Post by Johnkenn on Mar 13, 2015 9:34:44 GMT -6
Thought this was good info from Ethan Weiner's page...
realtraps.com/art_small_rooms.htm
UNDERSTANDING NODES, MODES, AND STANDING WAVES
Many people believe that room acoustics is a complicated subject, understood only by those with a Ph.D. in math or physics. However, the behavior of sound waves in small rooms is actually pretty simple, at least for the purpose here of solving problems common to recording studios and control rooms. As explained in the main text, all acoustic anomalies are caused by reflections off the walls, floor, and ceiling. However, I distinguish between problems caused by reflections at low frequencies - below about 300 Hz - and those at mid and high frequencies. Above 300 Hz reflections are perceived mainly as echoes, ambience, and reverb. Below 300 Hz the skewed frequency response typified by Figure 1 above is a much bigger problem. In all cases, waves bounce around in a room much like a cue ball on a pool table. In this case, though, they can bounce around in three dimensions.
Click to see a larger version
Figure 2: A node occurs when direct and reflected waves that are out of phase with each other collide in the air. Click the image for a larger version.
Nodes, modes, and standing waves are three key properties that apply to all rooms, and they are closely related.
A node is just a fancy word for a place in the room where a null or dip in the frequency response occurs. A node is caused when two waves meet in the air and combine out of phase, as shown in Figure 2. In a typical case, waves emitted from a loudspeaker reach a wall and are reflected back into the room. At some distance from the wall, the original wave will have a positive pressure while the reflected wave is negative, or vice versa. When the reflected wave is exactly equal in level and also exactly 180 degrees out of phase with the original, the waves will cancel completely at that particular location. At other levels and phase relationships the waves will cancel less. (And when they're in phase, they increase in level by some amount instead of canceling.) In practice, total cancellation never occurs because no wall is 100% reflective at any frequency.
A mode is simply a natural resonance that occurs in a room, and the frequency of the resonance depends on the room dimensions. For a typical rectangular room, there are three fundamental mode frequencies. One for the length, another for the width, and yet another for the height. Sound travels at a speed of approximately 1130 feet per second, so the resonant frequency between two opposite walls can be determined by this formula, where Feet is the distance from one wall to the other:
1130
Frequency =
Feet x 2
Twice the distance is used because a wave travels from one side of the room and back to complete one cycle. In truth, each dimension has a series of modes because higher frequencies can also occupy the same distance. That is, wall spacing that exactly fits one cycle of 70 Hz also accommodates two cycles of 140 Hz, three cycles of 210 Hz, and so forth.
The most common type of mode is the axial mode, which occurs between two opposing surfaces, such as the floor and ceiling. There are also tangential and oblique modes, which are weaker and thus have less impact on the room's response. Tangential modes complete one or more cycles after bouncing off four room surfaces - literally like a cue ball going around a pool table in a diamond shape. Oblique modes are weaker still and bounce off all six surfaces to complete one or more cycles.
A standing wave is a wave that's not moving - it is literally standing still. Standing waves occur when two equal yet opposite waves arrive from different directions and collide. A few inches away the waves are traveling toward each other. But at the one precise location where the wavefronts meet, there's no motion, much like the isometric exercise of pushing your hands together. Some people wrongly consider modes and standing waves to be the same thing because standing waves can occur at modal frequencies. But they are not at all the same because one is a wave and the other, a mode, is merely a propensity to vibrate. Further, opposing waves can create peaks and nulls at nearly any frequency in any room, not just those frequencies that correspond to the room dimensions.
realtraps.com/art_small_rooms.htm
UNDERSTANDING NODES, MODES, AND STANDING WAVES
Many people believe that room acoustics is a complicated subject, understood only by those with a Ph.D. in math or physics. However, the behavior of sound waves in small rooms is actually pretty simple, at least for the purpose here of solving problems common to recording studios and control rooms. As explained in the main text, all acoustic anomalies are caused by reflections off the walls, floor, and ceiling. However, I distinguish between problems caused by reflections at low frequencies - below about 300 Hz - and those at mid and high frequencies. Above 300 Hz reflections are perceived mainly as echoes, ambience, and reverb. Below 300 Hz the skewed frequency response typified by Figure 1 above is a much bigger problem. In all cases, waves bounce around in a room much like a cue ball on a pool table. In this case, though, they can bounce around in three dimensions.
Click to see a larger version
Figure 2: A node occurs when direct and reflected waves that are out of phase with each other collide in the air. Click the image for a larger version.
Nodes, modes, and standing waves are three key properties that apply to all rooms, and they are closely related.
A node is just a fancy word for a place in the room where a null or dip in the frequency response occurs. A node is caused when two waves meet in the air and combine out of phase, as shown in Figure 2. In a typical case, waves emitted from a loudspeaker reach a wall and are reflected back into the room. At some distance from the wall, the original wave will have a positive pressure while the reflected wave is negative, or vice versa. When the reflected wave is exactly equal in level and also exactly 180 degrees out of phase with the original, the waves will cancel completely at that particular location. At other levels and phase relationships the waves will cancel less. (And when they're in phase, they increase in level by some amount instead of canceling.) In practice, total cancellation never occurs because no wall is 100% reflective at any frequency.
A mode is simply a natural resonance that occurs in a room, and the frequency of the resonance depends on the room dimensions. For a typical rectangular room, there are three fundamental mode frequencies. One for the length, another for the width, and yet another for the height. Sound travels at a speed of approximately 1130 feet per second, so the resonant frequency between two opposite walls can be determined by this formula, where Feet is the distance from one wall to the other:
1130
Frequency =
Feet x 2
Twice the distance is used because a wave travels from one side of the room and back to complete one cycle. In truth, each dimension has a series of modes because higher frequencies can also occupy the same distance. That is, wall spacing that exactly fits one cycle of 70 Hz also accommodates two cycles of 140 Hz, three cycles of 210 Hz, and so forth.
The most common type of mode is the axial mode, which occurs between two opposing surfaces, such as the floor and ceiling. There are also tangential and oblique modes, which are weaker and thus have less impact on the room's response. Tangential modes complete one or more cycles after bouncing off four room surfaces - literally like a cue ball going around a pool table in a diamond shape. Oblique modes are weaker still and bounce off all six surfaces to complete one or more cycles.
A standing wave is a wave that's not moving - it is literally standing still. Standing waves occur when two equal yet opposite waves arrive from different directions and collide. A few inches away the waves are traveling toward each other. But at the one precise location where the wavefronts meet, there's no motion, much like the isometric exercise of pushing your hands together. Some people wrongly consider modes and standing waves to be the same thing because standing waves can occur at modal frequencies. But they are not at all the same because one is a wave and the other, a mode, is merely a propensity to vibrate. Further, opposing waves can create peaks and nulls at nearly any frequency in any room, not just those frequencies that correspond to the room dimensions.
