Room Is Chamber
What is a room? It has four walls, a floor and ceiling. It is a box or chamber. In this chamber, we introduce sound energy when we speak or play instruments. All of the sound energy sources within our room have a specific low frequency energy and a highest frequency energy that they produce. This is called their frequency response range. Once this energy is interjected into our room, it must all fit into the room or chamber. Lets look at each surface of our chamber.
Two Parallel Surfaces
The two side walls of our room or chamber are part of a resonating system. in fact, they are a resonating system all by themselves. The sound energy that enters our chamber strikes one wall and that energy then in turn, strikes the opposite wall. After the sound energy strikes the second wall, the process is repeated with third, fourth, and fifth order reflections. As these waves and rays of energy make this journey of reflection multiple times, they start to combine. If the wavelength of the sound is a factor of the physical distance between the walls, a resonance is created. This resonant is termed a standing wave.
Maximum / Minimum Pressure
Each standing wave and there as many of them as there are distances between room walls has a maximum sound pressure region and a minimum sound pressure region. The maximum region of pressure will be that area that is closest to the wall boundary surface. The area of lower pressure or null will be middle distance between the walls. If we double the chosen frequency, we will have another area of maximum pressure and another area of minimum pressure created just as in the first example. If we triple the example frequency, the same thing happens. Now, we have three areas of maximum pressure and three areas of minimum pressure.
These examples are for one set of parallel surfaces and are termed axial modes. We have two other mode producing energy situations that we must take into consideration. The oblique and tangential resonating systems involves energy moving from different surfaces other than a single, parallel neighboring wall. These two new modes produce their node creating energy in the same way as axial modes but involve different surfaces.
Tangential / Oblique
A tangential mode occurs as a result of four different surfaces. We can have a tangential mode created with the two side and front and rear walls. The oblique modes occur when the energy travels between six different surfaces. All of these surface areas assist us in reducing the intensity of each reflection and thus mode. An axial mode only has to travel between two surfaces, so axial modes have the highest pressure of the three modes in our room or resonating chamber and are the most troublesome to deal with.
Our low frequency energy waves must fit into our chamber. At 40 cycles, our low frequency wave is almost 30′ long. If we have a room that is 30″ long then we have a room that can receive the full length of the wave without it striking a room boundary surface. Most rooms do not have a 30′ dimension to work with. Most rooms are much smaller. Smaller room sizes forces the low frequency wave to “cramp up” and cause resonances within the parts of the room that the wavelength does not fit.
Vocals Fit Well
Our vocals have a less difficult time. Our vocal range is 75 Hz. – 800 Hz. With higher frequencies we have shorter wavelengths. Shorter wavelengths will fit more easily into smaller room dimensions. Vocal resonances within a room are rare unless one is recording a large number of individuals such as a choir or other larger mass amount of individuals and the room is not large enough. Vocals also do not produce the energy that say a kick drum would produce. Vocal cords are much smaller moving diaphragms than a kick drum head.
Pressure build up at certain positions within our room or chamber. The corners of our room or chamber are areas of higher pressure build ups. Along with the corners, we have any floor/wall or ceiling/wall intersection where energy likes to accumulate. Throughout the room’s center area these pressure piles build their way along and carry through the room or chamber center. Each resonance or pile of energy has its own bandwidth or frequency range.
All of these pressure piles produce colorations to our sound energy that enters into our chamber. Room modes produce resonances that can exaggerate certain frequency ranges that are part of the frequency response of the resonance. If your microphone is placed in the pressure maximum area of the mode, it will be smothered with resonances and you will be not recording certain frequencies at the microphone position. If that resonance’s frequency response is in the same range as an instrument or vocal you are trying to record, you will be fighting the resonance through your whole mix. If you place your microphone in a pressure minimum area, you will not hear everything that is produced by your chosen sound source.
A resonating chamber or room that has the length, width, and height all equal would be cube. With a cube all axial, tangential, and oblique modes will coincide along with their fundamental frequencies. Low frequency pressure build ups within the “cube” would be almost impossible to control. One could make the cube larger to increase pressure handling qualities or one could make one or two room length,width,or height dimensions even smaller provided the appropriate type of low frequency sound absorbing technology is used.
Our rooms are resonating chambers. The amount of energy created from our instruments and vocals must be taken into consideration and the correct size of the room must be coordinated with the intended use. Too much of the wrong type of energy within the chamber sets off a series of resonances. Modal issues can impact the information received at the microphone position. Proper room acoustic treatment can assist in minimizing room resonances, but there is no substitute for getting the room size correct from the beginning.
Reverberation times can be balanced throughout the church with proper treatments placed on the correct surface areas.
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