© Strojni{ki vestnik 45(1999)7/8,287-293 © Journal of Mechanical Engineering 45(1999)7/8,287-293 ISSN 0039-2480 ISSN 0039-2480 UDK 648.23:534.83 UDC 648.23:534.83 Strokovni ~lanek (1.04) Speciality paper (1.04) Zmanj{anje hrupa pralnih strojev z uporabo du{ilnih materialov The Reduction of Washing Machine Noise by the Use of Muffling Materials Nikola Hole~ek V članku je najprej prikazan pregled modeliranja vibracij pralnikov Gorenje. Sledi prikaz eksperimentalne metode določanja zvočne moči in potek zniževanja hrupa pralnika z uporabo dusilnih materialov. © 1999 Strojniški vestnik. Vse pravice pridržane. (Ključne besede: stroji pralni, modeliranje vibracij, hrupnost strojev, materiali dušilni) This paper begins with an overview of the modelling of the dynamical behaviour of Gorenje washing machines. Subsequently there is a presentation of the experimental method of defining sound intensity and the procedure for the reduction of washing machine noise by the use of muffling materials. © 1999 Journal of Mechanical Engineering. All rights reserved. (Keywords: washing machine, vibration modells, machine noise, muffling materials) 0 UVOD Razvoj velikih gospodinjskih aparatov se v zadnjih letih vedno bolj usmerja k zmanjševanju hrupnosti strojev. Od leta 1996 je v veljavi priporočilo evropskega združenja 92/ 75/EWG, ki predpisuje razglasitev emisijske ravni hrupa pralnih strojev in informiranje porabnikov z ustrezno nalepko, ki vsebuje podatek o ravni zvočne moči pri pranju in ožemanju. Svetovni trg terja mirnejše, tišje in varčnejše pralne stroje z večjo vrtilno frekvenco bobna pri ožemanju. Dinamične obremenitve se večajo s kvadratom vrtilne frekvence, kar se kaže v manjši mirnosti teka pralnika in v višji ravni hrupnosti. Sedanja priporočila za raven zvočne moči predpisujejo celotno raven manjšo od 65 dB pri ožemanju, ne glede na vrtilno frekvenco ožemanja. Metode za merjenje zvočne moči so standardizirane (ISO 3745), prav tako je predpisan način delovanja naprave, ki jo merimo (IEC 704-2-4 za pralne stroje). Poudariti je treba, da se mencanje in centrifuga glede problematike zmanjšanja hrupnosti povsem izključujeta. Medtem ko na 0 INTRODUCTION In recent years one of the trends in the development of large household appliances is the reduction of noise. In 1996 the Directive of European Community 92/75/EWG came into force, according to which the noise levels of individual washing machines have to be declared, and customers have to be informed by means of an appropriate label containing information about the level of sound power during washing as well as during spinning. The world market demands quieter and more economical washing machines with higher rotation speed of the drum during spinning. Dynamic loading is increased by the square of the rotation speed which can cause vibration of the washing machine and higher noise levels. The present recommendations for the level of sound power define the total level to be lower than 65 dB during spinning, regardless of the rotation speed of the machine. The methods for measuring the sound power are standardised (ISO 3745), as well as the way in which the appliance is operated, which for washing machines is IEC 704-2-4. It should be pointed out that tumbling and spinning are, as far as the problem of noise reduction is concerned, mutually exclusive. While the gfin^OtJJIMISCSD 99-7/8 stran 287 |^BSSITIMIGC N. Hole~ek hrupnost pri mencanju vpliva predvsem elektromotor, vpliva pri centrifugi, zaradi problemov z neuravnoteženostjo perila, na hrupnost konstrukcija pralnega stroja in elastični deli, ki blažijo vibracije in udarce. V prispevku je v nadaljevanju najprej na kratko prikazan potek modeliranja vibracij pralne skupine v domačem strokovnem okolju. 1 PREGLED MODELIRANJA VIBRACIJ Modeliranje dinamičnega obnašanja pralnika ter predvsem njegove pralne skupine se je odvijalo v okviru Laboratorija za dinamiko strojev in konstrukcij Fakultete za strojništvo v Ljubljani. Na začetku je bil postavljen osnovni ravninski model pralne skupine s tremi prostostnimi stopnjami. Pri preučevanju zagona smo uporabljali izmerjeni časovni potek vrtilne frekvence bobna pri ožemanju. V nadaljevanju smo preučevali ujetje sistema v resonančnem področju ter vpliv sile teže rotorja na dinamično obnašanje pralnika. Sledila je razširitev modela z vključitvijo gibanja okrova pralnika, kar je pomenilo upoštevanje sedem prostostnih stopenj. Izkazalo se je, da so prednosti tako razširjenega modela premajhne glede na obsežnost dela. Nato je sledila poglobitev teoretičnega postopka modeliranja z uporabo modernih metod iz teorije nelinearnih dinamskih sistemov. Obravnavo dinamičnega obnašanja pralne skupine smo razširili na fazni prostor, tako na pravi fazni prostor na podlagi integracije gibalnih diferencialnih enačb kakor tudi na rekonstruirani na podlagi meritve ene same spremenljivke. Dobili smo dobro ujemanje pri oceni korelacijske dimenzije med modelom in preskusom. y electromotor influences the noise during tumbling, the design of the washing machine and the elastic parts, which reduce vibrations and strokes, due to problems with unbalanced laundry, influence the noise level during spinning. In this paper, first of all, a short overview describing the modelling of the dynamical behaviour of the washing machine in Slovenian institutions is given. 1 AN OVERVIEW OF MODELLING VIBRATIONS The modelling of the vibrational behaviour of the washing machine was conducted in the Laboratory for Dynamics of Machines and Structures, Faculty of Mechanical Engineering in Ljubljana. Initially, the basic planar model that consisted of three degrees of freedom was set up. Startup behaviour was studied by employing the measured drum angular velocity. The next step introduced the study of spin-up through resonance including the influence of the drum weight. The basic planar model was also extended into a system of two planar rigid bodies with seven degrees of freedom. However, the benefit from such an extension was not sufficient when compared to the increased level of complexity. The next important step in theoretical modelling was orientated towards modern methods from the theory of non-linear dynamical systems. The description of the dynamical behaviour of the washing complex was expanded into the phase space of the system. The real phase space was based on the integration of the governing differential equations of motion while the reconstructed one was based on a single measured time history, employing the embedding theorem. In the estimation of the correlation dimension, good agreement between the model and the experiment was found. Sl. 1. Model pralne skupine Fig. 1. Model of washing group 99-7/8 grin^SfcflMISDSD I ^BSfiTTMlliC | stran 288 Zmanj{anje hrupa pralnih strojev - The Reduction of Washing Machine Noise V zadnjem času je bila izvedena tudi bispektralna analiza dinamičnih odzivov pralne skupine pri ožemanju. Vrednosti kvadratične bikoherence so dokazale kvadratično sklapljanje faz med posameznimi frekvenčnimi komponentami dinamike pralne skupine ter s tem povezano nelinearnostjo sistema. Glavnina povzetega dela pri modeliranju dinamike pralnika je opisana v [1] do [3], sam model pa je prikazan na sliki 1. Ne glede na obseg modeliranja vibracij ter na njihov nadzor pri realnem pralniku se vibracijam ne da izogniti. Zato pri praktičnem zmanjševanju hrupnosti pralnika uporabimo tudi dušenje strukturnega hrupa z dodajanjem izolacijskih materialov. 2 AKUSTIČNE MERITVE Pri meritvah smo uporabljali metodo merjenja zvočne moči. Zvočna moč da informacijo o zvoku tako po smeri kakor po vrednosti: Just recently, the bispectral analysis of the vibrational responses of the washing complex at spinning was performed. The values of bicoherence squared showed some quadratic phase coupling between spectral components and thus confirmed the non-linearity of the system. The major part of the described theoretical modelling is shown in [1] to [3]. The model itself is shown in Fig. 1. Irrespective of the extent of vibrational modelling as well as their control on a real washing machine, the vibrations cannot be avoided. Consequently, the noise as a result of the remaining vibrations must be tackled also in a passive way. In the following, the use of insulating materials in order to reduce structure-born noise is described. 2 ACOUSTIC MEASUREMENTS The method of measuring sound intensity was used. Sound intensity provides information about the sound direction as well as its value: I WF A A v = p- v (1). V nasprotju z meritvami zvočnega tlaka (merimo ga neposredno z mikrofoni), zvočno moč določamo v dejanskem času z merjenjem zvočnega tlaka in hitrosti delcev. Odjemnik, ki hkrati meri zvočni tlak in hitrost delcev, je sonda za jakost. Sonda ima dva nasprotno si ležeča mikrofona za zvočni tlak. Mikrofona sta na razdalji Dr (sl. 2) [4]. Unlike noise pressure measurements (it is measured directly by microphones), noise intensity is determined in real time by measuring the sound pressure and the speed of the particles. The receiver, which simultaneously measures sound pressure and the speed of the particles, is the probe for intensity. The probe has two microphones for sound pressure which are placed opposite each other. The distance between the microphones is Dr (Fig.2) [4]. Ar PlTt) Sl. 2. Sonda za merjenje zvočne jakosti Fig. 2. Probe for measuring sound intensity 2.1 Merjenje hitrosti delcev na podlagi razlike 2.1 Measuring the speed of particles on the basis tlakov of pressure difference Ce je povprečni zvočni tlak p = vzdolž merilne sonde [5] do [7]: je jakost p1+p2 2 , In=p-vn = p1 + p2 If the average sound pressure is p = p1+p2 , the intensity along the measuring probe is [5] to [7]: p1+p2 1 p1 -p2 Dr 2pDr 2r0 \(p1-p2)dt dt (2). stran 289 | \^B©TTIMD[rS N. Hole~ek 2.2 Sonda za merjenje zvočne jakosti Sonda ima dva nasprotno si ležeča mikrofona, ki sta fazno prilagojena. Med mikrofonoma je vgrajen ustrezen vmesnik, glede na frekvenčno območje, ki ga merimo. Za področje 50 Hz do 6300 Hz se uporablja 0,5 palčni mikrofon, za višje frekvence so bolj primerni 0,25 palčni mikrofoni. Iz merilne sonde vodimo signal na analizator zvočne jakosti. Pomembno vlogo za natančnost meritev ima fazna prilagojenost mikrofonov in ustrezni vmesnik [6]. 2.2 Probe for measuring sound intensity The probe has two microphones placed opposite each other and phase adjusted. There is an appropriate interface built between the microphones, according to the frequency range which is being measured. For the range from 50 Hz to 6300 Hz a 0.5 inch microphone is used, whereas 0.25 inch microphones are more suitable for higher frequencies. The signal is fed from the measuring probe into the sound intensity analyser. Phase adjustment of the microphones and an appropriate interface play an important role in precise measurements [6]. tlak jakost ¦ 6* ADP ADC ^ f--------------------{ 1/3-octave digital filters ) / / Ar • 1/3-oktavni digitalni filter t> ADP ADC 1/3-octave digital filters Sl. 3. Shema izračuna zvočne jakosti Fig. 3. Scheme of calculation of sound intensity 3 ZMANJŠANJE HRUPA Hrupnost pralnega stroja je odvisna od: konstrukcije, elektromotorja, gumijastih delov, ki blažijo vibracije, ter od zvočne izolacije. Vsak od navedenih delov ima svoje lastnosti, ki prispevajo k zmanjšanju ali povečanju hrupnosti. Čeprav je zmanjševanje hrupa na poti njegovegaširjenja tehnično najbolj učinkovito (uporaba dušilnih materialov ali materialov, ki dušijo prenašanje vibracij - kompozitna pločevina), smo svoja prizadevanja usmerjali tudi na zmanjševanje hrupa na izvoru, ker je to z energetskega in ekološkega vidika najbolj upravičeno. Po deležu seveda k zmanjšanju hrupnosti največ prispeva izolacija. Zato smo ji tudi posvetili največ raziskav. Ker smo si zadali tudi cenovno omejitev, je bilo potrebno s čim manj izolacije in s čim cenejšo izolacijo doseči našo ciljno vrednost. Opravljena je bila množica meritev v polgluhi komori laboratorija za akustiko in vibracije. Z merjenjem zvočne moči po vseh petih sevalnih površinah smo določili mesta največjih sevanj in se je temu primerno oblikovala tudi izolacija. Kot izolacijske materiale smo preskušali: - kompozitno pločevino, - poliuretansko peno (gladko in piramidno), - cefrana odpadna tekstilna vlakna bombaža, - težke mase. 3 NOISE REDUCTION The noise of a washing machine depends on: the design of the washing machine, the electromotor, rubber parts for absorbing vibrations, and sound insulation of the washing machine itself. Each of the mentioned parts has its own characteristics which contribute to the increase or reduction in noise. Although noise reduction during its transfer is technically the most effective (use of muffling materials or materials which muffle transfer of vibrations -composite sheet metal), our efforts have been directed also to the reduction of noise at its origin, which is the most justified from the energy and ecology point of view. Insulation is certainly the most important method for noise reduction and this is why most research has been dedicated to it. As we also took into account cost limitations it was necessary to reach our target value with the cheapest insulation possible. Numerous measurements were done in the half deaf chamber of the laboratory for acoustics and vibrations. By measuring sound intensity along all five radiation surfaces the places of the highest radiation were localised and insulation was created according to this. We have tested the following as potential insulation materials: - composite sheet metal, - polyurethane foam (smooth and pyramid), - frayed waste textile cotton fibres, - heavy masses. 8 Sšnn3(aul[M]! ma stran 290 Zmanj{anje hrupa pralnih strojev - The Reduction of Washing Machine Noise magnituda jakosti v dB intensity mag. in dB frekv./freq. 100-10.0kHz nad/above 64 63-64 62 -63 6] -62 60-61 zgoraj/up ? u r 59 - 60 58-59 57 - 5S 56-57 55-56 54-55 53-54 52 - S3 51-52 50-51 levo/left spredaj/front pod/below 50 Sl. 4. Ozemanje - delno izoliran stroj - zorni kot spredaj-levo Fig. 4. Spinning - partly insulated machine - angle of sight front-left zgoraj/up magnituda jakosti v dB .. • s- jj^ "* *-..... intensity mag. in dB •-¦....;¦¦ frekv./freq. 100-10.0kHz J < ^^ i-V^S^C "";::¦- ¦ nad/above 64 ^^ ¦ «3-64