VO Studio Acoustics: Let’s Get Ready to Rumble! Low Frequency Issues in the VO Studio

Identifying issues in your voiceover recording space is key to getting competitive sound. We usually hear reflections, but it’s easy to miss low frequency rumble.

Last week’s Tuesday Tech Tip talked about “Reflection”, but it would be imprecise to talk about acoustic reflection as though it were one simple thing. Those waves bouncing around your room don’t all act exactly the same. Some of them have a bit more “oomph” and may not care about padded surfaces or even walls. Meet Reflection’s stealthy cousin, Rumble. Rumble is low frequency or deep bass noise that lives in your voiceover recordings.

Unless you have acoustically isolated your recording space – basically floated it from the rest of your structure – you will have some level of low frequency rumble in your recordings. It’s extremely common in any home VO studio.

Rumble often continues to show up in your voiceover recordings even after investing in commonly recommended mid-density eggshell, wedge, or pyramid foam treatment panels. After covering all the walls with this material, things should sound perfect, right?

Not necessarily… I’ve had clients whose sound actually got worse after they took over a clothes closet and lined things with foam. Especially if they used the lower budget foam sometimes promoted on the internet. What happens is that someone will attach that foam to their wall and figure they are done.

But that foam (especially the stuff that looks OK but isn’t actually made from good material), will generally only reduce reflections in part of the frequency spectrum. Even the better quality open cell acoustic foam will generally do its best work above 250 Hz.

Let’s put some numbers on that –

2” Auralex “pyramid” foam has a reduction coefficient of 0.21 at 125 Hz, where their LENRD bass-specific trap has a reduction of 1.30 at the same frequency. (Higher is more effective at reducing that frequency). Auralex is actually quite transparent in supplying this information through this page on their site.

One benefits of your original closet-full-of-clothing was the variety of material densities distributed at odd distances or angles. When you remove the clothing and attach foam to the wall, the new material will filter out some of the reflections, though typically the low frequencies are not affected. Basically, lower frequencies are penetrating the foam, hitting the wall and bouncing back out into your recording space.

Another way of thinking about acoustic energy

It’s like a boat harbor with a breakwater around it. Even if the shorter, choppy waves are held back by the harbor wall, you can see the docks and boats rising and falling on a much slower cycle – reacting to long period waves which made it through and maintained their energy.

Soft, absorptive acoustic material is better at breaking up small waves – the higher pitched sounds. However, it tends to let the “swells” through – Rumble’s long/low frequency waves. That’s one reason we tend to have low frequencies build up in our booth space. Light foam and even heavy blankets may not prevent those low frequency waves from rolling through.

The other thing about low frequency waves is that they tend to penetrate walls. Because the wavelengths are so long – a single wave cycle of a 20 Hz wave is about 56 feet in length – you can think of them as having the momentum of a long freight train. It takes an awful lot to slow them down

None of this used to be a big issue. Back in the days of actual magnetic tape recording, those low frequencies did not get captured by the medium. Recording tape essentially filtered out those lower frequencies. But, since we’re all recording digitally these days, those frequencies get captured by our microphones, and then rendered directly into our recordings. We often don’t notice this, as those low notes tend to resonate in our bodies more than our ears. Our voices – even with the deepest voices – are operating somewhere north of 70-80 Hertz (OK, maybe this guy goes a little lower than that).

What’s the Fix? High Pass Filters!

What we hear is a certain lack of clarity in our voice. Using a High Pass Filter (either in software or a switch on the microphone) can reduce the Rumble and make us sound better. It’s one of the tools I use on everything I send out. I discuss filters and EQ in this post.

This information recently went out to my email community. If you would like to join in, please take a moment to share your contact information through this sign up form. Thank you.