Resonances within our rooms are largely determined by room dimensions and the associated room volume. Resonances are also located and caused by room boundary surfaces and the material they are constructed out of. Resonances can exaggerate certain frequencies which cause their presentation to be more than intended by the engineer in the recording. Resonances can also smother a frequency range and act as a “black hole” where time stands still and nothing is heard.
Our first set of resonances are termed room modes. Room modes occur as a result of our room dimensions. Certain frequencies need a required amount of real estate to live in and if they don’t get it, they act like a disgruntled renter. The largest and most powerful of the resonances occur between the side walls of our room; these are termed axial modes. Resonances can occur at just one frequency, but most bring their fundamental cousins along. One can have a 40 cycle resonance and the resonance could be restricted to just 40 Hz. One can also have other fundamental family members present with 80Hz. and 120 Hz.
Tangential and Oblique
Resonances that occur between four wall surfaces are termed tangential modes and resonances that occur between six different room surfaces are called oblique modes. Even though axial modes are the most powerful and most predominate, tangential and oblique can still add their signature to our presentation. Tangential and oblique modes are more predominate with smaller room sizes where the room surface boundaries are closer together.
Comb filtering is another resonance. I know, technically, it is not considered a resonance, but it has the same negative impact on our music signal. Comb filtering is called comb filtering because of the way it looks to an analyzer. If you see a series of hatch marks that are grouped closely in magnitude and frequency, you have a comb like looking graph with the “teeth” of the comb representing the physical reality of the signal presentation.
Underneath this series of hatch marks is our music. Not all of our music but the part that counts. In a comb filter, we miss the undertones and harmonics of our signal. Yes, we hear the energy that has the highest magnitude but underneath the “teeth” is our vocal sustains, harmonics, attack and decay. It can not be heard because it is masked by the series of reflections that are so close together. Move farther away from the wall. There you go.
Closets And Windows
Closets and windows produce resonances that bury our music. Each closet in our room has a physical dimension, just like our room. This interplay between width, height, and length produces a fundamental frequency or resonance for the closet or window opening. A bay window is a classic example of a “room” that only needs three surfaces of the six needed in a room to produce a resonance. Bay windows can produce resonances that can go as low as 100 Hz. A closet can be a resonating system in the same way as the window. Hopefully, the closet has lots of heavy winter coats hanging in it.
Walls in our rooms are huge moving diaphragms. The large sheets of material our walls are made of does bend and flex with sound pressure placed inside the room by our loudspeakers. Accelerometers when placed upon the wall surface will show you this vibrational signature. Obviously, the wall moves most where it is least secured to the structure. What impact does all this room boundary surface have on our audible signature. A lot, if they all start moving and most of the time, they do.
Walls that move, have an impact on the rate and decay of sound energy within our room. I do not know if the rate and decay is in any particular frequency range, but there is an audible difference that is not small; one can hear it immediately. Rooms that do not have walls that move, sound different than rooms that are made out of materials and constructed in a way that can encourage their movement. A 2×4 framed, interior room, dividing wall, will go diaphragmatic and move like a huge sound piston with a big surface area quickly, especially with bass energy. A 8″, poured concrete, wall with steel horizontal roof beams has another “sound”.
Internal Wall Resonances
Inside our walls can be part of the room resonances. Poorly fitting, unsealed electrical outlets, and air conditioning vents that are not sealed and connected correctly can be the conduit in which all this resonance is introduced into the room. If the surface of our walls move enough, the inside resonances of our wall’s cavities will start to generate sound at and around its resonant frequency. Proper wall fill material will minimize internal cabinet resonances. Securing the wall correctly, adding mass, and then adding activated carbon as a cavity fill material, will turn your whole wall into a bass absorber.
The 90 degree intersection of the wall and the floor is another source and location of more sound pressure and thus resonances. Sound pressure levels are always higher at the floor/side walls and the side wall/ceiling intersection. Higher pressure levels will excite room modes at these stated intersections and also the corners of the room. All room modes end in the corners of our rooms and any pressure increase can get everything moving that resides in the corners. This is one reason one does not place a sub woofer in the corner of a room. Yes, you get more reinforced energy and gain but the sonic costs of activating all the corner modes is not worth the trade off. Get another sub woofer. Don’t place this one in the corner either.
There are danger zones in all of our rooms for our music. Our room dimensions give us predictable 2, 4, and 6 surface resonances that smother and blur our musical presentation. This blurring and smearing is especially evident in the middle frequency range where our vocals lie. Other portals such as windows and closets act like cavities of resonances that can start talking when we interject sound pressure into them. Our corners and floor/wall and wall/ ceiling intersections, act as pressure chambers with two walls and can create zones of different pressures which can exaggerate certain frequencies and completely ignore others. Even our sitting proximity next to a side wall, will produce a “comb filter resonance” that literally can tear our music to pieces. With resonances, we need to take the room out of our music, especially when it starts messing with our vocals.