What do you do about a room that was simply not made for high-end audio reproduction? This week we explore some of the approaches people have taken, including the myths and the truth. In a few short weeks we will be setting up acoustic control systems in dozens of rooms that will fit this bill – at the PAF 2023.
The following excerpt from a paper written in 1993 for the Hong Kong HiFi Show by ASC founder, president and TubeTrap inventor, Art Noxon, PE Acoustical walks us through the fundamental concepts.
Old Wives’ Tales
The first idea the HiFi person usually has about bass these days is the idea of room modes or room resonances. There have been articles about this in the press. The basic expectation is if the room has the correct ratio it will sound great. Another popular idea is to build a room that has no parallel walls, even the ceiling could be at an angle. This is supposed to eliminate standing waves. Another often heard notion is that of killing resonance by adding bass traps or an equalizer. Amazingly, all of these too common ideas about bass and rooms are simply incorrect. Not one thing about these notions is true, but the people are not at fault. They have been taught these ideas by the sci fi writers of the HiFi press whose imagination exceeds their training or experience in acoustics.
The search for the perfect room dimension is a very old one and it is based on the good idea of evenly distributing the resonances. This means each room resonance is as evenly spaced away from adjacent resonant frequencies as possible. Originally this concept belonged to the art of building reverberation chambers. Reverb chambers are used to test acoustic properties of materials and they need to have evenly spaced resonances to give good results. Reverb chambers have their speaker located in one of the tri-corners (floor/wall/wall) and the microphone in the opposite tri-corner (ceiling/wall/wall). Every room mode can be stimulated by the speaker in the tri-corner and the mic will hear every mode loudly. This very unique property only belongs to the corners of the rooms.
There are certain room ratios that give evenly distributed resonant frequencies. However, the HiFi listening room is not an acoustic test chamber. Hifi speakers are almost never sitting in a corner except for Klipsch and the listeners are surely never sitting in a tri-corners of the room. In a HiFi room the speaker is located away from the corner and from there it cannot stimulate all the room resonances. Which room resonances are stimulated depends on where the speaker is located in the room. Only modes stimulated by the speaker need to be well spaced apart. There is absolutely zero guarantee that a room with reverb chamber quality well spaced modes will also have well spaced modes when the speaker is moved out of the comer.
The non-parallel wall is another magic carpet concept in audio. It is true that non-parallel walls keep the flutter echo down but that is a treble effect. Sound is energy. Put this energy into a room and it stays there until one of two things happen. Sound can be absorbed by bass traps and wall friction or it can be leaked out of the room. Sound energy is stored in the room in either of two forms, organized or disorganized. Disorganized sound dies out faster in rooms than does organized sound. Organized storage of acoustic energy is a room resonance. Because the room resonance is so efficiently stored, it is the sound in the room we mostly hear. It causes “room boom” and we wish it would go away. No matter what shape the room has, the room still has room resonances. If a room has walls it has resonances. The only effect the shape of a room has on resonance is which frequency it is and how the resonance is laid out in the room. Resonance is not eliminated by changing the angle of walls.
An equalizer is irrelevant to room resonances. What is more is that they are almost never used in high-end audio. A parametric equalizer changes the loudness of sound at a specific frequency. If a room resonance is “equalized”, then what used to sound loud now sounds normal but the room is physically the same. The long decay of the resonance still fills the room – it is just not as loud.
Bass Traps actually improve bass in rooms. Designer built, pro sound recording studios have the bass traps usually built into the walls and they are not usually visible. Only in HiFi and the smaller project or midi studios are bass traps being set up in front of the existing walls and corners of the room. Here, bass traps have become quite visible. Regardless of their location, bass traps can never “kill” room resonances. Properly designed bass traps do not weaken bass in the room. They actually increase the bass power delivered to the listener. Bass traps do not effect the direct wave from the speaker. They do reduce the strength of the room resonances that interfere with the direct wave. Bass traps reduce the sound of the room and let you hear more of the sound from the speaker.
Rooms have resonances because rooms have walls, floor and a ceiling. This is a fact of nature. Bass traps do change the sharpness or “Q” of the room resonance, no different than adding resistance to a resonant electrical circuit. No matter what the resonance, there will always be an intense sound pressure in the corners of the room. That is why pressure zone bass traps loaded into the corners of the room are so effective. By converfing 1% of your listening room volume into highly efficient corner loaded bass traps, an amazing conversion, a stability in the room acoustic sets in. Bass trap all four vertical corners and the ceiling perimeter comer with a soffit bass trap. This is the best way to control the behavior of bass in the room and causes as little visual impact as possible.