If you're planning on setting up a studio where people are going to be recording vocals, it's important that you fit in a proper vocal booth. While it's possible to record vocals without a booth, the results you get will be no where near the level they would be if you got one made professionally. You're going to be able to hear background noise, your levels are going to be inconsistent, and overall you're going to produce a poor quality product.
While it's often best to hire a professional to install a vocal booth for you, it's still a good idea to have some knowledge on how to build a vocal booth yourself. That way if you're a bit adventurous you can give it a try yourself, or at the very least you will know what's happening when the fitter is talking the lingo to you. It'll also allow you to better understand what you want from your booth.
So, below we are going to look at some specs of a vocal booth. If you are considering putting one together yourself, this is the information you will need to know.
Before we go further though, please note that this guide has been contributed by Mike Sorensen at www.AcousticFields.com. It's ideal for people who want to build their own professional studio rather then hiring one out.
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Considering The Vocal Frequency Range When Planning The Vocal Booth Size
When building a vocal booth, you will need to think about the vocal frequency range of the people using it. This will impact on the way your booth it built.
So, what is the frequency range for human vocals? Well while there are of course exceptions, it's between 150 Hz and 300 Hz for males, and 200 Hz and 500 Hz for females. Having said that, if we asked 10 different engineers, we would probably get 10 different answers back. If you average their responses together however, you will be somewhere close to this range.
For discussion sake, lets use 150 cycles as our bottom range and 500 cycles as our top room design frequency. A 150 Hz wave is about 7 plus feet, and a 500 Hz wave is a little over 2 feet long. If we want to make absolutely sure of no room resonance at these design frequencies, we must have a minimum 8 foot dimension in our width of the room. The 2 foot length of our 500Hz wave is not a issue with respect to our room's dimensions.
So a good size to go with is 8 foot wide, 8 foot tall, and 10 foot long. We can make the room smaller, but care must be taken in the dimensions used to avoid room resonances in our vocal frequency range. This is the same as what we'd do when soundproofing a room in a house.
Adding Your Booth Window
The main reason a vocal booth will need to have a window is so the control room engineer can have visual contact with the singer or rapper. This window must be clear for visibility to occur, but it also must employ barrier technology. The window must be seen through, but it must not let any sound through. On top of that, it must also keep the sound from the vocalist from entering other parts of the studio.
The window should be made of a laminated glass of at least half an inch thick. Laminated glass is a mass damped assembly that will assist you in reducing vibrations on the glass itself.
Make sure you get this right, as the last thing we want in our vocal room is our window moving in response to sound energy and in effect turning into a speaker.
Putting The Window Together
This is an important part of building a vocal booth. You will need two pieces of the laminated glass we talked about in the previous section. Before putting them in, you will need to mount both pieces in their own frame just like you are framing a picture. Make sure you leave 2 inches of air space between each of the individual glass pieces.
Use an acoustic glue for the windows to frame contact area, and use an acoustical sealant around the inside and outside edges of the glass / frame contact area. Place the frame and window assembly and then line the window opening in the chosen wall with a damping compound. Insert the window assembly into the opening using an acoustical sealant.
This will ensure that no sound is getting through your vocal booth window, yet the engineer will be able to see their client.
Specifications Of The Vocal Booth Door
The door on our vocal booth can be made using two standard solid core doors, with a viscoelastic damping compound acoustically glued between each one. You'll need to glue the damping compound to the side of one door and then let it dry.
Place glue on the damping compound, and place the other door on top of it. Frame around the door edges, making sure you select the correct hinge assembly depending on the weight of the door. Screw with 2 inch deck screws through both the solid doors and the damping compound. Place your screws 12 inches into the center so it covers the whole door surface.
Use an industrial strength weather strip to seal all air spaces that will occur around the new door. This will most likely happen at the sides, the top and the bottom. Use a longer door handle on both sides, since the weight of the door may require two hands to pull open. Do not cheat on the weather striping, as you want to make sure you have an airtight seal.
The wall construction of our vocal booth is all about sound and vibration isolation. We must keep all external noise from entering the vocal booth, and we must keep all vocal energy within the room from leaving. We must achieve a STC rating of at least 60 to achieve this goal. We can achieve this sound transmission loss number, but it will require mass to do so.
The easiest way to start this process is to build two, 2′ x 4′, framed walls at least 4 inches apart. This room within a room concept must be built so that each framed room is constructed in a way to insure maximum vibration isolation. If frame construction methods are not your forte, then you can instead use block construction.
Make sure to have a solid floor to build upon, and make sure each layer of block is sealed together. Fill the blocks with a sand / activated carbon blend of 50/50. This block method will give you the same vibration and sound isolation numbers as our previously discussed dual wall construction.
Now, we must look at the inside of our vocal booth with an ear on reverberation times. Reverberation times are caused by numerous reflections from our vocal booth room boundaries. The rear wall is of particular interest, because rear wall reflections at the mic position are not allowed. Their time delay signal at the microphone must be controlled. All vocal booth walls must be treated for reflection control, but the rear wall needs special attention. The goal of the inside of our vocal booth must be reflection control at the mic position, and achieving the proper reverberation times.
We need to strive for reverb times between .13 and .20 seconds. This can be accomplished through the use of acoustical foams. Acoustic foams are lightweight and economical. One can choose from open celled acoustic foams or closed cell foams. Closed celled foams offer a more consistent absorption curve with the proper rates and levels of absorption. In a vocal booth, absorption rates and levels are critical, especially in our vocal booth design parameters of between 150 Hz – 500 Hz for both male and female vocals.
Start with a minimum of two inch thickness and go from there. A single layer of 2 inch closed celled foam on all room walls and ceiling will achieve the necessary room reverb times in our chosen room size.
About The Author
This guide was written by Mike Sorensen a structural engineer, master cabinet maker, and the author of the www.AcousticFields.com audio blog. Check it out for more of his articles and services.