The room: the equalizer can not be excluded

Speakers installation method


Premise
Two the factors on which the listening environment weighs heavily: the acoustic masking and frequency response, the two sides of same coin: the environmental resonance modes. But there is also something else that affects the frequency response: an equalizer can not be excluded, as we shall see below.

The room
In theory, all that adds direct wave is a loss of fidelity ( an add sounds that are not present in the record) but since no one would listen to full-range speakers in the middle of a field, it must live with distributed environments affected by resonances and their reverberation times: the our home. Each environment is affected by resonances of various types, axial, tangential, and oblique distributed in a more or less irregular mode as a function of the proportions of the room and that cause either a masking effect on the original sound and has a strong influence on the frequency response. It was defined a limit frequency, said Schroeder frequency, different for each individual environment and sensitive to the degree of acoustic treatment of the same, which distinguishes the low-frequency behavior of the region other than the high frequency of the room. At low frequencies, ie below the Schroeder frequency, the acoustic is in its "modal area" that is dominated by the presence of standing waves, for which the sound pressure level detected in sinusoidal permanent regime fluctuates widely moving from point to point in ' environment: for this reason the frequency response is strongly nonlinear with excursions that can be up to exceed 30dB polluting drastically listening. At frequencies higher than that of Schroeder the acoustic behavior is more regular and the level fluctuations are reduced much is a function of frequency is moving inside the room, from which it derives a much more linear response and independent from listening point. The Schroeder frequency depends on both the size of the room that the reverberation time (decreases as the size of the room and to decrease the reverberation time) but, unless you do not want to change the proportions of the room with the use of walls plasterboard or appropriate dividers to optimize relationships, we must be satisfied that the effect furniture and large sofas may have to break the stationary modes. As for the proportions of the room, if you are about to build the house, it may be helpful to look at the graph Bolt on best areas: starting on the right foot helps a lot in the work. The worst solution we know to be one in which the proportions are 1: 1: 1, or the infamous cubic room. In 1979 Giancarlo Gandolfi signature was written an interesting article on this issue when the complex relationship aimed at establishing the best proportions of the room was put into a computer and the result was the following proportion: 1: 1.4: 2.1 (however in the Bolt area).
That feature that we perceive as "sound definition" is strongly influenced by the reverberation time T60 (ie the time in which a sound is to reduce the intensity of 60dB) especially as regards the mid-high range and being the energy of ' reflected wave is proportional to the proximity of the walls to the speakers (and from the walls to the listening position) and considering the characteristic of dispersion which normally has a speaker in the mid-high range, it is advisable given the choice, arrange them on the wider side of the room, as much as possible away from the side walls which however should not be left bare but normally furnished. Also for this reason, an almost indispensable element, if you have carpet, it is a wide belt no less than the distance between the speakers, preferably something more, and it gets to possibly cover the distance between speaker and listening position. In the case where the speakers should be installed in the vicinity of the bottom wall (as we will see later) it will be necessary to also treat this acoustically with heavy curtains or without glass panels: both are flexible materials with a gap from the wall and have a good damping effect also for medium and low frequencies. Not as pleasant listening is treatment with polyurethane panels or the like, more absorbent from about 400Hz to rise but much less low frequency if glued to the walls or without having a gap.
Another good rule in order to obtain the wall opposite the absorbent yield, so if you have a floor carpet to avoid to make absorbent not "turn off" the sound is to leave not acoustically treated low reverberation times but at the same time the ceiling, if you have the back wall of absorbent avoid making this one behind you, always giving priority treatment to the nearest wall, and on the other of the two opposing limit yourselves to hang a few paintings or even a large poster hanging on two points, a gimmick that the latter would seem futile and instead has an influence coming to change the vibrational mode of the same object. Because the resonant modes are finding maximum energy at the intersections of walls, are very useful absorbent cylindrical elements placed in the corners of the room. Alternatively the tube traps may be used as solution "custom" low cost of the foam cylinders of about 30-35 cm in diameter cut made ad hoc (it is not advisable rolling of sheets of such material as the cells are to compress / expand unevenly) and stacked one over the other; course can be covered with fabric to also make them aesthetically acceptable. Their flexibility combined with their diameter makes them very effective even in the damping of low frequencies. If you like modern artwork by Schroeder diffusers can be for you contributing with their reflection randomized to a better distribution of resonances although more effective in the midrange. Also the entrance door to the room itself behaves like a floating bulkhead, the influence to listening the note is quite clearly in the low range in which this function is open, closed or half-open and in the case of significant problems due to standing waves could be a good solution is to simply leave it open. Generally a decor so abundant and no bare walls will result in definite advantages to listening: from the graph you can see an example of how varied the reverberation of a room with or without acoustical treatment to medium risk. As we have seen so far should lead to a slightly downward reverberation curve upward with T60 times which could reach 0,3sec midrange thus obtaining an excellent definition. The ideal would be to have this reverberation time of the entire audio range but being generally the most effective absorbent materials to the rise of the frequency, this is in fact not feasible.

 

The equalizer can't be excluded: the reinforcements of the walls
But there is another problem to be addressed that may be less evident on frequency response than it is the influence of the standing waves but that is systematic and independent of the proportions of the room and not restricted to certain modal frequencies but the whole range low so it becomes crucial to position the speakers. From the graphs drawn from Allison work it is possible to observe three different environment in response curves: one that takes account of the reflection of a single wall (A) and those which demonstrate the trend that has in front of two (B) and three (C) walls equidistant from the speaker.


 

You can be drawn from this graph four basic information:
1) For sizes less than 0.1 wavelengths (λ) in front of each wall determines the reduction of the angle of an increase in energy radiation which tends to 3dB (to which we will call hereinafter with the abbreviation of " reinforcement").
2) in front of two or three walls equidistant from the sound source are determined strong irregularities that reach up to 20dB excursions with three walls to approximately 0.3 wavelengths.
3) The sound pressure variations for magnitudes greater than 0.5 wavelengths are much more mild to the point of being negligible.
4) With a single wall nearest the partial cancellation due to the same moves slightly higher, about 0.35 wavelengths with respect to the condition with three equidistant walls and the attenuation that follows is much smaller: just 1dB negligible.

From these cornerstones then we know that, compared to conventional speakers, calculated to be linear in the anechoic chamber, there is probably wrong thing to put a woofer at the same distance by three walls and be taken into account that the same rules also apply to the listening point (ie our ears) as a function of how far away from the floor and from the wall behind us. So if for example there is the 40cm woofer from the floor would be wrong to get to listen to 40cm from the wall behind us.
The other key aspect is that at the bottom of those frequencies for which the wavelength is about ten times the distance between the speaker and the wall will always equalization can not be excluded by 3dB for each wall.
In fact we can neglect the reinforcements that would occur at such low frequencies to be outside the audible band. And 'reasonable to limit this to 20Hz for which in the face of a wave length of 17,2m we can consider negligible the reinforcement of walls that are located at distances greater than or equal to 1.7 m from the speakers is that from our pinnae and then the ceiling is not normally considered. In any type of room, though huge, we will get to have more and at least one reinforcement: the floor. However, since the common listening environments are often far from huge, we will have to do with at least two or three reinforcements until reaching a maximum of five possible reinforcements useful in small rooms, excluding ceiling. Assuming an installation on the wide side of the room, if we assume no reinforcements from the side walls of these also pose a greater distance than or equal to 1.7 m, however, we must always avoid to have them both at the same distance from the listening position, for that reason We will avoid to put this on the axis of symmetry of the room but still a little moved, 20-30cm, sideways (see depictions types rooms). Considering that on average we listen to music while sitting in a chair or on a couch we will have a partial reinforcement due to the same session (and at the back). Partially because a sofa in part absorbs the wave energy accident and partly reflected. This will not be a kind of reinforcement that will be among those calculable, but still will make a contribution distributed more certainly than what you would get from the reflection of the floor if we were sitting in a regular chair, but also moved to the midbass since the reflective distances are lower on the couch. When replacing a chair with a sofa, listening generally has the feel of a fuller sound and full-bodied.
How then to choose the most appropriate speakers in your room? And once it made such a choice how to properly install the speakers using its reinforcements caused by the walls?
Since these reinforcements are not excluded and must therefore be distributed, as well look to do so in order to obtain a partial recovery of the natural roll-off at low frequency of our speakers and trying to get at the same time a curve in an environment with a next trend in range low at Møller curve. To this end, if we want to proceed with a good precision, we need to know the frequency of resonance of our speaker. It would also be advisable to know the quality factor Qt but being what not to everyone (but not decisive) we will see how we can get around it.
 

Speaker-environment
Some brief reference to the existence of this type of speakers that are born in front of Roy Allison work. Placement of a speaker-environment is very simple for the user because it simply place it is necessary to follow the manufacturer's directions. Unfortunately the speakers-environment are rare and the forced placement near the bottom wall is considered (erroneously) by many deleterious audiophile for which no one has ever taken an adequate commercial success. Are still several manufacturers who have proposed, especially in the 80s, this type of approach. On Allison speaker, the woofer is placed as close as possible to two (Allison One) or three walls (Allison Three) to exploit the increase in efficiency (6 dB for two walls, 9dB for three walls) achieved in the face of the decrease of 'angle of radiation. At the same time are energized so more homogeneous modes of resonance, and is cut off from the band assigned to the woofers the area to 0.3 wavelengths which is therefore to be above the crossover frequency (fixed at 400Hz on Allison ). The midrange are placed at a certain distance from the woofer and is instead located in the neighborhood of a single wall. The level entrusted to the woofer is adequately attenuated to take account of the reinforcements mentioned above. In low range this is the solution that achieves the best result in terms of the environment in response regularity.

Room types
In order to provide some concrete indication of installation we will refer to the rooms of a classic rectangular proportions. Here are the four most common cases that occur for the locations that prefer the wide side of the room in which we will assume the side walls at a distance> = 1,7m from the speakers. In the case in which the side walls are at a smaller distance it is necessary to consider the additional reinforcements that these walls will: for example it may be useful to exploit one extreme audible (25-28Hz) placing one of the two speakers to 130 cm from the nearest side wall .
Apart from the side walls we will have three potential reinforcements as follows:
1) Cross-woofer Wall
2) Floor-woofer
3) Points of listening-back wall (behind us)
The three distances in the game that will establish:
X = shorter distance
Z = intermediate distance
Y = greater distance

The distance X or Z depicted in the various "cases" is a function of the fact that is attributed to the lower or intermediate distance in alternation to that of the floor. In the four most common types of installation the "case A" and "B case" involving three useful reinforcements, the "case C" is for a type of room with only two reinforcements while in "D case" we are faced with the case limit of enormous rooms, in which there is only the reinforcement of the floor.

 

 

Distance from the side walls
A further good reason to have the system on the wide side of the room lies in the possibility of natural compensation of the speaker-dip or the partial cancellation that is created between the two speakers by distances equal to 0.5 wavelengths, perhaps the 'only real drawback of the stereo. To this end, if the size of the room so allow, it is simply necessary to choose the distance between the diffuser and the farthest side wall equal to the distance between the two speakers. For example for a 5m wide room you could put a diffuser to 130 cm from a side wall resulting in reinforcement helpful to start from 26Hz, the distance between the speakers equal to 185cm (speaker-dip 93Hz) and the other speaker distance 185 cm away from the side wall (in compensation). The possibility to arrange the two speakers in a decentralized manner with respect to the axis of symmetry of the room also allows the excitation of resonance modes different from each of them making less irregular the frequency response of the complete system.

Conventional speakers
In most cases we will find ourselves having to install speakers in the conventional environment, ie designed for maximum linearity in an anechoic chamber, for which the rules are already seen above. So considering a room type "A" or "B" with three useful reinforcements and having set: X =  shorter distance, Z = intermediate distance, Y = longer distance, we will distribute these distances according to
Z2 = X Y relationship as recommended by the Allison , in contrast to which, however, we place as the third reinforcement to be considered not relative to the side walls (suppositories here at considerable distance) but that relating to the distance between the listening point and the rear wall (behind us), totally equivalent as It relates to strengthening in the low range.
Once calculated, these distances are interchangeable with respect to the reinforcements, or can easily choose whether to use for example the Z intermediate distance between the woofer and floor or between the listening point and the rear wall, while they are not at all as regards the different excitability of the ways of low frequency resonance that instead can vary considerably and is the reason why it is good practice in the first installation step to exchange them between these three distances (or two of the three if the speaker is from the floor) to understand which combination best It suits proportions of your room / decor.
Having previously made the resonance of the speaker, we will calculate (assuming a Qt equal to 0.7 so a -3dB roll-off at resonance) the Z intermediate distance at the fr (the speaker resonance frequency) for which Z = ( 344 / fr) / 10
Z = 3440 / fr expressed in centimeters.
We will set at this point the shortest distance X to a shifted frequency of ½ octave higher in order to simulate the trend of a curve of Møller, then:

X = Z / 1.5 and, finally, we will derive the greatest distance Y = Z² / X

There will then know the resonance frequency fr and the merit factor Qt of your loudspeakers. See here .
If you don't want calculate Qt but approximate it to Qt = 0.7 does not create major in practice if you find a low range too full, you can move further down in frequency the three reinforcements inserting as fr a lower frequency and repeating the calculations, conversely if it is too light. In the case of rooms with four or five of these reinforcements will be deployed to follow towards the lower part of the spectrum if the size of the still allow room, if this is not possible and / or there were two or more very similar distances would become the important irregularities. Whereas the high-pass of a diffuser in the air suspension has a slope of 12 dB / oct, we see that in front of four votes reinforcements located at about ½ octave apart are recovered 12dB in two octaves to which it extends in fact so obvious the frequency response in the low range of the speaker used. For the same reason it is not possible to use a conventional full-range speaker in an environment with more than two reinforcements worth obtaining a clear exaltation of the low range.

enter "fr" and "Qt"
Results:
Frequency reinforcement

fr (Hz)

Z (cm)
fz (Hz)

       Qt

X (cm)
fx (Hz)
Y (cm)
fy (Hz)

Conclusions

Select a speaker regardless of the room where it will be placed exactly how to reckon without his host. This article will make tangible this aspect and provide real assistance in the placement of speakers in ambient air suspension in order to facilitate proper consistency and extension of the lower range using an ever-present aspect that, if not considered, is often deleterious. Happy listening!

 

The current version of the method is the same as published in n.385 of Audio Review magazine (italian magazine) with the title "the equalizer can not be excluded", here are added only a few Java tables  in order to possibly make the relief independently and the calculation of Qt and distances.                       

 

Bibliografy

Roy F. Allison, “The Influence of Room Boundaries on Loudspeaker Power Output”, JAES, June 1974"

Beranek, L.L., “Acoustics” New-York 1954

Olson, H.F., “Acoustical Engineering” 1957

 

modello Grundig SL1000 Grundig box850a Grundig box1500a Grundig box2500a Grundig box703 IMF ALS 40 IMF Super compact II Rogers LS3/5a 15Ohm
fr (Hz) 94 77 74 65 57   61 91
Qt 0.65** 1 1.15 1.21     1.05 1.25*

* With regard to the LS3 / 5a the measured Qt is 0.61 but the filter simulates an exaltation on the low comparable to that which would be obtained with a Qt of approximately 1.25 so that is the data to be entered in order to obtain the distances of the method.      

**regarding speakers with very limited extension on bass frequency, that needs more than three reinforcements, it is appropriate to place the speakers always near to bottom wall and arrange the remaining reinforcements available to climb of not less than half an octave to prevent irregularities .

                                    

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