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Sound­check for heat­pumps: Psychoa­coustics for minimum noise gener­a­tion

Air/water heat­pumps with heat outputs from 3 to 30 kW enjoy a high level of accep­tance in heating tech­nology. However, they do not work completely silently and the more dense the build­ings are, the more the neigh­bors can get disturbed by noise pollu­tion. It is by no means just the measur­able sound pres­sure level that is crucial, but human noise percep­tion, which is where psychoa­coustics and its exam­i­na­tion methods come into play.

WHAT THE TECH?! How do heat­pumps work?

Heat pumps can heat and cool homes. They transfer heat from one place to another. You can find out exactly how this works here:

Simple, under­stand­able and with a bit of humor!

In prin­ciple, a heat pump works like a refrig­er­ator, which with­draws heat from the food stored in it and trans­fers it outside. Air/water heat pumps extract heat from the surrounding air and transfer the heat to the heating system, which warms the living areas or is used to heat water. Fans ensure the neces­sary external air flow through the device’s evap­o­rator and inevitably generate more or less noise during oper­a­tion. This also applies to the fans with Green­Tech EC motors that are partic­u­larly quiet. Where they are installed can also have a nega­tive impact on noise gener­a­tion. Noise gener­a­tion always needs to be consid­ered by those who use air/water heat pumps. As a rule, it is not usually suffi­cient to merely observe the limit values of the local stan­dards and noise protec­tion ordi­nances.

Psychoa­coustics: Why does a noise sound unpleasant?

The values defined in the guide­lines and stan­dards, which can be measured on the test stand, have little to do with indi­vidual human noise percep­tion. Until now, stan­dards and direc­tives have not adequately dealt with tonality, i.e. the rela­tions between tones. Different psychoa­coustic tests are currently looking into this. Psychoa­coustics aims to define why we perceive noise as pleasant or unpleasant. For example, trumpet music and an exca­vator at a construc­tion site have approx­i­mately the same measur­able sound power, but our psychoa­coustic assess­ment of them is completely different.

Fig. 1: Das Psychoakustik-Labor für Testhörer:innen bei ebm-papst in Mulfingen. (Photo | ebm-papst)

Motor and fan specialist ebm-papst studied this issue early on and set up a special psychoa­coustics labo­ra­tory for test subjects to demon­strate the oper­ating noise of heat pumps and the fans installed in it in various config­u­ra­tions (Fig. 1).

Mini­mize oper­ating noise of air/water heat pumps

Devel­opers ques­tion the subjects after­wards to build up a scien­tif­i­cally founded data­base, for example based on the following key psychoa­coustic para­me­ters: loud­ness [sone], sharp­ness [acum], pitch [mel], rough­ness [asper] and fluc­tu­a­tion strength [vacil]. In addi­tion, tonality and stim­ulus are also signif­i­cant vari­ables. Tonality is present when single tones are notice­able within a noise, which increases the inter­fer­ence effect. Noises that contain rapid changes in level, e.g. knocking or ramming noise, are iden­ti­fied with impul­sive­ness.

The results from the psychoa­coustics labo­ra­tory are incor­po­rated into fan devel­op­ment.

Both impul­sive­ness and tonality can be measured with micro­phones and compared with the test subjects’ state­ments. Assess­ments by the test subjects are analyzed using statis­tical and psycho­log­ical methods. The results are incor­po­rated into in-house fan devel­op­ment, but also provide infor­ma­tion about the tested air/water heat pumps and which fans are best suited to the indi­vidual instal­la­tion scenario. Ulti­mately, the aim is to ensure that the oper­ating noise of a high-quality heat pump that is already very low is perceived as pleasant by the widest possible group of test subjects.

Metric for psychoa­coustic eval­u­a­tion

Research on psychoa­coustics is in full swing and we can look forward to more results. As part of a doctoral thesis carried out at ebm-papst, for example, it has already been possible to develop a metric, which is now used for psychoa­coustic eval­u­a­tion in the end device.

ebm-papst acoustic quality model

ebm-papst also exam­ines fans in instal­la­tion scenarios from a psychoa­coustic point of view and has devel­oped its own metric for acoustic quality. For this purpose, subjec­tive dimen­sions of percep­tion with measur­able psychoa­coustic vari­ables (objec­tive dimen­sions of percep­tion) are brought into play. The closer the corre­la­tion coef­fi­cient r is to 1, the more suited the model to depicting the subjec­tive aspects of percep­tion through objec­tive (measur­able) ones. The acoustic quality Q, which then repre­sents the fan’s perceived noise, should be as high as possible. (Grafic | ebm-papst)

The aim of this metric is to create a corre­la­tion between the subjec­tive perceived noise quality, which presents itself in different “dimen­sions”, and objec­tively measur­able vari­ables. During the exten­sive hearing tests carried out, different dimen­sions of percep­tion were compared to one another and to phys­ical measured values. A total of 123 test subjects between the ages of 19 and 60 were played 89 sounds in three series of exper­i­ments in real­istic condi­tions.

Fig. 2: In this measuring device, noises are recorded via a variety of micro­phones and then played to the test subjects for eval­u­a­tion. (Photo | ebm-papst)

For each series of exper­i­ments, 30 to 40 of these test subjects assessed the noise recorded (Fig. 2) in terms of perfor­mance (slow/strong, high-qual­i­ty/low-quality), sound level (hissing or whooshing), time struc­ture (fluc­tu­ating), quality (pleasant/disruptive) and pitch (humming, dark/light). In addi­tion, different noises were directly compared and noises were checked against one prop­erty for a rating scale.

Quieter heat pumps thanks to psychoa­coustics

Addi­tional inter­views from the test subjects revealed that the noise char­ac­ter­is­tics dark, deep, slow, monotone, consis­tent, soft and light were consid­ered pleasant. On the contrary, rattling, flut­tering, clashing, drip­ping, humming, changing, high, bright and hissing prop­er­ties were consid­ered unpleasant. Overall, it was quite accept­able for the test subjects that oper­ating noise was gener­ated, but they must be perceived as pleasant. Finally, however, the eval­u­a­tion of the hearing tests showed above all that the “distur­bance level” can be largely combined with objec­tive psychoa­coustic para­me­ters.

Fig. 3: ebm-papst fans are not only ideal for use in heat pumps with flam­mable refrig­er­ants. They are also aero­dy­nam­i­cally and acousti­cally opti­mized. (Photo | ebm-papst)

Based on this, two follow-up objec­tives can now be defined: In future, the aim is to also use psychoa­coustics in addi­tion to the phys­ical para­me­ters for assessing noise in fans (Fig. 3). The aim is also to work towards the intro­duc­tion of an inter­na­tional stan­dard based on stan­dard­ized psychoa­coustic vari­ables. This would then be an impor­tant prereq­ui­site for ensuring that air/water heat pumps with as pleasant an oper­ating noise as possible help to avoid any nuisance in the neigh­bor­hood due to noise pollu­tion.

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