Home Cinema Anatomy: Acoustic Treatment
What is Acoustic Treatment?
Acoustic treatment comprises any material used to improve sound quality in a space. This can include standard soft-furnishings, but the greatest impact will be realised by implementing purpose built acoustic panels. Acoustic Treatment is often confused with ‘Acoustic Isolation’, also known as soundproofing. While some acoustic panels do absorb an amount of sound, they do not decrease the overall volume (dB) in a significant way, and therefore are unsuitable to be considered effective in soundproofing.
To achieve the desired sound quality improvements in a room two primary types of acoustic panels are utilised; absorption panels, and diffusion panels.
Absorption Panels
Absorption panels are typically made of high-density foam, fiberglass, or mineral wool, which convert certain portions of sound to heat by way of friction. Absorption panels come in many creative shapes and sizes, and depending on their design will be most effective at treating a specific range of sounds, e.g. Bass traps are triangular, fit in the corners of a room, and are most effective at absorbing bass. A clever combination of absorption panels minimize sound reflections across a broad range of frequencies within a room, resulting in clearer and more balanced acoustics.
Diffusion Panels
Diffusion panels are typically multi-faceted shapes and are made of solid materials like wood or plastic, due to their purpose for reflecting and scattering sound energy. Diffusion panels help to create a more natural sound environment, helping to relieve the build-up of rebounding sound energy, and if implemented correctly will make the space sound larger and more open than it is.
Why is Acoustic Treatment Critical in a Home Cinema?
The sound you hear in an untreated room will be vastly different to the intended sound created by the artists involved in the content you are watching. Further to this, the sound heard at each seat, will be vastly different. Controlling frequency response and reflections in a room by intelligently implementing absorption and diffusion panels will be the single biggest improvement to sound quality in your space bar none, regardless of how good your electronics or speakers are. These improvements will be most noticeable to even the untrained ear when observing:
Dialogue Clarity
Sound-field Immersion
Bass Performance
How Do We Provide & Test an Effective Acoustic Treatment System?
In large spaces, such as auditoriums, the most prominent acoustic measurement taken to judge the success of an acoustic treatment system is reverberation time (RT), expressed as the time it takes for a sound to decay by 60dB (RT60) . While it isn’t the best method to judge small room acoustics, its simplicity and the existence of useful predictive tools make it useful in approximating the amount of acoustic treatment required to sufficiently treat a small room.
Prediction: The Sabine Equation allows us to predict the RT60 measurement for the room with as little information as the room volume, surface area, and manufacturer provided acoustic data of wall types and treatments being used. The Sabine Equation does not provide a room average, nor should it; it is specific to each frequency. As such, this equation is applied to each octave band between 125Hz and 4kHz, and a performance chart is generated for comparison against the performance goals of the room.
Testing: As noted, there is a better method for measuring the success of an acoustic treatment system in a small room, which is the reflection decay time (RdT). RdT is measured using audio software and a high quality portable microphone. Measurements are taken at each seat, allowing each seat to be compared to the performance goals of the room, and against the predictions made using the Sabine Equation.
In a private entertainment space, the goal should be to control RdT and early reflections across the frequency range of approximately 32 Hz to 8 kHz, and should be balanced against the need for some amount of reflective surfaces to create the sense of space in the room. If RdT is too low, the room will uncomfortable to spend time in, seeming dead and suffocating. A longstanding guideline is to target RdT (RT60) between 0.2 - 0.5 seconds, but high speaker count systems can benefit from slightly lower RT60.