NVH testing, analyses and solutions

NVH is an abbreviation for Noise, Vibration and?Harshness. NVH analyses are used today in various industrial sectors (development of household appliances, various means of transport). However, they were originally developed mainly for the field of automobile construction with the aim of ensuring acoustic and driving comfort for the vehicle's occupants.

In relation to solving NVH problems in automobiles, their noise and vibration sources can be divided into the following categories:

- aerodynamic (air flow around the vehicle body, HVAC systems, etc.)

- mechanical (combustion engine, drive mechanism, brakes, interaction of tires with the road surface, etc.)

- electric (drive motor in electric cars, various servo motors, etc.)

Technical equipment for ensuring NVH measurements

Acoustic measurements in vehicle development are usually carried out in so-called semi-anechoic chambers. These chambers provide protection against external noise and absorb reflected sound waves. This creates the prerequisites for precise acoustic measurements. From the point of view of solving problems of aerodynamic noise, testing in the wind tunnel is also used. In NVH analyses, various technical equipment is used, such as microphones, accelerometers, DAQ systems, calibrators, artificial head (HATS), shakers, modal hammers, acoustic cameras, laser vibrometers, sound sources (see Fig. 1). The chambers are also equipped with dynamometers that allow real driving conditions to be simulated. To investigate specific sounds, test tracks with different surface profiles are also used to ensure the necessary excitation energy at certain vehicle speeds (excitation of different types of noise).

Experimental and computational methods used in vehicle development

In the development of vehicles and their components, various computational and experimental methods are used to solve NVH problems. As for calculation methods, these include, for example, multibody simulations, dynamic and acoustic analyses using FEM or BEM, simulations using the SEA method, etc. The choice of an appropriate calculation method depends on the analyzed frequency range, type of system, boundary conditions, etc. Due to the fact that NVH properties must be balanced with other vehicle features (such as modern design, safety, reliability), so-called multiphysics simulations are also used in their development. These enable the simultaneous solution of problems of various physical disciplines (e.g. acoustics, flow, heat transfer, etc.). It is also important that computational models correlate well with experimental testing (use of calibrated computational models).

Among frequently used experimental methods (measurements on prototypes) it is possible to include experimental modal analysis, analysis of transmission paths, analysis of operational deflection shapes, measurements and analyses using an acoustic camera, artificial head (HATS) and so on (see Fig. 2). This is a traditional approach to addressing sound quality. In the past, when modern calculation methods were not yet available for virtually verifying the design of various structural units or even the entire vehicle, this was also the only method to check the suitability of the solutions used, as well as whether the vehicle met the prescribed requirements in terms of noise and vibration, etc. Due to the fact that solving the NVH problems often requires substantial changes in the construction, this approach is less effective and is associated with high costs for modifying or changing the used construction solutions.

The most modern approach to solving sound quality in vehicle interiors, which combines simulations and real-world testing, is the use of a driving simulator. The sound simulation system, practically of the entire car, allows for virtual verification of the impact of various modifications to individual vehicle components in different driving modes. This system is usually installed in the interior of a similar vehicle and, in addition to generating sound in the driver's headphones, for various modifications of car components, it also generates appropriate vibrations in the points of contact between the driver and the vehicle (steering wheel, pedals, seat, etc.).

Possibilities of optimizing the NVH properties of vehicles

Sound is transmitted from dominant sources (combustion engine, chassis) to the vehicle interior through the air, as well as through vibrations transmitted by the structure, which are subsequently, under certain conditions, radiated as sound waves. The traditional solution consists of structural modifications to vehicle elements, from the excitation point through various transmission paths to the radiating surfaces. It is used at that e.g. correct modal tuning of components, application of tuned mass dampers, double flexible mounts (insertion of auxiliary frame) with optimized characteristics, etc. It is optimal to deal with the reduction of noise directly at the source, that means using high-quality aggregates, components with optimized dynamic properties, etc.

Vibration isolation by means of increasing damping of structures

In addition to using structural materials with high internal damping, it is possible to increase the damping of the structure by adding a damping material with a high loss factor to the free surface of the structure (by gluing, spraying). They are used exclusively for damping thin oscillating sheets, up to a maximum thickness of about 2.5 mm. They are an important means of damping, especially transverse mechanical vibrations of the structure.

Reducing airborne sound transmission

It is about reducing the flow of acoustic energy from the noise source to the protected space. The most effective measure from this point of view is covering the noise sources. ?From this point of view, it is important e.g. design of the partition between the engine and the interior of the vehicle (fire wall), high-quality sealing of the openings in this partition, etc.

Use of sound-absorbing materials

In order to reduce the level of interior noise, various absorbing materials are also used in the vehicle interior. These materials are characterized by the sound absorption coefficient alpha, which is defined as the ratio of absorbed acoustic power to the total incident power. Various coverings (roof panel, door panels, carpets) are most often used to absorb acoustic energy, but the share of used seats is particularly significant in this regard.

New approaches to solving sound quality

- the use of the ANC system (Active Noise Control)

- the use of active beds of the combustion engine

- active suspension of vehicle wheels (use of adaptive dampers)

- active sound system (possibility to select different sound profiles)

NVH testovanie, analyzy a rie?enia

NVH je skratka pre Noise, Vibration a?Harshness. NVH analyzy sa dnes vyu?ívajú v r?znych priemyselnych odvetviach (vyvoj domácich spotrebi?ov, r?znych dopravnych prostriedkov). P?vodne sa v?ak vyvíjali hlavne pre?oblas? kon?trukcie automobilov s?cie?om zabezpe?i? akusticky a?jazdny komfort posádky vozidla.

V?súvislosti s?rie?ením NVH problémov automobilov je mo?né ich zdroje hluku a?vibrácií rozdeli? do nasledovnych kategórií:

- aerodynamické (obtekanie karosérie vozidla vzduchom, HVAC systémy a?pod.)

- mechanické (spa?ovací motor, hnací mechanizmus, brzdy, interakcia pneumatík s?povrchom vozovky a?pod.)

- elektrické (hnací elektromotor pri elektromobiloch, r?zne servomotory a?pod.)

Technické vybavenie pre zabezpe?enie NVH meraní

Akustické merania pri vyvoji vozidiel sa oby?ajne vykonávajú v?tzv. polo-bezdozvukovych komorách. Tieto komory zais?ujú ochranu pred vonkaj?ím hlukom a?absorbujú odrazené zvukové vlny. Tym vytvárajú predpoklady pre presné akustické merania. Z?h?adiska rie?enia problémov aerodynamického hluku sa vyu?íva i?testovanie v?aerodynamickom tuneli. Pri NVH analyzach sa vyu?íva r?zne technické vybavenie ako sú napr. mikrofóny, akcelerometre, DAQ systémy, kalibrátory, umelá hlava (HATS), ?ejkre, modálne kladivá, akustické kamery, laserové vibrometre, zdroje zvuku (vi? obr. 1). Komory sú tie? vybavené dynamometrami, ktoré umo?ňujú simulova? skuto?né jazdné podmienky. Pre skúmanie ?pecifickych zvukov sa vyu?ívajú aj?testovacie dráhy s?r?znymi profilmi povrchu tak, aby sa zaistila potrebná budiaca energia pri ur?itych rychlostiach vozidla (vybudenie r?znych druhov hluku).

Experimentálne a?vypo?tové metódy vyu?ívané pri vyvoji vozidiel

Pri vyvoji vozidiel a?ich komponentov sa z?h?adiska rie?enia NVH problémov vyu?ívajú r?zne vypo?tové a?experimentálne metódy. ?o sa tyka vypo?tovych metód sú to napr. multibody simulácie, dynamické a?akustické analyzy vyu?itím FEM príp. BEM, simulácie vyu?itím metódy SEA a?pod. Vyber vhodnej vypo?tovej metódy závisí od analyzovaného frekven?ného rozsahu, druhu systému, okrajovych podmienok a pod. Vzh?adom k?tomu, ?e NVH vlastnosti musia by? v?rovnováhe i?s??al?ími vlastnos?ami vozidiel (ako sú napr. moderny dizajn, bezpe?nos?, spo?ahlivos?), sú pri ich vyvoji vyu?ívané i?tzv. multiphysics simulácie. Tieto umo?ňujú sú?asné rie?enie problémov r?znych fyzikálnych disciplín (napr. akustiky, prúdenia, prenosu tepla a?pod.). Je tie? d?le?ité, aby vypo?tové modely dobre korelovali s?experimentálnym testovaním (pou?itie kalibrovanych vypo?tovych modelov).

Medzi ?asto vyu?ívané experimentálne metódy (merania na prototypoch) je mo?né zaradi? experimentálnu modálnu analyzu, analyzu prenosovych ciest, analyzu prevádzkovych tvarov kmitania, merania a?analyzy vyu?itím akustickej kamery, umelej hlavy (HATS) a?pod. (vi? obr. 2). Ide o tradi?ny prístup k rie?eniu kvality zvuku. V minulosti, ke? e?te neboli k dispozícii moderné vypo?tové metódy na virtuálne overenie návrhu r?znych kon?truk?nych celkov alebo i celého vozidla, to bola zároveň jediná metóda ako preveri? vhodnos? pou?itych rie?ení, tie? ?i vozidlo plní predpísané po?iadavky z h?adiska hluku a vibrácií a pod. Vzh?adom k tomu, ?e rie?enie NVH problematiky si ?asto vy?aduje podstatné zmeny v kon?trukcii, ?je tento prístup menej efektívny a je spojeny s vysokymi nákladmi na úpravu, ?i zmeny pou?itych kon?truk?nych rie?ení.

Najmodernej?í prístup v oblasti rie?enia kvality zvuku v interiéri vozidiel, ktory kombinuje simulácie a reálne testovanie, je pou?itie jazdného simulátora. Systém simulácie zvuku, prakticky celého automobilu, umo?ňuje virtuálne overovanie vplyvu r?znych modifikácií jednotlivych komponentov vozidla, pri r?znych re?imoch jazdy. Tento systém? byva? obvykle in?talovany do interiéru? podobného? vozidla a okrem? generovania zvuku do slúchadiel vodi?a, pre r?zne modifikácie komponentov automobilu, generuje i príslu?né vibrácie v miestach kontaktu vodi?a s vozidlom (volant, pedále, seda?ka a pod.).

Mo?nosti optimalizácie NVH vlastností vozidiel

Zvuk je od dominantnych zdrojov (spa?ovací motor, podvozok) do interiéru vozidla prená?any vzduchom, ako aj vibráciami prená?anymi ?truktúrou, ktoré sú následne, za istych podmienok, vy?iarené ako zvukové vlny. Tradi?né rie?enie spo?íva v kon?truk?nych úpravách prvkov vozidla od miesta budenia cez r?zne prenosové cesty a? po vy?arujúce plochy. Vyu?íva sa pri tom napr. správne modálne naladenie komponentov, aplikácia dynamickych tlmi?ov, dvojitych pru?nych ulo?ení (zaradenie pomocného rámu) s optimalizovanymi charakteristikami a?pod. Optimálne je rie?i? zní?enie hlu?nosti priamo pri zdroji, to znamená vyu?íva? kvalitné agregáty, komponenty s optimalizovanymi dynamickymi vlastnos?ami a pod.

Vibroizolácia pomocou zvy?ovania tlmenia kon?trukcií

Okrem pou?itia kon?truk?nych materiálov s vysokym vnútornym tlmením je mo?né zvy?i? tlmenie kon?trukcie i pridaním tlmiaceho materiálu s vysokym stratovym sú?inite?om na vo?ny povrch kon?trukcie (nalepením, striekaním). Pou?ívajú sa vyhradne na tlmenie tenkych kmitajúcich plechov, maximálne asi do hrúbky 2,5 mm. Sú vyznamnym prostriedkom na tlmenie najm? prie?neho mechanického kmitania kon?trukcii.

Zní?enie prenosu zvuku ?íriaceho sa vzduchom

Ide o zní?enie toku akustickej energie od zdroja hluku do chráneného priestoru. Najú?innej?ím opatrením je z tohto h?adiska krytovanie zdrojov hluku. ?Z tohto poh?adu je d?le?ité napr. prevedenie deliacej prie?ky medzi motorom a interiérom vozidla, kvalitné utesnenie otvorov v tejto prie?ke a pod.

Pou?itie zvukovopohltivych materiálov

Z h?adiska zní?enia úrovne vnútornej hlu?nosti sa v priestore interiéru vozidla pou?ívajú i r?zne pohltivé materiály. Tieto materiály sú charakterizované sú?inite?om zvukovej pohltivosti alfa, ktory je definovany ako pomer pohlteného akustického vykonu ku celkovému dopadajúcemu vykonu. Naj?astej?ie sa pre pohltenie akustickej energie vyu?ívajú r?zne obklady (stre?ny panel, vyplne dverí, koberce), ale vyrazny je v tejto súvislosti najm? podiel pou?itych seda?iek.

Nové prístupy k rie?eniu kvality zvuku

- vyu?ívanie systému ANC (Active Noise Control)

- nasadenie aktívnych l??ok spa?ovacieho motora

- aktívne zavesenie kolies vozidla (pou?itie adaptívnych tlmi?ov)

- aktívny systém zvuku (mo?nos? zvoli? r?zne profily zvukov)