you mention distortion on the ss site? either your a super 'picky' bass head or im not? ive never  experienced distortion in a sub? ever anywhere? i cant see how your wo would distort? unless somehow a clipped signal is getting to it? solid subs with solid amplification in a solid enclosure SHOULD play solid below 100hz? maybe not as 'accurate' as some, but not distorted?

Hey b,

I won't make an attempt to describe how distortion sounds using words, but I could write hate mail about it for days. Keep in mind that low frequencies are produced by a combination of driver and enclosure and distortions can be produced by either.

According to this old 4T (My definition)
Distortion = any tone or lack of tone that is added to, subtracted from, or in any way different from the original. I also expand that to include any deviation from the original shape of the waveform caused by numerous compromises made during manufacture of a so called low frequency transducer system.

To me, the most annoying distortion and the most audible is InterModulation (IM) distortion, which means that a tone is added to a complex signal as a result of two tones reproduced at the same time causing a third tone to be heard which is not musically related to the other two. This is usually caused after the amplifier, since most amps have decent IM specs.

The same is true with harmonic distortion (THD) except that it occurs with some harmonic consistancy and is musically related to the original tone. The second harmonic of a tone is exactly 2x the frequency of the original and follows a simple arithmetic progression (multiples: 2x,3x,4x,5x). this doesn't sound too bad as long as the 2nd is softer (less power) than the fundamental and the 3rd is softer than the 2nd and so forth, like a musical instrument. Musical instruments are usually harmonic on the even order and we are used to those. Mostly high order harmonics can become louder in a distorting driver and that increases fatigue on the listener and is unnatural sounding. Most people seem to hear odd order harmonic distortion sooner than any other type, since it so non-musical.

Nonlinearity distortion comes in many flavors. The term usually describes the inability of a driver to respond to rapidly changing voltages in the signal. A driver sometimes moves in one direction "easier" or "farther" than in the other and this causes all sorts of problems. The driver must also stop moving when there is no signal voltage, but many will continue to "ring" when they are supposed to shut up.

Power compression* is also a form of distortion, in my book, and is audible on powerful peaks as it lops off the peaks of the waveform by being too stiff to actually keep up with the changes in the signal voltage. This causes a kind of mechanical clipping and is very common. Generally power compression is heat related and increased power causes more heat and more compression. Any respectable driver should reach its Xmax without any power compression.

For that matter, any decent driver should not introduce any distortion within its rated parameters, but this is not reality. Truly excellent drivers can add as much as 5% distortion and be still be considered acceptable by current standards. Many high excursion drivers add as much as 15% or even 25% of their own anomalies to the sound. That is one of the reasons I tend to use powerful large diaphram Musical Instrument drivers with large magnets and small Xmax, in multiples, instead of one large Xmax driver to achieve the desired SPL.

Of course, it is much more complicated than this in practice, but that's a brief description of some of the basics. These types of distortion are the most noticeable to me and my ears, which have many years of use and abuse on them, but are still very sensitive for an old 4T and were well trained at one point in time.

*MOD:
Just to be more clear, power compression is an inability of a driver to get louder as you turn it up. Any driver will reach a point where they can't increase acoustical ouput predictably no matter how much more amplifier power you put into them. The rules are: if you double the power input, the driver output will increase by 3dB. A driver that is in compression due to overpowering (heat increases resistance in the voice coil) or its mechanical limits (Xmax) will no longer follow the rules. Unfortunately many drivers (Power) compress long before the rated Xmax is reached.

I think I fixed all of my typos.

b,

Do you always ask essay questions?

I can't imagine why anyone would tell you that inductance is not "all that important". As you know, all the T/S parameters that you see published are simply a starting point to try to predict what will happen in the application of that particular driver. You can never change one property without all others being affected and you can never say one property is more important than any other. Many times a lower inductance voice coil is a sign that the windings are a larger guage than an otherwise identical voice coil and that could mean higher power handling, assuming that the increased heat generated is adequately dissapated. Larger wire guage means fewer turns in the same space and that would also indicate a lower resistance. Fewer turns means decreased inductance, but that means less interaction with the magnetic field, resulting in decreased efficiency. Decreased efficiency means you have to use more power to reach the same SPL output. That's fine ..... you can handle more power, now, but have you really gained anything?  :'(

You mentioned scaling and to avoid controversy, I really "don't want to go there", but your concept of scaling for Volume Displacement of a driver may be valid in many cases. Do you scale for the maximum that a driver is capable of or the maximum that a driver is capable of without intolerable distortion? How much distortion is intolerable? How will you know until you try it?

Many here have built scaled WOs and used different methods. I used Steve's recommendation when I built a 12" WO and only scaled one dimension, using the diameter of the diaphram and it worked out fine. Some have suggested that the only way to do it correctly is to scale 2 dimensions, using the area of the diaphram as a guide. Others have scaled 3 dimensions, similar to what you suggest. Who's right? ..... I don't know ..... probably Steve when we are talking about his WO design. I do believe that scaling for a sealed or a BR cab the displacement is an important consideration, however in the WO I think the relationship between the chamber volumes is most important. I would try to keep the ratio beween the sealed and the vented chambers the same. I probably just instigated a skirmish, but additional input could be beneficial to you.

I use pro audio drivers for two reasons: I have them and I'm very sensitive to distortion. JBLs are my favorite, but I have used most top brands with good results.  There are many well made drivers available from most name brand manufacturers, but there are not really any secret bargains. You usually get what you pay for. The Goldwood comes to mind as one I have not tried. I have heard that they are a Chinese imitation of a JBL, whatever that means. Lots of people use them and are satisfied, but I often wonder if they would continue to buy a cheaper driver for half the money if they heard a real JBL in the same application and discovered, first hand, the 4 to 6 more dB of efficiency available to them. I haven't heard them at all much less side by side with what they are supposed to emulate, so my comment may be out of line. I would enjoy demonstrating my point to someone, since that's how I used to sell a few PA systems. Set up my old stuff and blow away whatever the potential buyer had and go get me new stuff.

You're wanting response down to 25Hz and that is what most would consider to be "sub" audible, so you need a subwoofer. If you're only seeing Fs in the 40s you haven't found the subwoofers, yet. Cerwin Vega, JBL, Eminence and Beyma all make subwoofers that I have used and I know sound great. There are many PA subwoofers available with a low Fs if you really want to try for a Pro Audio sound. Most people find that sound quite "dry and "flat", but I think that's the whole point. The Pro mind doesn't usually want the wetting and fluffing of a sound, making it seem more presentable and polished, coming from his speakers. (I just started another argument.)

To answer your question about the mechanical limits of many pro drivers you have to think of how they will be used ..... hours of mellow, almost restful, performance followed by moments of sheer destructive fury. They have greater mechanical limits so they won't fly apart when their powerful motors are at their peak power.

I hope some of that helps.

No, just started earlier.