Room Acoustics

Room acoustics is a huge subject in it’s own right, but I wanted to share some basics. A basic level of knowledge will help if you are trying to reduce the noise levels in a workshop or position your speakers in the optimal location. The trend of creating large open living areas that are decluttered can increase noise levels significantly. This is where curtains can have a significant effect, a small amount of absorption in an otherwise bare room will significantly reduce noise levels.

The general acoustic requirements of a room depend on the use. The basic characteristics are:

  • Background noise level
  • Frequency response
  • Reverberation time
  • Sound break-in from external sources


The basic parameters that can be controlled when designing a room are:

  • Room size
  • Room shape (cavity acoustics)
  • Wall transmission loss (acoustic barriers) and in addition you must consider Flanking Sound Transmission
  • Floor, wall and ceiling acoustic treatment
  • Positioning of sound sources
  • Positioning of receivers or seating arrangements


The acoustic response of the room can be broken down into four regions depending on wavelength:

  1. wavelength larger than room dimensions – “acoustically dead”
  2. wavelength in proportion to dimensions – room modes
  3. transitional band where diffusion and diffraction are important
  4. wavelength considerably smaller than room dimensions – sound propagation considered as rays.


Things to avoid in the design are:

  • An echo is a strong reflected sound that is sufficiently delayed from the direct sound that it can be heard as a separate entity rather than as a continuation of the original sound.
  • Flutter echoes, a series of echoes that occur in rapid succession. Flutter echoes usually result from reflections between two parallel surfaces that are highly reflective.
  • Focusing of sound can be caused by reflection from large concave surfaces. Certain sounds will be heard too loudly near the focus of a curved surface.
  • Sound shadows under balconies at the rear of an auditorium there may be insufficient early sound, since most of the reflections from the side walls and ceiling do not reach this area.

There are rooms designed to be very acoustically reflective (reverberation chamber) and completely echo free (anechoic chamber). These acoustic chambers and rooms are designed for very specific engineering purposes.

References

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