Acoustic Absorption Meaning
Acoustic Absorption is one of the primary principles of Sound Theory. As sound is created, it naturally propagates away from its source in a process known as diffusion. As the sound energy travels away in a hemispherical shape, it hits walls or obstructions along the way. As this occurs, the sound is either absorbed, reflected, or transmitted through the obstruction.
Graphic: Acoustic Absorption Coefficient
If no walls are encountered, then the sound is slowly absorbed by the atmosphere. Any Room Acoustics model should take into account the walls and room finishes, the size and shape of the room, and the absorption value provided by each surface and the air within the space.
Acoustic Absorption and How it Affects You – Classrooms, Restaurants, and More
Absorption: When sound is absorbed, it is really the kinetic energy of the sound wave being transferred to heat energy. You cannot feel the heat energy, typically, but as the sound is absorbed and converted, the sound level in the room dissipates.
Echo: A reverberation of 1.5 seconds or longer, such that the initial sound has completely decayed and the reflected sound is heard distinctly
Reverberation: Event when a signal is reflected off of a number of surfaces, and then decays as it is absorbed into objects in the space (including air, walls, furniture, and drapery). Soft porous materials are most effective at absorbing reverberation.
3 things that contribute most to echoes and bad reverberation
- Large volume space that does not have any partitions. The larger the volume of a room, the longer the reverberation time.
- Walls, floors, and ceilings made of dense, hard materials. When more reflective materials are used, the reverberation time increases.
- Parallel surfaces (these support standing waves and allow waves of similar frequency to superimpose). For small rooms with parallel walls, you may often experience a type of echo known as “flutter echo”
- NOTE: If you have these 3 elements, then you are likely going to experience bad acoustics in your space. This often occurs in auditoriums, cafeterias, restaurants, clubhouses, and other similar spaces that typically require some level of acoustic treatment.
- Your desired Reverberation time depends on the intended use of the space. Below are a few
- 0.3 seconds: “Dead” Space
- 0.5 seconds: Ideal for Office or Classroom Environment – this level is mandated by LEED design
- 0.7 seconds: Upper Acceptable Limit for Commercial or Residential setting
- 1.0 seconds: Speech Intelligibility begins to deteriorate (This is why it’s difficult to converse in noisy restaurants)
- 1.5 seconds: Distinct Echoes develop
- 1.8-2.2 seconds: Concert Halls and Gymnasiums actually benefit from “lively” spaces that allow reverberation in this range
Sabins Formula: The calculation that governs how much reverberation is inherent in a give space. The larger the space, the longer the reverberation time. Developed in the early 1900’s, Sabins Formula provides a direct relation between the Volume of a Room, its Surface Area, and the Acoustic Coefficient of each Surface. In the video below, we demonstrate how to use Sabin’s Formula to determine reverb time in different spaces.
By understanding the principles of Acoustic Absorption 101, you may design a space that is acoustically pleasant and that will meet and exceed the basic reverberation requirements demanded by many clients.
