A balancing act for heat pumps: the noise level
In the sound chamber of the Bosch Home Comfort Group
Noise level: A balancing act between cost, acoustics and size
The noise level of heat pumps is regulated by law in almost all European countries. In Germany, outdoor installation is regulated by “TA Lärm” (Technical Instructions on Noise) issued by the Federal Environment Ministry to protect the general public and neighbors from noise pollution. Noise levels are measured in dB(A). This unit of measurement of the acoustic pressure takes into account the human perception of noise. For example, a maximum of 35 dB(A) is permissible at night in spa areas, while as much as 70 dB(A) is permitted in industrial areas. These regulations and people’s individual perception of noise present the team led by Alexander Merzkirch at the Bosch Home Comfort Group’s heat pump development center with a wide range of tasks – even though the team could already achieve the inaudible today: “It is possible to build a heat pump that is virtually inaudible – but it would be very large and heavy. The noise level must be in proportion to the cost of production, size and weight. So we always try to find the best possible compromise. The current units from Buderus and Bosch meet this compromise in the best possible way,” explains Merzkirch.
Silencing the compressor and fan
To comply with laws and avoid neighborhood disputes over noisy heat pumps, 19 microphones are placed in a semicircle around the heat pump in the test lab. The team is particularly interested in two components: “The compressor and the fan are the main noise sources. To influence the sound of the fan, for example, you can try to install as few resistors as possible in the air path, which reduces the effort the fan has to overcome. As far as the compressor is concerned, it makes sense to keep its vibrations away from the rest of the structure of the appliance, i.e. to decouple it as much as possible,” Merzkirch says to explain the measurement setup.
The optimized sound bath in the front yard
But the developers in Wernau are not only interested in the sound of the heat pump hardware. With the help of test persons, they are investigating how physical and psychoacoustic parameters influence the acceptance of heat pump noise and its perception as annoying. “Sound level is not the only parameter for evaluating the acoustics of a device, but also the pleasantness of a sound. Take a tonal or rhythmic sound, for example. This is more “noticeable” to most people and is therefore perceived as being more annoying than, say, a constant whirring sound. We optimize our heat pumps so that their sound remains in the background and is not even noticed in the first place.”
Out of sight, out of mind...
A noise level is also a matter of perspective. For heat pumps, this means that larger units or those with visible fans are perceived as louder, even if they operate more quietly. Scientifically speaking, visual stimuli can increase the subjective perception of annoyance, regardless of the actual noise emissions. Alexander Merzkirch explains what this means in practice: “We have optimized the appearance of the front cover of our new generation of heat pumps in terms of noise perception: The cover has a very good flow geometry, which means that the air flows through the front grille with very little resistance, and the grille also hides the fan. This means that no rotating part is visible. This is also perceived by the customer – or it isn't. They don’t even ‘notice’ such a device. Even here in the laboratory, we have already been asked if we could switch on the heat pump after all, even though it had already been running for some time.”
