What is bottom?
An attempt to make sense of what 'big bottom' really equates to
A long time ago someone plugged a CD player into their new bass combo and was very puzzled at how unbassy the sound was. How could this be, surely a bass amp is all about bass? Or are we suffering from the same problem that afflicts anyone who dares head up to the dusty end of the neck, that risk of assuming that because it's a bass guitar and a bass amp that it's all about the bass?
So does saying a cab has big bass mean that it has big lows or that it makes our bass guitar loud?
This seems to be the crux of the matter. If you can play a bass guitar through a cab and it sounds loud and still sounds like a bass then it is (often incorrectly) deemed to have big lows. But what would happen if we played some recorded music through it? All too often we'd find there was very little depth to the music with at best the bass drum having a punchy kick to the sound but little feeling of size or scale to the mix and very little depth and rumble in the lows. Boom yes, size no. If you tried to use a bass cab as a home cinema subwoofer you'd run into a similar problem - most half-decent hi-fi speakers actually go lower than most bass cabs. But the bass cab goes way louder.
Firstly, let's split the lows into three manageable segments:
20Hz - 40Hz = rumble
These are the kind of lows that are felt more than heard and add a certain rumble and sense of scale to movie soundtracks, pipe organ music and some electronic music. The sub 30Hz realm is very much sound effects territory - explosions, thunder, et al. There is very little content in music below 30Hz so club systems tend to cut off at this point if not a bit higher, whilst live music systems as seen at major festivals / stadiums tend to cut off at 35Hz (which makes the systems a bit more efficient so you need less gear to reach a given SPL than with a system with a 30Hz cut-off). A low tuned bass guitar does have output below 40Hz but it takes a seriously potent speaker system to reproduce that at high SPL. Sub 40Hz output adds a certain size and weight to music. It is almost more felt than heard and is rarely perceived independently of higher bass output (because any low frequencies will have higher overtones). A good word for it would be rumble. A bad word would be boom (boom happens much higher up).
40Hz - 80Hz = depth
This is a very important region for lows in music. The depth of a bass guitar tends to come from having a good wodge of output in this region, either as second harmonics of the lowest notes (B0 to E1, i.e. 31Hz - 41Hz so second harmonics are 62Hz - 82Hz) or as first harmonics (fundamentals) of the higher low notes (F1 to E2, i.e. 44Hz - 82Hz). Often these lower harmonics tend to exhibit less sustain than the few harmonics above, and in fact it's the quick decay of the fundamental that causes that characteristic P-bass thump. This is not really where the fatness of the bass guitar is found, this is very much a region that makes the bass guitar sound big but not fat or round. Depth would be a good word, as would big. Bad words would be fat or boom, both happening higher up. Also inhabiting this realm is the bottom of the kick drum - often centred around 80Hz - that 'punch you in the chest' bass feeling is higher than most think.
80Hz - 160Hz = fatness and boom
The third and final part of the bass register is the mid and upper-bass realm, the latter being a region that causes more sonic annoyance than any other register (cars with blue neon sill lighting...) whilst also being critical to getting some bass happening from most TVs, radios, iPOD docks and so on. That '80s classic, the Boombox, specialised in thumping out bass boom in this region. If you're trying to make something small produce loud bass then you have to focus your effort in this register - although it does sound relatively boomy (and horribly boomy if there's too much of it and/or the THD is high) this kind of 'fake' bass is where it's at for making something small and cheap yet loud and apparently bassy. If most of the lows you're producing are at 120Hz rather than 60Hz then you only need a quarter as much 'speakerage' to reach equal SPL, because the Volume displacement (Vd) requirement quadruples for each octave you go lower.
Fatness well describes the lower half of this realm whilst boom suits the upper half. Many vintage bass tones have a lot of content in this realm, and many a thin bass sound will be sorted by a wodge of boost at ~80-100Hz. One of the perils of cheaper woofers and/or undersized enclosures is a large hump in output in this region, which can sound great when it's simply fatter and louder but can also sound terrible if it's too boomy and 'slow' (this describes poor transient response whereby a woofer/port system tends to ring rather than starting and stopping precisely). Also many rooms will have a floor-ceiling standing wave in this region which exacerbates any mid-bass boom.
This is a region which we have to share with many other musical instruments - not only do keyboardists, pianists and lower horns make noise down here, but so too do larger percussion instruments, lower voices and guitars (mostly when distorted in a metal stylee). It's all too easy for volume wars to start in this region and then bleed over to the lower bass and lower midrange due to frequency masking.
It's not a competition! We are all individuals!
Although we all play bass, and as such we all require some bass in our bass sound, we certainly do not have to like the same kind of lows in our sound, nor should we feel we have to conform with what the latest fashion or (even worse!) the latest self-proclaimed technical expert says is good bass. If you tend to go for an old school flatwound soul sound most of your lows will be happening higher up than someone playing an active 5-string with roundwounds. If you're playing reggae you might have your lows happening somewhere in between those two players, but in greater quantity. If you're slapping you might like a kickdrum like thump to your sound and thus the transient response is more critical. If you're in a really heavy band you might go for a big rumbly and slightly dirty bass sound that underpins the kick drum whilst also barking through above the chunky bottom of the distorted guitars, so your lows happen in two parts of the bass spectrum.
"So you say that such and such a speaker can't reproduce 30Hz - well I've tested it and it can"
One of the perils of modern technology is that it's all too easy to read a bit of wikipedia, download a sine wave generator, play it through a bass amp and declare said system to be able to reproduce 30Hz because you can hear a low sound coming out. Clearly if it couldn't reproduce 30Hz then we wouldn't hear anything - right? Wrong! Push a 30Hz sinewave through a speaker whose response plummets below say 55Hz and all you end up doing is synthesising the 60Hz, 90Hz, 120Hz etc overtones because the speaker exhibits so much harmonic distortion (harmonic distortion is a form of distortion where new harmonics are generated from the original source sound) when trying to operate at such a low frequency. So you then hear these overtones because the speaker is efficient at reproducing these frequencies but because few people know what a clean 30Hz sinewave sounds like it's all too easy to mistake these higher overtones for the pure 30Hz signal that went in.