This week we discuss the effect of room modes. Acoustic Sciences Corporation founder, president and TubeTrap inventor, Art Noxon, PE Acoustical shares his knowledge with us this week on this topic taken from an article in Home Theater Acoustics magazine. Enjoy!
The most common enemy of subwoofers is room modes. A room mode is the organized way that sound is stored in a room. An organ pipe becomes stimulated into resonance when a thin sheet of air is blown across a hole at the bottom of the pipe. That resonance, called a pipe mode, sounds great. A listening room, like the organ pipe, is an acoustic chamber that can be stimulated into resonance — but this time it’s done by the air pumping action of the subwoofer. Room modes cause the subwoofer to sound very loud for one note, and fairly quiet for another.
In the scientific study of room modes, there has developed a very unique type of room, a reverberation chamber. This room is designed for the testing of sound absorbing materials. A good reverb chamber has very thick, slick, and heavy walls. Sound is stored for a long time in such chambers. If you shouted in one of these rooms, you would hear your voice echoing around for 15 to 20 seconds.
There is a measure for how well sound is stored in rooms. It is called the RT-60 and seconds are the units of measurement. RT means reverb time and the 60 stands for 60 dB. RT-60 means the time it takes for the sound to die away over a range of 60 dB. The dB, of course, is decibel, the unit of sound loudness. It is no coincidence that the range of 60 dB corresponds to the range of loudness between a shout and a barely audible whisper.
The loudspeaker that drives the reverb chamber is traditionally located tight into a corner of the room and for good reason. The corner of a room is the single most efficient place to locate a low frequency driver for the development of room modes. The speaker can stimulate more resonances from the corner of the room than it can if located in any other part of the room. This has to do with the efficiency aspect of how speakers couple to room modes.
There is another, somewhat significant, reason that the speaker is located in the tricorner of the reverb chamber; it is the “horn loading” effect of the tricorner walls. There is no news in this concept as nearly all musical instruments have a similar but higher efficiency exponential type horn which couples their sound generating system to the air of the room into which they play. Can you recall listening to someone playing nothing more than a tuba mouthpiece? It isn’t very interesting at all. But plug that same mouthpiece into a spiral wrapped, exponential horn and that noise is turned into beautiful sounds.
For the purposes of testing sound absorbing materials, the acoustical engineers want to stimulate as many room modes as possible. They also want the mode tones to be as evenly spaced along the frequency scale.
This is not too strange. For example, the notes of the musical scale are very evenly spaced. There happens to be particular ratios of room dimensions that promote evenly spaced modes. This only holds true if the speaker remains located in the tricorner of the room.
If the speaker is moved away from the corner, only some of the modes are able to be coupled to the speaker and their spacing becomes anything but uniform. The “golden ratios” for room dimensions are only good if the speaker is located in the tricorner of the room. Almost no one listens to a good stereo with speakers located in the corners of the room. So, on a practical basis and especially for high end audio, where speakers are carefully positioned away from the corners, these golden room ratios serve little or no functional purpose.
One of the more popular tales in the folklore of high end audio stems from a basic misunderstanding of the purpose and limitations of reverb chamber design. Reverb chamber ratios are all too often quoted as being “ideal” room dimension ratios because they will “smooth out the bass.” Home theater does have some roots in high end audio and this tale will eventually begin to circulate in the world of home theater. It is important for those of us who work at and enjoy quality audio to avoid being charmed by magic numbers, unless, of course, they work.