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Optimal Position of a Loudspeaker in a Rectangular Room

Final student paper of Fabrice Ducomble, electronics engineer

Introduction

The position of a loudspeaker in a room, rectangular or not, influences in a sensitive way the quality of a sound to such a point that you can get better results with a bad loudspeaker judiciously placed than with a good speaker placed without planning. In fact, the imperfection appearing in the loudspeaker response curve can be attenuated by the resonance created in the room or, on the contrary, be amplified by them. It should be noted that the position of the listener is almost as important as the source of the sound.

The problem then is to find the optimum positions for both the source and the listener.

We are particularly interested in the behavior of the low frequencies in the room with the presence of the loudspeaker. It is, in fact, in this range of frequencies that the interaction between these two components is the most noticed. So, a study in depth modal behavior has been carried out, from their formation to their influence on the distribution of the sound field.

From this study we devised a model that permitted us to predict the behavior of low frequencies in a rectangular room. This model takes into account another very important phenomenon : The influence of the distance from the walls on the power of the emitted sound.

The program carried out is based on this model. It calculates the response curves relative to all possible positions regarding the source-listener, and chooses, with the help of two tests, the best among them. The optimum position of the loudspeaker, and of the listener, is then the positions corresponding to the best response curve.

This research can be divided into three parts :

  1. The study of the influence on the proximity of the walls on the sound power emitted.
  2. The study of modal behavior and the establishment of a model permitting the determination of the influence on the sound field.
  3. The presention of the results.

First Part - The influence of the distance of the boundaries on the emitted power

According to hypotheses used by K.O.Ballagh [3], a rectangular room can be modelled with the three walls closest to the sound source. These hypotheses impose a frequency limitation and mean we must be careful with the results obtained for the low frequencies using the formula derived from this model.

This formula, developped by Waterhouse, presents the power emitted by a source placed within the proximity of one, two or three walls in function of the power emitted in the free field by this same source. In naming the distances from walls as X, Y and Z, the Waterhouse formulas for one (1), two (2) and three walls (3) are :

(1)

(2)

(3)

where (4)

These three functions are represented in figure 1 respectively by the curves A, B, and C, for a source placed symmetricaly to the walls.

The formula relative to the three boundaries presents the advantage of being easily put to use, but has a major inconvenience : it doesn't take into account the Fundamental phenomenon of resonance. Concerning this subject, Morse shows in his work [2] that resonance has a strong influence on the power emitted by a source. This phenomenom is shown in figure 2. We can observe peaks with each resonance frequency.
Fig. 1 - Representation of the three Waterhouse formulas


Fig. 2 - Radiation impedance as a function of frequency

Conclusion

The Waterhouse formula gives good results as long as the hypotheses are respected. We must therefore, expect some inaccuracies from results obtained using this formula for low frequencies. During the presentation we will see from the results that these inaccuracies are the main source of limitations to the program's being carried out.

Second Part - Modal Behavior and Establishment of a Model permitting the Determination of the Influence on the Sound Field

Eigenmodes (or simply "modes") are resonance phenomenoms : discrete phenomenoms consisting in the amplification of an initial periodical phenomenom. Every resonance phenomenom is caracterized by an establishment duration. So the amplitude of a particular mode depends on the time that the source radiates. The establishment duration depends on the absorbtion factor of the room's walls. The more reverberant a room is, the longer this duration is (til 10 sec. for a very reverberant room). Figure 3 presents the establishment of resonance in the theoretical case of two parallel walls.


Fig.3 - Establishment of resonance in the theoretical case of two parallel walls

Eigenvalues and eigenfunction

A mode is caracterized by an eigenvalue and an eigenfunction. With the eigenvalue, we can obtain the phenomenom's eigenfrequency. The eigenfunction gives the spatial distribution of the mode's amplitude. These two expressions can be found by solving the following eigenvalues problem :

(5)

where has to satisfy the boundary conditions imposed by the walls.

In the general case (absorbing walls), the boundary condition imposes that the vibration speed of the walls' particles equals the vibration speed of air :

(6)

So, by solving the problem, we obtain :

(7)


(8)


(9)


With

(10)

(11)


The resonance pulse is given by the following equation :

(12)


where corresponds to the resonance pulse and represents the damping factor.


Then the resonance frequency is expressed by equ. (13).

(13)


In the case of perfectly reflecting walls, this formula corresponds to the Rayleigh's well-known formula :

(14)


For an absorbing room, there can be differences up to 3 Hz between Rayleigh's formula and (13). Thus the formula (13) is really useful for determining exactly the resonance frequencies of a real-world room.

Determination of a mode's maximum value

The eigenfunction determines the spatial distribution of the pressure relative to the maximum value of the mode. Four methods have been tried. All four give similar results. The first three formulas assume that the energy is uniformely distributed throughout the room. This is generally the case when the energy is distributed through a great number of modes. The fourth method is less restrictive, because it takes the energy distribution inside a mode to directly determine the maximum pressure, without using established formulas in the case of a uniform energy distribution. Here are the formulas for the two most interesting methods (2 & 4) :

(15)


(16)

where is the term in square brackets of the next formula (17).

(17)


None of these formulas perfectly reflect reality. The values measured values are sometimes higher, sometimes lower than the ones obtained mathematically. While we obtain similar results for all 4 methods, the imprecision must come from the estimation of the power emitted by the loudspeaker. In fact, Waterhouse's formula conditions are difficult to fulfill in practice. It would be better to calculate the exact radiation impedance of our loudspeaker in a rectangular room to obtain more accurate results, but this is outside of the scope of this paper.

Frequency distribution of modes

Though modes are discrete phenomenoms in the frequency space, they look like steep peaks instead of vertical lines. A mode can be modeled by the following formula :

(18)


Figure 4 compares the calculated and the measured characterisitics of a mode.


Fig.4 - Calculated and measured characterisitics of a mode

The final model

By combining formulas (7), (15) (or (16)) and (18), we obtain the following model :

(19)


where represents the pressure's maximum value in the mode and "fact_pond" is the ponderal factor (formula (18)).

So to obtain the pressure in one point at one frequency, we must add the contribution of all modes. In practice, the amount of calculation involved would be much too high. The method to solve this is simple : we first calculate the mode's contribution from the nearest mode to the furthest one, and we stop the calculation when the contribution becomes negligeable. Remarks :

Conclusion

This model allows to determine the spatial distribution of the pressure in a rectangular room, at any point and any frequency. Part 3 will show how the optimal position is determined.

Third Part - Results

The program's goal is to determine the optimum positions for the loudspeaker and the listeners. From equation (19), the loudspeaker position plays the same role as the listener position. So the perceived sound quality depends from the loudspeaker position and also from the listener's position.

Principles of the program

To calculate the frequency responses, several parameters are taken into account : walls characteristics, loudpseaker response, spatial and frequency steps. The optimum position is caracterized by the most linear frequency response. So the best way to determine this is to calculate the standard deviation of the curve. The lower the value, more linear the curve is. The program also calculates a standard of the deviations to eliminate the curves that present big peaks.

To optimize the calculation duration, room symetry is taken into account and preselected regions are determined both for the loudspeaker positions and the listener positions. A few particular cases have been chosen to achieve this : listener's position known in advance, loudspeaker position known in advance, calculation for a specific loudspeaker-listener couple, and so on.

A simulation using the "OptiSpeak" software (which is based on the techniques developped here)

The dimensions of the simulated room are :


The room parameters are illustrated in figure 5. The results are shown in figures 6, 7 and 8.


Fig. 5 - Room parameters



Fig. 6 - Information on the simulation case



Fig. 7 - Results obtained (first position highlighted in blue)



Fig. 8 - Results obtained (third position highlighted in red)

References

[1] ROY F. ALLISON, The influence of room boundaries on loudspeaker output, Journal of the audio enginneering society (June 1974).
[2] MORSE & INGARD, Theoretical acoustics, Mc Graw-Hill
[3] K.O.BALLAGH, Optimum loudspeaker placement near reflecting planes, Journal of the audio enginneering society (December 1983)

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