Most of you will be familiar with the idea of ducking, a process by which the level of one signal is controlled by that of another. For example, this is how the background music is dipped automatically in level when a DJ speaks. But ever since the inception of spectral processing, it occurred to me that a ducker that affects the levels of only conflicting frequencies should be possible. This concept has gained traction in recent years and it’s what lies behind FireSonic’s Firespacer, which is marketed by United Plugins and protected by a user key that allows installation on all the user’s machines. All the common plug‑in formats for Mac and Windows are supported, including AAX, and a trial mode is available so you can try before you buy.
Firespacer can be used in any DAW that supports side‑chaining. The plug‑in is inserted on the channel or bus that needs ducking and the control signal is received through the external side‑chain input. There are four modes of operation: two Spectral, which introduce some noticeable latency; plus two latency‑free modes that use minimum‑phase filters. The Spectral modes are the most accurate, as the minimum‑phase filtering modes may carve out some frequencies that don’t need trimming. As to which filter or spectral mode you choose, that’s down to listening to see what works best for the material being processed. A Precision control, only active for the Spectral modes, sets how tightly the spectrum of the source sound is followed, so at lower settings the effect is rather like smoothing the response curve of the spectral filters.
The resizeable GUI offers mono, L/R or Mid‑Sides operation, with metering and separate gain controls for the input, output and Sides levels. The Sides also has a solo listen button. Two controls set the minimum and maximum processing frequencies so that the user can opt to exclude lows and highs from the processing, and there’s also a variable release time that’s best adjusted by ear to suit the material. The large Amount knob in the centre sets the depth of the ducking and there’s a frequency display running across the top of the window that shows the main and side‑chain signals as well as the signal level after ducking. The power button functions as bypass.
By pulling down only overlapping frequencies, spectral ducking sounds far more transparent.
A typical application might be to duck the spectrum of pad or distorted rhythm guitar sounds in the presence of vocals, where there are likely to be significant overlapping frequencies, especially in the midrange. Similarly, you might use it to keep a lead instrument clear of whatever murk is going on in the background. There’s also potential for untangling conflicts between the bass and drums. Doing these jobs with a conventional ducker often results in very obvious gain changes to the track being processed. By pulling down only the overlapping frequencies, spectral ducking sounds far more transparent and the higher latency of the spectral modes isn’t an issue when mixing. Where zero added latency is important, the minimum‑phase filter modes still sound more benign than straightforward ducking, though some tonal modulation of the target track may be evident at more extreme settings with some material.
In summary, Firespacer is a simple‑to‑use and very effective mix tool that helps bring more clarity to key parts by reducing conflict due to overlapping frequencies in other parts of the mix.