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The formula for psychoa­coustic quality

The quieter a heat pump is, the better. However, reducing noise is not the only factor. It also matters how pleasant the sound is. And that can now be calcu­lated.

Dr. Marc Schneider is respon­sible for Pre-Devel­op­ment Aeroa­coustics at ebm-papst. (Photo | ebm-papst)

Direc­tives, such as “Tech­nical Instruc­tions for Protec­tion against Noise 6.1” in Germany, regu­late how loud a heat pump is allowed to be. In a resi­den­tial area, for example, it can operate at 40 dB (A) at night. These are strict limits and the following applies as a basic prin­ciple: the fewer deci­bels, the less trouble is caused with the neigh­bors. But there are other factors that make all the differ­ence in the end. In stan­dard­ized exper­i­ments, psychoa­coustics inves­ti­gates how pleasant different people find a noise and trans­lates this into objec­tively measur­able values. The final value we are talking about here is the psychoa­coustic overall quality of a sound, QOA. The higher the QOA, the more pleasant the noise is to human ears. ebm-papst main­tains its own psychoa­coustics labo­ra­tory and has tested various compo­si­tions of heat pump noises on over 100 subjects.

Four para­me­ters are rele­vant for the QOA in this context: N is the loud­ness, measured in the unit sone. It indi­cates how loud a noise is for human hearing. S is the sharp­ness, measured in the unit acum. It indi­cates how many high frequen­cies a noise contains. T is the tonality, measured in the unit tuHMS. It indi­cates how disrup­tive indi­vidual tones in a noise are perceived as being. H is the Shannon entropy, measured in bits. It indi­cates how much a noise changes over time and how randomly this happens, e.g. swelling and subsiding.

Happy neigh­bors thanks to quiet heat pump

From the hearing tests, it is then possible to deter­mine the pref­er­ences and incli­na­tions of the subjects and to weight them statis­ti­cally. The statis­tical calcu­la­tions give a value to the prelim­i­nary factors k1, k2, k3 and k4. k0 is merely a quality constant, which defines the QOA so that a value of QOA = 0 corre­sponds to a volume hazardous to health.

If the psychoa­coustic quality Q (y-axis) of a noise increases by the value 2 (green compared to the red curve), the sound may also be a whole 8 dB (A) louder (x-axis), and is never­the­less perceived as equally pleasant or equally as disrup­tive. Put in a different way: the higher Q is, the more pleasant and quieter the noise is perceived by humans. (Graphic | ebm-papst)

The values obtained exper­i­men­tally and statis­ti­cally for the prelim­i­nary factors can be referred to phys­ical, i.e. objec­tively measur­able, para­me­ters – and so finally to the design of a heat pump and the fans installed in it. The usual starting points for noise reduc­tion include air duct design, turbu­lence or inflows at the fan. The tests show that, if we improve the QOA of a specific heat pump noise by the value 2 at a constant noise level, it is perceived as being as pleasant as having made it quieter by a total of 8 db (A)!

That is why a high QOA means a nicer time spent in your own garden, and a better rela­tion­ship with the neigh­bors.

Read more about heat pumps:

Sustain­able energy supply with heat pumps

ebm-papst offers effi­cient and quiet prod­ucts suit­able for indoor, outdoor or hybrid heat pump appli­ca­tions.

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