32957
From: james, [h-68-165-65-210.snvacaid.covad.net]
Subject: Distortion in different enclosure types
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I am interested in knowing more about the low frequency distortion caused by different types of enclosures. Particularly sealed and ported enclosures, but TL as well. Also I am particularly interested in very low frequencys around and below the f3 and f10. Where does the distortion occur? What happens as you go lower in frequency? How do the different enclosures compare? Thanks for your answers |
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32958
From: James Davis, [n8u1n3]
Subject: Re: Distortion in different enclosure types
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Distortion essentially only occurs when the Xmax is being exceded (which can be done withou damage). If you have a ported box, and you play it below the Fs, then you unload the port and air is allowed to flow through it, and the cabinet loses its compression/rarefication properties, it also loses the ability to control the speaker, and it will begin to move much further, when it hits the end of its travel it stops, thus creating the distortion. DO NOT however think that this means speakers wont die when you do this, but it won't be immediate, but... the voice coil gets EXTREMELY hot when the speaker stops at an extended or retracted point, and it will burn up the coil after not too long. In a sealed box, distortion occurs if you play it too loud. Also you asked about frequecy drop. As the frequency decreases, the wavelength increases thus calling upon the speaker to move farther. Hence why tweets are hard to destroy. 5k doesn't require much distance from the diaphragm, only speed, which is why they are small. Now... playing below the F3 point. no way around it, if you don't filter it, the speaker WILL be playing below the F3 point, BUT, that is meaningless, because, for anyone who wasn't sure... F3 came from at what Frequency (F) the sound would be 3dB less (3). Hence F3. 3dB is a detectible gain or loss to the human ear. So what makes the F3 point significant is not that it plays no lower, it just becomes so much quieter than the rest of the sound you can't hear it. After the F3 most subs roll off at 12dB per octave. By multiplying the power by 10, you double the output volume (10dB gain) In THEORY. THEORY. THEORY... If you could build an autosetting amp that would compensate logrythmically for the F3, and double the power every octave below the F3 your bass extension would be limitless. But, we're back to excursion. no sub could take it. But if a guy had a magic sub that had xmax infinite, and the magic amp that self adjusted below the F3, you could boom as low as you want. But since hearing stops at 20Hz, you wouldn't care. So if you had a huge sub... Kicker Solo X 18 inch, 10,000 watt... if you were playing at 40Hz, and say 20Hz was F3. At 40 Hz, if you were pumping in 100watts, at 20Hz you would need around 1,200 watts (12dB), at 10Hz 14,400 watts. WAIT! your thousand dollar Solo X just burnt to a crisp, and the surround is laying in the floor! Oops! Even if your sub could take 100,000 watts, play it at 5 Hz, you need 172,800 watts, Crap, I just blew up a sub so powerful it doesn't exist, just because i tried to go 3 octaved below F3 and still hear it. Continue the example 2.5Hz 2,073,600 watts 1.25Hz 24,883,200watts .625Hz 298,598,400watts .3125Hz 3,583,180,800watts THREE BILLION WATTS?! Wow, that kind of power doen't exist in any amp I've seen... hope I helped answer your question. |
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32964
From: James, [a17-206-41-214.apple.com]
Subject: Re: Distortion caused by more than xmax
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Hi James I am re-reading and thinking about some of what you said: --------------------------------------------- James Davis said in part: Distortion essentially only occurs when the Xmax is being exceded. ---------------------------------------------- This seems to me an over simplification, and not true. I mean there are many causes of distortion which are not directly connected to the over travel of the x-max. Subwoofers tend to be distortion heavy devices. I mean they are not infinitely stiff, infinitely light, nor do they accelerate or decellerate without delay. But I don't want to go off topic into drivers just now, interesting as that is. I would very much like to better understand what causes distortion in standard enclosures. For instance, in a port enclosure, below resonance, the woofer is uncontrolled right? Whether xmax is approached or not, there seems to be some funky stuff going on. Also the stuff coming out of the port, how clean is that? James |
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32968
From: Adrian Mack, [dialup-196-208.wasp.net.au]
Subject: Distortion is caused by much more than just excursion limits
URL: My website, Peerless XLS 12 Dual PR Sub Project, 18LW1400 vented sub, Horns, etc.....
|
Hi James! A while ago I made a webpage about distortion. You can view it at http://www.geocities.com/adrian_mack/distortion.html. But I'll copy and paste it below anyway for those that are interested. ------------------------------------------------------------------------------------------------------------ Loudspeaker DistortionWhat is distortion I define distortion as ALL artifacts that are reproduced by the loudspeakers which are not in the original recording. This is the equipment's imprecision to properly reproduce the signal presented. Distortion is just plain bad, and you should avoid all types of distortion. Not only does it sound bad, but it can be damaging to your speakers too, so avoid it at all costs. Distortion can arise from the driver itself, the cabinet it is placed in, passive and active filters, and the amplifier. If you go to the Vented vs Sealed and Damping/Q sections of this website, you will see more information about distortion and how to minimize it. THD, or Total Harmonic Distortion is a common type of distortion, and is the one usually used to measure it. Usually expresed as a percentage. Electronics equipment such as amplifiers usually have very low THD levels, usually 0.1% or lower, which is WAY lower than what speakers are rated, and are of no audible difference. Hence amplifiers arn't usually a source of distortion. Amplifiers can have a different sound from each other, but distortion specs are all very low. Really, an amp is a signal multiplier, and nothing more. What makes amplifiers different from each other is its features, like signal-to-noise ratio, power ratings, resistance to clipping, additional crossovers, etc. Better quality amps have bigger internal power supplies, so they perform better and clip less at high power levels than an amplifier with a poor power supply. All these really are aspects of THD and S/N measurement, but thats what makes amplifiers different from each other. But overall, amplifiers should not sound different from each other or at differnet power levels until it approaches clipping, because amplifiers should not add, or subtract anything from the signal - all it should do is multiply what is already there. Matching the impedance of the speakers to the amplifier is essential too so that energy is transferred properly to them. It is reasonsable to say that thet lower the impedance, the higher the harmonic distortion, because too much power is drawn from the amplifier without properly transferring the signal, which can also lead to premature clipping. Tube amps have different sound of course, and are "famous" for their even-order harmonic distortion, which tube guys likes, and these sorts of amplifiers can provide a more "smooth" and "warm" kind of sound. Some people prefer this, some dont, but beware that only speakers with low maximum impedance should be used on tube amps due to their low damping factors and low power output. Mismatching a tube amp to a loudspeaker can increase distortion. Distortion in the crossovers/filters is another factor, and can introduce fair amounts of distortion if poor quality parts are choosen. Really, thats what it is, the quality of the parts that will determine how much distortion your crossover will have, and expensive parts are usually lower in distortion. High quality parts means using heavy guage air core inductors, polypropylene capacitors, and non-inductive resistors. Distortion from various crossover designs with many components in the signal path is rather low that it can be ignored. And of course, incorrect crossover points can introduce distortion and also some crossovers produce signal distortion (ie: a response peak or dip caused by the crossover) but that will be described in the crossover section of this site. Loudspeaker distortion is the biggest factor, which originates from the motors of these speakers. If the motor cannot effectivily control the cone, then distortion is enevitable. Motors with low magnet strength (BL) and high Qms figures dictates poor cone control. Once Qms starts getting high, around 10, thats where the cone starts getting loose, and if the motor cant control it then we have higher total harmonic distortion. We can use motor braking from the amplifier to regain some control of the cone, but motors with really high Qms, like above 20 just tend to sound really bad, and have more overring and undefined sounds. If short bursts are applied, then it must be reproduced as shorts bursts - not one long burst, which is what overring is. Since the cone isn't controlled, it will have more overring, and more distortion. Although overring and distortion are two seperate things, an increase in overring will still increase harmonic distortion. What is also important in cone control and minimizing distortion is having a perfectly symetrical magnetic field with the same number of conductor windings within it at all times. Motors with underhung voice coils will have less distortion and higher linearity, because the total length of the coil is always within the gap unlike an overhung voice coil. Of course once xmax is reached, the motor will then become very unlinear very quickly, but this is not too much of a problem because usually the entire driver is designed so that xmax wont be exceeded until other parameters are reaching critical levels as well. Underhung voice coils are more rare, more expensive, and need larger and stronger magnets. This is what a lot of pro drivers use, which is discussed below. We can achieve minimal distortion if we choose high quality motors, such as JBL, which have very low distortion, typically in the area of 0.3%. Commerical home drivers have THD levels of around 3-4%. These are all measured in midband areas, which means in the region from ~50 to 200Hz. A good commercial woofer will at its best produce about 3% THD with 1 watt. But a high quality pro woofer will produce 0.3% THD with 60 watts, yes 60 watts. The pro drivers will basically produce so much less distortion, in fact, as much as 10 times better distortion performance at 10 times the power level. What you get with a pro driver at its worst is still better than what a comerical offering can do at its best. One thing to point out is that distortion starts to rise dramatically from above a half octave above resonance, and way more more below resonance. Even the most expensive drivers are like this, and THD can run up to 10% - 25% or more. Now imagine how a commerical driver will perform in this area, it is often 100% THD, and theres no way around this. Commercial drivers simply arn't manufactured to the same level as pro drivers. But dont discount commercial woofers, because there are some out there which are good for 99% of people. The most expensive JBL components can run to $1000 just for the driver itself, then you still have to add the costs of the enclosure, amplifier, and crossover. Its all a cost vs performance thing too, and for a lot of people paying thousands of dollars for something that does 0.3% THD is simply not an option. Speaker designers should be aiming for 3% THD, which is very good. But the fact still stands that the pro drivers do perform better with less distortion than commercial drivers. Distortion also rises as maximum excursion is reached. This is better dealt with in vented boxes. From about an octave above woofer resonance (usually Fh) and down to resonance (Fo), damping makes the system more rigid because the port is tightly coupled with the woofer. Distortion is reduced because excursion is reduced. Obviously in boxes that are larger and tuned lower than optimal introduce more distortion to the system than the best box would. This is the trait in "EBS" vented boxes, and also the same with oversized sealed boxes. In this case, the response is extended and has gradual rolloff in the bottom octave(s), but the system is allowed to be used under resonance and for this fact distortion is raised. Use of a subsonic filter can reduce this problem. Enclosure design is a very important part in how much distortion the system will exhibit. As said before, operating a driver below its resonance is a bad thing, and distortion increases 10 fold. The same is true for the resonant frequency of the system, or the port in case of a ported system. When a subwoofer is operated below its resonant frequency, distortion rapidly increases, and furthermore, the cone becomes very uncontrolled - in both vented and sealed systems. A sealed system has a resonance too, and is just as prone to overring as vented systems are. But I wont go into detail about this here, you can see more on this in the Vented vs Sealed section of this site. So to achieve low distortion, we can summarize this into the following sentence: "Choose only quality professional series parts, good quality crossover components, and use the driver in a properly tuned vented box". That will ensure you have very minimal distortion in your system. Why have I said to use a vented box? When tuned properly, they exhibited less overring than a sealed box can. This is descibred in the Vented vs Sealed section of this site. ------------------------------------------------------------------------------------------------------------ Obviously the links in that article dont refer to diysubwoofers.org, because its just an article I copy and pasted from my website. Those links arn't even up yet anyway, but I think you get the idea. Adrian |
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33143
From: Esmond, [ip176.bb35.pacific.net.hk]
Subject: Re: Distortion is caused by much more than just excursion limits
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Apart from distortion, group delay shall be considered as well. The subwoofer element inherently poses non-flat group delay across the spectrum, irrespective of what the enclosure design is. On top of that, the enclosure with vented duct would make the case worse. Those enclosures designed with totally hidden driver element, like high order bypass system, would make the performance even worse. The above straightly comes out from the result of “Linear System” theory. It shows the physics that gaining certain advantage(s) must sacrifice others in return. For signal with non-constant group delay, sound will be smeared timewise. Short burst of bass signal will be sounded unclear, not sharp. The other thing that needed to consider is the reactance of the subwoofer and the inter-reaction with the power amplifier. The reactance of the subwoofer itself is not constant against frequency. The enclosure design will also alter the reactance curve (against frequency), and the driving voltage and current is not even in phase. It should be seriously to re-think whether the subwoofer is voltage drive or current drive, as they are not in phase, and whether the power amplifier feedback is taken from output voltage or output current. Ultimately, what we need is a constant group delayed signal, or sound pressure (of course flat frequency response is a must as well) heard at a reference distance from the subwoofer (with referred to sound pressure measured / received by recording mic as played by performer). Of course, some of the non-flat group delay is out of our control (as already there as recorded in CD for example), and some of them are out of the scope of this forum. If we focused at the input of the active subwoofer (either voltage or current), and we can use feedback or servo technique on the power amplifier greatly improve the performance. This is something like Velodyne does, but not exactly the same. In the case of active subwoofer, as power amp and subwoofer are under one’s control, measurement signal could be taken from - Voice coil movement with co-winded detection coil (current dual coil speaker element could do, but just relatively heavy and create unnecessary weight on the moving part). - Voice coil position detection by laser - Diaphragm position detection by laser - Diaphragm position detection by multi-beam laser (to even out detection error from possible diaphragm vibration in high order mode) - Ultimately, place a sound pressure sensor (could be a mic) at the vent output to monitor the output sound. Such active subwoofer might be available in next two or three years. |
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33159
From: Adrian Mack, [dialup-196-92.wasp.net.au]
Subject: Acoustic Suspension vs. Bass Reflex
URL: My website, Peerless XLS 12 Dual PR Sub Project, 18LW1400 vented sub, Horns, etc.....
|
Hi Esmond! This is getting to an interesting discussion now, that is good. This gives us a chance to discuss the so-called phenomenon of group delay. And we can now discuss and quantify the mechanism "at play" at resonance. No speaker can be time linear at resonance. And what should be understood is that this is not just limited to bass reflex cabinents, and it is just as much of an issue in sealed. They'll both have just as much ringing at resonance after a half cycle being presented, and in fact, a properly tuned bass reflex box will have less of an artifact. Lets examine a half cycle sawtooth wave having a fundamental frequency exactly at cabinet resonance. Most of the waveform is at a very high frequency, relative to the fundamental. You can draw this as a single (half cycle) sine wave, and then to the left of it is a full cycle harmonic, and several cycles of the next harmonic, and so on. It is formed by ringing harmonics that have initial attack at the same time as the fundamental, but then decay. And some of the harmonics sum to cancel each other, and some of them combine to augment each other, until the total summed waveform is a sawtooth. But the point is, if the fundamental is not at resonance, then there is no phase issue to consider. And the higher order harmonics will also not be at resonance, and theres no cabinet induced phase change to deal with whatsoever. Only the fundamental will have the anomoly - and it is that the half cycle will be followed by another attenuated half cycle of the same phase. A frequency burst in the resonant frequency of a bass reflex cabinet will have an initial half cycle that has less amplitude than the remaining cycles and that after the final cycle is presented, has a residual but attenuated half cycle "still coming." The only difference between this and a sealed box is that the sealed box will have overring, but no increase in amplitude after the first half cycle. This is a resonant frequency burst phenomenon, and not a transient issue, because as shown - transients are at a much higher frequency. And both the bass reflex and the sealed cabinets will suffer from overring at their resonant frequencies. But both are just as unwanted as each other - but it is much better attenuated in a ported box than a sealed box can. A sealed box is mechanically reactive because the woofer is a spring/mass device. Theres no way around that, the cabinet can assist in dampening it but is helpless to remove the reactive nature of it. And you'll see the same sort of "ringing" at resonance in a sealed box as with a bass reflex box. The more dampened the system is, the less it will ring, and is much better dealt with in a ported box. A sealed box can only assist in dampening the motor, in essence, decreasing compliance. However the bass reflex box can take advantage of port dampening, which counterweights to accelerate dampening at resonance. You'll find that while both systems "ring", in a sealed box, we are powerless to attenuate this artifact and in a bass reflex cabinet, we've purpose designed it attenuate the very artifact that this document was written to discuss. Its not hard to tell the difference between a well dampened cabinet and a poorly dampened one, and its pretty easy to visulize whats happening too. It's perhaps a bit more difficult to describe without becomming a little bit technical. The issues of anomolies generated by speakers at resonance are caused by their generation of an "overring" - and that is all. The phase consequences are simply that cycles are added, and is more troublesome than "just phase". If you've added three half-cycles then one could, I suppose, describe this in terms of phase or "group delay." But it truly places the discussion in a counterproductive direction because it describes the phenomenon with ill-chosen terms. It's like describing speed in "furlongs per fortnight" when most of us are comfortable talking about "miles per hour" or "kilometers per hour." An engineer can calculate using any of these methods, but even he is likely to try and visualize the speed in miles per hour - which is what he is most comfortable with. So describing this phenomenon is best done by visualizing it as an overring - which is exactly what it is. It's decaying partial cycles added to an excitory input signal. It's what makes a bell ring when you strike it. It's what makes the tuning fork sing. To describe this using phase relations is to put it in an unnecessarily technical realm, and one that really masks the true issue. I think Servo inventions are good, but not necessary. A properly tuned bass reflex cabinet using a professional series JBL, 18-Sound or TAD driver is very good, and also has very low distortion which is what the original discussion was about. Essentially, we can sum this up by the statement: "All resonant systems vibrate with excitory energies, and phase shifts occur only for frequencies in the bandwidth of resonance. These phase shifts are of secondary consequence, and the real offender is resonance itself, which causes a tendency to overring." Adrian |
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32987
From: mikeported, [border0.tekotago.ac.nz]
Subject: good text.
| I want the links on sealed vs ported :P |
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32983
From: Brian Owens, [page.paccar.com]
Subject: Re: Distortion is caused by much more than just excursion limits
URL: my webpage. . .
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Hey, how about the Tumult? Less then 3% distortion @ 28mm one way excursion!!!! Thank you Adire for XBL^2!!! I will throw in with prosound drivers that many of them are low Xmax with large high strength motors made to handle extreme amounts of power which is helpful. But, it is funny that a lot of prosound applications actually like to drive things to a point of a little distortion, because distortion many times can be mistaken for louder by the human ear. . . I like that post Adrian. . .I printed it for future reference. Brian |
32998
From: Adrian Mack, [dialup-196-142.wasp.net.au]
Subject: Re: Distortion is caused by much more than just excursion limits
URL: My website, Peerless XLS 12 Dual PR Sub Project, 18LW1400 vented sub, Horns, etc.....
|
Hi Brian! I just took a quick look at XBL2 technology, and it seems Adire has made an excellant way to keep linearity and high levels of excursion at the same time. > I will throw in with prosound drivers that many of > them are low Xmax with large high strength motors > made to handle extreme amounts of power which is > helpful. Most prosound drivers do have rather limited xmax in comparison to commerical drivers, but the best ones have more than enough excursion for subwoofer applications. They are also higher in sensitivity too, and they really do belt out the bass and sound very clean. An average JBL Professional Series woofer generates just 0.3% THD at 10% of full rated power for its two midband octaves, which is 60watts for a 600watt woofer or 80watts for an 80watt woofer. The point is that this is extremely good performance, and unrivaled by commerical offerings. As we get closer to resonance, distortion increases of course, and the best JBL components still generate with under -45db harmonics which is less than 1%, and near resonance this is still very low. Use of copper shorting rings (faraday) greatly decreases harmonic distortion also, and JBL have done a great job at balancing motor strength and cone mass to achieve low frequency reproduction with absolute minimal distortion. The Eighteen Sound drivers also use similar technology and generate very little distortion and minimal power compression. TAD (TAD-Pioneer) also make some excellant bass drivers. In any case, Tumult looks like a great driver and 3% THD is very, very good. Maybe one day I'll get a Tumult and do some distortion tests against the JBL and Eighteen Sound drivers just for fun! Adrian |
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33047
From: Jordan, [216-237-196-136-dslam1-hmc.northstate.net]
Subject: Re: Distortion is caused by much more than just excursion limits
| Post them here because I'd really be interested to see your results! |
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32973
From: james, [h-68-165-65-210.snvacaid.covad.net]
Subject: Well just rock my world then
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lead me to a broken link! : ) Great stuff Adrian! You have my attention big time. I desperately wish to read some of the accompaning articles which are linked to on the page, Alas I find the links are broken... (In particular the vented vs sealed...but also the damping Q...) This statement you made really rocks my world... "Choose only quality professional series parts... and use the driver in a properly tuned vented box" Can you clarify this just a bit? (I am fascinated really. ) Lets look at very low frequency drivers (since this is the subwoofer forum and all...) The rage in commercial subs are the long throw low Q super xmax drivers ...(Tempest, Stryke, Dayton... etc) Do you think that application of pro-subwoofer drivers would lead to better results? (Lower distortion?) Can you give me some examples of how you could approach this application? Thanks, James |
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32980
From: Adrian Mack, [dialup-196-59.wasp.net.au]
Subject: Re: Well just rock my world then
URL: My website, Peerless XLS 12 Dual PR Sub Project, 18LW1400 vented sub, Horns, etc.....
|
Hi James! > You have my attention big time. I desperately wish > to read some of the accompaning articles which are > linked to on the page, Alas I find the links are > broken... (In particular the vented vs > sealed...but also the damping Q...) Those pages arn't actually written yet though. I originally planned on making a website with lots of technical stuff, and I still do plan on that, but it will have to come later when I have the time to write them up. I'll keep you posted though. > Lets look at very low frequency drivers (since this > is the subwoofer forum and all...) The rage in > commercial subs are the long throw low Q super > xmax drivers ...(Tempest, Stryke, Dayton... etc) Do > you think that application of pro-subwoofer drivers > would lead to better results? (Lower distortion?) In a word, yes. The best pro drivers have very low distortion, such as the JBL ones, and they do sound very clean. Thats described in my last post. But they do come at a much larger cost, and unless your willing to spend lots of money or you are determined on having absolute minimal distortion, then the commercial drivers would be the way to go. But look for second hand - you can usually pick up quality pro drivers for a lot less. Some of my favorite 18"ers are the JBL 2245H, 2241H and 2242H, and also the TAD 1602 and Eighteen Sound 18LW1400. All have very low distortion and sound real good. As for 15"ers, JBL 2235H, 2234H are both very good bottom octave performers. JBL 222x series are also good for midrange and bass applications. I personally use the 18LW1400 for subbass, and also I have a pair of JBL 2225H's that I'm using for midbass, but they can be used for subbass applications too. Both are very clean and capable drivers. You can hear the difference too. One part of their reduced distortion is the use of copper shorting rings or faraday rings which highly contribute to reduction in harmonic and intermodulation distortion. But they are also very good motors, and have other features which make them an excellant choice for low distortion subwoofer applications. But again, dont think that commerical drivers are bad, because they arn't. But the fact is that pro drivers are made to more bleeding-edge levels and do offer much better performance than commerical offerings. Adrian |
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32970
From: James Davis, [n8u1n3]
Subject: Re: Distortion is caused by much more than just excursion limits
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I agree whole heartedly. I mostly said that to keep things simple because most distortion that causes a real problem comes from people overdriving their speakers, or overgaining their amps. You saw the length of my relpy, could you imagine how long it would be if I covered all distortion? Yes, distortion can be caused by standing waves, cancelation, phase misalignment, small wire, poor connections, bad airflow even. But most of that isn't a huge problem unless you are going for a perfect setup, otherwise, it is usually no difficulty to overlook. I hope I did not mislead anyone to think that distortion is ONLY caused by overdriving the system. It's just more commonly a trouble area that way. |
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32974
From: james, [h-68-165-65-210.snvacaid.covad.net]
Subject: Re: Distortion is caused by much more than just excursion limits
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Hi James, no problem... I guess I am trying to probe a bit deeper. James |
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32963
From: James, [a17-206-41-214.apple.com]
Subject: Re: Distortion in different enclosure types
|
Hi James, thanks for your answer. You make a very interesting point. However it seems one minor detail is wrong. (Shouldn't distract too much from your point however.) --------------------------------------------------- f you were playing at 40Hz, and say 20Hz was F3. At 40 Hz, if you were pumping in 100watts, at 20Hz you would need around 1,200 watts (12dB) --------------------------------------------------- If the F3 is 20hz, wouldn't you only need 3db to match the 40hz? Below that it would be 12db, of course. So it doesn't really affect your point. |
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32971
From: James Davis, [n8u1n3]
Subject: Re: Distortion in different enclosure types
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You're right, I missed that. I was just on my way to bed and must have been closer to it than I thought. Thanks for bringing that to my attention. :) In any case, after I wrote that example, something has been atop my mind... I wonder if it would be possible, physically with construction technology today to build a 3.5 billion watt system...? Sure it would be the size of the bloody chrysler building, but wouldn't it be awesome, just to say it was dome, and I had a hand in it. 3.5 billion watt amp, and say a pair of 18,000 inch subs (500 yards). Sigh... to be a rich man... lol |
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