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.

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.

Characteristics of Sound and the Decibel Scale

There are two important characteristics of sound or noise - frequency and loudness.

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.

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)

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.

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.

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).

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

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.

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.

Hand-held breakers should be:

Mufflers can reduce exhaust noise and body radiated noise by up to 15dB (A) and 6 dB (A) respectively.

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.

Equipment with Internal Combustion Engines include stationary and mobile plant, examples include:

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:

It is essential to regularly maintain and service all mechanical plant and equipment:

Typical noise sources for percussive piling include:

Control measures for piling include:

Noise mitigation measures that can be fitted to a joint cutter and stone saw include:

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.

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:

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.

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.

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.

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:

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.

In designing a construction noise mitigation measure for a particular situation, following factors should be considered:

It is necessary to take into accounts the constraints applicable to a particular situation when noise mitigation measures are being considered.

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.

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

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

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.)

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

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.