Bass shakers, tremblers or whatever you want to call them, were really conceived & designed for home theatre, gaming and simulators.
As such, it’s all about somewhat “over the top” and rather theatrical serious body shaking !. Fine (maybe) for explosions while playing Tomb Raider or landing on a passing asteroid with Bruce Willis, but not ever, by any bodies standards, remotely musical.
Left to their own devices, bass shakers themselves, will bang away, producing a relatively linear vibration. But hook them up to a piece of furniture, platform or floor area !. A raft of mainly mechanical resonances take over, modifying the response to something with a number of spikes and over-hangs. Since your room will also be doing this very same thing acoustically, your going to end up with a real mess !.
A shame, as the whole point of “direct bass” by body vibration is to lessen room derived bass response and timing irregularities. So, dealing with bass shaker mounting and mounting substrate mechanical resonance is a must.
Subjectively, a bass shaker on a single sofa for example, without any mechanical resonance control, will produce quite annoying and sometimes unexpected vibrations seemingly completely divorced from the music.
If you thought good integration of a sub was difficult – you should try a shaker !
Frequency response is another issue … Allowing a bass shaker to vibrate at too low a frequency results in the same “divorced”, unnatural and unbelievable performance. What works well is high pass filtering to reduce really subsonic low frequency (that’s generally the stuff that was recorded by accident anyway !), and low pass filtering to the same kind of frequency as a traditional sub. In fact, when running bass shakers in tandem with sub speakers, almost exactly the same pass band on both systems is by far the most believable subjectively.
Running a potentially quite expensive “add on” to a system with such a tiny pass band and therefore seeming to be “not doing a whole lot” may well seem wasteful. But, never has “less is more” been more true.
Timing and phase, much like a traditional sub (but more so) are arguably the most important parameters. Even in a small listening room, bass shaker derived bass is likely to hit your brain 6 or 7 milliseconds before you hear bass from your speakers. Just the same as the untamed resonances issue, perceiving bass twice with a short delay between them is nothing less than bass quality destroying, along with being easily detectable as “wrong” !
Volume or level wise bass shakers are definitely at their best if your not really aware of their presence !. Surprisingly, it seems that the smallest of vibrational energy on “one’s bottom” translates to quite a lot of information in “one’s brain” – for those of us lucky enough to not already have our brains in our bottoms…
Just at the onset of “obvious feeling” there’s a massive increase in performance scale, full range perceived level, and, even more surprising, a notable increase in “all dimension” image fluidity.
I would imagine this has to be all down to the brain “equating” the physical and the acoustic – like adding 2 and 2 and coming up with an absolutely technically correct 4, but with the subjective performance sounding more like 6 or 7.
Of course keeping somewhere near to the onset of “obvious feeling” is another control issue. Without which, at higher listening levels, (and / or bass levels), elevated transducer excursion can (will !) still produce too much vibration for comfort or realism. Very fast attack “infinity ratio” limiting works well, and can almost be viewed as “acceleration limiting” rather than “volume limiting” given the vibrational nature, as against traditional “acoustic” sound.
Like so many “effects” it’s very easy to over do it, and render it detrimental if not destructive to music, while, exercising reasonable restraint by doing “less” and offering transducers sufficient control, there truly is a whole new dimension to be had. Done properly, a very musical “new dimension”, which it’s all too easy to get used to, and miss instantly when you take it away.
To sum up, I would say, “direct bass” delivered by a bass “shaker” transducer is actually a must. But, must be set up correctly if it’s going to add to a musical performance rather than be destructive.
When considering transducer control ALL these parameters need addressing …
* Delay – to achieve coincidence arrival time of both mechanical and acoustic sources.
* Pass band – to remove sub-sonics and hand over to the main system at a suitably low frequency.
* Frequency response – to null mechanical system resonances by matched Q notch filters (potentially 3)
* Phase – to achieve exact phase match with main system.
* Compression – to limit high level “unreal” excursion at higher levels / certain music genres.
Go forth … add a whole new dimension to your music – shake, tremble and vibrate ! Or is that, shake rattle n’ roll …