INTERIOR NOISE CONTROL - TRYING TO UNDERSTAND ACOUSTICS
Acoustics is a very complex subject and the science behind it far to difficult for most. But designers need to know the basics, so that designs and surfaces are capable of achieving desirable acoustic outcomes and meeting Building Code (NZBC) requirements.
Although very detailed or critical requirements should be left to experts, this article tries to provide a simplified, big picture, non-expert explanation of noise control that hopefully will help with the simpler project issues related to sound barriers, sound insulation, noise absorption and Building Code (NZBC) requirements.
New Zealand abbreviations
| NZBC G6/VM1 |
| Airborne and impact sound - Verification Method |
| NZBC G6/AS1 || Airborne and impact sound - Acceptable Solution |
| dB || Decibel |
| NRC || Noise reduction coefficient |
| STC || Sound transmission class |
| IIC || Impact insulation class |
| CAC || Ceiling attenuation class |
Decibel dB is the unit of magnitude for sound pressure (10dB is quiet, 90dB is very loud)
NRC measures absorption of sound for a material or surface, range between 0 and 1.0. NRC 0.0 is non-absorbent and NRC 1.0 is the theoretical most absorbent, useful for reverberation control. For historic reasons it is possible to get NRC materials greater than 1.0.
STC is the amount of airborne sound transmission loss through a complete construction like a wall or floor, measured in decibels.
IIC is the amount of impact sound transmission loss through a complete construction like a floor, measured in decibels.
CAC is room to room sound attenuation of suspended ceilings sharing a common ceiling plenum over. CAC <25 is low performance, CAC >40 is high performance
Note; Overseas NRC is being gradually replaced by the Sound Absorption Average (SAA) which virtually the same but uses frequencies slightly differently.
Generally compliance requirements hinge around NZBC clause G6 Airborne and Impact Sound from other occupancies or spaces to habitable spaces of household units (multi unit houses, apartments etc). G6.3.1 says STC of walls, floors and ceilings to be min 55dB. G6.3.2 says IIC of floors to be min 55dB.
NZBC G6/AS1 provides examples of some limited building element constructions that would be Acceptable Solutions. However these details do raise some big questions about practicality, performance and long term compliance.
NZBC G6/VM1 provides the test standards to be met for Airborne Sound Insulation Field Tests and Impact Sound Insulation Field Tests. Field tests must be at least within 5dB below performance requirements. Field testing is usually carried out by an acoustic consultant on behalf of the supplier/contractor or the owner. Subject to any rectification required and retesting, the results are supplied to the Contract Administrator and possibly the BCA.
- although these apply only to household units, the figures are often used unofficially for other uses such as hotels, hospitals etc.
- IIC should sometimes be considered for wall elements as fittings and services can cause impact noise transmission (ie. kitchen sinks with impact on tops and water hammer in pipes)
- IIC should also sometimes be considered for structural elements as any impact noise could transmit long distances.
- Field (site) testing is very important, final installations must be proved as there are so many things that, if not done right, can undermine performance.
Noise Level Recommendations
Typical Noise Control Recommendation Table
|SPACES ||TYPICAL DESIGN LEVELS |
|Open plan office ||Ceiling NRC 0.70-0.80+ |
|Office to office ||Standard STC 40-45, High STC 46-50+ |
|Office meeting room, conference rooms, classrooms ||Standard STC 40-45, High STC 46-52+, Ceiling NRC 0.55-0.70 (other surfaces can contribute to NRC) |
|Apartment to apartment, house to house, hotel room to hotel room, some inter-tenancy walls ||Minimum STC 55 walls and floors, High STC 65 Minimum, IIC 55 for floors, High IIC 65 (Refer to NZBC G6 for minimum code requirements) |
|Bedroom in house ||Standard STC 40-45, High STC 46-52+ |
|Home theatre, home garage/workshop ||High STC 55-62+ |
- Specific project requirements may be higher or lower than the above table.
- Type and location of noise source and source room can affect the requirements.
- NRC can be affected by size of room and height of ceiling.
- Flanking paths, ceiling spaces, gaps, doors etc. will need appropriate treatment.
- IIC will sometimes need to be considered for walls, structure and services.
Barriers to Airborne Sound (Walls, Ceilings & Floors)
In building terms a wall or a floor can be considered a barrier to air-borne sound stopping or reducing sound created in one room travelling to another. The effectiveness of the barrier is measured by its STC rating, how much it reduces the sound by. A wall or floor can be solid (concrete etc) or composite (timber frame with insulation & plasterboard both sides) - they all perform differently. Dense thick materials like concrete usually perform best, but separation between elements can also help, like the timber framed wall. Some manufacturers provide STC levels for wall and floor/ceiling systems.
Whatever type of barrier you use, the important thing to ensure is that there are no flanking paths, holes or other routes that the sound can travel through. The barrier is only as good as its weakest link. Ensure all edges, holes, penetrations etc are sealed to the appropriate level and ensure all other paths like Doors, Air Ducts etc are acoustically treated to maintain the sound reduction.
Isolating Impact Sound (Floors)
Usually impacts like footsteps, chairs moving etc, transmit through the structure (floor) to the space below. To overcome this any hard floor surface needs to be isolated from the impact and/or the space below. The effectiveness of this isolation is measured by its IIC rating. Broadly speaking, hard dense materials transmit impact noise easily (concrete, steel), softer materials reduce impact transmission (air, rubber, insulation).
A hard floor surface can be isolated from the impact by putting a soft surface (carpet, acoustic vinyl etc) on top of it, or isolated from the rest of the structure (ie. tiles over slab) by putting a soft surface directly underneath it. Another method is to isolate one structure from another, like a separate floor above from a separate sound rated ceiling below, but it has many problems and has to be very carefully detailed to avoid flanking paths.
Issues to consider are:
- Soft flooring has a limited life if it is replaced with inferior product or hard flooring - it may be a problem in the future (this has been an issue in apartments).
- A hard surface floor isolated from the floor below must also be isolated from other surrounding structures and their hard finishes (walls, columns etc), and possibly services, fixtures and other elements penetrating the flooring.
- Impact sound may come from other sources not related to the floor; door slams, plumbing (water hammer), fittings and fixtures (ie. kitchen sink) etc, may need to be isolated.
Reverberation control is important to improve the acoustic quality of a space and/or to aid in the reduction of sound. This relies on absorption and the amount of absorption is expressed by the NRC.
The acoustic quality of a room can be improved by controlling the unwanted reflection of sound, so the listener hears the direct sound (speech or music etc) with less reflected delayed sound. This is important in classrooms, lecture rooms, music rooms, theatres etc.
Noise generated in a space (room, duct etc) can increase in level with reflective surfaces or decrease with absorbent surfaces. This is important in spaces with large numbers of people like classrooms, cafes, open plan offices etc. It may also be necessary with high levels of noise either with direct exposure in work places such as, workshops, machine rooms etc, or in helping reduce STC requirements by treating the source space.
Reverberation control is a very complex science so where acoustic performance is critical experts will need to be involved.
Absorption materials are used in a number of different locations, the most common is ceilings, particularly for large spaces with a relatively low ceiling height (classrooms, open plan offices, etc). Higher ceilings may mean the walls should also be considered (lecture rooms, halls, auditoriums, etc). Absorption requirements may involve only a certain percentage of the surface, in these situations separate absorption panels such as Acoustic Panels or Reverberation Control Panels can be used. These panels can be horizontal or vertical and fixed, free standing or suspended, depending on the situation and performance required.
Reducing Noise Travelling Through Ceilings
Particularly in office buildings, many walls stop at the suspended ceiling rather than extending up to the floor or roof above. As a result, noise from an adjacent space can penetrate the ceiling and move unimpeded throughout the ceiling space. Some portion of this sound can pass back down through the ceiling into adjoining rooms.
An obvious solution would be to carry sound rated walls up through the space, and this is probably the best thing to do for high STC rated walls. However for lower STC ratings in large projects this might not be economical. If the walls only go up to the ceiling, then the STC of the two (or more) ceilings involved and the NRC of the ceiling space would need to be considered. This is usually done with sound rated ceiling linings or tiles and acoustic insulation. For suspended ceilings CAC is used to measure this performance of the ceilings, and the manufacturers can usually supply CAC figures for their ceiling tile systems.
Another tool is the use of Baffle Stacks, which are simply stacks of insulation pads in the ceiling cavity over partition walls to the underside of the floor/roof to reduce noise transmission that can travel over the wall. Generally they are installed to manufacturer's requirements, with sufficient compression (usually 30%) to maintain stability above the wall below. Over or under compression can affect performance. Ducts, piping and other services can run through but ensure that there are no gaps or openings allowing sound to travel. Adequate ventilation is required around electrical fittings and some cables due to overheating.
Ceiling spaces in commercial or institutional buildings can often have Ventilation systems, Air Conditioning systems and/or Fire Sprinkler/Alarm systems - these must not be compromised.
The little things also count
As we discussed before any barrier is only as good as its weakest link, so it is vital that these are thought through and dealt with. Edges, penetrations, gaps, holes etc must be appropriately sealed. Isolation of elements must be maintained and not accidentally compromised. Consistency is important as well as following the acoustic system manufacturer's details.
Once you have dealt with the barrier requirements of the spaces consider any linking elements like doors, windows, mechanical ducts, pipes etc and make sure they are appropriately acoustically treated to match.
Don't forget to consider background noise, for instance noisy air conditioning in a noisy room may not be a problem, but it would be a problem in a quiet bedroom.
The terms and abbreviations used in New Zealand (and Australia) are virtually all American terms, however, some materials sourced from Europe may use different terms. Some are directly translatable and some are not. The following are the main ones and how roughly equivalent they are:
EUROPEAN TERM APPROX EQUIVALENT
R'w - Weighted Sound Reduction Index
STC = R'w (Subject to frequency)
L'n,w - Weighted Normalized Impact Sound Pressure Level (field)
IIC = L'n,w (Subject to frequency)
αp - Practical Sound Absorption Coefficient
NRC = αp (Subject to frequency)
A Sound Absorption Class (SAC here) can be assigned to αw - Weighted Sound Absorption Coefficient which is directly related to αp.
This allows a comparison to NRC.
NRC to SAC (Subject to frequency)
NRC 1.0 = SAC A
NRC 0.8 = SAC B
NRC 0.6 = SAC C
NRC 0.4 = SAC D
NRC 0.2 = SAC E
Dnc,w - Weighted Suspended Ceiling Normalized Level Difference
- "=" in this case is used to mean, measurements roughly equal but usable as a guide, however, not sufficient for critical calculation purposes.
- For compliance situations manufacturers should confirm performance in New Zealand terminology and measures, or seek expert advice.
References: The above article was sourced from numerous references and industry documents including
As it is intended as a guide and is not definitive, this article should only be used in conjunction with the NZ Building Code as well as other related legislation and Standards. If you are not sure about an application or product, specialist advice should be sought.