15 Reduction of Sound Produced by Air Source Heat Pumps

Introduction

Air source heat pumps (ASHP) are becoming a more commonly used to both heat and cool buildings. In Germany the use of ASHP has increased by 61% from 2021 to 2025 [1]. Although ASHP have the potential to dramatically reduce green house emissions that result from building conditioning up to 96% in Germany ASHP are noise polluters [1]. This case study will examine how perceived sound emitted from ASHP can be reduced.

Rationale for studying the case

Motivation

Noise produced from mechanical systems including ASHP can be loud, repetitive, constant, and irritating. The use of air ASHP is becoming more common as a means of both heating and cooling including residential areas. The sound produced by ASHP is more easily perceived in otherwise quiet environments including residential areas. Finding a means of mitigating sound produced by ASHP especially in areas with lower noise levels to begin with would make for more pleasant built and natural environments.

Purpose

The goal is to determine in how to reduce the amount of perceived sound produced by ASHP for the wellbeing of those in environments with ASHP. There are several ways to reduce the noise emitted from ASHP including optimizing the machine itself, the placement of the machine, and how the sound of the machine is shielded.

Case Study Facts

The three cause of sound created by ASHP are the fan, the compressor, and the vibration of the machine. Generally the sound made by ASHP are tonal meaning that they produce a narrow band of frequencies. Tonal sounds tend to be more easily perceived especially when there are not other sound in the environment in which the ASHP is placed. When an ASHP is operating the fan and any sound created by vibration will be constant except for when the machine turns on and off. At the time of turning on and off there will be variability in both frequency and amplitude of the sound created by the machine. However the compressor makes noise intermittently through out operation as it releases air from the machine.

Data Collection

In an interview with Erik Miller-Klein an acoustic engineer and founder of Tenor Engineering Group these insights were gained about ASHPs.

The first step in choosing an ASHP is to know the conditions of the site. Two major factors must be known about the site. One is the points at which people are most likely to hear the ASHP. The second is the qualities of the existing sound of the site. The reason the existing sound conditions of the site must be known is because they affect how the sound of an ASHP will be heard.

The next step in reducing the sound perceived by an ASHP in a project is to select the particular unit or units to be used and the location of those units. These two decisions are often made in tandem because the size or sound of the machine may rule out certain locations while limited choice in location may rule out certain machines.

The selection of an ASHP depends on several factors including the dimensions of the machine, the energy efficiency, and sound emitted. The qualities of the sound emitted must be examined. There is the amplitude of the sound, the frequencies, and then the rate and degree to which the amplitude and frequencies may change.

ASHP that have lower amplitude sound, emit a broader range of frequencies, and change in both amplitude and frequency slowly are perceived less. The further away from any point in which an ASHP may be heard the less it will be perceived.

The once machine and location is selected, placing shielding around the machine is the next step to reducing perceived sound. The placement of the shielding is determined relative to where the points at which the ASHP is most likely to be heard. A material that is solid and one pound per square foot is at the point of dimension effectiveness at blocking sound. Wood, metal, concrete, brick, or any other material that meets the qualifications will have similar effectiveness in blocking sound.

ASHP will continue to become more prevalent in the future due to their energy efficiency. There is no way to completely eliminate the perceived sound of ASHP in most instances and thus there will be more noise pollution in the future. There

Manufacturers often do not have information about the sound produced by their machines. Manufacturers may lack information about amplitude, frequency, rate of change of those two qualities and how these qualities change with different temperature conditions.

Another issue is the lack of understanding by those that install ASHP. Designers must be clear on both the orientation of the machine as well as the exact quality and quantity of vibration dampening material that the machine rests on.

Patterns/Theories

Placement of ASHP has the most affect on the sound perceived. Use of existing obstructions between the ASHP and the points at which the sound my be heard at the farthest distance from those points will most dramatically reduced perceived sound. Putting up shields that target the points where sound is most likely to be perceived is the next most effective tool at reducing perceived sound.

Connection to larger scheme of things

The modern world is filled with unpleasant sound created by machines that are suppose to make our lives more pleasant to live. Society has yet to tackle the noise that counter acts the conveniences that we may be gaining. ASHP are a rising cause of unwanted sound and being able to reduce the perceived sound of these machines will contribute to a quieter environment for people to live in.

References

1

Noise Immissions by Air Source Heat Pumps: A Case Study in Germany

https://www.sciencedirect.com/science/article/pii/S0360132325005189

2

Heat exchanger design aspects related to noise in heat pump applications

https://www.sciencedirect.com/science/article/pii/S1359431115011047

3

The noise spectral characteristics and noise reduction schemes of screw air-source heat pump: A case study

https://link.springer.com/article/10.1007/s12273-023-1085-2

4

Personal Interview with Erik Miller-Klein of Tenor Engineering Group