Tuesday VO Tech Tip: Rumble Revisited

All kinds of sounds lurk in the low notes of our recordings. Much of the time, we are blissfully unaware of how much “extra stuff” hides below our voice. Part of the issue is that most people do not readily hear sound down in that range of the frequency spectrum. We tend to “feel” the low bass notes in our body more so than with our ears. When we focus on that part of the spectrum, we realize how messy things are down there.

These sounds sometimes become more prevalent during the winter months, when our furnaces and HVAC systems keep humming. It could also be our wonderfully sensitive microphone. Things may sound decent when we are speaking, but we notice that our room tone is full of … well, “something.”

It can be horrifying to find how much is going on when you employ an analysis tool. When using a “spectral” view tool such as Izotope’s RX or Adobe Audition, you’ll notice a bright layer near the bottom of the screen. With a frequency analyzer such as Blue Cat plugin, you’ll see a hefty lump to the left of the frequency plot. (Both types of tools are shown in the screenshot above). How did all that sound get in there?

This is not something a few more acoustic foam tiles will fix. Low frequency noise – meaning those sounds below about 60 Hz (cycles per second) – is a powerful, penetrating bit of acoustic energy. A 60 Hz sine wave is approximately 18 feet long – and goes through us 60 times a second. It’s kind of like trying to stop a freight train. Any structure connected to the earth will tend to resonate with lower frequency sounds. That means that we are basically recording inside of subwoofers.

A High Pass Filter is Not “Cheating”

To actually solve rumble at the source, the most direct solution would be to isolate your studio from the earth. This can get very expensive, as that type of construction requires floating rooms or floors and isolated structures.

More commonly, we can reach for a High Pass Filter of some sort. This is a specific type of frequency filtering that removes low frequencies and allows higher frequencies to “pass” through. You may also hear people refer to this as a “Low Cut Filter”. That’s simply a different term for exactly the same process. These High Pass Filters (HPF) may appear on the microphone itself, as a hardware device you can plug into your input signal chain, or will appear as a software Effect in your audio plugins. The benefit of the software effect is that you can tune the “cutoff” frequency where the filter starts to do its work, as well as the “curve” affecting how aggressively it will remove the lower frequencies.

Unless you are blessed with an unusually deep voice, setting a High Pass Filter with a 60 Hz cutoff for most male voices and 80 Hz for most female voices would be a safe place to start. Depending upon the curve of the filter, you could adjust from that point to tune it for your specific conditions.

Using a High Pass Filter has the direct benefit of making you sound clearer. With all that hard-to-hear rumble removed, it’s much easier to hear the vocal range in your recordings. That’s a good thing!

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