LawrenceH

[quote name='ficelles' post='1323782' date='Aug 2 2011, 02:11 AM']Interesting perspective, not sure the accepted acoustics viewpoint is the same but then maybe that's just my perspective Splitting hairs I know but is the study of the hearing mechanism really biology, isn't it physiology?[/quote] Well, physiology is part of biology. At least the physiologists I work with would count themselves as biologists! Physiology as an active research subject often goes by the name neuroscience these days. From a biological/physiological perspective, you can break human hearing down into three major components for study. First, soundwave propagation through the ear/body. Second, signal transduction. Third, signal processing - a complex area. The reason this is the hot topic is because if you can't hear it then it doesn't matter. It's comparatively easy measuring physical characteristics and, say, comparing them to a reference. Harder to tell what's going on in the head and appropriate tests employing sufficient subjects are complex and expensive. [quote name='alexclaber' post='1324104' date='Aug 2 2011, 12:27 PM']When it comes to the specifics of loudspeakers and acoustics I'm essentially self-taught. But I have a BEng from Bristol in Mechanical Engineering (and obviously the relevant A-levels before that) and the general principles of engineering apply directly to the specifics of loudspeakers.[/quote] It's interesting, at least to me, that the behaviour of speakers from a physical perspective is a relatively mature subject, with the issues more in the application of that science through engineering. Whereas the science of hearing is much less developed, with lots of fundamental issues outstanding. Most designs are, ultimately, subjectively measured because even though we can characterise their output we can't completely model the ear/brain side of things. I think a lot of people don't actually understand the (blurred, but definitely substantive) difference between an engineer and a basic scientist. A scientist can understand the principles behind how a speaker works...but designing a good speaker based on those principles is another matter entirely requiring a different tool set. Btw I'd certainly agree with Alex that the output advantages of a properly ported cab make them easily worth choosing over sealed units for high-power PA/bass cabs. I don't think anyone has claimed otherwise here. But it is possible to royally cock up a cab with haphazard porting, as ficelles has already discovered - and some problems will only manifest themselves as you reach higher volumes. Thinking back to your problem before ficelles, when you added the pipe it's possible you introduced a port resonance which would be a harmonic sitting above the actual port tuning frequency. This can sound a bit 'orrible. [quote name='Bill Fitzmaurice' post='1323793' date='Aug 2 2011, 03:33 AM']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] I don't think what I said disagrees with this, though the hearing aid industry is not really very concerned with deep bass. One of the major aims of study for medical research into hearing is indeed generation of data that can be used to improve hearing aid design (though actually, according to my colleagues there is a often a bigger lag than you might expect between research publication and market product. Medicine is inherently conservative where possible). As for the home theatre buffs, I think for them it's not so much about hearing per se, they just like being vibrated you can certainly feel it when the floor is vibrating at 5Hz I would be fascinated to know how many speaker design companies are actually employing or regularly consulting with researchers into audio perception. I'm sure it's more common these days but historically it is a subject that has been dominated by engineers and physical scientists.

[quote name='ficelles' post='1323578' date='Aug 1 2011, 09:37 PM']I am intrigued as to why this topic has caused such dissent though! Am I right to assume that no-one in here has actually studied acoustics at college or whatever, or more specifically the acoustics of speaker cabinet design? ficelles[/quote] I have certainly studied the basic physics sufficient to give a reasonable description of soundwave propagation and the low-frequency behaviour of these boxes. A lot of that side of things is really no more than first-year undergraduate science, if that. I think the dissent is more in the area of acoustic perception, i.e. how we hear. There is lots understood and lots more ongoing research in this area, including that by my colleagues over in Glasgow. Why is this so controversial? My take is that in addition to areas of active debate within the field, there is a lot of stuff floating around that's out of date, not to mention claims that are likely unfounded where financial interests of exotic hi-fi companies/marketing men are concerned. 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. It's also worth bearing in mind that a lot of the research into hearing has been geared towards the interest of medicine rather than loudspeaker designers, and consequently perception of frequencies within the normal range of speech is prioritised. Add to that the fact that bass presents more practical complications to work with, read gets more expensive, and you see why it isn't an area that's focused on so much.

[quote name='stingrayPete1977' post='1323360' date='Aug 1 2011, 06:29 PM']Thats the first make of class D amp anyone has mentioned on BC as not being able to run on 120v.[/quote] Not exactly, the Markbass F1 requires a jumper (and fuse of course) switching round when changing between 240 and 120v

[quote name='Bill Fitzmaurice' post='1323280' date='Aug 1 2011, 05:13 PM']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] That is rather rich given that you were the one who managed to construct an argument from nothing, by putting words into my mouth. I struggle to see why you do this. ficelles, as has been said before a port without an additional pipe is still tuned. The effective port length can usually be considered the width of the baffle with the hole in. In general for a given box size, a port tuning is raised by either enlarging port surface area or decreasing port length. So you can have a short port with a deep tuning by keeping the diameter small. But there is no free lunch as the maximum air flow before it starts to get very noisy through this port is reduced. It'll stop behaving itself at higher volumes and you won't reach the full potential of the loudspeaker driver, which is why cabs for bass ideally have large, long ports. As BFM has suggested the easiest way to estimate how different ports will behave in your cabinet without actually riddling it full of holes is to plug the measurements of the cab into appropriate modelling software and play around with the numbers. Your port length before probably resulted in a tuning that was too high, giving a hefty bump in the lower mids/upper bass - increase the length and the speaker will behave a bit more smoothly and go deeper.

[quote name='Bill Fitzmaurice' post='1322712' date='Aug 1 2011, 02:49 AM']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] Tough guy. There are plenty of subwoofer configurations out there that have audible group delay, 30ms or more - not great designs (though to call them bad really depends what you are trying to achieve, you can't defy physics) but they are commercially available so LF group delay is a real world issue. You have taken a comment I made, that group delay exists and the audibility threshold is disputed, and managed to pointlessly manufacture an argument out of it by putting words into my mouth and atttributing me a stance that I haven't taken. Nice. Fortunately for me, I am a research scientist and know perfectly well without your 'help' how to read AES papers, design and evaluate experiments, and assess evidence (and see through BS). Oh yes, and understand basic, not even undergraduate-level physics. Because that is what I do for a living. Incidentally, as part of that living I've attended seminars at the MRC Institute for Hearing Research and dealt directly with their scientists. 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 name='Bill Fitzmaurice' post='1322181' date='Jul 31 2011, 03:10 PM']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] 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 name='ficelles' post='1321881' date='Jul 31 2011, 12:36 AM']Indeed, but isn't it in phase but a wave behind at and above the resonant frequency? I.e. the backwave phase is inverted by the port so in phase with the frontwave but with a single wave time lag? ficelles[/quote] Hello my fellow Devonian. At the port's resonant frequency nearly all the output will be from the port rather than the speaker. The threshold of audibility of group delay (frequency-dependent delay) is a bit of a controversial topic. It's important to remember that even a non-ported speaker exhibits group delay in the low register. I suspect that audibility thresholds show considerable individual variability and can change with practice, but I don't know if this latter point has ever been tested properly.

Segues are awesome. Medleys...hey, why not suggest to the band leader an even better idea - he could program the backing parts into a machine, that way it'll be really consistent night after night and the rest of the band could put their feet up. In fact, why not take the actual recordings, mix those into a few choice medleys, then simply take that out to gigs with a light show, a soundsystem and a laptop. That way no-one has to worry about singing, plus he saves a ton of money not having to pay a band. And he can call himself Disco Dave and get a banner made.

Worth considering something with a graphite neck, no?

[quote name='MoonBassAlpha' post='1319906' date='Jul 28 2011, 11:52 PM']Is that a big problem if you also have a tweeter, as in the picture?[/quote] It is if you like mids in your sound. Instant smiley face graphic for those who like that though.

All this stuff about the load being more on the strap if you move it further down is incorrect, and the science behind what's actually involved is very simple. It's about the centre of mass around a pivot point. In this case the pivot point is on the strap, at the point where the load is balanced equally. Assuming the strap is free to travel along the pivot point, the point of equal loading is dependent on the anchor points of the strap relative to the object being suspended. To make it clear which way this works, consider the extreme cases of a bass with an infinitely small distance between anchor points - ie suspended from the bridge end alone or from the horn alone. Pretty obviously, a bass suspended from the bridge alone has the worst neck dive possible (pointing at the floor). So Ou7hined is basically correct, apart from a few special cases with unusual body contours. But, if the horn strap pin isn't sufficiently neck-wards, even suspending from that alone will still leave the centre of mass too far up the neck. In those cases altering the bridge strap pin won't help much either way. Having said all that...this simple model makes the assumption that the strap is free to slide around the pivot point. In actual fact, there is considerable friction between the strap and the player's body, plus the player's body gets in the way of the instrument preventing it hanging freely and probably takes some of the weight. If less of the weight is taken by the player's body eg because you alter the strap position so that the guitar hangs at a different front-back angle, then it will improve neck dive, potentially even if that move takes the pin the other way to normal.

[quote name='lanark' post='1319071' date='Jul 28 2011, 10:18 AM']Well, it's accepted that Stradivarius used crap wood that instrument makers would nowadays rather burn than make an instrument from. Guess what, the explanation for the tone moves from wood, to the "special" glue or varnish or something, whatever. It can't just be that he was exceptionally good at making instruments and that the magic comes from a) the maker's skill and b ) the listener's brain *after* being told it's a Strad.[/quote] Well no, the magic can't come from the maker's skill because it's not actual magic, the skill has to have resulted in some physical property of the instrument. Psychological effects are almost definitely involved as well, but it's not like all Stradivari sound brilliant and every other violin total arse, its just that good violins sound incredibly different from bad ones. But a braced hollow structure like a violin is incredibly different to a solid body guitar. People often misinterpret that to mean that wood is more important for the acoustic instrument, but actually what it means is the structure as a whole is more important - of which wood type is just a part, hence the importance of bracing. Shape of a solid body instrument within reasonable boundaries is by comparison much less critical. As far as I can see, this discussion always goes on and on for 3 reasons (not pointing the finger at anyone in particular, just an observation of what always happens). First, insufficient grasp of the physics involves. Second, insufficient understanding of how to formulate a scientific hypothesis and test it. Third, insufficient understanding of statistical analysis to interpret results. The second reason is the most insidious IMO, because people often end up at loggerheads when really they'd probably agree on the evidence, just interpret the question differently.

[quote name='Mr. Foxen' post='1318791' date='Jul 27 2011, 10:23 PM']Trace already did stuff with front resonant chamber:[/quote] I wondered if these would get a mention. Bit of an oddity in the bass guitar world, and I recall them polarising opinion a fair bit too! Neat idea though, 4th order bandpass but a port that radiates directly and modifies directivity at the same time. Scoop city when you fired them up, it wasn't very hifi! The contribution of chamber resonance to the output of a foam blocked cab is by comparison going to be negligible by guitar/bass cab standards though...

[quote name='Bill Fitzmaurice' post='1318471' date='Jul 27 2011, 05:35 PM']Yes, though while it seems the opposite of the beam blocker it isn't. It's just two different implementations of the same phenomena.[/quote] That really depends on the foam used. This 1" thick auralex foam has absorption coefficients approaching 1 above 2000Hz. [url="http://www.auralex.com/testdata/test/1wedge.pdf"]http://www.auralex.com/testdata/test/1wedge.pdf[/url] A good acoustic foam at even 1/2" will absorb energy more than reflect or diffract in the mid-high frequencies giving a very worthwhile reduction in output, while as Bill says being essentially transparent at low frequencies. In those cases that image is a good visualisation for a single high frequency wave.

[quote name='Marvin' post='1318382' date='Jul 27 2011, 03:52 PM']Unfortunately the detractors, of what is somewhat basic acoustic engineering, put up little in response apart from to say they're talking rubbish because no one else builds cabs like that.[/quote] Lots and lots of people and companies build cabinets like the BF ones, ie reflex boxes. A fair number of manufacturers in the PA world have also build horn-loaded cabinets like the BFM designs. Their strengths and limitations are pretty well characterised - I built a set of Jack 10s, measured them and while I was at it modelled the design in software. The design behaved as the software predicted.

[quote name='cytania' post='1317680' date='Jul 26 2011, 09:45 PM']Somewhere out there I remember a research picture of the resonant parts of an electric guitar and the body hardly vibrates at all compared to the neck. So these experiments with lumber are revealing that in a sense the body and the body/neck joint are as much a damper as they are a resonator. Clearly the lumber used was part of the mid-spectrum of useable woods between the balsa/leadwood poles.[/quote] This is really interesting...after much swapping of pickups, necks, bodies, tapping of woods and pondering I have come to the conclusion that the role of the neck is a bit under-appreciated and would probably look to change that before body wood if I wanted to make tonal alterations.

[quote name='EdwardHimself' post='1317726' date='Jul 26 2011, 10:24 PM']but I think it sounds pretty good. Of course i've never tried a barefaced or bill fitzmaurice cab before so for all I know it might be totally rubbish.[/quote] That's because it is, the B&C drivers used are very capable indeed and the cab I've seen inside used decent wood. Some people don't seem to like the character of the sound but that's neither here nor there really.

[quote name='Bill Fitzmaurice' post='1317393' date='Jul 26 2011, 05:50 PM']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.[/quote] But that's not an alternative view that is what was described in the original link and by me. Hence the existence of a hole is essential for the design. [quote name='Bill Fitzmaurice' post='1317393' date='Jul 26 2011, 05:50 PM']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] Yes, I nearly wrote that before actually but thought that since a lot of slot tweeters are designed with horns/waveguides as well it confuses things. Anyhow a slot design is fine if you want narrow vertical distribution, but for the Beam of Death (which was why I brought up those foam things in the first place), I'd say it's better to have a more uniform dispersion to get it in the guitarist's ears!

[quote name='Mr. Foxen' post='1316945' date='Jul 26 2011, 01:00 PM']I've already read it, and the dissection of it with various engineers, including BFM. Just because something works doesn't mean that the explanation of why it works is true.[/quote] Wave transmission through an aperture at different wavelengths relative to the aperture width is secondary school physics, and pretty easy to visualise graphically. The problem in description is not in the foam donut but on the misconception where circular beam blockers are described as 'blocking' a 'beam' from the cone, when there is no such beam but instead a central point of symmetry where wave summation is maximally coherent. They actually 'work' by reflecting and as BFM has said diffracting the sound waves. 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. Graded density foam can be used to act as a waveguide but that is relying on wave propagation through the foam and far greater thicknesses are required, ie a stuffed horn, to give sufficient distance for the induced delay to alter directivity appreciably. This however is a refractive effect - as explained on one of Geddes' own patents for a stuffed waveguide. Foam stuffing is also used to absorb sound in waveguides/horns, which [i]corrects[/i] for [i]problems[/i] introduced by diffraction. It is not a diffractive effect in its own right.

[quote name='Bill Fitzmaurice' post='1316462' date='Jul 25 2011, 11:22 PM']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] Careful, or you'll reinvent the phase plug With flat reflective blockers though you'll create a horrible mess of variable HF path lengths. With a theoretical totally absorptive round flat blocker you will create a ring-shaped aperture which will give quite a strange diffraction pattern at high frequency but do very little other than a small reduction in volume in the mids and lower. In the context of the foam donut thing, the opposite edges of the loudspeaker cone are no different to two smaller spaced drivers, with the same solution presenting itself ie get them closer together.

[quote name='Bill Fitzmaurice' post='1316393' date='Jul 25 2011, 10:11 PM']Not if you do it right, which is with varying foam thickness across the cone. See the Geddes model. 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] 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 name='Bill Fitzmaurice' post='1316285' date='Jul 25 2011, 09:15 PM']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] I didn't claim it was new or novel - just that it works and is quite neat - that one happens to be a good cheap implementation. But in that design the hole in the middle IS what makes it work, otherwise you just attenuate the sound without modifying directivity. The foam with hole is just an HF absorber to reduce effective cone diameter. Some of the designs use different density foams to alter the speed of sound through the material at different points to make a waveguide but that's not what's going on here. EDIT - oh yeah and a standard beam blocker doesn't use the same principle at all as it doesn't work, at 'blocking' the beam that is

[quote name='Bill Fitzmaurice' post='1316341' date='Jul 25 2011, 09:36 PM']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] Yes and it'll become more and more common for larger acts I think - you need bloody awesome monitoring to pull it off though, way beyond what's 'average' in the UK and it still doesn't sound the same on stage. One functions band I used to do sound for went down this route, but in the end they were never satisfied with the stage sound and regressed back to big amps (sigh)! I suppose those same limitations inherent in a big stack also contribute to a unique stage sound.

[quote name='thodrik' post='1316257' date='Jul 25 2011, 09:00 PM']They see something that looks like a Fender and when it doesn't handle or sound like one, they go away disappointed and dismiss the bass as average/generic/rubbish.[/quote] Yeah this is definitely part of it. Though that's fair enough given what it looks like and Sadowsky's pedigree in the world of Fender repairs/mods! If they had a custom shape then even with the exact same pickup configs then people would see them as different. But, sound-wise, I honestly haven't heard anything that would distinguish their own 'signature' sound as distinct from any old jazz with aftermarket pickups/pre.

[quote name='ficelles' post='1316280' date='Jul 25 2011, 09:11 PM']I sense a beam blocker discussion approaching... ficelles[/quote] Oh dear, I hope not! Because they clearly don't work as designed... Mr Foxen, what I mean is that the problem of the guitarist not hearing himself is more down to the fundamental limitations/directivity of a single large diameter speaker than the comb filtering effect from stacks. Even if your stack is set vertically the sound will be highly directive just due to the speaker. Btw vertical stacks will still comb filter, just in one plane - but audience members at different heights will get just as different a sound as those in different positions around a horizontal stack. Vertical stacking is not an ideal solution and in, say, theatres or any gigs with balconies then it will actually be more problematic. Bill makes a very fair point that guitar stacks are overkill and a single speaker would do the job, though personally 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".