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Live sound: how to get the most out of your subwoofer's

As a sound engineer, I love having maximum control; and I’ve found that during a live show I’m often unable to control certain variables at the mixing console, leaving me to get creative. I recently stumbled upon the Idea of routing subwoofers as auxiliaries, which for my situation as a live sound engineer for mostly small venues and festivals, has been a game changer. Lets take a look at what I’m doing to have full control over my low end frequencies.

The 'aux fed sub':

The standard practice of routing subwoofers in a front of house P.A system is connecting them within the main outputs of the front of house mixing console – whether that be linked or in cross-over connection. This form of routing works fine, and requires you tune the subwoofers in terms of volume, and then through some form of equalisation connected to the main outputs.

The ‘aux fed sub’ is a method in which the subwoofers of a front of house P.A system are connected via an auxiliary output, where you can choose to send certain channels to this auxiliary output. This means that you can now control the subwoofers independently from the ‘mid-high’ speakers, and have volume control at the desk. This method also opens up a great deal of opportunities for the creative engineer, especially one with lots of tools.

Setting up the subwoofers:

We have established the general form of connection, now lets get to making noise. For this guide, I am mainly going to be refering to tools availiable to those operating a digital mixing console – which is really the standard of todays venues.

See this setup in action over on my Instagram in upcoming events:

1.

Plug your subwoofer into a spare auxiliary output/sub-mix output. If using multiple subwoofers – link them together via the speakers linked outputs (which are on almost all speaker boxes).

2.

Assuming that you have already connected your ‘mid-high’ speakers to your mixing console via the main outputs, you then want to set the level of the subwoofer(s) as per usual, but remember to have the auxiliary master volume of the auxiliary output that the subwoofer is connected to at unity.

3.

Apply a high pass filter to the auxiliary that the subwoofer(s) is connected to. This pathway will be refered to simply as the sub. This high pass filter will be to control what frequencies are being reproduced by the sub, and thus allows us to control where the low frequency cross-over point in the front of house system is. I usually set this high pass filter around 100hz.

This step should also be taken in reverse for the ‘mid-high’ speakers, in that you then need to apply a low pass filter at the frequency you set the high pass filter at on the sub. Once this has been done, listen for a build up of low end information, which can occur around the frequency of the filtering. If a build up is present, you want to respond to it via a graphic EQ of sorts – only taking the minimum of dB out of the frequency to ensure the speakers are set as flat as possible in terms of frequency reproduction.

4.

After equalising the front of house, you could sit back and relax, but thats boring – so lets add some compression to the sub. I want my low end frequencies to remain as still as possible, so typically I will set a moderate attack and decently quick release, 8:1 ratio compressor over the sub auxiliary, and dial in the threshold enough to make it that the low end becomes ‘still’ at the extremities – whilst allowing some room for dynamics.

This compression is also unrelated to any main output compression you may apply to the rest of the mix, meaning that you shouldn’t need to mess around with sidechaining to avoid low end pumping.

5.

After adding compression to the main mix, it’s best to adjust the volume once more to ensure the right balance between low and high frequencies. Remember to be on the lookout (or ‘listenout’) for any resonances in the low/low-mid region.

With great power...

The aux fed subwoofer is a great tool for those wanting ultimate control over the mix, but using this method can present some problems. Be sure to have a decent understanding of balance and to not go overboard with any compression added to this aux. This method when done correctly provides seperation in the low end, and prevents unwanted amounts of low frequencies from interfering with the mid-to-high frequencies when it comes to main mix compression.

You should probably stick to the traditional method of routing the subwoofers within the main mix outputs if the mix does not require a strong and vibrant low end, or you may want to avoid excess processing for acts that don’t require that extra magic on the engineering side – acts that have an acoustic feel. This method can really make an impact in a venue, the low end becomes quite powerful whilst remaining in its ‘pocket’ (it does not interfere with the other frequencies).

I have noticed on occasions that the low end can become overpowering even at a lower volume when using this method – this can make the mid-high frequencies lose impact. Ensure that balance remains the priority, and don’t emphasise one frequency range over another. The treat of using this method is that if the low end is too present in the front of house, you can simply turn it down at the desk and re-adjust the processing that you may have applied.

 

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