Bill Fitzmaurice

[quote name='Chienmortbb' timestamp='1410452743' post='2549614'] Of course unless you play bass flute or whistle through your system, you will not be putting a sine wave through your system. [/quote]Nor then. All instruments save electronic have harmonic content, and no one I know of plays a synth patch that's a pure sine wave. The only instrument I can think of offhand that might play pure sine waves is the Tannerin (that's what's on [i]Good Vibrations[/i], not a theremin). I say might, as how pure the wave is depends on the quality of the oscillator and the attendent circuitry, and the speaker as well.

[quote name='Magic Matt' timestamp='1410451921' post='2549598'] 100Hz square wave would require the speaker to move over that 4mm distance instantly, then stay there motionless for half a cycle, then instantly jump back, stay motionless for half a cycle, etc... [/quote]That's part and parcel of the myth. It doesn't happen. Where a speaker is concerned there is no such thing as a square wave as seen on an oscilloscope. [quote] I'm not sure what you're saying by "breaking down a square wave into its basic components"... square waves are the basic component![/quote]That's also part of the myth. A square wave is a sine wave with all of its harmonics present with equal power density. Run a square wave through a brickwall low pass filter, its corner frequency set slightly above the fundamental, and you are left with the original sine wave. Further reading: http://forum.qscaudio.com/forum/viewtopic.php?f=29&t=2736

[quote name='Sonic_Groove' timestamp='1410437959' post='2549412'] If you could convert the SPL of an average acoustic guitar into watts how may approx would it be.[/quote]Assuming a level of 90dB at 1 meter that would be 0.001 acoustical watts per square meter. [quote]More specifically if you want to duet with an acoustic with bass what is the fewest watts you can get away with? [/quote]Impossible to say. First, watts are not a measurement of sound levels, they're a measurement of power consumption. To convert to what sound levels are measured in, decibels, you must consider a number of factors, not the least of which is that loudspeakers have very poor electrical to acoustical power conversion efficiency. From a practical standpoint the watts don't matter, the speaker size does. A typical ten inch driver loaded twenty watt combo will have no problems matching an acoustic guitar in output. For that matter an eight inch driver loaded combo will have adequate output, but those tend not to have very good low frequency response.

[quote name='Magic Matt' timestamp='1410422288' post='2549201'] Also, speakers can't produce proper square waves...It's going to have to accelerate, and decelerate, at each "end" of the waveform, and trying to stop instantly is probably going to introduce artifacts into the sound. [/quote]Logical, but that doesn't occur. On reason why is because voice coils are inductors, and like all inductors they are low pass filters. That being the case, a goodly portion of the harmonics that make a square wave square are filtered out. When you break down a square wave into its basic components it's not intrinsically different than any other complex wave form, and drivers have no more issues with them than they do with any other complex wave form.

[quote name='Magic Matt' timestamp='1410382508' post='2548928'] [color=#282828][font=helvetica, arial, sans-serif]Are we talking harmonics that are still lower than the fundamental frequency of the square wave[/font][/color] [/quote]Harmonics are multiples of the fundamental, so they're always higher in frequency, never lower.

[quote name='Magic Matt' timestamp='1410377714' post='2548836'] if the speaker can handle more power than the amp can put out, how is that any different to playing music through it that's been hard-limited, or a sound from a keyboard that's basically a square wave?[/quote]It isn't. [quote]Seems to me that this whole under-powering thing is just a load of nonsense... so what I am I missing? ...or aren't I? [/quote]Clipping can toast tweeter voice coils, because what makes a square wave square is harmonics not present in a clean signal. Those harmonics can increase the power seen by a tweeter tenfold over normal, and that's what toasts the voice coil: overpowering, not under-powering. BTW, this document is the source of the under-powering myth: http://www.jblpro.com/pub/technote/lowpower.pdf With the re-telling of the tale, probably by the first wag to do so, the key phrase 'high frequency components' was omitted, and the myth was born.

[quote name='Magic Matt' timestamp='1410365239' post='2548616'] I've bought a set of Mr. Fitzmaurice's rather excellent looking Simplexx 12 plans. I will probably follow those, but possibly vary a little to add batons in the corners, just purely for the sake of compensating for the lower quality ply. [/quote]Battens are a waste of wood and weight, as the corners of the cab are the strongest point of the cab. Suggest you join my forum to post any questions, and be forewarned that if you suggest any alterations there you will be told in no uncertain terms that there are three rules that you must not violate. They are: 1. Follow the plans. 2. Follow the plans. 3. Follow the plans. They'll also be able to tell you where to source your plywood.

[quote name='Magic Matt' timestamp='1410301280' post='2548088'] I couldn't see what drivers I'm supposed to buy for them...? [/quote]None, until you have the plans in hand and are able to weigh the options. There's very little difference in the cost of two 112 versus one 212, the only major one being the need for more jacks with two cabs.

I'd always go for two twelves over one fifteen, and for that matter two 1x12 over one 2x12. If you're thinking of a course change you should consider my Simplexx 12, as all of the R&D is done, as is the forty years of experience that went into that R&D. You just have to put it together.

[quote name='Magic Matt' timestamp='1410264681' post='2547594'] So, I'm looking to get the most output I can over 50Hz, but avoid getting the 41Hz low E dropping too far down that steep slope, otherwise it will be much weaker than the other notes... yes? [/quote]No. While the low E note has a fundamental of 41Hz most of its content is in the harmonics. For reference, a Fridge f3 is 58Hz. No one complains about how it handles the low E.

Low 'E' is 41 Hz. Most amps have a high pass filter built in to prevent over-excursion below 40Hz. Maximum output above 50Hz is what matters.

[quote name='Chienmortbb' timestamp='1410215722' post='2547311'] Got to also say the AC30 bass was a POS. [/quote]So were 60s Fenders that were basically guitar rigs without reverb. Best guitar amp ever? '59 Bassman... with outboard reverb.

[quote name='Magic Matt' timestamp='1410136424' post='2546529'] Ok... but if tuning the cab lower gets rid of the hump, and doesn't seem to make a huge difference to SPL on the graph... why is that bad? ...can't I just turn the amp up a tiny bit more to compensate? (I expect I'm missing something crucial here, but I don't know what) [/quote]Look at the Maximum Power and Maximum SPL charts.

[quote name='karlfer' timestamp='1410170694' post='2546682'] I always thought a bit more speaker watts than amp watts, so with the Carvin I would have looked min 800watts cabs. [/quote]Most cabs reach maximum driver excursion at no more than half their rated power, so that's all you need to get full output from them. You can have more power on tap if you wish, but you simply won't use it. The same applies to PA. Where PA differs to some extent is with respect to high frequency drivers. They can be harmed by clipping, so plenty of amp power used to be a necessity to prevent that, but clipping is very effectively prevented with electronic limiting, something which was not an option when this was written: http://www.jblpro.com/pub/technote/lowpower.pdf As for the link's take on guitar speakers, he misses that by an even wider margin. Most guitar drivers have such short xmax, 1mm or less, that they reach maximum uncompressed output in the lows with only 10-15w input. The power required where guitars get most their decibel output, between 500 and 2kHz, is quite low, and doesn't require massive driver capacity to handle it. Anyone doubting that has never been on the receiving end of a Vox AC-30.

The problem is that it's incorrect, because like most laymen he's considering watts and that's not what matters. Voltage swing and driver excursion are. Suffice it to say that where watts are concerned if your amp is rated anywhere between one half and twice what your speakers are rated for you're fine.

[quote name='Magic Matt' timestamp='1410126455' post='2546474'] Ok, so I downloaded WinISD Pro and tried to put the parameters in direct from the spec sheet.... same as I had in WinISD... [url="http://www.eminence.com/pdf/Basslite_C2515.pdf"]http://www.eminence....slite_C2515.pdf[/url] This happens... ...oh. Bugger. Now I don't know what to do. [/quote]That's because WinISD is more accurate in the spec calculation than Eminence, going to more decimal places. Only put in as many specs as required, let WinISD calculate the rest. That means Qes,Qms,Vas, Fs, Re, Le, Sd, Xmax and Pe. As for eliminating the hump, you do that by using the correct size cab. Since you're undersizing the box you'll have to live with the hump.

Why tune so low? Maximum output demands, even for a low F# six string, lie between 40 and 70Hz, so you should be looking at the maximum SPL chart to find the tuning that gives the best result in that range. Tuning lower than 40Hz robs you of maximum output in the 40-70Hz bandwidth, and it makes the cab larger than it needs to be. If you have a 4 string you should be tuning even higher, 45-50Hz.

[quote name='Magic Matt' timestamp='1410084748' post='2545885'] It'll be a simple box, rectangular bass port, 130 litre, tuned to 25Hz Recommendations please? [/quote]How did you arrive at that? The C2515 models to the flattest possible response in 412L tuned to 33Hz. That's silly large, of course, but points out why one should think thrice about using the C2515. It can work in 130L, but you wouldn't tune it at 25Hz. 40Hz is the lowest you'd want to go. 25Hz would make the port unmanageably long, and would cause a serious loss of maximum output, while there's no benefit to be gained from tuning that low.

[quote name='bonzodog' timestamp='1409911065' post='2544474'] Very glad I started this thread as there is clearly difference of opinion which I find interesting. [/quote]Well, there's science and there's speculation. Science will win that battle every time.

[quote name='Marky L' timestamp='1409836488' post='2543796'] I currently run ..a 2x10 cab ...rated at 8ohms. I fancy adding a 4x10 8ohm to the 2X10. I can't see what probs I would have. [/quote]The drivers in the 210 would see twice the power as those in the 410, making the excursion capability of the 210 drivers the weak link in the chain. Looking at it from another perspective, what you'd be able to get out of the system wouldn't be all that much different than what you would get out of a matched pair of 8 ohm 210s. The additional output capability of the 410 wouldn't be of much use with the 210 holding it back.

[quote name='mart3442' timestamp='1406894967' post='2515679'] I'll tell that to my friend with the fried amp...... [/quote]His amp may be fried, but it had nothing to do with the lack of the speaker. When in standby mode there is no current flowing to the tube plates. That's why no sound comes from the amp when in standby. If there's no current there's no potential for damage. He might as well have claimed that he blew the engine in his car by pressing the gas pedal to the floor...whilst the ignition was off.

[quote name='spacey' timestamp='1409599335' post='2541410'] The trouble is a lot of of 2x10 are 4ohm and end up doing the lions share of the work if the 4x10 is 8ohm [/quote]That's because of the manufacturers conceding to the "I want to get all the watts out of my amp" syndrome. A 16 ohm 2x10 would be best, allowing you to use up to four of them if you wish, or to pair one or even two of them with an 8 ohm 4x10. But even though it would be the dodgy amp that couldn't drive a 16 ohm 2x10 to full output no manufacturer is going to produce them, because the average player doesn't know that, and would never buy a 16 ohm cab.

[quote name='budget bassist' timestamp='1409581179' post='2541105'] if I end up with something that sounds similar to my Ashdown kit [/quote]Setting your sights way too low you are. DIY isn't about equalling off the shelf, it's about kicking the snot out of off the shelf.

[quote name='budget bassist' timestamp='1409576681' post='2541035'] If what you're saying is true, then surely I can't do much worse than an ill-considered mass-manufactured box would be? [/quote]True, but what most newbies set out to do is to basically build the equivalent of an ill considered mass manufactured box, with the intent of doing so at a lower cost. In the end they don't save any money, while mass manufactured boxes aren't worthy of emulation. For that matter I can count on the fingers of one hand the number of boutique manufacturers whose cabs that I'd consider owning, and one of those I did the design work for.

[quote name='budget bassist' timestamp='1409524872' post='2540616'] But how are all the manufacturers using such tiny enclosures and still getting a good sound? [/quote]They aren't. One doesn't have to go to the full optimal size for a driver, but one also should be judicious in how small one goes, as it does affect the result. OTOH most manufacturers aren't concerned with getting the best possible result, they're interested in sales. Average customers don't know anything about Hoffman's Iron Law, and think f3 is a note, not a marker of speaker performance. What they've been led to believe is that one gets the best results from stuffing as many drivers as possible into the smallest box, so that's what said manufacturers give them. Since these customers have never used a truly well designed and built cab they don't know the difference, so the cycle remains unbroken. [quote]half the satisfaction will come from having designed the thing myself! [/quote]One might say the same about a hang glider. Making an error with a speaker may not have as disastrous a potential result, but still you are asking questions that one fully prepared to design their own speaker would not be asking. If you're bound and determined to reinvent the wheel, and repeat the same errors made by all those who came before, you should at least avail yourself of resources such as this: http://techtalk.parts-express.com/showthread.php?219617-The-Speaker-Building-Bible