Decidamp CLD Conquers Urban Condenser Noise

The challenges

Residents of a newly developed apartment block made complaints about excessive noise emitting from the rooftop. Resonate Acoustics were commissioned by the body corporate to do a noise study and report on the findings. The NSW Industrial Noise Policy provides guidance on setting criteria for industrial noise sources based upon the measured continuous level (known as LAeq). Noise levels have been established in order to determine the potential for intrusive, industrial noise sources within the community.

When conducting a site inspection and testing, Resonate Acoustics discovered that the condenser units mounted on top of the apartment block were generating tonal noise. Often the sound energy from a noise source is spread over a wide range of frequencies, but sometimes a noise source will emit noise that is concentrated in a narrow part of the spectrum, or contain a high proportion of energy at a single frequency (a pure tone). This is referred to as tonal noise.

Sources that can cause tonal noise include fans, compressors, motors and transformers - most having moving parts that rotate or vibrate at a given, audible frequency. Tonal noise is generally more noticeable and can be perceived as more annoying than non-tonal noise at the same level. After a noise survey was conducted on the condenser units, it was found that the condensers had a tonal frequency at 200 Hz, which generated 70 dB(A).

The solutions

Once the source and radiation mechanism was identified, Resonate Acoustics recommended their client use a vibration damping product to reduce the level of the tonal noise.
Resonate Acoustics selected Pyrotek’s Decidamp® CLD, a constrained layer damping product as the solution.
Decidamp® CLD relies on the viscoelastic properties of the inner layer constrained by a rigid metal layer, reducing structural vibration and sound transmission within lightweight materials.

The results

The condenser units emitted a tonal noise during operation, illustrated on the graph, peaking just above 70 dB(A) at
200 Hz. After applying Decidamp® CLD, the tonal noise at a frequency of 200 Hz was reduced to just below 50 dB(A).
The tonal noise at 200 Hz was effectively eliminated, as a result of Decidamp® CLD damping the vibration in the panelling of the condensers. The residents can now enjoy the improved peace and quiet.

We consider that if all units were treated in the same manner, there would be a noticeable subjective change.