5 Noise emissions from heat pumps

Every source of noise, be it a heat pump, a car or an airplane, emits a certain amount of sound. The air around the noise source is set in vibrations and the pressure spreads in waves. When it reaches the human ear, this pressure wave causes the eardrum to vibrate, which then triggers the process of hearing.

The sound field sizes are used to describe this so-called airborne sound. Two of them are sound pressure and sound power. The sound power is a theoretical quantity that is typical of a sound source. It can be calculated from measurements. The sound power is the total sound energy radiation in all directions. Sound pressure is understood to be the change in air pressure as a result of the air caused to vibrate by the sound source. The greater the change in air pressure, the louder the noise is perceived. The sound pressure is what is perceived at the ear of a listener or the microphone of a measuring device.

Physically, sound is the propagation of pressure and density fluctuations in a gas, a liquid or a solid. Sound is generally perceived by humans in the form of airborne sound as noise, tone or bang. Pressure changes in a range of 2 * 10^-5 Pa to 20 Pa can be detected by the human ear. These changes in pressure correspond to vibrations with frequencies of 20 Hz to 20 kHz and represent the audible sound or the audible range of the human being. The individual tones result from the frequencies. Frequencies above the audible range are referred to as ultrasound, and frequencies below that as infrasound.

The sound radiation from noise or sound sources is specified or measured as a level in decibels (dB). This is a reference value, with the value 0 dB roughly representing the hearing limit. Doubling the level, e.g. by using a second sound source with the same sound radiation, corresponds to an increase of +3 dB. For the average human hearing, an increase of +10 dB is necessary so that a noise is perceived as twice as loud.

The sound propagation can be divided into two types.

Structure-borne noise
Mechanical vibrations are introduced into bodies such as machines and parts of buildings as well as liquids, are transmitted in them and finally partly emitted as airborne sound elsewhere.
Airborne sound
Sound sources (bodies excited to vibrate) generate mechanical vibrations in the air that spread like waves and are perceived by the human ear.

5.1 Sound pressure level and sound power level

The terms sound pressure and sound power level are often confused and incorrectly compared with one another. In acoustics, sound pressure is understood to be the measurable level that is caused by a sound source at a certain distance. The closer you are to the sound source, the greater the measured sound pressure level and vice versa. The sound pressure level is thus a measurable, distance- and direction-dependent variable that is decisive for compliance with the immission-related requirements according to TA-Lärm, for example.

The entire change in air pressure emitted in all directions by a sound source is referred to as the sound power or the sound power level. With increasing distance from the sound source, the sound power is distributed over an ever larger area. If you consider the total, radiated sound power and relate it to the enveloping surface at a certain distance, the value always remains the same. Since the sound power emitted in all directions cannot be precisely measured, the sound power must be calculated from the measured sound pressure at a certain distance. The sound power level is therefore a sound source-specific, distance and direction-independent variable that can only be determined by calculation. Based on the emitted sound power level, sound sources can be compared with one another.

5.1.1 Emission and Immission

The entire sound emitted by a sound source (sound event) is referred to as sound emission. Emissions from sound sources are usually specified as sound power levels. The effect of sound on a specific location is called sound immissions. Noise immissions can be measured as the sound pressure level. Fig.5.1 graphically shows the relationship between emissions and immissions.

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Fig. 5.1: Emission and immission

Noise immissions are measured in dB (A), these are sound level values that are related to the sensitivity of the human hearing. Noise is the term used to describe sound that can disturb, endanger, significantly disadvantage or annoy neighbors or third parties. Guide values for noise at immission locations outside of buildings are specified in DIN 18005 "Noise protection in urban development" or in the "Technical Instructions for Protection against Noise" (TA Lärm). The requirements according to TA-Lärm are listed in Table 5.1.

Table 5.1: Limit values for noise immissions in dB (A) according to DIN 18005 and TA-Lärm

Tab.5.2: Typical sound levels

5.1.2 Sound propagation

As already described, the sound power is distributed over a larger area with increasing distance, so that the sound pressure level is reduced as the distance increases. Furthermore, the value of the sound pressure level at a certain point depends on the propagation of the sound.

The following properties of the environment have a decisive influence on the propagation of sound:

• Shading by massive obstacles such as buildings, walls or terrain formations

• Reflections on reverberant surfaces such as plastered and glass facades of buildings or the asphalt and stone surfaces of floors

• Reduction of the level spread through sound-absorbing surfaces, such as freshly fallen snow, bark mulch or the like

• Increase or decrease through humidity and air temperature or through the respective direction of the wind

Calculation of the sound pressure level The sound pressure level of the heat pump at the receiving location can be determined using the following formula: Formula:

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with:

• L._Aeq = Sound pressure level at the receiving location

• L._WAeq = Sound power level at the sound source

• Q = guideline factor

• r = distance between receiver and sound source

The guideline factor Q depends on the installation of the heat pumps. There are three different variants:

Fig.5.2: Free installation of a heat pump (Q = 2)

Fig.5.3: Heat pump or air inlet or outlet (for indoor installation) on a wall (Q = 4)

Fig.5.4: Heat pump or air inlet or outlet (for indoor installation) on a house wall with a reentrant corner (Q = 8)

For each of these set-up variants, there is a different decrease in the sound pressure level the further you are away from the heat pump.
Example: Sound power level LA 9S-TU: 5360 dB (A) The following diagram shows the decrease in the sound pressure level for the three different installation variants for an air / water heat pump LA 9S-TU.

Fig. 5.5: Sound pressure level decrease with different installation

5.2 Sound propagation from heat pumps

5.2.1 Indoor installation

Like any boiler, a heat pump should be connected using separating fittings. For the connections between the heat pump and the heating flow and return, it is advisable to use pressure, temperature and aging-resistant, elastic hoses to avoid the transmission of vibrations. Most heat pumps also have a vibration-decoupled compressor base plate. This means that the compressor is mounted on a separate base plate that is placed on rubber buffers to decouple structure-borne noise. Furthermore, the heat pump should be installed on the SYL 250 sylomer strips, which are available as a special accessory, to further reduce the transmission of structure-borne noise.

Especially with indoor air / water heat pumps, the use of air ducts and bends available as accessories leads to a reduction in noise emissions at the air intake and outlet. The inside insulation made of mineral wool and laminated glass fiber fleece not only prevents condensation, but also significantly reduces the sound radiation at the weather protection grille (air intake and exhaust) of the air duct. As a guideline, the following apply:
Straight air duct
A sound reduction of ~ 1 dB (A) per meter of air duct.
Air duct arch
A sound reduction of ~ 2 to 3 dB (A) per arch.

5.2.2 Outdoor installation

Structure-borne noise decoupling is only necessary if the foundation of the heat pump is in direct contact with the building. Flexible hoses make it easier to connect the heat pump to the heating system and at the same time prevent possible transmission of vibrations.
In addition, most heat pumps installed outside also have a vibration-decoupled compressor base plate, as already described for the units installed inside. When installing heat pumps outdoors, the sound propagation must be taken into account. It should be avoided that the sound emissions are reflected on walls.
Blowing directly onto house walls etc. should also be avoided, as this can lead to an increase in the sound pressure level. The propagation of sound can be reduced by structural obstacles. The outlet side should be aligned towards the street if possible.

Fig. 5.6: Integration example of a heat pump for outdoor installation

Vibration decoupling through compensators
All Dimplex heat pumps are internally decoupled from structure-borne noise. However, if further structure-borne noise decoupling is desired or necessary on site, this can be implemented as follows. Double bellows rubber expansion joints are used to decouple the heat pump and heating system. The expansion joints absorb vibrations and movements caused by circulation pumps, compressors, fittings, etc. Furthermore, they reduce noise and compensate for tensions (axial and lateral differences) from assembly inaccuracies.

Fig. 5.7: Integration option for compensators Exchange heat pump image

In order to ensure the functionality of the expansion joints and not to shorten their service life due to additional stress, some rules must be observed:

• Compensators must be installed in such a way that their position and movement are not hindered.

• During assembly and after installation, make sure that no offsets and twisting (torsion) are transferred to the bellows.

• Protect the bellows from damage caused by external mechanical, thermal or chemical influences.

• Bellows shafts must be free from contamination.

Noise emissions from air / water heat pumps installed outside
Fig. 5.8 shows the four main directions of sound propagation. The suction side has the direction number "1", the discharge side the number "3".

Fig.5.8:Sound directions for air / water heat pumps of the LA ... S-TU (R) series installed outdoors

With the help of the tables, the directed sound pressure level of the air / water heat pumps can be read off. The values at a distance of 1 m are actually measured values. The values at a distance of 5 and 10 m are calculated using a hemispherical spread in the free field. In practice, deviations caused by sound reflection or sound absorption due to local conditions are possible. As can be seen from the values in the table, an air / water heat pump has the highest noise emissions in the discharge direction, followed by the suction side. Significantly lower emission levels occur on the sides.

Tab.5.3: Sound propagation LA 22TBS Air / water heat pump installed outdoors (approx. 22kW)

If emissions from heat pumps are to be determined or calculated in advance, the use of the BWP sound calculator has established itself in the industry. Experience shows that the results are accepted by all authorities.

5.3 Example for a sound calculator

LA12S-TU at a distance of 8m in a general residential area with installation close to the wall (<3m)

Calculation results with explanations:

Fig.5.9: Sound calculation according to BWP sound calculator

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