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Wednesday, September 19, 2012

Article Preview :: How To Craft The Perfect Bottom End

Mixing Bass

Article Preview :: How To Craft The Perfect Bottom End


Avoid all the low-frequency pitfalls and learn to achieve the perfect foundation for any mix, with our bass-mixing masterclass...
Mike Senior
How do I mix bass? It’s a simple question, but compare a dozen records picked at random and you’ll see that there’s no simple answer. When it comes to instruments, ‘bass’ can mean (at the very least) guitar, upright, drum or synth. Each can perform many musical roles, and every genre has different conventions for low-end sonics. In this article, I’ll help you make sense of all that, whatever instruments or genre you’re working with.

Cancellation Insurance
A bass ‘sound’ is often a combination of several similar signals: for example, electric bass can be multi-miked; a DI signal may be captured; and you might introduce MIDI-triggered layers to fill things out further. Such shenanigans give you tremendous power to refine your sound, but also enough rope to hang yourself, because the layers don’t always reinforce each other when mixed. In fact, they can cancel gruesomely at certain frequencies if there are polarity or phase mismatches — so you need a clear understanding of phase and polarity! There’s an in-depth article on the SOS web site (www.soundonsound.com/sos/apr08/articles/phasedemystified.htm) but I’ll run through the basics.
Phase differences are caused by one signal being delayed relative to another; and polarity differences are caused by one waveform being inverted relative to another. If you’re unlucky, the phase/polarity relationship between a pair of similar signals can result in tonal carnage when they’re combined, and you must tackle such issues as early as possible.
With multi-mic/DI recordings, a good way to start is to zoom in on their waveforms and try to match them up as closely as possible, so that phase and polarity differences are minimised and you get the strongest reinforcement. Sort out any obviously polarity-inverted waveform first — by either processing the audio region or hitting that channel’s polarity-inversion switch — and drag the audio regions to line up better. If judging things visually is tricky, hunt for transients, which tend to be more easily identifiable.
Now to start refining things by ear. Put the first two tracks out of polarity with each other, fade them up to equal levels, and adjust the timing offset between them to achieve the strongest cancellation. Returning to a matched polarity will then give you the fullest composite sound. Repeat this process, adjusting the timing of each new layer in relation to those you’ve phase-matched.
It’s by no means ‘wrong’ to deliberately mismatch polarity and phase settings to radically transform what was captured (this is art, after all) but creative phase-cancellation is something of a lottery, and there’s a tendency for it to mess with the relative balance of different note pitches, thus introducing musical irregularities.

Phase Me Baby, Right Round...

It’s often hard to judge the relative polarity and timing offset of mic and DI bass signals by looking at their waveforms, (upper pair). It’s easier if you focus on transients, such as the note onset (lower pair). Even then, though, you need to use your ears.
A specialist ‘phase rotation’ device allows you to delay different frequencies by different amounts (for links to affordable phase-rotation plug-ins go to www.cambridge-mt.com/ms-ch8.htm#links-phase.) Phase rotation won’t change a channel’s frequency response in isolation, but it will change the way one layer of a multi-channel sound interacts with others.
I find it more time-efficient to grapple with polarity and timing adjustments before faffing with phase-rotation, and there’s no point in trying to finesse exact phase relationships if they don’t stay consistent (as in the case of most multi-miked acoustic bass parts, where instrument movements will alter the relative path-lengths to the mics, and hence the time-offset). But I do use phase rotation a lot when mixing processed and unprocessed versions of the same bass sound — something called ‘parallel processing’.
Most DAW systems auto-compensate for a plug-in’s processing latency, but some plug-ins (equalizers and amp emulators in particular) generate additional time/phase shifts, and a phase rotator or simple delay line can help to compensate for this.
There may also be hidden phase gremlins between the left and right channels of stereo bass-synth patches, which you’ll only hear when the channels are mixed to mono. The worst-case scenario is that the low frequencies will cancel badly, and won’t make it out of club and PA systems, or single-subwoofer home/car systems. If the phase mismatch is static, adjusting the polarity, timing, or phase response of one channel may help, but if the bass is seriously flaky in mono, you might as well filter it out and layer in a mono sub-bass synth.

EQ: The First Two Octaves
The 20-100Hz frequency region presents probably the most difficult challenge, as it includes the fundamental frequency of most acoustic/electric bass notes, and maybe a harmonic or two besides for the most seismic of synths. Studio monitoring has a lot to answer for here (see the ‘Bass Under Pressure’ box), but it’s also a question of EQ technique.
Be cautious with low-shelving boosts if your monitoring system (including your room as well as your speakers) struggles to convey information below 40-50Hz. Lots of rubbish like traffic rumble and mechanical thuds can be lurking at the spectrum’s low extremes, and you don’t want to boost this. If you must apply a shelving boost, also use a 20-30Hz high-pass filter for safety. LF shelving filters also continue acting, to some degree well beyond their specified frequency, so if you find you’ve collected excess low mid-range baggage while trying to boost the true low end, a compensatory peaking cut at 200-400Hz may be in order.
Beyond broad-brush decisions, the most common job is compensating for unhelpful resonances. Acoustic bass tracks always seem to feature one or too fundamentals that boom out awkwardly, but room resonances can also afflict miked amp recordings, aided and abetted by the cab’s resonant structure. Even the recording mic can play a role, especially if it’s one with a frequency response heavily tailored to rock kick-drum sounds.
The simplest remedy is to deploy well-targeted narrow-band peaking cuts. Find a pitch that consistently booms undesirably, and loop a representative note. Then sweep around with a narrow peaking filter in the sub-100Hz region to see if you can bring the errant frequency back into a better balance. Boosting with the filter first can assist with finding the right frequency, as can a high-resolution spectrum analyser. A Q value of eight is a reasonable starting point, but be prepared to adjust that by ear: some resonances may affect several adjacent pitches, requiring a wider bandwidth, but otherwise, try to increase the Q value as much as you can (without making the cut ineffective!) to avoid messing with the spectral balance of other notes.

Low-end Interactions

Amp simulator plug-ins (those from Aradaz, Acme Bar Gig, and IK Multimedia are shown) are often useful for processing bass parts at mixdown, but be careful that phase shifts incurred by the processing don’t introduce unwanted phase-cancellation side-effects, especially when using them for parallel processing.
No matter how solid your subs in isolation, they won’t do you much good if the rest of your arrangement clouds them over, or if they interfere with the low end of other important tracks. For a start, if there’s more than one bass part (perhaps a bass guitar layered with a synth bass), I’d usually choose only one as the main low-end source, and high-pass filter the others around 100Hz, to avoid insidious phase-cancellation nasties between their long-waveform LF components, which would be pretty much unfixable with mix processing.
The low-end level modulation inherent in some detuned multi-oscillator synth patches is similarly undesirable if you want an absolutely solid low end, so if you can’t switch off the patch’s detune directly, I’d suggest filtering off the synth’s lower octaves and replacing them with a more reliable static sub-bass synth.
With multi-mic or ‘mic + DI’ recordings, you’ll often find that one signal provides a clearer low-end than the other(s), and high-pass filtering can again help add focus and definition to the final product. The subjective timbre of the combined sound is heavily dependent on the mid-range, so as long as you don’t move your filtering too far above 100Hz, you shouldn’t need to worry.
High-pass filtering is also handy for removing low-end junk from other instruments in your arrangement, to help the low end of your bass part pop though more cleanly. Full-range keyboard instruments such as synths, pianos and organs warrant special attention, as may orchestral overdubs, found-sound snippets or sampled mix loops, any of which could conceal a lot of unwanted rumble. Doing this has an extra benefit if you’re working under less-than-ideal monitoring conditions: if you dramatically undercook your mix’s overall LF levels, it’s then easier to correct using mastering processes without dredging up a bunch of underlying sludge at the same time.

Sub Warfare
The most critical sub-100Hz conflict in modern mixes is that between bass and kick drum: their low frequencies are normally responsible for the lion’s share of the mix bus’s output level, and therefore present the primary headroom bottleneck at mixdown and mastering. The engineer’s task is to divide the available headroom appropriately between these two main LF sources.
If your bass line needs to relieve people of their fillings (think Nero’s ‘Guilt’ or Pendulum’s ‘Watercolour’), you’re unlikely to have the headroom to put much real low-end on the kick-drum channel: you’ll have to move up into the 100-200Hz zone to salvage any beef. Alternatively, if your kick’s threatening to wake Godzilla (as on Rihanna’s ‘Umbrella’ or Pussy Cat Dolls’ ‘When I Grow Up’), you’ll have to be sparing with your bass channel’s super-low frequencies.

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