The room: the equalizer can
not be excluded
Speakers installation method
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.
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
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
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
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
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 /
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
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.
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 "
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
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
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
Partially because a sofa in part absorbs the wave energy accident and partly
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
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.
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.
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
1) Cross-woofer Wall
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
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.
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
= 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
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
your loudspeakers. See
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.
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
Roy F. Allison, “The
Influence of Room Boundaries on Loudspeaker Power Output”, JAES, June 1974"
“Acoustics” New-York 1954
“Acoustical Engineering” 1957
IMF SC II
Rogers LS3/5a 15Ohm
* 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 .