RWS Window System
Secondary glazing is the best all round approach for noisy windows but if the costs are prohibitive or you are renting a property then the following suggestions may be helpful.
You need sound absorbing foam in combination with a heavy sound blocking layer (foam/barrier composites) to reduce traffic noise through windows.
As simple way to achieve this is to build your own removable window plug.
The plug ideally needs to seal well against the battens or frame so sealing tape is a good idea. Its also a good idea to keep the wood sheet from directly touching the frame by using more neoprene strips. The plug needs to fit as tightly as possible. You can increase performance by adding thicker acoustic foam or by adding more weight to the wood sheet by using further layers of our soundproofing mat
Accoustic Windows
The two main types of glass that can be used in studio construction are:
Plate/Float - which is typical glass ranging from 3mm to 25mm thick and
Laminated-in - which an interior layer of very thin viscous plastic is sandwiched between layers of glass.
The noise reduction produced by a barrier is proportional to its mass, area, limpness and air-tightness. As the glass thickness increases the mass will increase. However, at a certain point there will be no more increase in transmission loss due to resonance effects. As the window opening gets bigger, extra glass thickness is more desirable since it will vibrate less in a control room.
Laminated glass performs better acoustically because of its increased limpness for the same thickness. Although laminated glass is more expensive than float glass, a thinner laminated glass will equal the performance of a considerably thicker float glass equivalent.
SINGLE SHEET LAMINATED GLASS
Fig2. shows a guide to the soundproofing performance of standard 6mm laminated glass.
The transmission loss is dominated by the damping at the critical frequency. In Fig2 the major difference can be seen between 6mm plate and 6mm laminated glass at 2.5kHz. This is because of the laminated damping interlayer. The bending waves transmitted through the glass cause shear strains within the viscous interlayer material which in turn transforms the bending waves into heat energy. The laminated glass is less susceptible to excitation by the incident sound waves at the critical frequency.
It should be noted that although damping controls the transmission loss at the critical frequency it is the materials stiffness, mass and density that dictates at what frequency the critical frequency will be.
It can be seen that laminated glass has significantly improved transmission loss without a significant increase in glass thickness or mass. 6mm laminated glass has a transmission loss performance nearly equal to that of plate glass having nearly twice its surface weight. 6mm laminated glass has a rating of 35 which is only 1dB less than that of 12mm plate glass.
STANDARD DOUBLE GLAZING
The sound transmission loss characteristics of air-spaced glass (double glazing) are more complicated.
Fig3 shows a comparison between the performance of 6mm plate glass and insulated air-spaced glass composed of 3mm glass - 6mm airspace - 3mm glass.
Comparing the figures we see that the transmission loss performances are quite different for the same surface weight of glass.
The graph shows that the transmission loss of the monolithic glass is considerably higher, between 250Hz and 1kHz, than that of the double glazing. This is because of the resonances associated with the 6mm air-gap. Between 1kHz and 4kHz the critical frequency of the 6mm single pane glass causes it to have a considerably lower transmission loss than the double glazing. At about 4kHz the critical frequencies of the individual 3mm panes causes the double glazing to be slightly lower than that of the 6mm single pane alone.
This resonance effect, for 3mm glass, is further illustrated in Fig3b.
Near the resonant frequency (250-500Hz) the transmission loss of the double glazing is actually LESS than that for a single layer of the same glass.
At lower frequencies the double glass has higher transmission loss which is due to the overall doubling of the mass of glass.
Only at frequencies appreciably above the resonant frequency does the air-gap provide significant acoustical benefits.
The graph also shows the critical frequency dip that occurs around 4kHz which is due to the coincidence effect NOT the resonance effect.
Doubling the air-gap thickness in double glazing can increase the sound rating between 2 to 6 dB (per air-gap thickness doubling).
LAMINATED DOUBLE GLAZING
As with plate glass, the sound isolation performance of double glazing can be increased significantly through the use of laminated glass.
Fig4 shows the performance between 25mm double glazing configurations, one using two 6mm standard panes and the other using two panes of 6mm laminated glass.
The laminated glass consists of two panes of 3mm monolithic glass laminated together with a 0.75mm thick viscous plastic layer. As a reference point, the performance of 12mm plate glass is included.
It can be seen that the critical frequency dip has been greatly reduced. However, the laminated sections still suffer from resonance effects, which reduce the sound transmission at 150Hz.
SECONDARY AIR-SPACED GLAZING
In general, the larger the air-space between two layers, the higher the transmission loss through them.
It is therefore preferable to use as large an air-gap as possible between panes of glass to achieve high transmission losses.
Fig5 illustrates the data for two layers of glass with different spacings.
There is a practical basis for setting a minimum spacing between panes. The lower curve, for the 6mm air-gap in Fig5, shows a sharp dip just below 500Hz. This is due to the 'mass-air-mass' resonance. This is caused by the air trapped in the cavity acting as a spring transferring vibrational energy from one layer to the other. This particular energy transfer is significant only over a small frequency range, where it causes a sharp lowering of the transmission loss.
The frequency of the 'mass-air-mass' resonance depends on:-
- the mass of the layers
- the distance between the layers
The larger the air-gap or the heavier the glass, the lower the resonant frequency.
To maximise the improvement due to air-space, the window system should be designed so that the 'mass-air-mass' resonance is as LOW as possible below 80-100Hz.
The resulting deficiencies, should these criteria be ignored, are poor low frequency performance.
The overall transmission loss of a window system with large air-spacing strongly depends on the characteristics of the air-space separating them.
ACOUSTIC WINDOW REVEALS
The transmission Loss of a window can be further enhanced by lining the periphery of the space between the panes with a sound absorptive material. This helps reduce the acoustic coupling, particularly at the mass-air-mass resonance frequencies, between the panes of glass.
The air in the space between the panes acts like a spring. At the resonant frequency of this sprung mass-air-mass system the vibrational coupling between the panes is very strong and at this point the system behaves very efficiently when it comes to transmitting sound thus reducing the Transmission Loss at this frequency.
The mass-air-mass resonant frequency depends on the thickness of glass and depth of air-gap separating the panes. Increasing either one reduces the resonant frequency and reducing either one increase the resonant frequency. Double glazing constructed of 6mm glass - 12mm airgap - 6mm glass resonates around 200 Hz which greatly reduces the sound insulation at a frequency which is already typically low. There is a lot to be gained by reducing this resonance condition using sound absorptive reveals.
The type of material used in the reveal must have an NRC of at least 0.75.
WINDOW FRAME ACOUSTIC EFFECTS
Window frames play an important part in the overall sound transmission loss of windows and can often compromise performance that would otherwise be obtained if the glass were used alone in an unframed, direct mounting, situation.
There are no fixed standards for designing window frames that maximise the sound performance of the glass but the following points are worth considering:
- The higher the TL of the glass used in the frame, the more likely it will be that the window frame will compromise the sound performance.
- If acoustic glazing is mounted in a lightweight frame then the frame should be designed such that the the glass surface area is as large as possible, extending as deeply as possible into the window frame itself.
- Window frames that rely on glass positioning by means of blocking should be constructed from continuous blocks. These must extend up the sides, and across the top, of the window.
- Window frames should ideally have the same surface weight as that of the glass used. In the vast majority of acoustical situations this is NOT the case.
- Packing out the hollow sections of PVC frames can improve on the frames sound transmission loss.
- Window frames are often insufficiently damped and this can lead to a reduction in transmission loss below 500 Hz. Laminated glass can slightly reduce frame 'ringing' but it is often more effective to apply a damping compound to the frame components to more efficiently reduce damping.
- For critical window applications such as recording studios it is CRITICAL that the frames are separately anchored and sealed to the building walls to avoid any vibration coupling between them. Failure to ensure this would SERIOUSLY degrade the acoustic performance expected.
- It is CRITICAL to ensure correct and adequate acoustic sealing of ALL glazing components and frame joints to eliminate any sound leakage.
If a blockboard type window lining is used to frame a window opening it is essential that any gaps behind the blockboard be completely filled. As with doors, the frame must be firmly bedded in mastic and well sealed. It is common to see window frames reducing the performance of a window by as much as 5dB below that expected with the glass alone !
To increase performance:-
- separate panes of glass should be used with an air-gap of at least 100mm separating them.
- resonance effects can be damped by placing absorptive material in the cavity.
- the frames and panes should be completely isolated from each other using neoprene type material.
- the frames/liners must be firmly bedded in non-hardening mastic, often a U-shaped seal is specified for around the edges of the glass.
- the construction should be as air-tight as possible and any compression seals should only be compressed by 50% to function correctly and maintain a resilient seal.
THE 'ANGLED GLASS SYNDROME'
Studio windows very often have tilted panes of glass and sometimes they are constructed with three panes. Tilting the glass does eliminate resonances in the air cavity between the panes which would otherwise limit the sound transmission loss at the resonant frequencies.
IN THEORY this can be a valid concern. However, in actual construction, there is always a practical limit on the overall thickness of the wall into which the window is to be mounted.
Tilting the glass often results in an average air-space between the panes being half to three-quarters the size it could be if the panes were vertical and parallel. The sound transmission loss through the window construction is HIGHLY DEPENDANT on the width of the air-space. The acoustical benefits of tilting the panes can often be negated because of the reduced separation between the panes. Sound isolation is maximised by maintaining the largest overall air-space between the panes of glass.
Four Panes is Better than Two
In acoustical terms 'more' can often end up being 'less'.
Adding an additional pane/panes of glass, within a cavity, will divide the air-space into smaller segments. The low frequency sound transmission loss (which dominates the performance rating) will reduce accordingly. (The costs will also double !!)
Sound isolation is maximised by maintaining the largest overall air-space between the panes of glass.
Despite the widespread belief that 'the more panes of glass the better', triple glazing provides essentially the same noise reduction as double glazing, unless the air-spacing is very large.
Fig6 compares the data for double and triple glazed windows with the same total air-space.
Below the 250Hz resonant frequency the transmission loss of triple glazing is about 3dB higher than that of the double glazing because of the increase in overall glass mass.
At higher frequencies the two curves are almost identical and both windows carry the same STC rating.
The most basic example of a control room window, with two totally isolated panes, is shown on the right. The acoustic lining helps to reduce cavity resonances as well as absorb any sound that finds its way into the airspace.
