sort of sound (Fender P + flats etc) and it's difficult to prevent boominess. I normally stand the cab up for dispersement but was wondering whether reducing dispersement by laying flat would reduce reflectances and boominess.[/quote]Boom is caused by too much content in the midbass, in the vicinity of 100-150Hz. Many cabs are actually built to accentuate those frequencies, as they're perceived as louder (louder sells) and because accentuating the midbass at the expense of the lows results in a smaller cab (also a sales point). But if you're on a stage or in a room that resonates in the midbass it's just too much. Laying the cab flat has no effect on the midbass frequencies, and makes the situation worse by making the mids and highs harder to hear. Lifting it above the stage by two to three feet will help tame the boom, how high depends on the frequencies that are resonating. The best tool to cure it is a parametric EQ, which allows you to dial in exactly on the boom frequency and cut it without robbing the low end as well.

[quote name='Phil Starr' post='1307456' date='Jul 18 2011, 04:20 AM']I love these old cab designs, I don't think the designers knew themselves how they worked half the time.[/quote] Quite right. The best example of that is the Karlson. Karlson himself called it a horn, but it had not a single characteristic of a horn. It was actually a series tuned dual chamber bandpass reflex, but as it was introduced some 20 odd years before T/S theory became well known he could be excused for not knowing that. OTOH he made response claims for it that were nonsense, and produced SPL charts that could only have been concocted.

[quote name='Mr. Foxen' post='1306946' date='Jul 17 2011, 02:09 PM']Like with half open back cabs, is that a giant port tuned really high to the point where it makes no odds, or is there a point where it ceases to be?[/quote] It is technically a ported enclosure, but the resonant frequency is so high that its function is moot and the enclosure functions as an open baffle. As to how the enclosure in question really works that becomes definitive only when both frequency response and impedance sweeps are examined; the one or the other in and of itself isn't definitive. The lack of impedance sweeps in the patent abstract is most curious, and is one missing detail that should have resulted in outright rejection of the application.

[quote name='alexclaber' post='1306489' date='Jul 17 2011, 06:02 AM']Stevei, tsn't a reflex cab a Helmholtz resonator? Though I see your point, you're suggesting that the cab works by solely exciting the resonant system of the two masses of air in the ports vibrating against the spring that is the volume of air in the enclosure divided by the area of the port:enclosure opening. But what's happening with the woofer backwave, why wouldn't that act like the frontwave from a woofer in a normal ported design?[/quote] Exactly. All the parts are the same as a standard bass reflex, they're just rearranged. It would be easier to visualize as a bass reflex with the driver mounted in the usual fashion, but that's what one's imagination is for. Now if one has no imagination...

[quote name='escholl' post='1306283' date='Jul 16 2011, 06:16 PM']The ratio of harmonics does not, however, depend upon length.[/quote]I've found that it does, though I've only done very rudimentary testing just to satisfy my curiosity.

[quote name='fatback' post='1306026' date='Jul 16 2011, 12:35 PM']Presumably it's the longer scale length that's allowing for the different harmonics? How does that work?[/quote]The longer the scale the higher the fundamental to harmonic ratio in the waveform. A string doesn't reach its full potential for fundamental output until it's 1/4 wavelength long. At 30 Hz that's 9 feet, so you'd need one really big bass to create a strong 30 Hz tone.

[quote name='ShergoldSnickers' post='1305827' date='Jul 16 2011, 09:31 AM']Does the constriction of the hole immediately in front of the speaker on the top shell - and not shown properly in my rough from memory illustration - serve any purpose other than to speed up air 'squirted' into the resonant chamber in order to excite it? Surely this constriction would radically alter the behaviour of the cone as it is now impeded more by having to move air through a constricted hole? Edit: Also wondering about how much distortion this set-up would introduce to any otherwise pure signal. [url="http://www.avforums.com/forums/attachments/subwoofers-tactile-transducers/237169d1303471020-graham-holliman-infrasonic-generator-graham-holliman.pdf"]Link to original plans for this cabinet[/url][/quote]The size of the hole is part of the tuning mechanism. The size of the hole has far less to do with the driver excursion than the volume of the chamber. The THD of this box would be similar to a standard bass reflex with similar specs. BTW, the tapering of the ducts would seem to be an attempt at adding some horn loading, but duct length and exit area preclude that, both being far too small. As my colleague George Augsperger would say [i]"There are many ways to build a bass reflex. This is one of them."[/i]

[quote name='ShergoldSnickers' post='1305621' date='Jul 16 2011, 06:33 AM'][/quote]It's a bass reflex cab, nothing special or unusual except for the external driver mounting, and that serves no purpose, it could be inside with the same result. In current parlance it would be called an LLT, for Long (duct length) Low Tuning. The designer was totally in error with regard to the rounding of corners. Ten to forty foot wavelengths aren't the least bit hindered by hard corners.

[quote name='Phil Starr' post='1305381' date='Jul 15 2011, 07:49 PM']Fair comment, I should have said there is little point in doing this with your computer speakers. Even with decent hi-fi speakers you will get some audible harmonics but it does give you an idea of just how low the fundamental of bottom E is and it's usually a shock to people who hear it for the first time.[/quote]Not as much of shock as it is to have a speaker putting out 15Hz at 100dB and you can't hear it, nor would you even be aware of it if not for a meter telling you it's there. And that's why even for home theater LFE I don't even think about going below 20Hz.