The shorter and simpler version:
Transient response is the ability of a speaker to start and stop exactly when the sound going into should start and stop. A bass cab with excellent transient response sounds far more alive and responsive than one with poor transient response and makes it easier to both groove hard and be heard on the gig. It is particularly important that a musical instrument speaker has good transient response because it is how the player hears themselves and thus has a huge effect on how they play.
The longer and more complicated version:
The biggest causes of poor transient response in bass cabs are cheap drivers with weak motors, badly tuned enclosures which lack stiffness, and a lack of internal cab damping. Barefaced cabs have never suffered from this which is why even the bottom-heavy 2x15" Dubster could shame any mediocre sealed 8x10" in transient response (and sealed 8x10"s are by far the easiest cabs with which to achieve good transient response). But we can still do better... So the best way to explain how to optimise transient is to go through what we did to give the 12XN drivers incredibly quick transient response.
As we've said elsewhere, the goals behind the first and second generation Big Series cabs were to achieve the most accurate frequency and polar response possible whilst achieving very high maximum output. It wasn't that we ignored transient response but it wasn't the top priority.
Our most transient correct older model
The fact of the matter is that our Super Twelve cab outperforms all our older models when it comes to transient response, so why is that? The most obvious advantage is related to moving mass: The 12" woofers in the Super Twelve have just as strong a motor as the 12"s in the 2nd gen Big Series cabs or the 15"s in the Compact and Super Fifteen but the moving mass (cone plus voice coil) is lower. F=m.a so a=F/m. Reduce m (mass) and a (acceleration) goes up if F (force) stays constant - just like putting the engine from a big heavy car into something smaller and lighter makes it accelerate faster. And that affects both setting the cone moving and bringing it to a halt.
Our 12XN550 driver
Our first Barefaced custom driver has a better force to mass ratio than any of our old drivers which means it can accelerate and decelerate and stop even more precisely than the drivers in the Super Twelve. And there's more...
What may not be so obvious is that loudspeaker cones do not act like perfectly rigid pistons - at higher frequencies the sound ripples outwards from centre to surround. Those ripples will start when the voice coil moves but they won't necessarily stop when the voice coil stops moving - as the cone is rippling we're reliant on the self-damping of the cone, the absorption of energy at the surround, the absorption of energy at the voice coil / cone / dust cap glue joints and the efficiency of coupling with the air load on the driver (the better the driver couples, the more effectively the energy is taken out of the cone and into the air, thus both creating sound and damping cone ringing in the process). The lighter yet still stiff cone of the Super Twelve has better damped ripple modes than our 15"s (by a wide margin) or the 12"s in our second generation Big Series cabs (by a much smaller margin). The cone in the 12XN driver has better controlled ripple modes than any of our old drivers.
Inductive phase lag
Our next consideration is system inductance. Inductance causes the current to lag the voltage at lower frequencies. It is the current through the voice coil that generate the electromagnetic field which moves the woofers. So the greater the inductance, the more the transient response is delayed. In the case of the Super Twelve, the drivers have lower inductance than those in the 2nd gen Big Series cabs and there is no additional series inductance in the crossover - total inductance is about 20% of that in the 2nd gen Big Series cabs. The 12XN cabs have just as low inductance as the Super Twelve so match it when it comes to reducing the inductive phase lag.
Amplifier damping factor
Then we have to consider the ability of the amplifier to control the cone movement - the series inductance in the crossover of the 2nd Gen Big Series cabs adds resistance to the signal chain which lowers the damping factor at the woofer. The Super Twelve connects the woofer directly to the amplifier, which maximises the cone control. The 12XN models do likewise - your power amp gets to directly grip the driver (in an electrical sense), like pulling and pushing it with a steel rod rather than an elastic band.
Low frequency alignment
The final part of the puzzle is the low frequency alignment of the enclosure. The low frequency response of a loudspeaker is analogous to a high pass filter, and the lower the order of the filter, the better the transient response - this is mathematically inescapable. Also a filter with a sharp knee (an underdamped filter) will cause a greater rate of phase shift across the knee and this can be audible as increased group delay, whereby frequencies below the knee are delayed relative to those above the knee frequency. The extended unEQ'd low frequency response of the 2nd Gen Big Series cabs is achieved by opting for an enclosure alignment which flattens the LF response above the tuning frequency and has a fairly sharp knee and then steep slope below the knee, so less than ideal transient response and fairly high group delay in exchange for the huge bottom. The Super Twelve isn't designed to produce such big lows and instead rolls off smoothly and gently from higher up - and thus has better transient response. The Generation Three designs use an even better optimised slope which gives very good transient response - the Big cabs go lower than the Super cabs but all have better transient response than any previous Barefaced model.
Excursion related motor non-linearity
Just one more thing… As soon as a driver moves beyond its linear region (i.e. the voice coil starts leaving the front magnetic field) then the motor strength drops and the low frequency alignment shifts and the transient response becomes significantly worse. Our cab needs to stay linear in normal (LOUD!) gigging use otherwise all our efforts will have been in vain - and the huge linear excursion of our new 12XN woofers is key to that.
So in summary, to achieve excellent transient response we require:
- High motor strength : moving mass ratio
- Good control of non-pistonic cone output
- Low system inductance
- Minimum resistance in series crossover components
- Gentle LF roll-off slope
- Linear output at the required high SPLs
The Barefaced 12XN cabs, first launched in 2013 tick all six boxes and thus have the best transient response of any cabs we've ever made, and possibly of any bass cabs ever made. INSTANT tone!