“Image of a Failed Iso Booth Design,” Ben Wilson , CC BY 2.0.

TechnologySo I'm Building a VO Sound Booth

In looking at voice-over, perhaps I’ve jumped the shark. But, as a self-published author, I think being able to make my books available as audiobooks is a niche to explore. As an author, it’s not an insignificant outlay of funds. I’m going to focus today on the why and the plan.

A Foray into Voice Over

I first started looking into this last Winter. I bought a fancy mic, Avid’s Mbox and its accompanying GoPro tools. I lacked the cord, a stand and I wanted to focus on my writing goal, so I waited. The materials remained in their box.

On Friday, I had the cable delivered and a table stand. I thought at the very least I could use the mic and mixer to improve my dictation using Dragon Dictate. The mic-mixer is wholly incompatible with Dragon. However, playing with the mic-mixer revived my interest in voice over recording of audiobooks. You’ll notice that my word count for the past few days has diminished. That’s because of the research I’ve done on voice over this week.

Why Build an Isolation Booth?

According to what I’ve read, recording an audio booth is far more than a USB headset and a straight read of the text. The sound needs to be as clean as possible, lacking ambient noise and other sound artifacts. The voice has to be engaging but at an even meter. Amazon’s Audiobook Creative Exchange sets high standards. It feeds into the Audible website. My goal is to be publishable there. I need clean sound.

[What’s neat about being able to record audiobooks is that you can record other’s books (with contract) beyond your own for added revenue.]

I live near a major national airport. The takeoff/approach lane is less than one mile away. The planes operate every 2 to 5 minutes from about 630 in the morning until midnight. I also have children, so there’s always chatter in the house. My booth location is in the basement because it is the quietest corner of the house. Even with this, I cannot record clean sound without an isolation booth.

Sound-Proofing & Sound Absorption

My original research focused on finding a design that would work, with lists of materials they could just duplicate. The more I looked into it, however, I realized that I need a little more understanding of the theory. Once you can grok the theory, how to build your own booth becomes little easier. Isolation booths involve two principles: sound proofing (STC) and sound absorbing (RNC). Soundproofing involves preventing the transmission of sound from one space to another. Sound absorption involves managing the sound within a space.

Most of the articles and videos I ran into focused on how to build. They mixed both principles. Many had to do with vocal booths which are used by musicians who have the advantage of instruments to hide sound artifacts that a voice actor does not have. Many of the articles bought into the use of specialty acoustical materials with the appropriate surcharge. Once I start looking more into the theory, I started to see that more as buying into the marketing hype. I could be wrong, but I think that the approach I’m going to take, which minimizes use of those materials, will work for me. I want to go cheaper.

This article will focus on soundproofing, as that is the critical first step for voice actors. A later article will discuss sound absorption.

Stage 1: Sound-Proofing for an Isolation Booth

Because most articles mix the two concepts, there’s a lot lost in translation. The Wikipedia article on STC gives a pretty good guide of what you need to soundproof adequately. STC is roughly a decibel sound-attenuation.

The STC shows the decibel reduction from one space to the next. If there is a sound at 90db in one space with an STC 30 wall, then 60db pass into the other space. There are plenty of decibel loudness comparisons available. Most home theaters have a decibel rating of 100db; so an STC 60 would reduce the sound to 40db.

I have an HVAC unit in the area that may get as loud as 80db. My goal is to attenuate to 30db, so an STC 50 is a good target. To be certain of my target, I’ll use a USD 20 sound meter.

STC RangeImpact of rating
30–9Poor (Loud speech intelligible, normal speech not)
40–9Onset of "privacy"
50–990% of noise attenuated
60+99% of noise attenuated

Here are some common frame arrangements:

35Single-StudSingle-layer 1/2-inch drywall
40Single-Stud Resilient ChannelSingle-layer 1/2-inch drywall
40Single-StudSingle-layer 1/2-inch drywallInsulated
45Single-Stud Resilient ChannelSingle-layer 1/2-inch drywallInsulated
45Single-StudDouble-layer 1/2-inch drywallInsulated
45Staggered-StudSingle-layer 1/2-inch drywall
50Staggered-StudSingle-layer 1/2-inch drywallInsulated
50Staggered-StudDouble-layer 1/2-inch drywall
55Staggered-StudDouble-layer 1/2-inch drywallInsulated
60Double-StudSingle-layer 1/2-inch drywallInsulated

Staggered-stud uses a 2”x6” footer/header with 2”x4” studs. Double-stud appears to have about 1-inch air gap.

The following is a list of rough attenuations by method or material:

~db imp.Method
+6-8Increase airspace (3"-6")
+5Add Dense Mass-Loaded Vinyl (DMLV)
+5Double layer, double mass
+4Resilient Channel
+4Add insulation (fiberglass batts)

Green Glue claims +5db improvement per sandwich, based on my deciphering of their market literature. I was unable to find any objective data to validate, so I didn’t feel comfortable putting it in the table above. To attain that, you are asked to use two tubes per 4’x8’ sheet, which is about USD 40 per sheet.

My Framing Decision: Based on the above, I should be able to get away with a single-layer, staggered-stud insulated framing. I highlighted that row above. If it proves to be inadequate, then I have the option of adding another layer of drywall. I’ll verify the before-sound prior to finalizing this decision.

While I said the article would focus on sound-proofing, the basic frame of the structure has a major impact on absorption. Since I can’t soundproof without a frame, I have to at least have an answer here.

Smaller spaces create more bass, parallel walls create more reverb, and small square spaces are the worst of all. I looked at designs that had five sides like a princess cut diamond, but the problem was the dimensions meant that I would need more than one sheet of anything to cover it. They were wider than 48 inches on both sides. I then designed a smaller design look a little more like a coffin but could use one sheet. But that got to be too complicated; the image above shows that design. I’m going to go with a 4’x5’ rectangle.

To avoid flutter echo caused by parallel walls, and can shorten two of the walls by 5%, which means that one wall will be 46 inches, and an adjacent wall will be 57 inches. This should break the problems with parallelism. Because I’m doing this in my basement, which is not full height, the maximum height will be 80 inches. I will add a false ceiling later that will go from 80 to 75 inches, which is a 5% drop off; avoiding parallelism with the floor.

Next Step

Now that I have a basic idea (staggered, insulated, etc.), now all I need is a four-day weekend…like Labor Day 2014. Once the area is framed and the walls are complete, I’ll take a second sound-check

© 2014 Ben Wilson. All Rights Reserved