ACOUSTIC TREATMENT. 1 Reverberation. Inside a given internally reflective enclosure, the human ear receives:

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1 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. 1

2 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. 2

3 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 3

4 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. 4

5 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. 5

6 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 6

7 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 7

8 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 8

9 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. 9