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Room Modes

Room modes are resonances that occur when frequencies reinforce and cancel each other as they reflect back and forth between the hard boundaries of the room.  They are often referred to as “standing waves” because they represent areas of high and low pressure zones which “stand” within a room.  Room modes are an especially important consideration in audio rooms because they can cause severe low frequency distortions.  When room modes are not dispersed evenly, portions of the soundtrack can sound abnormally loud, or extremely soft, depending upon whether speakers and listeners fall in their sound pressure “peak” or “valley”.

A/V RoomService helps to insure that a room is properly designed to substantially reduce the effects of room modes on the listening environment.  Going beyond the typical room dimension formulas, A/V RoomService developed a computer model that analyzes five different criteria for modal spacing that factors in human perception to identify potential problems.  With this information, A/V RoomService can determine optimal room dimensions and proper speaker and listener locations to minimize modal problems. 

Room Mode Facts:

  • All enclosed rooms have modes.
  • Room modes are formed by sound waves reflecting between parallel walls such as the front and back walls, the two sidewalls and the floor and ceiling. In turn, each has a series of modes at integral multiples of the fundamental mode.
  • The room dimensions (length, width, height) determine the number, frequency and therefore location of modes within a room.
  • Room modes are most problematic at low frequencies when energy gaps are too close or too distant from each other. Small rooms are particularly troublesome because small rooms have fewer modes and greater separation between them.
  • Speaker and listener locations can accentuate or diminish modes.
  • Room mode bandwidths become wider and smoother with the addition of absorption by reducing reverberation times. 

In small room acoustics, the audible frequencies of 16 Hz.-20 kHz. can be broken down into four regions. The first region is for low frequencies below the lowest fundamental room mode. This region does not have the resonant support of the room boundaries, but still exists. The second region is where the troublesome room modes lay. These first two regions deal with wavelengths that are of the size of the room dimensions and larger and are governed by wave acoustics. The third region is a transitional region where the wavelengths are too short to be controlled by the room boundaries, or wave acoustics, yet they are too long to be controlled by ray acoustics. Here, diffusion and diffraction prevail. The fourth region is where ray acoustics take over, like super-balls bouncing off of room surfaces where the angle of incidence equals the angle of reflection. In the upper limits of this region, frequencies are short enough to be absorbed by room furnishings, which requires only ¼ of a wavelength. These four regions are governed by the acoustical characteristics of the room.

For comparison: 16 Hz. wavelength is about 70.4 ft. long, 100 Hz. is 11.3 ft., 1 kHz. is 1.13 ft., and 20 kHz. is 0.678” long.

 

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