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.
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Loudspeaker Distortion
What 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.
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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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