Table of Contents


2. Basic Acoustic Principle

3. Principles of Construction Noise Mitigation

4. Quieter Construction Equipment or Method and Mitigation Measures

5. Glossary of Acoustic Terminology

1. Introduction

Construction noise in a compact city like Hong Kong is always an issue of concern. Through the years, a lot of good practices have been implemented by the trade to reduce the construction noise. Most of them have been reported through various channels and organizations. The objective of this website is to establish a platform for the trade to share some good practices and innovative mitigation measures proved to be successful in tackling construction noise both in Hong Kong and overseas. Moreover, we welcome mutual sharing on other good practices or quieter construction equipment information with others. Any opinion and information could be directed to the email address of This email address is being protected from spambots. You need JavaScript enabled to view it. , the Noise Management and Control Section of Environmental Protection Department. Thank you.

1.1 What is Sound and What is Noise?
Sound is a form of energy that is transmitted by pressure variations which the human ear can detect. When one plays a musical instrument, say a guitar, the vibrating chords set air particles into vibration and generate pressure waves in the air. A person nearby may then hear the sound of the guitar when the pressure waves are perceived by the ear. Sound can also travel through other media, such as water or steel.
Apart from musical instruments, sound can be produced by many other sources - man's vocal cord, a running engine, a vibrating loudspeaker diaphragm, an operating machine tool, and so on. Here you can hear common sources of sound.
Noise is unwanted sound. Usually the sound of a violin is referred to as music - is something pleasing. Depending on other factors, the sound may be perceived as noise.
Noise perception is subjective. Factors such as the magnitude, characteristics, duration, and time of occurrence may affect one's subjective impression of the noise.
The majority of the source of construction noise is operation of mechanical machineries or tools at the construction site.

2. Basic Acoustic Principle

Characteristics of Sound and the Decibel Scale
There are two important characteristics of sound or noise - frequency and loudness.

2.1 Frequency of Sound
Sound is the quickly varying pressure wave travelling through a medium. When sound travels through air, the atmospheric pressure varies periodically. The number of pressure variations per second is called the frequency of sound, and is measured in Hertz (Hz) which is defined as cycles per second.
The higher the frequency, the more high-pitched a sound is perceived. The sounds produced by drums have much lower frequencies than those produced by a whistle. Here you can hear sounds of different frequencies.

2.2 Loudness and the Decibel Scale
Another property of sound or noise is its loudness. A loud noise usually has a larger pressure variation and a weak one has smaller pressure variation. Pressure and pressure variations are expressed in Pascal, abbreviated as Pa, which is defined as (N/m2) (Newton per square metre).
Human ear can perceive a very wide range of sound pressure. The softest sound a normal human ear can detect has a pressure variation of 20 micro Pascals, abbreviated as µPa, which is 20 x 10-6 Pa ("20 millionth of a Pascal") and is called the Threshold of Hearing. On the other hand, the sound pressure close to some very noisy events such as launching of the space shuttle can produce a large pressure variation at a short distance of approximately 2000 Pa or 2 x 109 µPa.
A simpler way is to use a logarithmic scale for the loudness of sound or noise, using 10 as the base. The following is a brief introduction of the common logarithm to the base 10. You may refer to the demo here to learn more about the relationship between linear and logarithmic scales.
To avoid expressing sound or noise in terms of Pa, which could involve some unmanageable numbers, the decibel or dB scale is used. The scale uses the hearing threshold of 20 μPa or 20 x 10-6 Pa as the reference level. This is defined as 0 dB.
Sound pressure level, which is often abbreviated as SPL or Lp, in decibels (dB), can then be obtained using the following formula.

SPL (in dB) = 10 log10 (Measured Sound Pressure/Reference Pressure)

Relationship of sound pressure in micropascals and in decibels

In the chart here, some sounds are expressed both linearly in µPa and logarithmically in dB. One can see how the logarithmic scale helps us to handle numbers on a wide scale much more easily.
One useful aspect of the decibel scale is that it gives a much better approximation to the human perception of relative loudness than the Pascal scale. This is because human ear responds to the logarithmic change in level, which corresponds to the decibel scale.

2.3 Adding Sounds or Noises together on the Decibel Scale
In real life, several sources of sounds often occur at the same time. One may be interested to know what results when one sound is combined with another, i.e. the addition of sounds.
Adding 60 apples to 60 apples results in 120 apples. But this is not the case with sounds when they are expressed in decibels. In fact, adding 60 decibels to 60 decibels gives 63 decibels. The following formula explains the general principle of adding sounds on the decibel scale from two sound sources.

SPL(Total) = 10 log10(10(SPL1/10)+10(SPL2/10))

Addition of sound levels can also be done simply using the chart here.
Addition of sound levels
2.4 The "A-weighting"
A normal human ear is able to hear sounds with frequencies from 20 Hz to 20,000 Hz. The range of 20 Hz to 20,000 Hz is called the audible frequency range. The sounds we hear comprise of various frequencies. The entire audible frequency range can be divided into 8 or 24 frequency bands known as octave bands or 1/3 octave bands respectively for analysis. A particular sound or noise can be seen to be having different strengths or sound pressure levels in the frequency bands, as illustrated here.
One single sound pressure level is often used to describe a sound. This can be done by adding the contribution from all octave bands or 1/3 octave bands together to yield one single sound pressure level.
The response of the ear to sound is dependent on the frequency of the sound. The human ear has peak response around 2,500 to 3,000 Hz and has a relatively low response at low frequencies. Hence, the single sound pressure level obtained by simply adding the contribution from all octave bands or 1/3 octave bands together will not correlate well with the non-linear frequency response of the human ear. This has led to the concept of weighting scales. The diagram here shows the "A-weighting" scale.
In the "A-weighting" scale, the sound pressure levels for the lower frequency bands and high frequency bands are reduced by certain amounts before they are being combined together to give one single sound pressure level value. This value is designated as dB(A). The dB(A) is often used as it reflects more accurately the frequency response of the human ear. Weighting networks are often incorporated in measuring equipment to give readings in dB(A).

2.5 Propagation of Sound
In air, sound is transmitted by pressure variations from its source to the surroundings.
The sound level decreases as it gets further and further away from its source. While absorption by air is one of the factors attributing to the weakening of a sound during transmission, distance plays a more important role in noise reduction during transmission.
The reduction of a sound is called attenuation.
The effect of distance attenuation depends on the type of sound sources. Most sounds or noises we encountered in our daily life are from sources which can be characterized as point or line sources.
If a sound source produces spherical spreading of sound in all directions, it is a point source. For a point source in free space, the noise level decreases by 6 dB per doubling of distance from it. The SPL at a distance from a sound source also can determined by the following formula using the sound power level (SWL) of the sound sources.

SPL = SWL(point) - 10log (4 π r2)
r is the distance from the sound sources

3. Principles of Construction Noise Mitigation

A construction noise issue starts with a noise source such as construction plant operation. The noise is transmitted through a path and then arrives at the receiver. The noise will be perceived as a problem when the noise is so high as to be a nuisance to the receiver.
The severity of the problem depends on the strength of the noise source (such as noisy construction plant) or the length of the path, that is, how large is the separation between the noise source and the receiver.

3.1 Construction Noise Mitigation Measure
A. Control at noise sources
It is always better to consider reducing the noise at its source. Whenever possible, quieter working methods or technologies should be used.
Noise emission from some products is also under controls. According to Noise Control Ordinance (NCO), hand held breakers and air compressors must be fitted with “Noise Emission Labels (NELs)” when in operation.
Certain advance and quieter technologies have enabled some construction works to be done much quieter as compared with conventional noisy equipment. For instance, some building demolition projects have adopted the more environmentally-friendly hydraulic concrete crusher instead of the conventional mounted breaker. In some projects involving installation of underground utilities, pipe jacking is used instead of the conventional open-cut method.
Enclosing the noise source is one of the typical construction noise mitigation measures. A noise enclosure for reducing machine noise is commonly made of an exterior metal skin, an interior perforated sheet, with some absorptive materials such as fiberglass filled in between.
Some examples of common measures for controlling construction noise are described in this section.

(i). Hand-held Breakers
Hand-held breakers should be:
  • Fitted with mufflers to minimize the exhaust noise (sudden release of exhaust air) through a longer exhaust passage and to reduce the body radiated noise that originates from the cylinder enclosure.
Mufflers can reduce exhaust noise and body radiated noise by up to 15dB (A) and 6 dB (A) respectively.
  • Fitted with dampening layer with steel collars to minimize ringing noise caused by the steel bit vibration and to keep the damping material in place and protect it from abrasion. About 3 dB (A) reduction can be achieved by using this control.

(ii). Excavator-mounted breakers
Excavator-mounted breakers are amongst the noisiest items of general construction equipment. To reduce noise from excavator-mounted breakers, use of a hammer bracket (the bracket is made of special alloy and the inside of it is lined with sound insulation material). A noise reduction of up to 10 dB (A) can be provided.

(iii). Equipment with Internal Combustion Engines
Equipment with Internal Combustion Engines include stationary and mobile plant, examples include:
  • Stationary Plant: compressors, generators, concrete pumps and welding sets
  • Mobile Plant: excavators, bulldozers, loaders and dump trucks
The natural of the noise emissions relating to this kind of equipment includes exhaust noise, cooling system noise and engine noise. Control measures for these include:
  • Exhaust Noise
    -Install suitably designed exhaust silencers
  • Cooling System Noise (including noise from water pumps, belts, pulleys and cooling fans)
    -Replace poor fan blade design or damaged fan blades with an aerodynamic model
    -Install a contoured fan shroud or cowl with a close fan tip-to-shroud clearance
    -Remove all obstructions such as bars and pipes from the inlet airflow
  • Engine Noise
    -Installing vibration isolators for individual components to reduce transmission of engine noise to the surrounding
    -Installing specially designed partial or full acoustic enclosures for individual noise generating components. Partial or full enclosures can result in a reduction in the overall noise level of up to 5 and 10 dB (A) respectively
    -Apply damping material to vibrating panels
It is essential to regularly maintain and service all mechanical plant and equipment:
  • Implement a preventive maintenance programme, such as regular checking and renewal, to ensure equipment is operating in good order and not emitting abnormal noise
  • Make sure stocks of regularly required spare parts are available
(iv). Piling
Typical noise sources for percussive piling include:
  • Ringing noise (radiated from the surface of the pile);
  • Impact noise; and
  • Exhaust noise (created by the release of pressurized gases from exhaust ports).
Control measures for piling include:
  • Use non-percussive piling techniques such as pre-bore, vibratory hammers or hydraulic hammer for driving steel piles
  • Use resilient packing and dolly
-For percussive piling, the head of the pile should be protected by a helmet fitted with resilient packing over the top of the pile and a dolly which cushions the blow of the hammer.
  • Use of a shroud
-Enclose the complete pile and the pile driver. The shroud should consist of a robust framework fitted with an acoustically designed cladding or acoustic panels.
-Alternatively, a flexible acoustic curtain of appropriate thickness can be suspended to enclose the whole length of the exposed pile and pile driver.

(v). Joint Cutter / Stone Saw
Noise mitigation measures that can be fitted to a joint cutter and stone saw include:
  • A metal hood to screen the operator from the machine;
  • Anti-vibration mounts between the engine/motor and the support frame; and
  • A small pad supplied with water (for cooling the blade) to serve as a viscous damping layer between pad and blade.

(vi). Demolition and Concrete Breaking Works
Noise mitigation measures for demolition and concrete breaking works include using non-percussive equipment such as a hydraulic concrete crusher, low noise hydraulic breaker and other quiet plant.
  • Hydraulic concrete crusher, low noise hydraulic breaker can replace excavator-mounted percussive breakers in demolition work resulting in noise reduction of around 8 dB (A).
  • Low Noise hydraulic rock drill with SWL can replace conventional crawler mounted pneumatic rock drill resulting in noise reduction of up to 20 dB (A).

B. Noise reduction at the transmission path
An obvious way of reducing noise is to separate the sources of noise from noise sensitive uses. This is however often not practical in a compact and high-rise city to rely only on distance attenuation to cut down the noise. Additional attenuation, which can be provided through screening by movable barrier and enclosure.
Two common options to control sound transmission path are:
  • Noise Enclosures – fully enclose the equipment
  • Noise Barriers – prevent sound path reaching receivers
The noise reduction performance of those controls depends on the materials, design, configurations and distance between noise source and receivers and between the noise sources and the noise barriers / enclosures.

(i). Noise Enclosures
An enclosure can be constructed from a variety of materials. To achieve desirable results, material with superficial density/surface mass of at least 10 kg/m2should be used generally. In addition to effective materials used, the enclosure should also be properly designed, constructed and maintained because small gaps, poorly sealed joints would have noise leakage.

(ii). Noise Barriers
Noise barriers reduce sound transmission by breaking the direct path between the noise source and the sensitive receiver. However, sound will still be transmitted at a reduced level by refraction and reflection. Noise barriers are most effective when they are placed close to the noise source, and totally shield the noise source.
Barriers can either be fabricated on site from readily available construction materials or can be constructed from proprietary acoustic panels to achieve the maximum screening effect.
Depending on the material used and the design of the effective noise barriers, in general, an overall reduction of between 5 to 10 dB (A) can be achieved.

C. Administrative Noise Control
Apart from employment of quieter powered mechanical equipment, quieter construction methods and mitigation measures, administrative noise control measures can further reduce the noise impact from construction sites. Those measures include:
  • Providing adequate planning with contingency to ensure that lengthy operations e.g. concrete pours, can be completed within enough and permitted hours
  • Planning routers for construction vehicles carefully in order to minimize noise affecting nearby noise sensitive receivers
  • Scheduling construction work carefully to maximize any required noisy work during less sensitive hours (e.g. lunch time, outside school hours and avoiding examination periods). For unavoidable night works with Construction Noise Permit, carefully schedule the noisy works at locations close to any sensitive receiver so as to minimize sleep disturbance
  • Minimizing the concurrent operation of noisy activities in order to reduce excessive cumulative noise level from various activities
  • Keeping nearby residents informed of what is being planned and what is going to do so that they can understand inevitable noise impact, resulting in fewer complaints
  • Establishing a communication channel such as a manned hotline to address concerns from the affected neighbours, so that immediate responsive actions could be taken to reduce adverse noise impact
  • Switching off any equipment when not in use
  • Locating noisy equipment as far away as possible from any noise sensitive receivers
  • Using electricity supply from public utility for all machinery if possible, to avoid generator noise
  • Using stockpiles of earth as an additional natural noise barrier whenever possible
  • Using the site office as an additional noise barrier whenever possible
  • Providing regular and effective maintenance for all powered mechanical equipment in order to prolong the life of equipment as well as to reduce noise emission
  • Promoting good site practice through regular site supervision and training in order to avoid unnecessary noise disturbance created from shouting, using loudspeaker for talking, colliding of materials or striking of steel bars due to rough handling, etc.
  • Reminding the workers for loading and unloading vehicles, dismantling scaffolding or moving materials to reduce unnecessary noise generation by gentle handling of these materials
  • Avoiding carrying out noisy operation, including delivering of noisy/ bulky equipment /material, in restricted hours or early morning, to prevent the noise affecting the nearby noise sensitive receivers. Schedule such operation and delivery after 9 am as far as practicable
  • Monitoring noise on site regularly. In case of noise exceedance, further mitigation measures may be necessary
  • Maintaining a good security system especially at the site entrance to avoid unauthorized entry of workers during restricted hours
  • Setting up a restricted hours patrol team to ensure compliance with the Noise Control Ordinance
  • Requesting sub-contractors to obtain prior permission (e.g. through a permit-to-work system) before carrying out work during restricted hours
  • Seek professional advice on noise control measures. Seek professional advice on noise control measures

4. Quieter Construction Equipment or Method and Mitigation Measures

Nowadays, many new technologies or quieter working methods are commercially available. With the use of such construction equipment or methods, noise generated from the construction activities would be effectively reduced.
These methods or equipment could be broadly categorized as “Quiet Construction Equipment”, “Quieter Construction Methods” and “Mitigation Measures”, which address different types of construction works including “Concrete Removal”, “Demolition”, “Renovation”, “Road Works”, “Tunnelling and Pipe Installation” and “Foundation Works”, and are presented one by one in this website.

4.1 Constraints
In designing a construction noise mitigation measure for a particular situation, following factors should be considered:
  • the amount of noise reduction by virtue of the construction noise mitigation measure;
  • the applicability of the quiet construction equipment for specific construction works;
  • structural/foundation requirement of the construction noise mitigation measure;
  • land/space requirement of the construction noise mitigation measure;
  • any obstruction to the road and pedestrian walkway due to the construction noise mitigation measure;
  • any potential hazard as a result of the construction noise mitigation measures, e.g. obstructing or restricting emergency access, blocking drivers' view and so on;
  • the capital and maintenance cost of the construction noise mitigation measure and how they compare with other alternative measures for achieving the similar noise reduction.
It is necessary to take into accounts the constraints applicable to a particular situation when noise mitigation measures are being considered.

4.2 Construction Noise Management in Hong Kong
As in many densely populated cities around the world, noise is a significant environmental problem in Hong Kong. The small geographical size of the urban areas means that buildings are always very close to each other, and that commercial and residential premises are mixed together. So when new construction or renovations are underway, noise can be very disturbing. Since the Noise Control Ordinance came into effect in 1989, the Environmental Protection Department has worked to reduce this problem by carefully managing construction noise. The aim is to strike a balance between the needs of the construction industry and social harmony.

5. Glossary of Acoustic Terminology

Noise is any sound which at the time of reception is unwanted or disturbing.

Decibel, dB
A dimensionless unit used to express logarithmically the ratio of one sound power or pressure to a reference value.

Sound Power Level
A measure, in decibels, of the total acoustic power radiated by a given sound source. It is independent of any reference distance or other extraneous factors. (Using light as an analogy, it is equivalent to the power of a light bulb expressed in watts.)

Sound Pressure Level
A measure, in decibels, of the sound pressure at a particular point. It is dependent upon distance from the source and many other extraneous factors. (Using light as an analogy, it is equivalent to the brightness or intensity of light at a particular point).

A-weighted Decibel, dB (A)
A The A-weighted decibel is a commonly used unit for measuring environmental noise taking into account the way human ear responds to noise.

The Equivalent Continuous Noise Level, Leq
The level of a constant sound having the same sound energy as an actual time-varying sound over a given period.

The number of repetitive variations of sound pressure per unit of time. Frequency is measured in Hertz (Hz), i.e. cycles per second. It is the characteristics of a sound which influences our perception of it is as high or low in pitch.