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Room acoustic issues 101

The characteristic acoustic regions of rooms

Different parts of the frequency spectrum have different room acoustic issues or distortions.

Acoustic distortions can be defined as sound quality degrading issues introduced by the interaction of a sound source such as a speaker and the acoustic environment it is placed in.

At Acoustic Frontiers we believe there are three main regions that are relevant to the types of rooms used in residential settings for high performance audio reproduction:

  • “Sparse” room mode region
  • “Dense” room mode region
  • Speaker off axis region

There isn’t really a clear transition from one region to the next, and the frequency across which the transition occurs also varies depending on room size. Room modes occur to higher frequencies in small rooms relative to large rooms. See the Transition Frequency article for more information.

The three regions each have different acoustical characteristics, which helps with analysis and solutions to room acoustic issues.

“Sparse” room mode region

In this region the response typically exhibits wild swings and dips, with 15dB or more of SPL variation being common. The response is dominated by room modes that are spaced far enough apart from each other that they do not interact.

For example there might be a room modes at 25Hz, 45Hz and 60Hz. Each one of these is far enough away from the next that a noticeable dip in the response occurs between each one.

The room modes in this region are also the most powerful of all room modes, being the major axial and tangential modes.

This area is hard to improve through acoustic treatment, since the wavelengths of sound are very long. Typical commercial bass traps, with a few exceptions like the RPG Modex Plate and Edge, become increasingly ineffective under 100Hz.

Our approach to dealing with this region involves:

  • Room dimension optimization, to space the room modes as consistently apart as possible
  • Structural optimization (yes, the wall, ceiling and to a lesser extent the floor can act as “bass traps”)
  • Speaker / listener position optimization, to change the interaction with the room modes
  • Multiple subwoofer arrays, to drive room modes destructively
  • Equalization, to flatten the response and reduce ringing in the time domain

Learn more:

“Dense” room mode region

This region is still dominated by room resonances but the individual modes are spaced close enough together in frequency that they interact.

It’s generally much easier to deal with issues in this frequency range than in the “sparse” region. Our approach to this area includes:

  • Acoustic treatment
  • Room dimension optimization

Learn more:

Speaker off axis region

In this region the response at the listening position in two channel listening spaces and home theaters is dominated by the off axis response of the speaker. On the face of it, this seems “crazy”…surely the direct sound dominates?

In recording studios or other nearfield listening situations the direct sound does dominate, but as Harman research shows the frequency response at the listening position above the room’s transition frequency can be predicted from the anechoic measurements of the speaker by combining the data together as follows: 14% direct sound + 44% early reflected sound + 44% late reflected sound.

Of course acoustic treatments or other materials at the reflection points on the walls, floor or ceiling can change the energy spectrum and timing of reflected sounds.

The main levers to improve this acoustic region are:

  • Speaker off axis – choosing speakers with good off axis response
  • Acoustic treatment, to manage the energy return from reflection points in the room and the overall level of late arriving energy

Learn more:

  • Speaker off axis response – we’ve written a comprehensive set of articles on speaker directivity that includes an examination of coaxial, cone / dome, constant directivity and some other designs
  • Early reflections – this article looks at the acoustics of “single bounce” reflections and the Energy Time Curve, the acoustic measurement most commonly used to examine these phenomena
  • Late reflections – this article looks the sound that arrives at your ear after multiple bounces. It’s more frequently known as reverberation, and often characterized by an acoustic measurement known as RT60.

 

Other acoustic distortions

There are other types of room acoustic issues that can occur at any frequency.

  • Speaker boundary interference, which is a phase based SPL addition or cancellation. These generally occur in the region between 50Hz and 200Hz.
  • Structural resonances. These are excitation of the wall, ceiling and floor of the room. We typically see them below 40Hz.

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