In physics, the pitch of a sound is expressed in terms of frequency, measured in hertz (Hz). The human hearing range is approximately between 20 (very low hum) and 20,000 (very high whistle) hertz. Pitches below or above this range are inaudible to us. In addition to pitch, the volume of a sound is also crucial to our perception. Volume is physically referred to as sound pressure level and measured in decibels (dB). Since the human ear perceives volumes very differently, there is an A-weighting of the sound pressure level. The A-weighting is specially adapted to the human hearing spectrum, which is why most values indicated on devices are followed by the designation dB(A).

The low-frequency noise range includes all sounds below 100 hertz, regardless of their volume. Since our hearing does not function as well in this range, it is more difficult for us to distinguish low-frequency noises by pitch, volume, and the direction from which they come. If a sound is lower than 20 hertz, it is referred to as infrasound, which we cannot hear, but can sometimes perceive as vibration or pulsation, for example. Other living creatures (e.g., elephants and whales) communicate in the infrasound range. 

Low-frequency noises are usually caused by mechanically moving sources, such as the compressor of a heat pump or the pump of a refrigerator. In recent years, the sources of low-frequency noise in residential environments have increased. Low-frequency noises are less attenuated with distance from the source than higher-frequency noises. In addition, standard exterior components such as windows or walls provide poorer sound insulation against low-frequency noise. This is why, for example, the bass notes of music coming from a neighboring room can be heard much more clearly than the vocals.

These noises are often more annoying because they are perceived as particularly threatening. In addition, low-frequency noises usually cannot be avoided because they occur throughout the entire living area. Due to the specific perception of low-frequency noise and the associated reduced sensitivity to pitch, low-frequency noise from different sources can subjectively sound the same, overlap, and thus amplify each other. The bottom line is that low-frequency noise in the living environment is difficult to locate and is perceived as unpleasant.

The Federal Immission Control Act (BImSchG) regulates the handling of harmful environmental impacts caused by immissions. The associated Technical Instructions on Noise Protection (TA Lärm⁠) enable the assessment of plant noise from a frequency of 90 Hz. For the "measurement and evaluation of low-frequency noise emissions in the neighborhood," the TA Lärm therefore refers to DIN standard 45680 and the associated supplement 1. The BImSchG divides the plants falling within its scope into plants requiring approval and plants not requiring approval. Stationary building services equipment such as air heat pumps or air conditioning and cooling units are systems that do not require approval within the meaning of the BImSchG. It is precisely these systems that are often the source of low-frequency noise in residential areas.

The noise emissions of some low-frequency noise sources that do not require approval (air heat pumps, room air conditioners, and comfort fans) are limited by law. In the European Union, many products are also required to be labeled with their noise emissions. This can be found, for example, on the product's EU energy label. This allows consumers to directly influence the noise impact of a device when purchasing it.

Low-frequency noise is a recurring topic in neighborhood disputes. To ensure that low-frequency noise pollution is kept to a minimum after installing a new air source heat pump or air conditioner, the following should be taken into account:

• If possible, visit a reference system under comparable acoustic conditions before purchasing.
• Agree on guaranteed sound emission values in the low-frequency range with the manufacturer/supplier in the purchase contract.
• Inform your neighbors about the planned installation of the device.
• Install the device in a suitable location, e.g., not directly in front of a sound-reflective wall.
• If necessary, install the device with sound insulation.
• Regularly check the device for unusual noises and perform maintenance immediately if necessary.

We initially associate the term "sound" with noises that we can hear. The infrasound range lies below this, at 0 to 20 hertz. It is lower than the lowest audible tone—at least when it comes to infrasound in everyday life. At very high sound levels, infrasound is also perceptible to humans because the fluctuations in air pressure are very strong. However, it is more noticeable as a vibration than as a sound. In everyday life, infrasound can be perceived, for example, when driving fast with the windows half open. Complaints about infrasound are mainly known in connection with the installation of wind turbines. Further information on this topic can be found in our list of links.