ACOUSTIC TREATMENT
Acoustic Treatment addresses the quality, intelligibility and level of sound INSIDE
a room and has nothing to do with how much internal sound is or is not heard
outside the given room, or how much external sound is leaking from the outside
into the room.
Reflective surfaces within a given finished room cause sound impacting these
surfaces to be reflected and bounce around within. This gives rise to four distinct
issues which can be the cause of problems:
1 – Reverberation
Inside a given internally reflective enclosure, the human ear receives:
a) Sound directly from the source in a straight line.
b) An undefined number of “copies” of that sound which have bounced off the
reflective walls, ceiling and floor before reaching the ear.
Because reflected sound travels a longer distance before reaching the human ear, it
arrives at a later point in time. The resultant out-of-sync overlap of direct and late
arriving reflected sound causes the human ear to percieve what is known as
reverberation (“church” or “bathroom” effect). In a normal enviroment,
uncontrolled reverberation causes lack of focus. This translates in increasing
inability to understand everything that is being said clearly in case of speech, and
makes music sound muddy. The larger the size of the room or enclosure, the more
distance the reflected sound has to travel before it reaches the ear, and the later it
arrives – therefore the larger is the reverberation effect, and the more problematic
is intelligibility. In smaller reflective rooms reverberation may not be that obvious,
but it is there, and the problem with intelligibility is still present to an extent that
may cause problems.
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2 – Room modes
Rooms with roughly parallel opposite surfaces have a length, width and height that
are multiples of half-wavelengths of a particular sound frequency. The affected
frequencies are low frequencies and are amplified by the room, causing
“booming”, or resonances which are more intense at particular points within the
room. The more reflective are any set of opposing parallel surfaces in a particular
room, the more emphasised will be the problem.
3 – Sound Quality
One of the properties of reflective materials is the reflection of different
frequencies of sound at different intensities. Even if a material is considered as
“reflective”, it tends to reflect lower frequencies more than higher frequencies
because high frequencies contain relatively very little energy and are therefore
much more easily absorbed. The addition of the reflected sound to the original
therefore exhibits a resultant sound which by ratio, is stronger in lower frequencies
than the original, or devoid of enough high frequency.
The articulation of consonants in speech (such as “F” & “S”, T” & ”D” and other
pronounced letters) contains mainly high frequency, which is suppressed by the
effects of reflection. As reflected sound increases, these letters start to get mixed
up for each other, and in more severe cases, are partially heard or not heard at all.
This effect is called “Consonant Loss”, and is the main cause of speech
intelligibility. It also causes music to lack in crispness.
0% Consonant loss – “She sells sea-shells on the sea shore”
40% Consonant loss – “She ell eee-shell on the eee-shore”
100% Consonant loss – “Eee eh eee-eh oh eee eee-ohr”
4 – Sound Level
The addition of reflected sound to direct sound causes the sound level (volume)
within a given room to increase. A reflective surface cannot distinguish between
wanted and un-wanted sound and reflects everything – not only voice, but noise
from office and other machinery, air-conditioning and all other external sound which
has penetrated the room. In an enclosed enviroment this can be rather annoying. It is
the cause of un-percievable stress and issues with communication, as people start to
have to raise their voices to be heard because of the high ambient volume.
Furthermore, louder sound within a room also increases the probability of it being
heard outside the room.
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Typical problematic scenarios caused by the combination of the above issues:
Meeting/presentation in a boardroom or conference room where the listeners
are not clearly understanding what is being said.
Attending a performance in a theatre and having a bad musical experience
because the sound is not right.
Sitting across a table in a restaurant and having to lean forward and talk
louder or shout to make yourself understood.
Inability to properly concentrate in an office enviroment because everybody
can hear what everybody else is saying or doing.
Inability to enjoy a supposedly quiet experience because of too much noise
resulting from excessive reflections and increased volume in the area.
Machines in a workshop or factory enviroment causing added sonic stress to
the operators, every day of the week.
Typical reflective and acoustic surfaces
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Treatment
Unlike soundproofing, acoustic treatment cannot be “hidden”. It must either
originally constitute your surface, or otherwise be applied in front of reflective
surfaces so that incicent soundwaves are absorbed. Every room with a particular
discipline, (meeting room, classroom, music room, lecture room, recording studio,
voice booth, office, theatre, concert hall, treatment room etc..) carries international
recommendations with specified values for reverberation time (RT60), where this
is the time required for the sound to decay by 60 dB. RT60 for a concert hall for
example, is around 1.8 s, and for a recording studio, 0.2 s.
In existing buildings, a physical test is usually carried out in order to determine the
reverberation time of the untreated area. Alternatively, and of course in the case of
un-built structures that are still at the design stage, RT60 is calculated virtually via
specialised software simulations, where a 3D model of the area including all
original material surfaces are specified and a value is obtained. This is compared
with internationally recommended RT60 figures defining the rooms or enclosures
in question, and further calculations are carried out to determine the acoustic
treatment and area of coverage required to reach specification. In professional
music applications many other parameters also need to be measured and analysed.
Normally, a specified percentage of total surface area ‘A’ within a given enclosure
to be treated is calculated mathematically, where ‘A’ will depend on the amount of
reflection provided by the employed materials in the un-treated room, and also on
the type and absorbtive strength of acoustic treatment used. In theory, the more
reflective the un-treated enclosure, the more surface area will need to be treated.
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One needs to understand that acoustic treatment needs space. Acoustic panels work
on the principle of sound being absorbed as it travels through the panel or
panelling structure, and therefore panels need depth to be efficient. The deeper (or
thicker) the panel, the more sound (and low frequencies in particular) can be
absorbed by the area constituting the panel. In a nutshell, thinner panels or panel
structures absorb less sound and therefore require more un-treated enclosure
surface area coverage than thicker panels in order to obtain the same result.
Acoustic wooden panelling showing rear structure
In commercial, industrial and domestic applications, best results are obtained by
treating a given percentage of at least one out of two opposite surfaces to eliminate
or reduce reflections continuously rebounding between the two surfaces (standing
waves). Therefore in a rectangular room (with four walls, ceiling and floor) one
would treat one of each of the walls directly opposite each other plus either the
floor or ceiling – (usually, but not exclusively, the ceiling). It is to be noted that
reflective ceilings are in principle the most problematic offenders. On the other
hand in recording studios for example, you need to treat every square centimeter of
surface available. Therefore in anticipation to a commonly put forward statement
(ie: “we don’t need recording studio specifications”), a commercial, industrial or
domestic building is not treated in the same way as a recording studio – far from it.
Acoustic treatment does not only constitute broadband or bass absorbtion, but also
sound diffusion, and in very rare cases even sound reflection. For this reason
panels which technically perform different tasks are available.
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Absorbers absorb most of the sound incident on them, eliminating reflections.
Feature with Fabric & Wood Absorbers
Flat Panel Absorbers with Custom Print
Polyester Fibre Absorbers
Wood Absorbers – Various designs
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Types of absorbers (design):
Acoustic foam (un-covered)
Acoustic foam (fabric covered)
Acoustic foam (wood-covered, with slots or perforations)
Mineral fibre (un-covered)
Mineral fibre (fabric-covered)
Mineral fibre (metal-covered, with slots or perforations)
Polyester fibre (solid) ** low absorbtion
Wood (slotted or perforated)
Wood wool
Multi-layer (fabric-covered)
Acoustic Gypsum (specialised installation)
Coated fibreglass weave (acoustic stand-alone fabrics for curtains and
blinds)
Acoustic absorbtion in the form of complete walls or ceilings requires mounting of
relatively thin panels over a structural frame, with acoustic cladding and suitable
air-gap at the rear (which is techically doing a large part of the work). In the case
of stand-alone absorber panels, this structure is integrated within the panel itself. In
any case, this translates into some loss of wall and ceiling space and a change in
original visuals imposed by the acoustic treatment. For this reason, architects and
designers should consult with sound specialists to determine changes to the
original visuals and the space required by the necessary acoustic treatment (and/or
any necessary soundproofing required). This should preferably be carried out prior
to construction of a new enclosure and/or internal design of an existing enclosure.
Bass Traps are required to treat room
modes and effectively control the stability of
low frequencies below 200 Hz, where
standard absorbers do not work. They are
most commonly used in the form of
treatment of corners, but are also available
as wall-mounting units. Bass traps are
mostly used in music enviroments, including
all types of studios, rehearsal rooms, music
rooms, media rooms, theatres, concert halls
and commercial and home cinemas.
Types of bass traps (design):
Acoustic foam (un-covered)
Resonating bass traps – fabric covered
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Diffusors scatter and diffuse sound in all directions. They improve projection
where required, create ambience, eliminate direct reflections and dead spots, and
work to improve the uniformity of sound at all points within a given enclosure.
One-Dimensional Diffusor
Polygon Diffusor
2D Skyline-Type Diffusor
2D Omnidirectional Diffusor
Media Room with Ceiling Diffusion
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Types of diffusers (design):
Wood (natural or RAL painted)
Wood (fabric-covered)
Thermoformed Plastic
Polyethylene / Polystyrene
Depending on the particular application, utilisation of absorbers, diffusors and bass
traps may be required as necessary. The large selection of materials, designs,
coulours and profiles available nowadays guarantee a finish which is in total
conformity to the design and style required – No, it does not have to look like a
recording studio at all.
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