Sep 182012

For three posts the Auria for iPad Blog will be taking a closer look at the ChannelStrip. The ChannelStrip was developed from the ground up specifically for Auria by PSPaudioware. The ChannelStrip is available on the FX panel of each channel in Auria. In this three-part series, we will be focusing on each component of the ChannelStrip: the Expander, the EQualizer, and the Compressor. Using the ChannelStrip, we will go through a step-by-step process of mixing a kick drum track. Specifically, we will be treating the kick drum from the Auria demo song, “The Approach.”

In this first post, we will be discussing the Expander. Before we get into specifics about the Expander, we will begin by covering some fundamental concepts behind expansion. If you are familiar with the technical specifics behind audio expansion, feel free to skip to the tutorial. Otherwise, (or if you need a refresher!) please read on.


An expander is used to increase the dynamic range of a recording by attenuating the quieter parts of the signal. Most often, expanders are used to reduce the volume of or even eliminate unwanted noise in a recording, while preserving the moments it was intended to capture. Such applications include removing hum on a guitar track, the rest of a drum kit bleeding into the kick drum track, or even ambient noise picked up by a microphone during an interview.

An expander works by setting a volume threshold, below which any audio signal will be attenuated. The amount that the signal is reduced is dictated by the ratio. For example, a ratio of 2:1 implies that a signal originally peaking at 3dB below the threshold will be attenuated to 6dB below instead. The same signal with a ratio of 4:1 would peak at 12dB below the threshold. As the ratio increases, the expander becomes a gate, which will drastically reduce the volume of signals falling below the threshold, or eliminate them completely.

Most expanders/gates also include attack and release controls, which are both generally measured in milliseconds. The attack parameter determines how quickly the volume reduction will be lifted after the threshold has been breached. Release, conversely, determines how quickly the attenuation will be reengaged after the signal has fallen back below the threshold.

These are the basics behind the dynamic range expansion. Below is an excerpt from the Auria User Guide detailing the Expander’s controls and their functions. Familiarize yourself with these controls before moving on to the tutorial, where we will apply the Expander in a real mixing situation.

The Expander

Threshold – Adjusts the threshold of the expansion or gating. If you need to set a very low threshold, press the -24dB button to lower the scale of the knob by 24dB.
Ratio – Sets the expansion ratio. You can select between 1:1 and five expansion settings, as well as switch to a dedicated GATE mode.
Gain Reduction Meter – This meter displays the immediate attenuation provided by the expander or gate.
ATK (attack) – Sets the attack (gate open) time. The attack phase in FAST mode is covered by an internal pre-delay, which makes this setting click free. This is the best setting for transient content like drums and percussion. Use medium and SLOW settings whenever you need a smoother fade-in on open.
REL (release) – Sets the expansion or gating release (gate close) time.
Low Pass Filter – Sets the cut off frequency of the low pass filter for the side chain (control path). Use the IN button to engage the filter.
High Pass Filter – Sets the cut off frequency of the low pass filter for the side chain (control path). Use the IN button to engage this filter.
MON (monitor) – This button allows you to listen to the side chain signal (control signal) including any activated filters.
RANGE – Sets the maximum attenuation of the expander or gate.
EXP – Engages the expander module when lit.

Now that we’re accustomed to the specifics of the Expander, let’s move onto the tutorial.


We will now start mixing the kick track from the Auria demo song, “The Approach.” Open the demo project and follow along! Let’s hear an excerpt from the unprocessed kick track:

Let’s get rid of that bleed! First, make sure the entire ChannelStrip is not in bypass mode (Bypass button not lit) and that the Expander is activated (EXP button lit).

Our goal is use the Expander as a noise gate to remove the bleed between each kick drum hit. To achieve a substantial reduction in volume between each hit, we’ll set the ratio to ‘GATE.’ Since we want to preserve the attack of each hit we need the gate to open quickly, so we’ll set the ‘ATK’ control to ‘FAST.’

Now we’ll adjust the ‘THRES’ control. It needs to be set at a low enough level that every hits will open the gate, but that any other sounds will not. We don’t want any hits being missed, so we’ll set it on the lower side After listening and fine-tuning, the appropriate level seems to be around -30db. It may open excessively now, but we can improve on this using the side chain filter later.

Next up is the ‘REL’ control. We don’t want any of the resonance of the kick drum getting cut off when the gate closes, so it needs to a high enough value to avoid this. The 440ms or 646ms setting are both in the ballpark, with them being on the short and long side, respectively.

The side chain filter is a handy feature of the Expander. Press the ‘MON’ button to enter monitoring mode, which will allow you to hear what the filters are doing. Let’s lower the frequency of the low-pass filter to isolate the kick drum. The high hat disappears at around 1khz, so we’ll set it there. Press ‘MON’ again to exit monitoring mode. If the gate was opening unnecessarily before, this will most likely have helped.

Finally, the ‘RANGE’ control sets the lowest level to which the material that doesn’t open the gate will be reduced. Some may feel that setting it all the way down to -∞ can be jarring. Feel free to leave a bit in, but setting it too high will negate the purpose of using a gate in the first place.

We should now have a clean, isolated kick drum. Let’s take a listen to our results:


Sounds great! Tune in next time for a tutorial on using the Compressor…

  10 Responses to “Closer Look: ChannelStrip (Part One): Expander”

  1. The expander definitely helped. I suspect I will be using it on other tracks within the Approach. The hi-hat? Some guitar comes through. The snare I expect. Guitar? Not as much, but I think it can be cleaned up!

    • You don’t have to gate it if you have the option of renivomg the bleed. After all, the purpose of gating is to remove/reduce the bleed. And yes you still need to tweak the eq and compression of the drums so it will really blend or sit with your song.

  2. This is a great idea to show in depth how to use all the features in Auria. I can’t wait for the EQ section. Kept them coming!

  3. Great, and helpful post. Definitely refreshed my understanding a bit. Looking forward to the next 2 posts on this. Thank you

  4. Words cannot describe the disapointment I felt after purchasing this app. I just bought the app and ordered an alesis io dock to go with it. That is a total of $250 that I completely wasted. I can’t even open this app without it saying a crash has occurred then kicking me out. I had high hopes for this app and so did many of my friends who I am now going to tell to steer clear of this app.

    • Jake–if you’re having trouble, please contact We will help you solve any issues you may be having, since Auria should be working quite well in conjunction with the Alesis I/O Dock.

  5. I’m going through the demo tracks and a question comes to me. Is it possible to record the kick drum withough snare and cymbals coming through? I remember back in the day an acoustic blanket in the kick drum next to the mic. Then gating if necessary. I wonder if that might have helped or were the mics employed so sensitive that bleed was unavoidable.

    • Yes, in a studio setting this is certainly the traditional way to do things–we recorded this song in more of a live setting, tracking Drums/Guitar/Bass all at once. We didn’t cover the kick drum, which is why there is bleed on the track. Good thing we have the Expander to deal with it!

  6. We are using Toontracks, ezdrummer and Superior dmmeurr. With these plug-in’s do you think we will need to use any gating, since we can already turn off bleed if we want to within the drum program? Do you think most of these plug-in’s need to be tweeked, or in general are they already EQ’ed, gated, and mixed pretty well as a default. I know there is a lot of opinion involved, but what is yours.I love what you are teaching us, I love the music and the message. Thank you for doing this.Ed

  7. Expanders are also great for noisy tracks, where simlpy editing out the space between tracks makes it sound too unnatural. An expander turns the noise DOWN without turning it off entirely. It can be a real life-saver for us home studio folks!

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