Small Studio Acoustics
Mike Craig avoids egg-box cliches and tells you how to do it properly.
With the recent large growth in the recording industry, in particular the small studio, proper studio design has become associated with the larger studios using design consultants. Since most small studio owners have limited resources, proper design is often neglected. However, designing and building a studio need not cost the earth, provided some basic rules are employed, with a sprinkling of logic and common sense. Indeed very reasonable results can be obtained for relatively low cash outlay, particularly if some second-hand building materials are used and a little time is spent: this first article will look at the basic fabric of the studio, while a second will concentrate on acoustic treatments.
The outside world is filled with noise, the level of which we are used to ignoring. But it can be quite high, contributed to mainly by road, rail and airborne transport. These sources are principally air-carried, but do contain some low frequency components which resonate the structure of a building. This means that, on the whole, single-shell buildings are not a practical proposition for studios, since sound insulation is generally a function of weight (mass), increasing by approximately 5dB for each doubling of mass per unit area. So a brick wall 5in thick would average 5dB worse than one of 10in thickness. However, external sound sources such as traffic can produce sound pressure levels of over 90dB at low frequencies, whereas instrument amplifiers (especially bass amps) are capable of outputs in the 110dB region. So it can be seen in fig 1 that a single leaf wall will give insufficient reduction; double leaf construction with an air gap between each leaf must be used.
But it's not quite that simple, as the air gap between the two walls will act as a 'spring', coupling the walls at a definite frequency, so the sound insulation by the mass of each leaf cannot be simply added together. This frequency, or resonance, is proportional to the inverse of the square root of mass multiplied by the cavity width. An increase of the cavity width, or each wall, will lower the resonant frequency by one octave if the increase is by a factor of four.
Fig 1 shows a theoretical reduction of a double leaf wall in comparison to single leaf walls, the reduction increasing by 10dB per octave above resonance, and giving 15dB improvement at 125Hz. Fortunately most small studios are built in houses or basements which are usually of double leaf construction, though this sometimes proves insufficient and a further 'floating' leaf is required. In practice the reduction of sound by a wall depends on its porosity and the amount of sound induced into it; an unplastered wall could give a reduction as low as 15dB (average); plastering the wall can raise this to about 40dB.
For control room/studio separation walls a double brick wall with a cavity of greater than 7in should be used, although a cheaper and perhaps easier solution is to use plastered breeze or clinker blocks with the same cavity width. The amount of reduction will depend on how much sound is transmitted from one wall to the other, the coupling being reduced with increase of the gap, or if space is a criterion, as it usually is, a blanket of 2in glass wool hung in the cavity. Since for mechanical strength the walls may need to be tied together, special butterfly ties should be used, although any bridge between the two structures should be avoided if possible. Mortar dropping in the cavity can cause bridging, and one method of avoiding this is to use a board that fits the cavity, supported by a string at each end, being drawn up as each course is laid, surplus mortar then being removed.
It is important to realise that however high the wall's insulation, a flanking path through the floor, ceiling or side walls to which the partition is built can reduce the overall separation considerably. Using a flexible material, at the edges, such as building mastic and perhaps a glass wool quilt overlaid with building paper on which to build the partition, will reduce the coupling, although care is needed to make sure that the wall is mechanically stable. This may prove impractical depending on the area of the partition.
Control room and isolation booth windows are acoustically weak links, and insufficient care over construction can almost eliminate the reduction of the surrounding wall: attention to detail pays off. Ordinary window glass gives a reduction of about 20dB which is insufficient, so 32oz glass (or heavier) should be used – see fig 2. With 8-10in spacing, two sheets of this can give 40dB isolation with the window aperture as small as possible, though maintaining enough visual contact with the studio. Improvement to this figure can be made by lining the sides of the air space between the glasses with an absorber, and by venting the air gap between the walls and eliminating any air leakage paths round the glass or frame. The mistake most widely made is to mount the two sheets parallel; the two then act like twin drum heads and develop resonances between them. By mounting one pane at a different angle to the other, this is virtually eliminated; making the panes of different weights helps as well. The glasses should be rigidly mounted so they can 'borrow' mass from the surrounding wall (see fig 3). It is advisable to make one pane removable so that the insides of the windows can be cleaned.
Another weak link is doors, since total seals are difficult and, again, mass determines effectiveness. There are two main approaches, best of which is a sound-lock that, like an air-lock, is a lobby as large as is practical, separating the doors to each area. The mass of each door can now be kept to a practical level.
Sometimes this solution is not possible, and the only answer is to make the door as massive as the frame and surrounding walls will allow, or to use a mini-lock with a minimum of 8in inner spacing and absorbent inner surfaces. Several door designs are shown in fig 4 – note that special attention should be paid to the seal of the frame to the wall, and of the door to the frame.
Compression type door handles of the style used on commercial freezer-rooms can help, as will fitting the bottom of the door with a drop-bar type of draught excluder. Alternatively the door can be arranged such that when it is closed it comes in to contact with the carpet by use of rising butt hinges. The remaining door surround is best fitted with sorbo rubber strips, rubber extrusion, or phosphor-bronze draught excluder.
Most existing ceilings will consist of plasterboard or lath and plaster, with boarding on joists forming the floor above. An average level of reduction for this type of construction is 35dB, but an increase on this can be obtained by filling in the ceiling void with 2in of dry sand, resulting in an improvement of about 5dB. The weight loading of this is about 183 lbs per square yard, so if the existing ceilings will not stand this, rough boardings supported by battens nailed to the sides of the joists will do.
A lighter ceiling fill, if sand is impractical, would be a 3in thick layer of Rockwool, though it is not so effective. Further improvement can be made by floating the floorboards above on a glasswool quilt, and, without contacting the floor joists, the boards are nailed to battens laid on the quilt and running the length of the joists.
Impact sound from footsteps and dropped objects is difficult to deal with, but given some underlay and a reasonably thick carpet, along with the foregoing, it can be usefully attenuated — see fig 5. The floating floor technique can be used as a general isolation treatment for studio/control room areas which have existing wood floors. An alternative treatment for concrete floors is a layer of concrete about 1½in thick, resting on a quilt turned up at the walls and covering the existing floor. A layer of waterproof paper overlay to stop the concrete sinking into the quilt is necessary, and ¾in chicken mesh directly on this as reinforcement is a good idea since it also prevents damage to the paper when the concrete is laid. The concrete mix required is one part cement, two parts sand, four parts small aggregate — any area thus treated should not exceed 18 square yards because of cracking (fig 5A).
This is often non-existent or inadequate in the small studio, and can therefore be a problem. Nevertheless it is an area of importance which is often overlooked — after all, good results are harder to obtain if people are not comfortable. The main problems associated with ventilation systems are noise from the fan, and duct transmission.
Fans are of two main classes, axial (or propeller), and centrifugal. The former normally operate at high speed, and introduce mostly high frequency noise; the latter run at lower speeds and cause lower frequency noise and rumble. Since it is easier to attenuate hf noise, the axial fans are preferable and, since these normally use plastic blades, are well balanced. As a general guide the noise output of both types increases more or less as a logarithm of the power, size, and speed of the fan. The fan unit is ideally mounted on or in a concrete block, which is then fixed to the building via anti-vibration mounts, such as rubber-to-metal as used in cars, or springs. The block is easily cast in an aggregate concrete with the use of some timber, and an old tin for the hole (see fig 6). The unit can now couple to the trunking with a flexible ducting such as canvas. Note that the intake and outlet should ideally be on the quietest side of the building and not on the main road!
Ducts which are unlined will attenuate very little noise and should be lagged with a material which has a high absorption coefficient, is fire- vermin- and rot-proof, and has a relatively smooth surface for low air friction. Glass fibre blanket is ideal for this application; further attenuation in small lengths of duct can be obtained by inserting splitters (consisting of plywood plates covered with absorber on both sides) into the duct lengthwise. This splits the duct into a number of channels (see fig 7).
Crosstalk between two rooms can be caused by sound travelling along the duct from one area to another, so it is better to run separate ducts for each room. Construction in wood, for more acoustic transparency, is better than metal, which can 'short-circuit' the isolation. The conventional louvre grills used for ventilation systems should be avoided as they buzz at certain frequencies; drum louvre diffusers or felt-damped grills are best. The cavities created by acoustic treatments can be used too — for example the space above a suspended ceiling can be used as a chamber to distribute air into the room below.
These basic structural techniques will probably need modification to suit individual requirements, and it may not be necessary to apply them all. But they outline the procedure required to give a solid base for the acoustic treatments which will be outlined in a future article.
Feature by Mike Craig
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