It’s a sad fact of life that getting a great bass tone through a P.A. system starts with a great bassist playing a good instrument. On second thought maybe that’s a happy fact of life. Given that starting point (or something close to it) it’s your responsibility as an engineer not to screw it up. While you can’t polish the proverbial turd, there’s a lot you can do to make the most of what you’ve been given, and these days there are plenty of tools to help you in the process. Beware: we’re about to open up a can of worms!
Bass Peddler?
I typically take the bass into a minimum of two channels; a DI and a microphone placed in front of the bass amp. If the bassist is using an extensive pedal board, I might take a DI before the pedal board (right off the instrument’s output) and another one at the end of the pedal board chain before it hits the amp. This gives me signals that are pre- and post effects, and then I’ll also have a channel for the mic on the amp. Typically I insert a compressor on the “clean” DI channel and on the microphone, but I use more compression on the DI channel. We’ll get to that later.
At sound check you want to listen to the similarities and differences between the signals from the DI and from the mic on the amp, and use them to build the bass sound. I always use a high-pass filter on both channels; the high-pass on the DI is there to clean up the low-end rumbly crap (LERC) that you can’t necessarily hear but soaks power from the P.A. and occupies space in your mix. In the past, I would high-pass the microphone channel quite high (say 250 to 400 Hz) and use that channel more for mid- and high-mid character, or growl if the bassist was producing it. Lately, I am opening that filter downward and allowing the mic channel to produce more extended low-end (say down to 70 or 80 Hz) — but still using the HPF to remove some of that LERC that stands in the way of clarity.
Here Come the Worms
The truth is, you can play with filters and EQ all day long, but if you don’t address the fundamental issue of phase, you are spinning your wheels. You need to assess and correct any inherent phase issues between the mics and DIs. Here’s why: the DI feed is strictly an electrical signal. It travels down the wire from bass pickup to DI to mixing console at an exceptionally fast speed (something like 186,000 miles per second — even faster that a Lamborghini Gallardo). Ditto even if the bass player is using a wireless, though there may be some latency in digital wireless systems.
I think we could say that audio traveling down a wire is pretty much instantaneous, and probably no one would be too upset. By comparison, the sound of a bass amp reaching a microphone is slow. The speaker converts electricity into acoustic energy and the microphone performs the reverse function. The sound leaving the speaker takes a fraction of a second to travel through the air and reach the microphone. How much of a fraction? Let’s say that under standard temperature, humidity and elevation conditions you are looking at (or hearing, as the case may be) a delay of approximately 1 millisecond (1/1000 of a second) per foot. If you place a microphone one foot away from the voice coil of a speaker, you have a delay of approximately 1 millisecond relative to the DI. One millisecond may barely be significant (even on an Olympic downhill run) but in audio, it’s huge — enough to screw up your bass sound. Fig. 1 shows a screen shot from a DAW, zoomed way in. It illustrates the difference between a note coming through a bass DI (top) and the same note captured through a mic on the bass amp (bottom). You can clearly see that the waveforms start at different times.
My Doc Says I Have Reverse Polarity
Many mixing consoles have a button on each input channel cryptically “Ø” — we often call this a phase reverse switch, but technically it does not change the phase of an audio waveform. It changes the electrical polarity of the signal. This is the equivalent of swapping pins 2 and 3 on an XLR cable and trading positive for negative, thus making a positive waveform negative and vice-versa. Regardless, in cases where the DI and the microphone are obviously “out of phase,” flipping the polarity switch may cause a change in tone when the two channels are combined. Note that if you listen to one of these channels and flip the switch while listening, you may not hear any change, because many humans are not sensitive to changes in audio polarity (a.k.a., “absolute phase”).
Phase really implies a timing comparison between two waveforms — in our case, the DI and the microphone. You need to have both channel faders up, flip the polarity switch and listen for a difference. Often times, flipping the polarity switch causes a marked increase in bottom end. If you hear a decrease in low frequencies, then the channels were already in-phase, or close to being in-phase. Whether you need all that bottom end in the mix is your business, but in either case, you must be aware of the phase relationship between the channels so that you can make an intelligent decision.
Flipping the polarity switch induces what amounts to a 180° change of phase. Fig. 2 shows two channels that are 180° out of phase: one waveform heads north while the other heads south at the same time (note the green line marker). This can be “fixed” with the polarity switch because reversing the polarity is the electrical equivalent of turning one of the waves upside-down. Alas, in reality the waveforms are usually not 180° out of phase; they may be more or less out of phase (say 90°), and the garden-variety polarity switch is too coarse a tool to fix the problem. There are modern tools available to correct this “out-of-phaseness,” tools with far more finesse than a simple polarity switch.
Some of these include Radial Engineering’s Phazer (radialeng.com/phazer.php) and Little Labs’ IBP or IBP Junior (littlelabs.com/ibp.html), both of which lets users adjust the phase of an audio signal continuously from 0° to 180° (and by inverting the waveform, from 181° to 360°). These are way more useful tools than a simple polarity switch.
You can also address phase issues via software, either using a plug-in intended for the purpose (Waves’ InPhase for example), or by using the channel delay on a digital mixer. Sometimes, adjusting the DI channel for a very slight delay (between 0 and 2 milliseconds) achieves the desired time-alignment required to cause the waveforms to crest at the same time.
Uh-oh… Sorry, but I gotta get on a plane. Next month we’ll discuss the relationship between the P.A. and the onstage bass rig, and get some tips on how to treat the bass input channels.
