Originally Posted by ssabripo
Kram...have you heard the new Tumults as compared to the older tumults? I have, and honestly, there were no "improvements" with the variances Dan introduced. I believe Bosso has or will have the new tummies, so he will be a better one to chime in, but taking out a pair of the older tumults in a sealed box driven by QSC1450s/each, and putting the new ones in, did not improve the SQ......dare I say, it was slightly worst?
Dan himself had driven this test a while back between the two, and you can see where the changes in Le and the other params came into effect (see attached graph). Surely there is more to this story, and I'm very interested to find out more myself:
before i sold it, i had one of the first of the second generation drivers released in Feb 06...with TC Sounds delaying the 4100Neo, i'm seriously considering a 15" Brahma...though the qts. is double what the tc3k is...seems like the biggest improvement is up top where you'd crossover to the mains...i'm mostly concerned by how much better the low Le voice coils deal with current changes which Dan states as a very important factor in transient response, citing his woofer speed article...
Interesting! This says that the change in acceleration of a driver - how fast it can change position - is strictly a function of the current through the driver. In fact, if you could make the current change infinitely fast, then the driver would accelerate infinitely fast, and we'd have infinite transients - zero time to change between states. Infinite frequency response.
So, now that we know that current is the driving force (pun somewhat intended) behind driver acceleration changes, let's look at what limits how fast we can ramp current through the driver. Because if we are not restricted in how fast we can change the current, then we are not restricted in how fast the driver can accelerate - transient response is not limited at all.
So, back to that loudspeaker model... A loudspeaker is a coil of wire wound on a former that attaches to the cone. The current flows through the coil, creating an alternating magnetic field that interacts with the static magnetic field of the permanent magnet. So, what could limit current flow? Well, what does a voice coil
How about an inductor? You know, those coils of wire (hey, isn't that what a voice coil is) that you use in crossovers? Guess what - a voice coil IS an inductor! In fact, an inductor stores its energy in the magnetic field (as opposed to a capacitor which stores its energy in the electric field). It is this magnetic field of the
voice coil "inductor" that interacts with the permanent magnet field we talked about above. Hey, a loudspeaker is an inductor hung on the end of a cone in a static magnetic field!So, what about an inductor will alter the way current flows? Well, inductors don't like to have the current flowing through them change. They like to hold the current constant. They will allow you to change the current flowing in them, but the bigger the inductor (or, the higher the measured inductance) the longer it
will hold the current before it starts to change (I'll leave it to the reader to go research inductance on their own, to learn why this happens).
So, the voice coil is an inductor. And we see that inductors don't like to change current. But we also see from equation 4 above that we need to change the current if we want to change the acceleration. So, the voice coil doesn't want us to change the current. How good is it at holding the current? Depends upon the
inductance! The higher the inductance of the driver, the longer it can hold the current flowing through it. Which means the more time elapses before it starts to respond to the amplifier's applied voltage. Which means we have slower transient response.
Guess what - we just answered the original question! It turns out that transient response of a woofer is not a function of the moving mass, as is commonly espoused (one of the most infamous audio myths). In actuality, it is based upon the inductance of the driver. And the greater the inductance, the slower the
driver - the lower the transient response.
and now the Stan White stance:
A simple electrical law states that the current in a loop is constant throughout the loop. In the operation of a loudspeaker, the voltage across the speaker terminals times the in phase current is real electrical power input and will be reflected in the output of the speaker. A voice coil has inductance. This inductance is a phase lagging electrical parameter. What it means is that when a voltage appears across the speaker terminals, there is a time lag (caused by the voice coil inductance) before real acoustic power appears. This means is that impressed transients suffer a time lag before acoustic power appears. Heavy cones create mechanical inductance and voice coils create electrical inductance. Both factors can inhibit transient response. This time lag is sort of an inertia. In reality, real sounds do not suffer from this inertia.
The POWRTRON "FF" model has made possible a partial cure for this inductance (inertia) problem. It is called field forcing. The technique is common in the electrical power field. A way has been worked out to apply this industrial echnique to loudspeakers. The FF amplifier nullifies the (inertial) effect of time lag caused by heavy diaphragms and voice coil inductance. There is a noticeable improvement in transient response when the new FF amplifier is used in conjunction with loudspeakers. In a real sense, the FF is not an amplifier but a servo mechanism. The FF senses the inertia and corrects for it. The improvement is noticeable no matter what kind of speaker is used. Of course, speakers like the Shotglass (with no modal breakup) will sound better than ordinary speakers, but that is to be expected.
These two articles really bring home the inductance-transient response correlation...so now i can deduce that utilizing XBL^2 or LMS coils in a critical Q alignment, with the high damping factor of a Crown K2 (>3000 DF/8 ohms), would produce some amazing transients, given an overbuilt cabinet...