Sound Absorbent Materials
Sound absorbent materials for the studio.
A cursory glance at photographs of sound studios reveals one major difference between these and normal rooms and that is the wall covering. It may be decorative but, unlike its domestic counterpart which is chosen for its appearance, it is chosen for its function, appearance being secondary. It is, therefore, pointless creating a home studio based on photographs, because what goes on behind the facade is not obvious and it is essential to understand what purpose the wall covering serves before attempting a room treatment.
The various coverings act as sound absorbers or dispersers. It has been pointed out in HSR previously that sound absorbers and sound insulators exhibit entirely independent characteristics and that the absorbers seen in studios are there to control the sound within the room and not from outside. There is little sense in making the effort to attain ideal studio acoustics if the recording made in it is marred by extraneous sounds - silence it first!
When describing a room's acoustics, words such as dead, live, warm and dry are regularly used to describe such characteristics as short or long duration reverberation and the dominance of certain frequencies. Measurements of reverberation time refer to the time taken for the original sound to die to one millionth (60dB) of its original intensity, the frequency at which this is measured is usually also specified. What is aimed for is a smooth decay of a duration suited to the sound source, room size and frequency being reproduced. The duration for acceptable recording must change as those factors vary. A large room of 15,000 cu.ft. may require a second whereas a 500 cu.ft. room may need only half a second. Music, particularly by stringed instruments, in order to sound 'richer' needs a longer reverberation time than speech which becomes unintelligible with excessive duration. Experience has shown that a 0.4 to 0.5 second decay is best suited to speech in a space the size of a large living room. This would, of course, need to be less for a smaller room perhaps half of this figure, a time not easily obtained in a home studio. In a professional studio, much attention is paid to the design in order to obtain the required smooth decay at all frequencies, though some advertising production companies do not bother about perfection as much as they should and their minute speech booths create an appalling sound. In these types of studios often only one sort of absorbent tile is applied to every surface, except the floor, which is usually covered in heavy carpets. This combination soaks up the high frequencies — the resultant recording often sounds as if it were obtained through a sock and renders the voices of highly paid personalities virtually unrecognisable. Listen carefully to those TV advertisements.
Until the amount of reverberation in the room is excessive, it is not too obvious to someone in the room when listening to a sound. This is because of the ability to 'focus' the hearing on the wanted sound and almost ignore the unwanted. The same person listening to a faithful recording of the same sound produced in the same room will experience something totally different - all the sound is reproduced as it really is at the position of the microphone — a real jumble.
The resultant sound is only too familiar to anyone attempting to record in a normal room - too much reverberation and usually 'boomy'. The first problem (reverberation) results from the ability of the sound to reflect from one surface to another for longer than the previously mentioned optimum time, the second problem results from the room resonance, when a several fold increase in sound level occurs whenever a note is produced whose wavelength is twice the width, length or height dimensions of the room. This increase in sound level also occurs at harmonics of the note. A spare storage room measuring perhaps 6' x 4' x 7.5' which could easily be pressed into use as a speech booth, would have resonant frequencies which would create a sound not unlike that of a telephone box, far better to use a larger room.
If there are such problems, how come anyone manages to produce a decent sound? The answer is that hardly any problem is insurmountable, but by appreciating the problems, the home studio constructor can avoid many of the pitfalls which lack of knowledge could easily cause.
A whole industry has evolved to tackle the problems encountered with acoustics, and a wide range of proprietary tiles, panels and absorbent treatments are available to tackle specific problem areas. Not all are cheap, so it is essential not to waste material by applying it in circumstances where it is ineffective. Manufacturers' recommendations should be observed, and it is worth bearing in mind that manufacturers making exaggerated claims for materials intended for professional applications are soon found out; however, those for which it is claimed are used by professionals, but in reality are intended for semi-pro/amateur use, are something else.
Different materials have different absorption characteristics, both in frequency range and degree. Performance figures are not widely available for all materials but in general terms the softer the material is, the greater the degree of absorption and the thicker the material is, the lower the frequency to which its absorption is effective. Large soft items such as 3 piece suites or beds are very effective absorbers, which raises a very important point about the acoustic treatment of dual purpose studio/rooms - if the walls etc. are treated according to theory, and then the furniture is returned to the room, the overall effect will change considerably, something to bear in mind at the planning stage.
So, where to start? Perhaps a good jumping off point is to establish the frequency range to be recorded.
Assuming for the moment that the studio is to be used mainly for vocal work then the range will extend upwards from 90Hz. This strategy solves one difficult problem, that of absorbing low frequencies. Apart from bass instruments there are few with a fundamental frequency which falls far below 90Hz. Absorbing frequencies from 90Hz to 380Hz (voice fundamental range) is easily achieved by panelling the walls with 5mm plywood which should be spaced from the walls on battens (approx. 50mm) at varying centres to provide a different panel resonance across any area. The higher fundamentals and harmonics fall into the range controlled by the readily available proprietary tiles, panels and absorbent materials such as those manufactured or supplied by Alpha dBk, Armstrong, Castle Associates, Fibreglass, Rockwool, Turnkey Two and Wilmex.
Alpha dBk are suppliers of all manner of studio materials for both sound absorption and sound insulation purposes. They sell the Ecomax tile system which is available in 70kg and 140kg per cubic metre densities and thicknesses up to 50mm. The economically priced Ecomax mineral fibre tiles may also be used for suspended ceilings and with additional absorbent material above the tiles to provide low frequency absorption. Prices vary but typically £12.00 to £15.00 per sq. metre.
Armstrong acoustic tiles have been used extensively for suspended ceilings in shops and offices for many years; which means there is a ready source of used tiles when shops etc. are refitted.
This type of tile is most effective around the 1kHz region but effectiveness may be extended downwards by the use of an airspace behind the tiles. One important fact to remember is that the performance deteriorates when painted, this applies to all similar tiles, so restoration should be limited to cleaning.
Castle Associates of Scarborough offer a wide range of surface treatment materials for use in speech booths etc. in addition to screens for isolation of areas requiring lower sound levels such as interview areas.
Fibreglass Probably the best known insulating material around today for thermal use and it has found its way into many loudspeaker enclosures. It does have value as a sound absorber and like most other material benefits from an air space. There is one drawback and that is the very nature of the material itself, it being composed of fine fibres of glass which can enter the skin causing considerable discomfort. Gloves and other protective clothing should always be worn when applying it. The treated area could be covered with a fine mesh cloth to retain loose fibres and prevent abrasion problems.
Rockwool which is used extensively in studios, is available in various densities and in thicknesses up to 200mm though usual maximum thickness supplied is 150mm. Further information from Alpha dBk.
Turnkey Two are well known for their range of studio equipment but probably less well known for the range of acoustic control panels which they supply. The panels are manufactured by Illbruck from flexible polyurethane foam in several forms to satisfy the noise control needs of domestic, industrial and studio customers. Of interest to the studio constructor are the 'Audio' high performance, maximum absorption panels and the 'Waffle' medium performance panels; the former costs about £35.00 per sq.metre, the latter about £24.00 per sq.metre. 'Infinity' panels are also available at £32.00, and are designed to provide effective absorption down to 250Hz, a frequency which may be lowered by constructing the bass absorbers detailed in the Illsonic Acoustic Cook Book which is available from Turnkey Two.
Wilmex Ltd. are the UK suppliers of the Spanish produced Ecostop, a polyurethane panel system available in several thicknesses ranging from 30mm to 100mm. The undulating surface presents an 'irregular' absorption due to the different thicknesses in one area. Ecostop is supplied in panels of 1 sq.metre and is applied to the wall with an adhesive available from Wilmex. Prices vary with thickness and finish, but basic grey at 50mm has a professional users list price including VAT of about £20.00.
Most treatments benefit from an airspace and since air costs nothing and the battens for the space cost very little, it would seem the sensible way of mounting panels is by this method. The alternative to achieve the same performance will cost much more because of the thicker absorber required. As mentioned previously, some bad recordings occur when one type of treatment, be it egg boxes or the real thing is applied everywhere. The ways of avoiding this are to treat only two non-opposing walls or to arrange the treatments in some alternating layout. Close inspection of some photographs of studios will reveal a checkerboard or banded effect where this has been done.
It is hoped that the foregoing has shed a little more light on acoustic treatments, a subject which will be dealt with in greater detail, both theoretical and practical in forthcoming issues. In the meantime, consult the suppliers listed below for technical advice and application notes etc.
Feature by Steve Taylor
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