Bill Fitzmaurice

[quote name='LawrenceH' post='1324723' date='Aug 2 2011, 02:27 PM']historically it is a subject that has been dominated by engineers and physical scientists.[/quote]
True, but when those engineers and scientists were those employed at Bell Labs they knew far more about how human hearing functioned than the medical community, who were late comers to the party. The history of Bell Labs, and what they accomplished from 1876 to roughly 1950, is a fascinating topic in and of itself.

[quote name='LawrenceH' post='1323765' date='Aug 1 2011, 08:19 PM']It does take a bit of time for that type of biological research to filter out and cross specialisms to the engineering/physics arena, and much longer before it becomes 'common' knowledge.[/quote]Indeed. If you want to see some screwy ideas check out the home theater sites, where some truly believe that there's benefit to speakers that run to 5Hz. That's where biology comes in. Below roughly 12Hz human beings lack the ability to hear anything, because 75 foot long wavelengths pass through muscle and bone with impunity. That fact, combined with the period of the wave, means that both sides of the tympanic membrane are exposed to the same pressure and phase. It does not vibrate, so no sound is heard, and as those wavelengths pass though the body they aren't felt either. What is heard and felt from systems capable of high output below 12Hz is harmonic content and air pressure fluctuation.
But it would be incorrect to assume that this is knowledge restricted to the medical community. Acoustical engineering isn't limited to loudspeakers and acoustics. One of the major sub-fields is the hearing aid industry, where knowledge of how hearing works is paramount.

[quote name='LawrenceH' post='1323079' date='Aug 1 2011, 08:52 AM']But apparently a so-called acoustic engineer knows better than my scientific colleagues at the cutting edge of hearing research, how the ears and brain work.[/quote]
An acoustical engineer must know how the ears and brain work. If he does not he isn't an acoustical engineer, he's a mechanic. If you've got nothing better to do than argue about the audibility of group delay got to DIYaudio and waste a few dozen pages in debate with Earl Geddes.

[quote name='alexclaber' post='1322743' date='Aug 1 2011, 03:13 AM']And why would you want that to happen? Just curious about the thinking - I think I might write an article about ports soon...[/quote]
He's not really off track. When I opened up my first bass cab in '66 the thought occurred that there was compression inside that was restricting the movement of the cones and that they might move more freely if I cut a hole in the cab. What I knew about how speakers worked at that point was precisely nothing, but as it turned out my hypothesis was correct, all that I lacked was the knowledge of how to properly carry it out. I did cut a hole in the baffle, and it seemed to work a bit better. I know it didn't work any worse.

[quote name='LawrenceH' post='1322704' date='Jul 31 2011, 08:41 PM']Bill, you seem a bit fond of putting words into other people's mouths and then gleefully contradicting them as a self-appointed "engineers' representative". I said absolutely nothing about 10ms. To take an extreme case, clearly a group delay of, say, half a second, is extremely audible well below 100Hz. But the exact cut-off is reported differently in different studies - that is the area of controversy.
From a neuroscientific perspective it would be very surprising if there wasn't a degree of individual variation in the threshold.[/quote]
If you somehow managed to obtain an electric bass cab with 50ms of group delay, let alone 500ms, the speaker would have to be so completely AFU that the group delay would be the least of its problems. As to being an 'engineers representative', it's what I do for a living. Deal with it.

[quote name='LawrenceH' post='1322000' date='Jul 31 2011, 05:51 AM']The threshold of audibility of group delay (frequency-dependent delay) is a bit of a controversial topic.[/quote]Not within the community of acoustical engineers, who are well aware that group delay below 100 Hz is moot. It does bother the heck out of those who see a group delay of, say, 10ms at 50 Hz on a chart, but it doesn't bother those who know that you can't hear 10ms at 50 Hz.

[quote name='DirkThrust' post='1321296' date='Jul 30 2011, 08:45 AM']Ashdown quote an extra 23Hz of lowend for the MAG115 below the 4x10[/quote]Quoting it is one thing, backing the quote up with objective data is something else entirely. They don't. Not that Ashdown is alone on that point, as no manufacturers do so. In any event 'there's no such thing as a free lunch' remains the operative caveat. If the 115 does go 23Hz lower than the 410 that extension is obtained at the cost of sensitivity, so it would take two to four of those 115s to match the output of one 410.
One of the laws of acoustical engineering is that power demand doubles for each octave of lower frequency extension. That being the case the notion that a single fifteen driven with the same power as four tens can significantly add to the low end just doesn't add up.

[quote name='sorrow' post='1321027' date='Jul 29 2011, 09:33 PM']has anyone tried tecnical pro power amps or seismic audio bass cabs?[/quote]
Both brands should be avoided.

[quote name='DirkThrust' post='1320906' date='Jul 29 2011, 05:27 PM']I've found that a 4x10 produces more thump, thud, thunk, whump or whatever other unscientific name you call the sound bass players like than a 1x15.[/quote]It's only logical, as the average 410 has more total driver displacement, higher sensitivity and for that matter very often the same tuning frequency as the average 115.
[quote]that extra 10 or so Hz of bass extension manufacturers quote[/quote]More often than not it's a rubbish figure anyway.

[quote name='fatback' post='1320641' date='Jul 29 2011, 11:53 AM']there should be a competition for the greatest number of acronyms (or whatever they are) in a BC thread.

Vd, Vb, Fb, EBP, QTC ????[/quote]
[url="http://www.eminence.com/support/understanding-loudspeaker-data/"]http://www.eminence.com/support/understand...udspeaker-data/[/url]
[quote]For front porting don't you get that to some degree anyway? The backwave is inverted but one wave out at the resonant frequency...[/quote]The port radiation is only out of phase below the tuning frequency, and in a properly designed cab it's moot as that lies primarily below the speaker operating bandwidth.

[quote name='ficelles' post='1320497' date='Jul 29 2011, 09:46 AM']Untuned ports i.e. just a hole... anyone in favour / not in favour? Any rule of thumb for size?

ficelles[/quote]
All ports result in a tuning frequency (Fb), which can be determined with any speaker modeling software. The duct length is the thickness of the panel.

[quote name='xgsjx' post='1320350' date='Jul 29 2011, 07:42 AM']Ahh, but what if you set the EQ to a sad face?



Would you get the mids back or would you just have a sad face???[/quote]You'd get some of the mids back, yes.

[quote name='MoonBassAlpha' post='1319906' date='Jul 28 2011, 06:52 PM']Is that a big problem if you also have a tweeter, as in the picture?[/quote]
Even best case there's an octave wide hole between where the useful high frequency response of a woofer stops and the average tweeter begins. This cab is a long way from best case.

[quote name='Beedster' post='1319915' date='Jul 28 2011, 07:08 PM']I can understand why the speakers are aligned as they are, but does it work?[/quote]
It does, but the widened dispersion of the mids was obtained at the cost of low end due to the reduced internal volume.

You'll get the best result with another cab loaded with the same driver complement as is the cab you have now, so that probably means an Ashdown 410.

[quote name='Mr. Foxen' post='1318791' date='Jul 27 2011, 05:23 PM']Trace already did stuff with front resonant chamber:

[/quote]That's taking it to the extreme, and what you end up with is a 4th order bandpass. I'd imagine the mids are pretty weak with that one.
I once did some experimentation with the effect of a slot alone, as opposed to a full horn. A slot lowered the effective Fs of the driver by as much as 25%, which is a plus as far as the low end is concerned. But it also lowered the upper corner frequency by the same amount.

[quote name='Mr. Foxen' post='1318530' date='Jul 27 2011, 01:40 PM']Bear in mind in that image, the hole is about one wavelength across.[/quote]Also bear in mind the saying 'There's no such thing as a free lunch'. When you place a barrier in front of a driver in that fashion you're creating a resonant chamber, and that resonant chamber acts as a low-pass filter, attenuating the same high frequencies that you're trying to improve the dispersion of. With foam as the barrier the effect won't be anything like with a piece of wood, but it will still be there. In acoustics you can never gain in one area without making a sacrifice in another.

[quote name='4 Strings' post='1318340' date='Jul 27 2011, 10:08 AM']Here's a photo of the tidal movement through a restriction from above.

Is this what the hole does (seems the opposite of the beam blocker)?[/quote]
Yes, though while it seems the opposite of the beam blocker it isn't. It's just two different implementations of the same phenomena.
What the pic shows is what happens to the short wavelengths that lack sufficient energy to vibrate the foam. But since the foam isn't a solid object it's not 100% accurate, as some of the shorter wavelengths will pass through it, some with an alterdd path, some not. And the longer wavelengths will pass through the foam as if not even there.
As for the beam blocker, it acts as much as a phase plug as it does a diffraction lens. That being the case it will be more effective in the upper mids than in the highs.
You can very closely duplicate various diffractive effects with the applets found here, and also emulate side by side sources to see why they should not be so placed.
[url="http://www.falstad.com/wavebox/"]http://www.falstad.com/wavebox/[/url]

[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.