On his site, Daniel A. Russell, Ph.D. illustrates how room modes are excited as the source location moves across the length of a room (simplified 1D). The key illustrations are the following which illustrate how the first three modes behave in a 5 meter room as the source (red dot) moves across the room (modes at 34.3Hz, 68.6Hz and 102.9Hz shown below).
The first question I have is what is the optimal location of a source in relation to the modes at the modal frequencies? Obviously it will be different for each mode, but am I right in saying that the ideal location is so that the pressure at where ever the listening location may be is as close to 1 as possible? I assume that as the graph drops below zero is the acoustic pressure equal but in opposite phase which would also be an acceptable location as long as it gets you close to -1?
Here are the first 4 modes layered on top of each other, would we want to be as close to the 0.5 point (in this case) at the listening position for as many modes as possible?
So, impossible to find a single speaker/listener location that is optimal for all above modes, and this is not even taking the Y and Z axis into account, let alone the oblique and tangential modes. Of course with using two speakers some of the modes will cancel out given the speakers are in locations of opposite phase of the modes. Now, in rectangular rooms the modal pattern that will exist will have the basically the same shape however differ in frequency depending on the room dimensions.
I am wondering for rectangular rooms if there cannot be a few 'rule of thumb' speaker/listener positioning options that minimise the most problematic axial modes the most and tend to be 'as good as your gonna get' options for positioning? I am sure something like this is possible but there seems to be no such guidance available anywhere. I have compiled a list of speaker placement methods but they all seem to be overly simplified and some cases completely meaningless when trying to optimise modal excitation.
Appreciate if any acoustics experts are able to help me out with this.
The first question I have is what is the optimal location of a source in relation to the modes at the modal frequencies? Obviously it will be different for each mode, but am I right in saying that the ideal location is so that the pressure at where ever the listening location may be is as close to 1 as possible? I assume that as the graph drops below zero is the acoustic pressure equal but in opposite phase which would also be an acceptable location as long as it gets you close to -1?
Here are the first 4 modes layered on top of each other, would we want to be as close to the 0.5 point (in this case) at the listening position for as many modes as possible?
So, impossible to find a single speaker/listener location that is optimal for all above modes, and this is not even taking the Y and Z axis into account, let alone the oblique and tangential modes. Of course with using two speakers some of the modes will cancel out given the speakers are in locations of opposite phase of the modes. Now, in rectangular rooms the modal pattern that will exist will have the basically the same shape however differ in frequency depending on the room dimensions.
I am wondering for rectangular rooms if there cannot be a few 'rule of thumb' speaker/listener positioning options that minimise the most problematic axial modes the most and tend to be 'as good as your gonna get' options for positioning? I am sure something like this is possible but there seems to be no such guidance available anywhere. I have compiled a list of speaker placement methods but they all seem to be overly simplified and some cases completely meaningless when trying to optimise modal excitation.
- Cardas Golden Rule Method - based only on back wall dimension (speaker to side wall = 0.276 x back wall length and speaker to back wall = 0.447 back wall length)
- Ethan Winer – The ‘38% rule’ - mathematical relationship between speaker (bass driver) and the width and length of the room.
- Room Mode Calculator - Distance of bass driver from side walls = width / 1.618^4 or width / 1.618^3, bass driver from back wall = length / 1.618^4 or length / 1.618^3.
- Nordost Method 1 – ‘Audio Arithmetic” Positioning relationship of Y^2 = X*Z where X,Y, and Z are the distances to the nearest boundaries
- Nordost Method 2 – “Voicing the Room” Based on Wilson Audio method. Subjective listening with the ear, the method aims to optimise for room gain/boundary reinforcement only, nothing to do with room mode positional optimisation.
- TAS System Set Up Guide
Appreciate if any acoustics experts are able to help me out with this.
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