Feeding Your Subwoofer

Text: Hugh Covill

FEED YOUR SUBWOOFER SYSTEM FROM A POST FADER/POST EQ AUXILIARY SEND

That’s it, in a nutshell. Instead of feeding your whole mix to a crossover, which then derives the bass signal from the entire signal, why not send your subs only the ingredients you deem appropriate to the mix?

In a conventional PA setup, the subwoofer system’s signal is derived from the main mix bus. Typically, a band setup will be mixed at front of house on a number of console input strips, subbed to groups and then combined in the stereo bus. This stereo mix is then usually sent to a loudspeaker management device – of which the DSP variety now firmly holds sway. The DSP box performs whatever frequency division is required by the speaker system: i.e. two-way or three-way splits etc. For the purposes of this discussion let’s assume a three-way loudspeaker system is in play, in which case we’re talking about three divisions of our sound: Low (subwoofer), Mid and High frequency.

DIVIDE & CONQUER

These frequency divisions across our PA loudspeakers are achieved by the use of crossover filters that allow each component of our PA to work most efficiently. Most loudspeaker manufacturers will specify ballpark crossover settings for their products and the bulk of DSP loudspeaker management systems will allow the end-user a vast array of crossover slopes and other variables from which to select. Endless debate surrounds the appropriate crossover settings for the optimisation of subwoofer systems, and it’s beyond the scope of this article to talk in any detail regarding just how these crossover settings are derived. Suffice it to say, correct crossover settings involve detailed measurement of the entire loudspeaker system to determine the most appropriate crossover frequency, slope and time component delay setting (where appropriate).

Crossover settings are chosen in an attempt to match phase and frequency response through the crossover region. At this point, it might be helpful to look at a couple of illustrations that will help represent these concepts.

LOOK BEFORE YOU CROSS

Figures 1 & 1(B) show the way in which a typical three-way PA system is divided using crossover filters. The blue line shows the desired output across the entire frequency spectrum. The red line shows the mid frequency band-pass filter and the areas where the red and green lines (hi- and low-pass filters respectively) intersect are called the crossover regions. The slopes and crossover points are specifically chosen so that, when summed, these two sounds theoretically produce a ‘flat’ frequency response through the crossover regions. The steeper the slope the better the rejection of frequencies into neighbouring bands, although, as a general rule the more gradual the slope, the better the phase response through the crossover region. Having said that, some contemporary digital filter designs like the Dolby Lake Processor, utilise very steep (100dB per octave or more) slopes while maintaining the phase response through the crossover region.

The point of this article, however, is not to debate the merits of DSP crossover technology, but rather to simply illustrate the fact the subwoofer system of most PAs is usually fed from a certain filter set of your DSP. This is typically extracted from the main left/right output signal emanating from the console, so that any ingredients of your mix that possess sub-octave frequencies will, by default, have their bass content delivered to the sub via the crossover.

DISCRETE SUBS

But what if you could choose specifically which instruments went to the sub, instead of being at the mercy of a system setup that sends everything there by default? Feeding the sub system via an auxiliary send gives you precisely this control directly from the console, offering distinct advantages over the Left/Right-derived method. Let’s look specifically at how this technique works.

CONTROLLING THE SUB MIX

The main advantage of using an auxiliary bus as an alternative means of providing signal to the subwoofer is that you can specifically select which instruments will be sent to the subwoofer system merely by punching in that channel’s auxiliary send – no different to how you might feed a foldback wedge. Those channels not selected are then, by choice, completely removed from the sub signal bus, thereby removing a swathe of unwanted low frequency trash/rumble. In Figure 3 I’ve used a separate crossover unit for simplicity’s sake, however, all that is really required is a discrete crossover in/out path.

In a typical band scenario, I’d probably only assign kick, bass, floor tom and maybe keys – depending on the nature of the content – to the subwoofer mix, which would be impossible, of course, if I was using the typical setup of Figure 2.

Any channel, for which you would routinely punch in the high-pass filter, will benefit from this control method, but probably where the greatest benefit is achieved is in those channels employed for distance miking such as drum overheads, choir microphones, horn section microphones and orchestral reinforcement situations.

As an example, let’s consider drum overhead microphones for a moment. Typically, two of these will be employed to capture the entire kit. They would usually be high quality condensers with an excellent frequency response that extends well into the sub octave band. Where the PA is conventionally set up so that all channels feed the subwoofer via the crossover, the first thing most engineers would do – almost as a matter of course – would be to punch-in the high-pass filters across both drum overheads. However, even with these filters engaged, a significant amount of low frequency ‘trash’ will still be passed to the subwoofer system by virtue of the fact that the filter slope, which is typically 12dB/18dB per octave, is instantly reduced by a factor of 3dB when the two microphones sum.

If you then extrapolate this to a scenario of multiple choir or orchestral microphones reinforcing a single large area, you can see that your HPF’s efficiency can be severely reduced. Remember: doubling the number of open microphones will decrease the potential acoustic gain by a factor of 3dB. (Each microphone is sampling the sound field. If we bring two mics up to the same level in the PA we then need to reduce the gain by a factor of 3dB to avoid feedback.) This is just plain ol’ decibels combining. There’s even a formula for this which is given as: NOM (number of open microphones) dB = 10logNOM. By plugging some numbers into this equation we can see that two mics = 3dB, four mics = 6dB, 10 mics = 10dB and so on.)

A HPF set for 120Hz (assuming a 12dB per octave slope) will measure –3dB at this cut-off frequency (sometimes termed the ‘shoulder’ or ‘corner’), and –15dB at 60Hz (an octave down). With two microphones producing +3dB more energy we can see that the high-pass response will now be flat at the filter cut-off frequency! If we then consider a choir scenario with eight microphones and extrapolate the numbers, we now have +6dB at the filter cut-off and –6dB at 60Hz. Not much of a cut off really is it? You could be forgiven for thinking that the concept of a ‘cut off’ might infer that there’s no extraneous low frequency energy passing through to the sub, when in reality, a lot of these unwanted frequencies are still marching on through!

PEAK & CHOOSE

Sending specific channels to the sub via an auxiliary is especially effective at reducing general very low frequency rumble associated with room and stage vibrations, low frequency wind noise at outdoor events, microphone proximity effects and vocalist plosive and breath noise. Much of this noise will be very low in frequency, from around 5Hz to 40Hz. Including this unwanted program material in your mix bus will consume a good deal of your sub amplifier’s power trying to reproduce it and only make your sub amplifier less capable of efficiently reproducing the signals that are important to your mix. Moreover, depending on the quality of your speaker system, really pushing these unwanted frequencies into your subs can lead to excessive speaker excursion and possible damage to your drivers.

When you think through a standard band’s input list, the majority of sources will have very little useful dynamic range in terms of reinforcement much below 80Hz. One of the real advantages of the ‘auxiliary send’ technique is that if you’re not sure about a particular input, you have the option at the mixing console to just punch it in or out of the sub-mix, and audition the difference. Fantastic!

SUB PRIME MARKET RE-IN-FORCES

There are very few scenarios that won’t benefit from the ‘auxiliary send’ technique. Most styles of music are prime candidates for an auxiliary-fed sub system; with outdoor events, orchestral and chorale mixes sitting at the top of the list. What’s important to note is that you really don’t need to do anything particularly different; the two set-ups are fundamentally the same. You can utilise the same gain structure and crossover filters that were derived from the system optimisation, all you need is a discrete DSP in/out for the auxiliary send. The crossover slope/delay etc settings that you would normally set for the L/R fed system are retained; they’re just accessed differently… Then, once your gain structure is set, it becomes very important not to alter it. Set your channel and master send aux levels to unity and then just use the auxiliary’s on/off switch at the desk for assignment (and if there’s no desk switch, just turn the pot up to unity).

One word of warning: I have seen many people adjust the aux send levels from song to song to create a more ‘dynamic’ sub sound and this might be appropriate in some very special circumstances – theatrical special effects, for example. However, when you’re dealing with applications where high fidelity reinforcement is paramount, doing this will actually seriously screw with the phase response of the sub system, negatively affecting its relationship to the rest of the PA, and no-one wants that! So whatever you do, don’t fiddle with the send levels once they’re established!
So there you have it. A pretty simple and easy-to-implement technique that will clean out your bottom drawer, leaving you space for all those important bits and pieces. Good luck and happy mixing!