In the last episode we have learned that E-MU Emulator 4 gives us three distinct compressors, rather than one. Sharp-eyed readers likely noticed that we didn’t mention one feature of the compressor—a setting that provides two additional options and is related to the compressor analysis mode, which can be either Peak or RMS based. This wasn’t an omission but rather a deliberate choice to avoid making the text too lengthy—and potentially tedious to read. However, in the case of a compressor, limiter and expander, using the correct mode can be a critical setting worth understanding.

RMS (Root Mean Square) compressors measure the average power or energy of the audio signal over time, which closely aligns with how humans perceive loudness. Instead of responding to short, sharp peaks, they consider the overall “energy” of the signal. Characteristics:

• Smoother, more natural compression.
• Ideal for leveling overall volume rather than reacting to transient spikes.
• Often used for vocals, bass, and mix bus applications where consistent loudness is desired.

An example usage would be reducing the overall loudness variation in a vocal track without reacting to quick peaks like plosives.

Peak compressors respond instantly to the highest signal levels or transients, regardless of the overall energy of the signal. They focus on capturing and controlling sharp, sudden peaks. Characteristics:

• Precise and reactive.
• Ideal for controlling sharp transients and preventing clipping.
• Often used for instruments with pronounced attacks, such as drums or percussive elements.

An example usage would be taming the attack of a snare drum or preventing distortion in a highly dynamic recording.

Armed with all this knowledge, let’s now get back to our Emulator 4 and run some more tests. This time, we will test the expander in all three threshold configurations: Below, Center, and Above.

The Expander
A dynamic expander is an audio processing tool used to increase the dynamic range of a signal by amplifying the differences between loud and soft parts. Instead of reducing the range by lowering louder signals, an expander decreases the volume of sounds below a set threshold, making quieter sections even softer relative to the louder parts.

This behavior is useful for noise reduction, as it can lower background noise levels when the main signal (such as speech or music) falls below the threshold. Expanders can operate in downward expansion, where the quiet parts get quieter, or upward expansion, where louder parts are made even louder. This is where our Emulator 4 comes into play as it offers all two options, plus additional one called “Center” which is in a way the combination of the two.

The key parameters of an expander include the threshold, which determines when the expansion begins, and the ratio, which controls the degree of expansion – for example 1:2 ratio doubles the dynamic difference. Unfortunately Emulator 4 does not provide us with rational numbers for the Expansion control (e.g.,1:2, 1:3, etc.) instead is uses Real numbers like 0.50:1 which in this particular case would be equal to 1:2 expansion ratio.

We will skip the explanation of what each column and row in this graphics above represents, since we have explained that part in the first episode. Now let’s start with the third (bottom) row, as it uses the expander in a threshold configuration set to “Below.” This is how your average expander is supposed to work. At around -6 dB, it starts applying dynamic expansion, and everything seems to look correct. However, when we examine the falling slope (the second half of the waveform), we can notice a strange feature: the expansion is not symmetrical.

In fact, it brings us back to the compressor “issue” or feature that we mentioned in the first episode. While I can understand a compressor having such a feature, I can’t figure out why this would be done for an expander, which is primarily a tool for “repair” rather than adding character. What’s happening is that the expansion on the falling slope is not being applied as it should. It misses almost 3 decibels of audio before starting the expansion in that part of the audio. One can’t help but notice the similarity in behavior to the compressor. But then, we have to keep in mind that this is the same compressor algorithm, which I guess explains this “feature”. I know the image is very small for detailed inspection, but in the high-resolution version, I can clearly see how the expander misses the -6 dB mark and starts expanding at around -9 dB. A bug? I guess we’ll never know.

Now, let’s examine the middle row. Remember when we mentioned earlier that some expanders make loud portions of the audio louder? That’s exactly what the expander in the “Above” threshold setting does. As we can see, it doesn’t affect any audio until -6 dB, after which it starts expanding, making the middle of the waveform (the peak) even louder. In this case, it results in overdrive. Therefore, I’d recommend never using normalized (maximized) audio in this configuration, as it will definitely produce distortion/overdrive.

Finally, in the top row, we have a rather exotic type of expansion—a sort of “extreme” expander. This mode combines the previous two methods by attenuating audio below -6 dB and applying gain above -6 dB. This could have potential for percussive or drum-type sounds buried in heavy noise, as this expander effectively removes that noise. Again, be very careful with the gain control, and don’t normalize the audio before using it in this configuration. Here is another example of expansion with different settings:

This time, the original audio was set to -1 dB to prevent clipping phenomena for the two expander threshold types that make loud parts louder. Slightly stronger expansion was used this time, with a 1:2 ratio at -6 dB. In hindsight, I should have set the source to -3 dBFS for even better preservation of the peaks in the middle of the wave (the loudest portion), but when zoomed out so far, it would have been difficult to see other details. So, I opted for this approach.

Let’s start with the third row. Again, the expansion is not symmetrical—something isn’t right in the descending part of the audio. I also noticed “leakage” in the low-energy area, as if the expander didn’t entirely reduce the volume in a 1:2 ratio but suddenly shifted to something closer to a 1:1.1 ratio (or less). I’m not sure why this happens; perhaps I’m missing something.

The vertical scale in all these graphs is logarithmic, which means the displayed volume reduction should appear linear—but it doesn’t. The other two threshold options behave as expected: the “Above” setting makes the area above the threshold louder, while the “Center” threshold option once again shows us a rather exotic, extreme expansion taking place. I guess one could create quite sharp percussive sounds using this threshold configuration. In the next episode, we will look into a limiter and conduct some tests with normalized audio.