Low Frequency Waves
Low frequency energy fills our rooms with waves of energy. These are long waves of energy. they can be as long as 56′ They are omnidirectional and radiate in a 360 degree pattern from the low frequency source in our rooms. Whether it is a kick drum in a recording studio or a sub woofer in our home stereo system, this low frequency energy wave is like an ocean wave rising up and down and splashing against the boundary surfaces of our room.
Sound Waves / Ocean Waves
As these low frequency energy waves move up and down, they have different distances between the peak of the wave and the trough or bottom of the wave. These waves are moving from the front of our rooms to the rear of the room. They are also moving from the ceiling to the floor and diagonally across all six surfaces of the room. The amount of time they bump into each other is determined by the room dimensions.
Total Room Response
The total room response we receive is a combination of all room modal responses. We have waves traveling from floor to ceiling, back to front, and along all six surfaces of our room all in virtually the same time. All of this wave movement creates pressure within our room. These energy waves are trying to get back to the ocean but they are contained, well some of them, in a box. Some are happy within the confines of the box or room, most are not.
Where are the pressure piles in our room located? For sake of discussion, we will use a rectangular room. A rectangular room has parallel sides with consistent and uninterrupted surfaces. This structural consistency lends itself to predictable and consistent measurements and consistent wave behavior. Seal your lips and puff out your checks. Your mouth is the room and your cheeks are your room walls. Collapse your checks and then expand them. That is what your room walls are doing in response to the energy waves. If your room walls move past certain vibrational thresholds, they begin to sing and add their own sound into the room.
The pressure in our rectangular rooms accumulates in certain parts of our rooms. Sound pressure begins to pile up at the places where all of this energy bangs into each other and at what places in the room it does. The room dimensions determine how much and where these waves will collide with themselves within the room.
Waves of energy pile up in the corners of a rectangular room. In fact, the first cousin of the wave, the ray, which represents higher frequencies than the wave energy, resides in the corners with the waves. We have waves and rays living together in the corners of our rooms. This is why you have heard people say that all room modes lie in the corners of our rooms.
When you see energy piled up in the room corners, the next place you see it is in the room middle section. All room modes may end in our room corners, but they also leave energy piles in the middle of our rectangular room. The amplitude or strength of these two pile locations does vary with room dimension. The strength of the room middle energy pile is always less than the corners, but it must be taken seriously because the room middle is where we sit and listen at.
Sound Pressure Minimum
If you think of a sound pressure wave like a ocean wave, you see the top or peak of the wave. This is the part that surfers like to ride on or just underneath the top curl. This would be our sound pressure maximum area of the wave. The valley or trough is the part that helps create the crest of the wave. This is sound pressure minimum area.
Sound Pressure Maximum
If we map the pressure areas we have just discussed in terms of pressure, and we assign them numbers, we can get a handle on how this pressure is distributed. The number 1 will represent the highest pressure reading. When we examine the corners of our rooms, we will see the number 1 in the very corner, at the wall/floor/ceiling joint. It will radiate out from this corner.
Pressure Plot Mapping
On top of this most powerful zone, will be an area of decreased pressure that will be reduced by approximately 20%. On top of that area, radiating out into the room, we have another pressure reduction occurring that is 20% less than the one underneath it. One more on top of that at a 20% reduction and finally the last one at another 20% reduction and the energy pile ends up with zero pressure about the locations of the room where our speakers in a two channel system would be positioned. This is why one never puts their speakers close to the corners of a room.
These pressure piles produce room modes. Room modes occur at frequencies and positions in the room that are dependent and dictated by the dimensions or space within our rooms. Waves that are certain dimensional ratios to the length, width, or height of a room do not like the rooms size. When they don’t get along with their surroundings because they are too long to fit into our box and keep hitting their heads on the ceiling, they produce distortion. This distortion can be heard by our ears and microphones.
These distortions are termed room modes. Axial modes occur between two parallel surfaces and are our strongest. Tangential modes occur between four parallel surfaces and are 50% weaker in strength than axial modes. Oblique modes occur between the six parallel surfaces of our room and are 50% weaker than a tangential mode.
Our goal in studio design is to see what modes we are dealing with, what frequency they are at, and what position in the room they are. We want to identify, measure them, and then treat them according to their strength. If the modes are spaced together in frequency, we have more energy issues to deal with. If our modes are at least 20 Hz. apart in the axial dimension and at least 5 Hz. apart in tangential, we will have a manageable situation to begin with.
Room modes add distortion to our music because these areas of high and low pressure can mask certain frequencies and over exaggerate others. If we place a microphone in a room mode, we may hear too much of some frequencies and maybe none of others. If we are sitting in a room mode, the mode may allow us to hear some things in our recording but miss others completely. Staying away from any room modes is the goal of any microphone or monitoring position.
Pressure Wave Colorations
Pressure waves fill our rooms whenever sound energy is placed or generated within it. Our rooms try and contain the waves but they want to be set free. Some make it to freedom and are able to leave their captor and go into the next room and create havoc in that room. Most are trapped by the dimensions of the room. The waves that are trapped within the room as a result of bouncing into each other and the room, locate themselves within certain physical positions within the room. These pressure piles occur at certain frequencies and are termed room modes. Room modes vary in strength and must be treated to avoid sound colorations.