Bass Amplification, Part 1

21 05 2007

Yesterday I brought my full bass rig (meaning not the little practice amp I use at home, but the full live setup with effects, amp, and speakers) to the studio for a performance-volume rehearsal. It was not loud enough. I spent about 5 hours yesterday and 2 hours today studying up on what’s out there, and I ordered something new today.

I realized today that I’ve read a whole lot about bass amplification over the years and had plenty of real world experience with different approaches, and I think this information is interesting to bass players at least, and probably rock music audiences and price/performance minded home audio enthusiasts too. So, I’ll start with some basic information about the state of live rock music amplification in Part 1, and then share some of my recently acquired knowledge with you in Part 2.

On the whole, rock musicians seem to be quite rational about selecting gear: it either sounds better, or it doesn’t. Except for carefully controlled studio environments, subtle nuances of tone at low volume don’t matter; the noise level in a live situation is pretty high. What matters is what you hear when everybody is amplified enough to be heard in conjunction with a live drum kit. So, I’ve never encountered anyone thinking like an audiophile and dragging a $250,000 4×10 guitar cabinet or $1200 guitar cable or $6000 platinum guitar pick around with their equipment to gigs.

Here are some examples of why there’s so much noise on stage. Put a drum kit anywhere near a bass amp, and the snare will rattle with every note and the toms will resonate slightly with each kick drum hit and with bass notes. Add a vocal mic and you’ve got a bit of everybody else’s sound on stage being picked up by that mic and amplified along with the vocals. It’s a mess. As a result, rock musicians tend to pay more attention to the subtle tone qualities of their instrument, and just buy amplification and speakers that do the job of making that great tone louder.

An exception is guitar preamplification: ever since the 60s, intentionally distorted guitar has been the norm, and that usually means the subtle and nuanced sound of vacuum tubes being overdriven like crazy, placed in between the guitar and the main amplification. The preamp and power amplifier are commonly combined in a single unit, but it is possible to buy physically separate preamps and power amps. So in this case, the amplification the musician chooses effects tone a lot.

An axiom of rock guitar is that it sounds better the louder it gets. Part of this is probably the nature of distortion (it really does sound very different when driven hard), but part of it is the psychoacoustic response that lets you perceive the same signal more clearly when it’s louder.

For bass players, that means that the wonderful pillowy yet crisply detailed tone you get from your 25 watt practice amp at home is going to get drowned out by the rusty jackhammer diesel chainsaw fine instrument that your guitarist so delicately wields. As a low frequency musician, you’re at a disadvantage due to physics: to achieve the same perceived loudness in the listener, you need to move more air, and that means more power shoving those chunky speaker cones around. For a handy real-world example, think of laptop speakers, a digital watch alarm, or a mobile phone ringtone. Tiny amounts of power can make really loud sounds out of a wimpy little speaker, if the frequency is sufficiently high. But if you want deep rumbling bass (such as the percussion in that hip hop ringtone, or the phat synth bass of a disco MP3 on your laptop), you need a lot more power.

That means that your gear is likely to cost more than you would expect. On the other hand, the guitarist has to spend a lot of additional money on a fancy preamp for tone reasons, while bass just sounds good naturally. :)

There are some corners that can be cut to get loud bass without actually spending big money. You can cut out all the midrange and treble frequencies, sacrificing detail and clarity to let the amp and speaker focus on driving the low notes. If you have a $200 bass, the thing probably sounds terrible anyway, so this isn’t really a sacrifice: the closer you get to a 100Hz sine wave, the better. But most bassists seem to be tone-obsessed, probably because of the fact that speedy flashy technique that sounds cool on a guitar or sax or violin or keyboard usually sounds terrible on bass (“where’d the bottom end go?”). So we aim to make a small number of important, valuable notes, and to make each one sound fantastic. (The guru of this style is Tony Levin, whose work you have almost certainly heard; go re-listen to Peter Gabriel’s “Sledgehammer” if you need an example of his mastery of the less-is-more approach.)

The other big corner that can be cut is in speaker cabinet construction. There’s an engineering trade-off that can be made between resonance and detail; consumer subwoofers lean heavily toward resonance at the cost of detail, and low end speaker cabinets do the same thing. You get a lot more low end rumble, but the precise attack of each note is lost in an auditory blur.

One very effective engineering trick that makes up for this is the use of “biamping”. That means that you use a crossover and two amplification channels, so low frequencies go into one amplifier channel which is connected to a big speaker in a boomy, resonant speaker cabinet. The other channel is midrange and highs only, and goes into a set of smaller speakers in a separate speaker cabinet. You get the clicky squeaky detail and the fat low end puddin’ all at once. It’s not very expensive to do this, so it’s a pretty common feature in bass amplifiers, and it’s what pretty much all computer speakers do: teeny speakers for detail, big boomy subwoofer for bass. For rock bass, it’s not too different. You just hook up two very dissimilar speaker cabinets (typically one with a single 15″ speaker and one with either two or four 10″ or 12″ speakers) and dial the crossover frequency knob until it sounds great. In my experience, 250-500Hz is the sweet spot.

Biamping is very effective, but it means you have to carry several big speaker cabinets around, and when used in conjunction with inexpensive low end speaker cabinets, it tends to lack “punch”; unlike a kick drum, there’s no “whump” at the beginning of each note. There’s just a “click” or “crack” from the high speaker; the “whump” gets lost in that cavernous low end speaker. The reason that’s undesirable is that the sound of each of your notes beginning is now trapped in the same frequency range as the guitarist’s low end, the snare drum, the toms, the singer, and the keyboard player’s right hand. That’s a crowded place in the mix, and it generally sounds pretty bad for the bass player to elbow in the midrange part of his sound where there’s already plenty going on.

So, if you don’t exercise good taste and tune your sound so that the band sounds good overall, which you really should be doing, your bandmates will do so. They’ll tell you to “turn down” and then “turn up the low end”, and then you’ll be back to providing puddin’ (which is why you have a place in the band in the first place), but without the detail that makes people dance and pay attention to your distinct and groovy bass part. So, given a fixed amount of power, biamping is better than not biamping, but it is still a pretty big trade-off.

So, what to do? Basically, buy more power. Instead of trading worse tone for more volume at a constant price, trade affordability for better tone. More on how to do that in Part 2.



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