A portable soundproof booth must accomplish two main objectives. First, it must isolate the inside noise levels from the outside noise levels. Isolation is function one. The next objective is the inside treatment of the portable soundproof booth. We have two choices of treatment for the inside of the portable soundproof booth. We have absorption and diffusion. https://www.collinsdictionary.com/us/dictionary/english/soundproof-booth Diffusion is an option for larger portable soundproof booths since distance is required for diffusion to work properly. Absorption is the treatment of choice for smaller portable soundproof booths. When using absorption for voice, we must be concerned with vocal resolution at the microphone position. In order to accomplish the highest resolution we can, we must focus on the rate and level of absorption used in order to achieve the highest resolution possible for vocals. https://www.merriam-webster.com/dictionary/soundproof
Not all absorption technologies work for the human voice. Most over absorb and drain the life out of the middle range frequencies in human voice. Whether you are treating a portable soundproof booth or a fixed vocal room, you must pay special attention to the rate and level of absorption that you will be using to manage the reflections from the wall surfaces that arrive in a delayed time signature at the microphone position. The microphone sees two types of energy. It sees the direct or straight line energy from the mouth to the microphone. It also sees the reflected energy from the wall surfaces. When the reflected wall energy mixes with the straight line or direct energy from the mouth to the microphone, phase occurs. Phase reduces resolution. It’s the same as noise in a recording. https://www.sciencedirect.com/topics/engineering/sound-absorption.
To minimize this phase interference, we must choose an absorption technology that has the proper rates and levels of absorption for human voice. Since voice is our standard communication method, our hearing mechanism is very sensitive to small tone and energy changes. We must respect this if we are going to produce a quality resolution recording. The most critical region for human voice is the 125 – 500 hz. region in the frequency response curve. This area of sonic quality is very easy for the human hearing system to detect subtle differences within. This area must be treated with respect or you will never be able to produce a quality recording of human voice. This is the frequency response region that building insulation destroys. You must choose technologies that are designed for voice. https://www.acousticfields.com/about/
When we are dealing with sound absorption within our portable soundproof booth, we must choose a technology that absorbs energy within this critical frequency response area that absorbs in a smooth and linear manner. There can be no spatial irregularities in this frequency response region. The 125 – 500 hz. region must be linear with no peaks or dips. At Acoustic Fields, we spent 8 years and over 2M to create the smoothest curve ever achieved in an open celled product for human voice. You can see in the data below how smooth and linear the absorption response is. This absorption curve treats the human voice with respect and allows for all the subtle tonal variations to come through at the microphone position.
Once you have the inside of your portable soundproof booth treated correctly with the proper rates and levels of absorption, you must position your portable soundproof booth within an area that does not have large amounts of low frequency noise. By definition, a portable soundproof booth will not have the necessary barrier technology to reduce the lower frequency noise transmission. Low-frequency barrier technology is heavy and will not be used in any portable soundproof booth. By definition in order for the vocal booth to be portable, it must be lightweight and easy to assemble and move around. This required feature will not allow for the portable soundproof booth to reduce the transmission of lower frequency noise. In our 40 + years of experience with voice, we find these technologies lack the ability to stop most energy below 125 hz. With low-frequency noise entering your portable soundproof booth, you will not be able to produce the required speech intelligibility. https://www.acousticfields.com/product/acoustic-foam/