When we look at the sound in a room and in particular the reverberation times and reverberation fields in a room, we seek to find true reverberation. true reverberation exists only in a diffuse sound field. All reverberation must be spread everywhere equally for true reverberation to take place. It must be equal energy and equal volume. It must also be 100 % diffuse. This is not an easy scenario to find.
Many Issues To Consider
There are many issues that prevent us from having a true reverberate field in our rooms. Room modal resonances can impact our reverberation uniformity requirement. Resonances are pockets of energy that lie in particular room locations. These positions are determined by the room dimensions and associated volumes.
Resonances radiate energy in certain directions and at certain frequencies which interfere with reflections which must develop into a true reverberate sound field. as resonance energy radiates from its center point it can add large pressure gains at their respective room locations. A microphone placed into a room mode, may “hear” some frequencies, but may miss others.
The furniture within our room minimizes our efforts to achieve a true diffuse reverberant filed. A console or back wall couch can add impediments towards our reflections equally spread out. The absorption coefficient of the piece of furniture coupled with its size, all add negative variables to our equation. Reflections from the furniture surface materials can also add to the positioning of reflections in areas not necessary towards our true reverberant field goal.
Direct vs. Reflected
Our room is a combination of direct and reflected sound. Direct sound is the sound energy that travels in a straight line from the source, say a loudspeaker, to your ears. Reflected sound is everything else you hear. Reflected sound comes from the room boundaries to your ears. The direct sound emits from the source or loudspeaker and travels directly in a straight line to your ears.
We know from basic physics that sound energy loses power as it travels farther from its source. Physics tells us that our direct energy will reduce by a factor of 6dB for every doubling of the traveled distance from its source. The reverberate field created by all room boundary surface reflections will still be spread out and evenly distributed within the room.
Low frequencies within our room are a different animal. Low frequencies radiate 360 degrees from any source producing low frequencies. We mean frequencies below 100 cycles. No matter what the low frequency producing source this radiation pattern exists. With this 360 degree radiation pattern, we have the low frequency waves striking multiple wall or boundary surfaces. All of this energy will impact itself upon our reverberate sound field.
Critical distance is finding that balance between the direct sound from our sources and the reflections from our room boundary surfaces. Where a person sits or stands within the room can make a large difference. There are paces within the room where the room sound or reverberate field would be 20dB or greater above the threshold of the direct sound. The area within the room where the two sound fields are equal is termed the critical distance.
Formula For Everything
To find this number we turn to physics and the associated mathematics. We look at two variables that impact themselves on our formula room constant number. They are the room constant or absorption coefficient and the directivity of the loudspeaker. We can even find the distance for our critical distance with both low and high frequencies.
Critical Distance Meaning
What does this critical distance mean in our listening or monitoring rooms. If we run the numbers and we get back say six feet for high frequencies and maybe 3 feet for low frequencies, we need to seriously address both low frequency control and reflection management from our room boundary surfaces.
Rule Of Thirds
Another good start, would be to divide the room into thirds. The middle of the room would be the middle third. The remaining thirds would be the front and rear portions of our room. start with your listening distance in the rear third of the room and move towards room center. You will immediately begin to hear the differences. You will also see the need for proper room boundary reflection management either through absorption or diffusion technologies.
We do not want the room sound to predominate at ant position within the room. We also do not want the direct sound to dominate. We do not want direct sound to dominate in our listening rooms. We do not want room sound to dominate at our monitoring or mix position. There needs to be a balance of direct and reflected energy for both room type applications.
We Have The Technology
One can treat the room with acoustic treatments to bring our critical distance more in focus with our acoustical needs. Low frequency management can be installed at room boundary intersections. Middle and high frequency absorption and diffusion technologies can be implemented upon individual room boundary surfaces. Today, we have the technology to maximize the critical distance formula and make it usable for the intended acoustic purpose.