Understanding noise and ways to minimise it

As a result of the changing landscape: the encroachment of residential development into formerly exclusive commercial territory; a decrease in distance between neighbours due to an increase in high density living; combined with an increase in air conditioner sales, more and more complaints are being lodged to city councils and subsequently the local courts, reinforcing the need for manufacturers and contractors alike to make a serious commitment to meet suitable noise standards.

Noise attenuation has emerged as an increasingly important issue, when noise becomes too loud it affects our behaviour and health and raises our stress and anxiety levels.

Temperzone is often asked the question: “how noisy is this unit?” Well, it’s a bit like asking "How long is a piece of string."

To understand this question, we must first understand sound, and only then can we address sound (noise) attenuation.

Sound (noise) is a form of energy; it appears as a pressure disturbance or fluctuation that we can actually hear. Sound travels in a “wave” motion, hence the commonly used term “sound waves”.

Noise can be thought of as unwanted sound that needs to be reduced or attenuated.

Without going into the mathematical formulas for sound (noise), an air conditioner’s sound level is typically measured in sound pressure or sound power.

Sound pressure (Pressure) (Lp) is what the human ear can hear, it is an audible disturbance within the atmosphere whose intensity is influenced by the surroundings and the distance from the sound (noise) source.

Sound power (Watts) (Lw) is the acoustically generated energy measured in “Watts” emitted by the sound (noise) source. It is a measure of the source’s (air conditioner’s) acoustical energy.

The relationship between sound power and sound pressure which can be heard, and the power of the object producing them depends on the nature of the area around the object and where you are in relation to the object.

Typically, we talk about the “A” Weighted Sound Level, which is the single overall value. The ear perceives sound at different frequencies. In order to give a single overall value for the sound output of an object, a weighted sum or average can be taken of the sound level in each frequency band between the lowest and highest that can be heard. The generally accepted bands are 63Hz to 8000Hz. These are called octave bands. Where you see the dBA – this is the A weighted calculated figure or the average.

While appropriate noise design criteria, standards and legislation exist to limit the effect of noise on neighbouring residents, the current challenge is where urban redevelopment places residents within earshot of existing commercial buildings meeting previously acceptable standards.

In order to minimise noise output, there are a number of simple questions to answer when selecting an air conditioner. Three easy ones that spring to mind are:

  • Is it sized correctly for the cooling and heating load? The last thing you want is for the system to operate 24/7, emitting unwanted noise.

  • Is it the right type of system? Would it be better to use a package unit located on the roof rather than a ducted system with the outdoor unit situated near a neighbour?

  • Where should you locate the outdoor unit? Sometimes it’s better to spend a little more on the installation and locate the unit as far away as possible from the neighbours.

Many people mistakenly think that moving the condenser or outdoor unit twice the distance away will reduce the noise by 50%. But if you double the distance from a noise source the unit’s sound level is reduced by approximately 6dB – not halved – which is a real trap.

Noise emitted from an air conditioner typically comes from three sources: and there are ways and means to treat each of them.

  • Noise emitted by the outdoor unit.

  • Noise reflection in a room due to fan or air noise.

  • Noise transition or vibration through a structure like a wall.

The environment and location also have an effect on the noise of a unit. For example, if it’s an extremely hot day, the unit’s compressor will need to operate harder, subsequently making it noisier. If an air conditioner is located in a quiet residential street then greater care and precautions are needed. Likewise, lesser treatment is needed if the unit is installed near a busy roadway or industrial complex. Each application is different and should be treated on a case-by-case basis.

To cope with such conditions some form of attenuation is needed, most commonly a louvre, silencer or barrier.

Acoustic louvres offer noise reduction with minimal airflow restrictions and suit any type of opening size. Besides noise reduction, other benefits offered by louvres include easy installation, cost effectiveness, and longevity.

In some cases, a properly designed and constructed barrier can reduce an outdoor unit’s noise levels by up to 19db. Barriers are often used to attenuate the noise created by the air conditioner’s outdoor unit containing the compressor and condenser fan (or fans). A tip when using a barrier is to locate it as close as possible to the noise (sound source) or the receiver (neighbour) to maximise performance. However, care must be taken not to locate the barrier any closer than 500mm to the sound source (condenser or outdoor unit) which runs the risk of noise coupling which can only make the noise situation worse. In noise sensitive applications it is always best to employ the services of a specialist noise consultant. Better to be safe than sorry.

Another trap people fall into is placing a condenser or outdoor unit inside a well or surrounded by walls to treat the noise. “Not only will this promote system performance issues due to air recirculation but in many cases this will cause the noise (sound) to reflect off the two surfaces and make the situation worse,” he says. “This is known as sound reflection which can be thought of as the same as light reflection.” The barrier construction material and texture also plays a big part in the noise attenuation process.

Other simple tricks designed to assist in noise reduction include:

  • Building a barrier to remove the line of sight noise transmission from a unit.

  • Correct air balancing in a noise critical room.

  • The use of spring mounts on indoor and outdoor units to assist in the omission of vibration through structures.

  • Ducting of return air paths and changing the air path’s direction.

One of the most significant changes in noise attenuation has been the development of new and quieter EC condenser fans. Fans speed can be reduced to minimise the noise, this is particularly beneficial outside of traditional work hours where residential and commercial zones merge.

Temperzone has experience in noise sensitive applications and offer pod-less silencers, compressor acoustic hoods, high static condenser fans for ducted applications and condenser fan speed control to reduce noise levels when ambient temperatures are lower, and the condenser fans don’t need to be operating at 100%.

Noise and its attenuation is very closely related to the application. Get the application correct and the rest becomes easier. Temperzone sales offices are equipped to offer you advice on noise sensitive applications.

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