How To Build A Vocal Booth (Part 3) - Acoustics

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This is part three of my vocal booth series. Today we will be learning all about the proper acoustic treatment for your booth. I will go over the downsides to small room acoustics, how we can make up for those downsides and the best approach I take when designing smaller rooms and vocal booths. Let's dive in! 

1) A Vocal Booth Will Not Have Perfect Acoustics - Ever

The physical properties of acoustics limit how effective a small space can be at attenuating low frequencies. For this reason, a vocal booth will never give you optimum acoustics. However, this should not discourage you as much as help you understand why you are making a vocal booth in the first place. The only reason for a vocal booth is to isolate the vocal from the rest of the environment you are recording in. It should not be to get that crisp perfect sounding vocal. 

Most professional studios may record a scratch vocal in a booth while a band is playing, but the final vocal is usually done in the control room or a dedicated vocal room that is not the size of a small closet. 

With all of this said, we also cannot leave our vocal booth entirely bare with just reflective drywall. So, the best course of action is to add as much absorption with as deep of an air cavity as possible behind those absorptive panels as we can afford space to give up. 

To learn more about why vocal booths make vocals sound bad check out my blog article here - https://www.soundproofyourstudio.com/blog/why-vocal-booths-make-vocals-sound-bad

2) How Absorptive Acoustic Panels Work

In our vocal booth the best method we can approach with such a small space is to use acoustic panels or walls that use mineral wool insulation to absorb sound waves. The insulation can be from many different brands, but I usually use Knauf ECOSE, Rockwool Safe N Sound, or Owens Corning Thermafiber. You can also use Owens Corning 703, but I tend not to use it because it is made of fiberglass and contains formaldehyde. 

Now, the basic concept around acoustic absorption panels is important to understand in order to design your vocal booth acoustics correctly. First, all insulation based acoustic panels work by converting sound to heat. The sound waves brush past the miniscule fibers in the insulation and the friction converts the sound energy into heat. This is very effective at absorbing mid to high frequencies, but not so effective at absorbing lower frequencies below 125 Hz. A 4" thick insulation panel with 3 lb/ft3 density has near perfect absorption down to 125 Hz (Everest and Pohlmann, 195).

This leads us to ask just how low does a human voice go down on the frequency spectrum? Michael Miller states in his article "The Voice: The Importance of Vocal Registers" that "The range of the male voice typically extends from about 85 Hz to 180 Hz for fundamental frequencies. However, professional bass singers can reach frequencies as low as 40 Hz" (Miller). This means that we should design our booth with the understanding that a large portion of the male voice will not be fully absorbed by traditional 4" thick insulation and that to gain greater absorption at lower frequencies we need to also understand something called the quarter wavelength rule. Here is a quick rundown of the 1/4" Wavelength Rule. 

The 1/4 wavelength rule in acoustics is a guideline used for the placement of absorptive materials in a room to maximize their effectiveness at reducing sound reflections. Here's how it works:

1. Definition: The rule states that to effectively absorb a particular frequency, an absorptive material should be placed at a distance from a boundary (such as a wall or ceiling) that is equal to 1/4 of the wavelength of that frequency.

2. Explanation: When a sound wave hits a boundary, it reflects back into the room. The point where the wave pressure is highest (called the pressure maximum) and the point where the particle velocity is highest (called the velocity maximum) are important in absorption. The velocity maximum for a given frequency occurs at 1/4 of its wavelength away from the boundary. Placing absorptive material at this point allows it to more effectively absorb the sound wave's energy.

3. Calculation:

   • Wavelength (λ): The wavelength of a sound wave is determined by the formula λ = v/f, where v is the speed of sound (approximately 343 meters per second or 1125 ft/s in air) and f is the frequency.
   • 1/4 Wavelength: Once you have the wavelength, divide it by 4 to find the optimal distance for placing the absorptive material.

4. Example:

   • For a frequency of 100 Hz:
     • Wavelength (λ) = 343 m/s / 100 Hz = 3.43 meters
     • 1/4 Wavelength = 3.43 meters / 4 ≈ 0.86 meters
     • Therefore, to absorb 100 Hz effectively, place the absorptive material about 0.86 meters (or approximately 2.8 feet) from the boundary.

5. Applications: The 1/4 wavelength rule is commonly used in the design of bass traps and other acoustic treatments to control low-frequency sounds, which have longer wavelengths and are more difficult to absorb.

Understanding and applying the 1/4 wavelength rule helps in creating more effective acoustic treatments and improving the overall sound quality in a room and is another helpful tool in getting our vocal booth to have maximum absorption across the entire frequency spectrum. 

3) Designing Our Vocal Booth (Putting It All Together) 

Now that we understand how absorption panels work we can now design our vocal booth to maximize absorption for the human voice. To do this lets think through our tools and constraints. 

1) We know that a 4" thick insulation panel will absorb down to 125Hz with almost perfect absorption and will absorb lower frequencies, but not as effectively. 

2) We know that by moving the panel off the boundary wall and ceiling we can increase absorption based off the quarter wavelength rule. 

3) We know that our vocal booth is mall and space is limited, so we cannot build panels that come off the wall too far or are too thick. 

With this information now we have to make sacrifices. This is up to you the designer to understand where to sacrifice acoustic performance for the sake of more space. My goal is to usually get 3-4" of insulation installed on all the walls and ceiling within a 2x4 frame. I then cover the insulation with acoustic fabric for a clean look. 

Now, if the client is willing to lose some space in the corners I straddle the acoustic treatment across the corner increasing the air gap behind the insulation. This will increase our low frequency absoprtion based on the 1/4 wavelength rule. 

Next, if the client is willing to give up some space on the ceiling I will lower the insulation off the ceiling drywall boundary to increase low frequency isolation. 

Conclusion: 

As you can see, a vocal booth has limitations because of its size with how well you can acoustically treat the space. This said, if you understand why you are building a vocal booth and that you can only do the best you can with respect to the acoustics, you now have a tool set with which to build your acoustic design for your vocal booth. 

As a side note, some people may be wondering about pressure based absorbers for lower frequencies. While these absorbers are effective at absorbing lower frequencies they are ineffective in smaller rooms because they require a lot of space to function. For this reason, in smaller rooms and most rooms for that matter, I stick to velocity based absorbers like fiberglass or mineral wool insulation. 

Works Cited:

"1/4 Wavelength Rule in Acoustics." ChatGPT by OpenAI, 15 July 2024, https://www.openai.com/chatgpt.

Everest, F. Alton, and Ken C. Pohlmann. Master Handbook of Acoustics. 6th ed., McGraw-Hill Education, 2015.

Miller, Michael. "The Voice: The Importance of Vocal Registers." National Center for Voice and Speech. National Center for Voice and Speech, n.d. Web. 15 July 2024. http://www.ncvs.org/ncvs/tutorials/voiceprod/tutorial/quality.html.