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
|
|