a. Glossary of Acoustic Terminology

Floating Floors

A floating floor is a floor which is supported by a structural slab but is completely isolated from the structural slab by resilient support members, so it is nowhere in rigid contact with the structure slab. The floating floor acts as a protective covering for the structural slab. Impacts, no matter in the forms of vibration or noise, on the floating floor will be absorbed substantially before reaching the structural slab. As a result, the room below the structural slab is much quieter than it will be if the same impacts directly strike the structural slab.

Figure 1 shows two types of floating floors which are supported resiliently on isolation pads and fiberglass insulation board. To prevent potential damage of a floating floor, it must be structurally designed for proper strength and the applied load should be within the design limits and uniformly applied. For any type of floating floor, the construction must be monitored carefully to ensure proper installation as it is usually very difficult to remedy defects after completion of the installation.

Vibration Isolators

Equipment with moving parts vibrates during operation. The method of reducing vibration transmission to other sensitive areas is to separate the equipment from the supporting structure by vibration isolators. Generally, there are two types of isolators, metal springs and isolation pads, that are widely used for vibration isolation.

(a)Metal Spring

Springs are particularly applicable where heavy equipment is to be isolated or where the required static deflections exceed 12.5mm. Static deflection of a spring is a value specified by the suppliers. Selection of appropriate springs is important as this may result in poor isolation efficiency or even amplification of vibration, especially in the case that the vibration frequency is extremely low.

The most important feature of spring mountings is to provide good isolation due to its ability of withstanding relatively large static deflection. Metal springs however have the disadvantage that at very high frequencies vibration can travel along the spring into the adhered structure. This is normally overcome by incorporating a neoprene pad in the spring assembly so that there is no metal-to-metal contact. Most commercially available springs contain such a pad as a standard feature. Figure 2 shows some common spring mountings.

(b)Isolation Pads

Isolation pads can be made of rubber, neoprene, glass fibre or combination of them. They are relatively cheap, easy for installation and replacement, and have the advantage of good high-frequencies isolation. However, attention should be given to the life of the isolation pads as some of them can be damaged by overload or low temperature. Figure 3 shows some common isolation pads.

Window and Glazing

The factors determining the acoustic performance of glazing and window are the glass thickness, pane spacing and gas filling of the cavity.

Glass thickness

Increasing the mass of glass, i.e. its thickness, increases the sound insulation. Laminated glass provides increased insulation compared with monolithic glass of the same thickness. Further improvement is obtained by using glasses of different thickness combination.

Pane spacing

For multiple pane system, the acoustic insulation is not affected by the pane spacing over the range normally found for hermetically sealed units, i.e. 6mm to 20mm. It should be noted that a sealed double glazing unit constructed of two panes of 4mm glass with airspace in the range 6mm to 20mm no better sound insulation than a single pane of 4mm glass. Therefore, at least 50mm airspace is recommended. Significant improvement is achieved with pane spacing of 100mm or above.

Gas filing of the cavity

Use of sulfur hexafluoride in acoustic performance (SF6) can improve the acoustic performance at certain frequencies but adversely affects it at low frequencies. However, argon, the gas most commonly used to improve the thermal performance of sealed double glazing units, does not provide any improvement in acoustic performance.

(a)Acoustic double glazed window

Double glazing is an insulating technique where there are two layers of glass in a single frame with an air gap in between. Based on the abovementioned factors, a well-designed acoustic double glazed window can have a superb acoustic performance. Figure 4 shows the section of double glazed window.

(b)Secondary glazing

Secondary glazing involves installing supplementary glazing on the existing single-glazed window inside the premises. For existing licensing premises, installing a secondary glazing window is the most cost effective way to avoid noise breakout through windows.

Air-sealed acoustic passage with double door arrangement

Acoustic passage / lobby

Two doors entrance of the licensing premises separated by a small lobby could avoid sound escaping through the opening door. The acoustic lobby act as a buffer zone in order to safeguard that one door is opened the other door still be shut. As two doors would not be always opened at the same time, sound is less likely to escape.