Just another line of thinking: my experience has lead me to believe that this "slam" factor is a function of energy over time, also the way the energy is focused over an area. I may be wrong, but more energy over less time is what is actually creating this phenomenon. A series of questions may arise from this. For example, does the signal in actual music contain such waveform? What about the
crest factor (link stolen from
@Holdt )
Further, if you are using multiple subs with dedicated amplifiers in your system, what gives when it comes to time alignment? Impulse and step response? Phase? As others have pointed out, I'm also wondering about the tradeoff of having multiple subs (also multiple amps) for the benefit of having smooth FR in the bass region, and time alignment? Music signals are quite different than sinewaves and steady state measurements don't tell us everything. There are other measurement methods for sure, but what would be the correct yardstick?
This form of tactile energy, the attack of it, is more often than not, very short in duration. Short enough that it really does not have a perceptible definitive pitch. It's just a "thump" and for high frequencies just a "click". As an real life analogy, in a real kick drum, there are modes of circular membrane involved:
"The short duration (fraction of a second) of the (0,1) mode means that this mode does not contribute greatly to the musical tone quality of a drum. In fact, when struck at the center a tympani, or other large drum, produces a "thump" which decays quickly and with no definite pitch."
Anyone who had close encounter with a real kick drum, may as well confirm it can pound your chest really hard, thanks to these couple of modes which have no definitive pitch, just a lot of energy. Other modes come later in time and are contributing to the timbre and pitch perception. Granted, many recordings of a kick drum are not really done very well, and are "beefed up" artificially with mixed in sinewaves. So the recording may not even contain information that would result in a "kick" from any system.
This is another example of electronic music artificial kick sounds and how it may be created:
I've posted this already a while ago, but this artist really is creating some good kicks which are produced with enough dynamic range so that you crank it as high as your system can handle. But on this video, if you follow his mouse cursor you may observe the following:
This is a pitching down sinewave sweep, ranging from as high as 17kHz, down to 44Hz, which is by no means flat in demand of acceleration.
Namely, the 17kHz to about 93Hz downward sweep is about only 2-3 milliseconds in duration, also with curved wave amplitudes (the lower the frequency the higher the amplitude). The lower frequency portion is also containing all frequencies down to 44 Hz or so, also with varying amplitudes towards lowest frequency. It is stretched in comparison to highs and in takes about 10ms for the lower frequencies to sweep, and then there's long sustain (a couple of hundreds of ms) and amplitude rolloff till settling down. Also, further EQ is applied, and then other tools to polish the sweep.
Even though the bass is slow and usually nothing out of the ordinary when it comes to voltage and current demands on your amplifier (or slew rate), I would agree with
@Duke that this is where the motor strength comes into play when it comes to acceleration.
You may draw your own conclusions weather time alignment is important, crossover, integration. I would say that it matters, in fact microseconds would count if you want to reproduce a signal such as described above. If I misalign my sub to the mains by some 10mm, it's not audible at all, but the punch of the whole system is less. The energy is somehow lost. Perhaps not in SPL, but simply the wave propagation is messed up in time domain with respect to the MLP. The transients are simply having less "punch" that can't be compensated by more SPL.
Finally, the question of where does the energy go? I've also found the benefit of decoupling the system from the floor. Again, it's not really audible, but the tactile sensation is affected by having less energy dispersed to the floor and walls, windows and solid objects. The system is more efficient and I have more energy in the Y axis (what's coming through air), rather than X (what's coming through the floor) and felt at the seat. Wave propagation through solids is many times faster and is dissipated and unfortunately audible by rattling various objects around the house. Not exactly hi fidelity. It also has complex phase relations and some precedence effects over the audible sound. Resonances may arise and even affect the very boxes and baffles your drivers are mounted on. IMO, yet another thing to consider. Your floor vibrations won't give you the chest slam (as already pointed out by
@neRok ) .
