Heat Pump Noise Enclosures - NoiseTrap® Blox Application

In climates with moderate cooling and heating requirements, heat pumps provide a greener alternative to air conditioners and boilers. 

An air source heat pump absorbs heat from the outside air and uses it to provide heat in your home; including radiators, the provision of hot water, underfloor heating systems or warm air convectors. It can also meet your home’s air conditioning needs. 

Heat Pumps are more environmentally friendly compared with traditional heating systems, as the heat it extracts is renewed naturally – hence the only impact on the environment is the electricity it uses.

Heat pumps continue to represent a small share of total residential heating equipment, as more than three-quarters of sales globally were for fossil fuel or conventional electric technologies in 2018.

At the same time, global heat pumps sales did rise by nearly 10% between 2017 and 2018 – double the 2016‑17 growth rate.

Nearly 80% of new household heat pump installations in 2017 were in China, Japan and the United States, which together account for around 35% of global final energy demand for space and water heating in residential buildings.

Europe’s market is expanding quickly, driven by a global trend towards urbanization – By 2030, 60% of the worlds population will live in cities.

HVAC equipment has become one of the most common noise complaints, affecting people all over the world. Higher density building increases these noise complaints dramatically.

This results in demand for noise attenuation due to excessive noise level generated by the units and close proximity of neighbours

where does the noise come from?

Heat pumps produce noise while working due to their components. The noise level might be quite high, and this factor usually affects the decision when buying such devices. It is important to have an overview of how the system works and what the common noise sources are.

Heat pumps generally produce noise levels of 50 dB(A) with larger units reaching up to 70dB(A), with the main sources of noise from a heat pump are the compressor and fan.

As a result of the rotating components during a heat pumps operation, they can produce low frequency noise - a constant hum which, can be the most disturbing aspect of the noise.

Regulations and government provisions

Some limits and regulations in order to control the noise level have been established. Guidance from the UK Government suggests that the noise should not be higher than 45 dB when being 1 meter away from the window of a neighbouring residential property. This can be a cause for concern with installing a heat pump.

Heat pump units are typically installed close to properties and windows, creating the potential for noise complaints.

Noise legislation across many territories have penalty factors that can be applied in certain measurement cases. Noise that contains audible tones can have a correction of up to 6 dB applied and when noise is intermittent and noticeable above the background level a penalty of 3 dB can be applied. A good example of where these penalties are applied are where the switching off and on of air conditioning system heat pump unit occurs.

Multiple heat pump units are often installed close together and on reflective surfaces, which increase noise levels.

In a lot of applications, multiple units are installed in close proximity to each other, so it is important to be aware that the noise levels add up. E.g. 50 dB(A) + 50 dB(A) + 50 dB(A) = 55 dB(A).

The installation location of a heat pump unit is also crucial, with distance to any neighbours maximised, and ensuring multiple reflective surfaces are avoided as this serves to increase noise levels further. If a unit is mounted onto hard ground the noise also travels further.

Heat pump NOISETRAP® BLOX enclosure - the challenge for Sonobex

We have been developing a ground breaking new product, based upon our proprietary acoustic metamaterial principles - NoiseTrap® Blox.

It is to be used for such applications as heat pumps, aircon units and chiller units’ that has the following properties.

  • Market leading low frequency performance where absorptive louvres are weak.
  • As much open area / airflow to equivalent louvre performance.
  • Modular system that can be assembled by hand with no mechanical lifting.
  • Lightweight module that can be transported by elevator to rooftop AC units.

We utilised state of the art finite element software to optimise the acoustic and airflow performance of the NoiseTrap® Blox and underwent a rapid prototyping phase through the use of our in-house FDM 3D printer facilities.

The acoustic performance of the NoiseTrap® Blox solution was benchmarked in our in-house acoustics facilities, in accordance with BS EN ISO 10140-2:2010.

The Merford “Silence Factory”. Our in-house reverberation chambers and horizonal transmission loss suite.

Following successful acoustic testing of the individual NoiseTrap® Blox units, we then proceeded to design dedicated noise enclosures for heat pump units, using the modular capability. As part of the testing and validation process, our NoiseTrap® Blox Enclosures for heat pumps were then tested in accordance with the following standards:

  • BS EN ISO 11546-1:2009 – Acoustics – Determination of sound insulation performances of enclosures. Measurements under laboratory conditions (for declaration purposes)
  • BS EN ISO 14511-3:2018 – Air conditioners, liquid chilling packages and heat pumps for space heating and cooling and process chillers, with electrically driven compressors – Test methods
  • BS EN ISO 12102-1:2017 – Air conditioners, liquid chilling packages, heat pumps, process chillers and dehumidifiers with electrically driven compressors – Determination of the sound power level. Air conditioners, liquid chilling packages, heat pumps for space heating and cooling, dehumidifiers and process chillers

This enabled Sonobex to deliver a solution that surpassed all our technical and commercial expectations.


NoiseTrap® Blox is a modular louvre system that has been designed to provide exceptional low frequency acoustic performance. Nearly all industrial machine enclosures require ventilation, either for cooling of the asset or to operate and this is especially the case for heat pumps. Providing adequate airflow whilst still reducing noise is a big challenge for acoustic engineers, and we offer the ideal solution that caters for both problems.

Modular NoiseTrap® Blox unit (left) and a NoiseTrap® Blox louvre constructed from multiple units (right).

NoiseTrap® Blox use a coupled resonator principle to achieve broad band low frequency performance. The addition of a thin weather resistant foam layer enhances the mid to high frequency performance. The open surface area has been chosen to allow equivalent airflow relative to a standard 300mm deep chevron style louvre.

Each NoiseTrap® Blox is a modular unit that can be assembled together to form flexible sized louvre sections. The lightweight design is easily installed by one person and is suitable for use on rooftops (chiller units for example), where weight is a crucial factor and access can be difficult.

An enclosure for a heat pump with NoiseTrap® Blox integrated into the design.

Key features include:

  • Low frequency performance – Transmission Loss @ 100Hz = 20.4 dB
  • Acoustic Metamaterial based design
  • Modular units allow for enclosures to be constructed
  • Large amounts of passive ventilation
  • Negligible effect on machine efficiency

Our solution for heat pumps addresses airflow requirements, noise planning restrictions and noise level requirements all in one technology.

If you’re planning to install an air source heat pump, Sonobex can effectively combat the noise to protect neighbouring properties using our innovative range of noise control solutions.