Are your room dimensions approximately 10' x 15' x 8' ?
Originally Posted by Aspen0220
Finally got my cap 1400. Ran Dirac on it and got a big null from 37hz to 55hz regardless of where I put my sub. I sit around 12 ft from the sub. Any idea what I can do to solve this? Can hardware cause a null like that?
You could have a Null
at those two frequencies because of a problem with Room Modes or Standing Waves caused by the shape of your room. These two issues are inextricably related to the physical dimensions of your home theater.
A room mode is essentially a resonance, an area of increased amplitude that results when a sound wave reflects off a boundary surface (wall, floor, or ceiling) and combines in phase with the original direct sound wave. What causes the direct and reflected waves to combine in phase is simply a whole-number correlation between the length of the sound wave and the length (or width, or height) of the room.
Antinodes and nodes
Areas of maximum amplitude, representing the maximum change in air pressure, are called Antinodes. Areas of minimum change in air pressure (essentially the zero-crossings between positive and negative halves of the pressure wave cycle) are called Nodes.
If a sound wave is exactly one-half the length of any single room dimension, its nodes will be at that boundary surface. The reflection off of that parallel surface will be in phase with the initial sound wave, the antinodes will line up in space/time and reinforce one another, and there will be an increase (Peak
) or decrease (Null
) in amplitude at that fundamental frequency.
This phenomenon not only occurs for frequencies whose wavelength is one-half a room dimension; it also occurs at any whole number multiples of that frequency…i.e., harmonics of that fundamental. Double that fundamental frequency and a full wave cycle now fits in the space between boundaries. The nodes will still occur at the room boundaries, causing the reflections to be in phase.
These sound waves whose wavelength is equal to one-half (or any whole number multiple of) a room dimension, and hence whose nodes occur at boundary surfaces, are called Standing Waves. There are three types of standing waves: Axial Mode
is the name given to standing waves that exist between two parallel surfaces (front and back walls, left and right side walls, or floor and ceiling). Other types of standing waves include the Tangential
Mode, where the sound wave bounces off of four distinct surfaces, and the Oblique
Mode, where all six room boundaries are involved. Since axial modes are the most troublesome standing waves in home theaters, they are the primary concern.
Axial Room Modes
Tangential Room Modes
Oblique Room Modes
How to measure and calculate
To calculate the axial modes of a rectangular room, one uses the formula
1130 / 2L = f
1130 is the approximate speed of sound in feet per second, and L represents the length of a room dimension in feet. The result f is the frequency of the axial mode in Hertz. So for example, if your home theater ceiling is 8'
1130 divided by (2 times 8), or
1130 divided by 16
...there will be a standing wave (between floor and ceiling) at 70.6 Hz. For any rectangular room, there will be a standing wave at each of the three fundamental axial modes (corresponding to the room’s length, width, and height) as well as at the whole number multiples of those three frequencies.
And this is only the effect of the first two axial modes of one dimension (height). You can calculate the fundamental axial modes for each of the three dimensions of your room using the formula 1130 / 2L = f. Multiply each of those three frequencies by 2 through 8 to figure out the first eight multiples of each axial mode. (You’re only concerned with the first eight multiples because above 300–400 Hz, standing waves have less of a destructive influence on sound.)
You should wind up with a list of of 24 frequencies (for each of the three types of standing waves) with a standing wave at each one of those frequencies. If they’re fairly evenly spread out, it should not be a problem. However, if any of those standing waves are within 5 Hz of each other, these will be problem areas in your room’s frequency response.
Axial, Tangential and Oblique Standing Waves
If your overall home theater dimensions are 15' x 10' x 8'
, plotting the axial modes would indicate potential problems around 37.7 Hz
(the 15' dimension
), . It also points to the likelihood of problems at 56.5 Hz
(the 10' dimension
). You can also use one of the room mode calculator's available on-line:
Room Mode Calculators
List - Frequency/Amplitude