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The Breathing Mode

This review of small-room acoustics begins with the lowest octave. Here, the wavelength is quite long as compared to the size of the playback room. The room as a whole experiences internal pressure changes. Acoustic activity in this region below the room’s so-called “cut-off frequency” remains quite audible. Here the speaker is acting on the room as if it were a pneumatic plunger, alternating between pressurizing it and pulling a partial vacuum on it. The walls, floor and ceiling react to what seems to be a rapidly changing “barometric” pressure in the room. Room surfaces billow out and then cave in with each cycle.

Major structural resonances are easily stimulated by breathing mode acoustics, a common problem in playback for the larger power systems of today. The surfaces of the room simply shudder in the bottom end as the speakers stimulate, then overpower the mechanical stability of the room. The result at high sound levels is a total loss of control for low-frequency musical reproduction, as if sound in the room “crumbles” when it is overloaded. This LF breakup of the room itself is particularly evident in the concussive punch bass beat attack transient.

Room Modes

As the tone from the speaker is raised in pitch, out of the deep bass octave and into the piano’s first bass octave (40-80 Hz), a new class of room acoustics develops, called Room Resonant Modes. The lowest frequency at which this can occur is called the long dimension axial (1,0,0) mode.

The fundamental room resonance is easily stimulated when the speaker is located at one end of the room and the wavelength of the tone played happens to be twice as long as the room. The wave from the speaker travels down the room only to bounce off the rear wall and return to the front of the room. During this time the speaker makes one full cycle of motion itself. It generates a tone exactly in step (or in phase) with its reflection. These two waves–the old reflected wave and the new one–add together exactly, without confusion. After a number of cycles the sound levels build, enveloping the room in resonance.

For a non-resonant tone, sound builds up in the room in highly disorganized manner. With resonance, however, the air is stimulated into a “sloshing” mode of behavior, not too unlike what can happen with a child in the bathtub if their to and fro movement happens to keep time with the water’s natural end-to-end slosh motion, called first harmonic.

Click the link to read Art’s entire article.