{"id":13975,"date":"2025-08-12T10:05:27","date_gmt":"2025-08-12T08:05:27","guid":{"rendered":"https:\/\/mag.ebmpapst.com\/en\/?p=13975"},"modified":"2025-11-14T09:56:37","modified_gmt":"2025-11-14T08:56:37","slug":"tracking-down-sound-sources","status":"publish","type":"post","link":"https:\/\/mag.ebmpapst.com\/en\/industries\/refrigeration-ventilation\/tracking-down-sound-sources_13975\/","title":{"rendered":"Tracking down sound sources"},"content":{"rendered":"\n
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Contents<\/h3>\n\n\n\n
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  1. Sound sources<\/a><\/li>\n\n\n\n
  2. Inter\u00adview with Marc Schneider<\/a><\/li>\n\n\n\n
  3. Sound effects<\/a><\/li>\n<\/ol>\n<\/div><\/div>\n\n\n\n

    Tracking down sound sources in fans is not an easy task. Whereas rela\u00adtively advanced aero\u00addy\u00adnamic simu\u00adla\u00adtion programs are now avail\u00adable and have become estab\u00adlished on the market, the simu\u00adla\u00adtion of aeroa\u00adcoustics is still very much at the research stage. The number of cells needed for the required spatial reso\u00adlu\u00adtion of small turbu\u00adlent struc\u00adtures is far higher than in the case of aero\u00addy\u00adnamic simu\u00adla\u00adtion. For a fan in partic\u00adular flow situ\u00ada\u00adtions the figure may even be as high as tens or hundreds of millions.<\/p>\n\n\n\n

    Another neces\u00adsity is high time reso\u00adlu\u00adtion \u2013 typi\u00adcally with inter\u00advals of around 10 microsec\u00adonds. This demands great compu\u00adta\u00adtional resources and the asso\u00adci\u00adated time and finan\u00adcial outlay. That explains why, in the case of fans for example, only the larger turbu\u00adlent struc\u00adtures (of rele\u00advance to acoustics) are resolved. Even with these limi\u00adta\u00adtions the outlay is consid\u00ader\u00adable, and work is currently in progress to find ways of reducing the amount of compu\u00adta\u00adtional work involved. Exper\u00adi\u00admental processes are helpful in this regard.<\/p>\n\n\n\n

    Beam\u00adforming with micro\u00adphone array<\/h2>\n\n\n\n

    One example of an exper\u00adi\u00admental method of local\u00adizing sound sources on a rotating fan used by the ebm-papst motor and fan special\u00adists to supple\u00adment complex aeroa\u00adcoustic simu\u00adla\u00adtion is the so-called beam\u00adforming process. At the heart of this is a circular micro\u00adphone array (Fig. 1) with 80 micro\u00adphones arranged on two levels.<\/p>\n\n\n\n

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    Figure 1: The circular micro\u00adphone array with 80 micro\u00adphones arranged on two levels is used on the intake side of the fan test stand.<\/figcaption><\/figure>\n<\/div>\n\n\n\n
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    Figure 2: The beam\u00adforming method (above) detects the same trends as aeroa\u00adcoustic simu\u00adla\u00adtion (below). In both cases the red areas show loud noise behav\u00adiour.<\/figcaption><\/figure>\n<\/div>\n<\/div>\n<\/div><\/div>\n\n\n\n

    The micro\u00adphone array is used on the intake side of the fan test stand to measure the differ\u00adences in prop\u00ada\u00adga\u00adtion time of the sound waves to each of the micro\u00adphones. Sophis\u00adti\u00adcated algo\u00adrithms then eval\u00aduate the data obtained over a period of 30 seconds at a known fan speed. The results show that the beam\u00adforming method detects the same trends as aeroa\u00adcoustic simu\u00adla\u00adtion (Fig. 2). The exper\u00adi\u00admental find\u00adings thus also permit checking and opti\u00admiza\u00adtion of the numer\u00adical simu\u00adla\u00adtion. For local\u00adizing sound sources ebm-papst uses the so-called beam\u00adforming process as a supple\u00adment complex aeroa\u00adcustic simu\u00adla\u00adtion.<\/p>\n\n\n\n

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    Figure 3: The noise level is increased by inflow turbu\u00adlence and the tip gap flow. Inflow turbu\u00adlence is partic\u00adu\u00adlarly an issue when the fan is enclosed.<\/figcaption><\/figure>\n\n\n\n
    <\/div>\n<\/div><\/div>\n\n\n\n

    The eval\u00adu\u00ada\u00adtions reveal two domi\u00adnant sources of noise for a typical axial fan: the tip gap flow between blade and fan housing and the so-called inflow turbu\u00adlence (Fig. 3). At the tip gap, the differ\u00adence in pres\u00adsure between the outlet and intake sides causes air to flow over the fan blades at the blade tip. The flow inter\u00adacts with the edges present there, in other words the blade surface and the surrounding housing wall. Vortices form which can increase the sound level by up to 10\u2008dB on sepa\u00adra\u00adtion.<\/p>\n\n\n

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    Figure 4: Vortex strings in a box-type inflow situ\u00ada\u00adtion.<\/figcaption><\/figure>\n<\/div>\n\n\n

    Inflow turbu\u00adlence is partic\u00adu\u00adlarly an issue when the fan is enclosed. A box such as those used for heat exchangers, for example, was chosen for testing with the micro\u00adphone array. Back\u00adflow areas with corre\u00adsponding circu\u00adla\u00adtion, in other words air turbu\u00adlence, occur at the housing walls. This is then drawn towards the points at which the gap between fan and housing wall is at its narrowest. The turbu\u00adlence from both sides merges here.<\/p>\n\n\n\n

    These \u201cvortex strings\u201d then produce great turbu\u00adlence (Fig. 4). As a result, consid\u00ader\u00adable fluc\u00adtu\u00ada\u00adtions in pres\u00adsure and velocity occur at the front edge of the blade, resulting in some\u00adtimes dramatic addi\u00adtional noise in the low-frequency range in partic\u00adular. This takes the form of both broad\u00adband noise and narrow band tonal sound compo\u00adnents, also known as blade passing noise. Everyone has at some point come across the unpleasant \u201chumming\u201d noise typical of this.<\/p>\n\n\n\n

    From finding the cause to tack\u00adling the noise<\/h2>\n\n\n\n

    Once the sources of noise have been local\u00adized, action can be taken to improve the aeroa\u00adcoustics of the fans: The size of the gap between the blade tip and fan housing was found to have a consid\u00ader\u00adable influ\u00adence on noise behavior. The noise level decreases with a smaller gap, but produc\u00adtion-related neces\u00adsi\u00adties mean that the gap dimen\u00adsion cannot be reduced beyond a certain point because of the risk of the blade tip catching on the fan housing.<\/p>\n\n\n\n

    Figure 5: Winglets (red) added to the blade tips influ\u00adence the tip gap flow and the vortices that form and so signif\u00adi\u00adcantly reduce the noise level.<\/figcaption><\/figure>\n\n\n\n

    This is where winglets can help. These curved end caps added to the blade tips influ\u00adence the tip gap flow and the vortices that form and so signif\u00adi\u00adcantly reduce the noise level (Fig. 5). This has a posi\u00adtive effect on the tip gap flow, thus reducing the inter\u00adac\u00adtion of the flow with the edges. The sound power drops by up to 10 dB as a result.<\/p>\n\n\n\n

    Geometric modi\u00adfi\u00adca\u00adtions to the fan alone are however not suffi\u00adcient to reduce the inflow turbu\u00adlence, as this results from the instal\u00adla\u00adtion situ\u00ada\u00adtion. Addi\u00adtion\u00adally insu\u00adlating the housing tends not to be partic\u00adu\u00adlarly successful either, as the insu\u00adla\u00adtion panels used normally only take effect as of higher frequen\u00adcies. A different approach is more promising: Improving the inflow of air to the fan lessens turbu\u00adlence and thus also the annoying low-frequency noise it gener\u00adates. For this purpose ebm-papst has devel\u00adoped a special air inlet grill (Flow\u00adGrid) that acts like a flow straight\u00adener on the intake air. It thus dramat\u00adi\u00adcally reduces noise-gener\u00adating distur\u00adbance in the inflow and is equally effec\u00adtive with both axial and centrifugal fans (Fig. 6).<\/p>\n\n\n\n

    Figure 6: Regard\u00adless of the struc\u00adtural condi\u00adtions and the instal\u00adla\u00adtion situ\u00ada\u00adtion in the housing, fans with an air inlet grill (Flow\u00adGrid) attain noise values compa\u00adrable with those for oper\u00ada\u00adtion under labo\u00adra\u00adtory test condi\u00adtions.<\/figcaption><\/figure>\n\n\n\n

    Regard\u00adless of the struc\u00adtural condi\u00adtions and the instal\u00adla\u00adtion situ\u00ada\u00adtion in the housing, the fans then attain noise values compa\u00adrable with those for oper\u00ada\u00adtion under labo\u00adra\u00adtory test condi\u00adtions. Aeroa\u00adcoustic testing has thus proven its worth as a means of opti\u00admizing fans. It will be inter\u00adesting to see what the future holds in store. One thing is certain, energy-effi\u00adcient fans from ebm-papst will become even quieter still.<\/p>\n\n\n\n

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    Sound\u00adcheck for heat\u00adpumps: Psychoa\u00adcoustics for minimum noise gener\u00ada\u00adtion<\/h3>\n\n\n
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    <\/a><\/figure>\n<\/div>\n\n\n

    Air\/water heat\u00adpumps<\/strong> with heat outputs from 3 to 30 kW enjoy a high level of accep\u00adtance in heating tech\u00adnology. However, they do not work completely silently. It is by no means just the measur\u00adable sound pres\u00adsure level that is crucial, but human noise percep\u00adtion<\/strong>, which is where psychoa\u00adcoustics and its exam\u00adi\u00adna\u00adtion methods come into play.<\/p>\n\n\n\n

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    Read the article <\/i><\/a><\/div>\n<\/div>\n<\/div><\/div>\n\n\n\n
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    \u201cNumbers alone say nothing\u201d<\/h1>\n\n\n\n

    The level of noise emis\u00adsions is a deci\u00adsive quality factor for fans. Dr Marc Schneider, ebm-papst\u2019s Group Leader for Acoustics, explains how the company keeps its prod\u00aducts quiet<\/em><\/p>\n\n\n\n

    At what point is fan noise perceived as disturbing?<\/h3>\n\n\n\n

    That is not an easy ques\u00adtion to answer. There are of course phys\u00adical char\u00adac\u00adter\u00adis\u00adtics like the noise level that you can measure on a test bench. But numbers alone often say nothing about how the human ear perceives this kind of noise. For a subjec\u00adtive assess\u00adment, what is impor\u00adtant is how \u201craw\u201d the noise is perceived. This percep\u00adtion can happen when the signal is given a temporal struc\u00adture through a change in the frequency or ampli\u00adtude.<\/p>\n\n\n\n

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    <\/figure>\n<\/div><\/div>\n\n\n\n
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    A lot of noises also have tonal compo\u00adnents which can be extremely disturbing.<\/p>\nDr. Marc Schneider, Leader for Acoustics ebm-papst<\/cite><\/blockquote>\n<\/div>\n<\/div>\n\n\n\n

    A lot of noises also have tonal compo\u00adnents which can be extremely disturbing. This percep\u00adtion differs from person to person, which makes assess\u00adment even more compli\u00adcated. One person reacts nega\u00adtively to low-frequency noises, another winces at higher-frequency noises.<\/p>\n\n\n\n

    How do you measure this personal percep\u00adtion?<\/h3>\n\n\n\n

    At ebm-papst, we have the \u201cAudiMax\u201d, a so-called psychoa\u00adcoustic labo\u00adra\u00adtory. In this noise-insu\u00adlated facility, we have space for up to eight test listeners to whom we can play the noises of our prod\u00aducts in different config\u00adu\u00adra\u00adtions.<\/p>\n\n\n\n

    How does this method help achieve usable results?<\/h3>\n\n\n\n

    After hearing the noises, our staff ask the test subjects about how they perceived them and thus create a data\u00adbase with scien\u00adtific consid\u00ader\u00ada\u00adtions. Using this data\u00adbase, we can eval\u00aduate together with our colleagues from produc\u00adtion which measures need to be taken and which not. Our overall aim is to create a fan that is perceived as comfort\u00adable by as wide a spec\u00adtrum of test subjects as possible.<\/p>\n\n\n\n

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    Sound effects<\/h1>\n\n\n\n

    Analyzing noise to provide figures relating to physics and people.<\/em><\/p>\n\n\n\n

    A product\u2019s quality in terms of its sound emis\u00adsion is usually deter\u00admined by its dB(A) value, which refers to the sound pres\u00adsure level gener\u00adated by an acoustic source at a certain distance. Depending on the level of the alter\u00adnating pres\u00adsures from which the sound pres\u00adsure level is deter\u00admined, expo\u00adsure to noise can cause effects ranging from slight reduc\u00adtions in mental perfor\u00admance to pain and even uncon\u00adscious\u00adness.<\/p>\n\n\n

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