Available online at ScienceDirect. Procedia Engineering 176 (2017 ) Dynamics and Vibroacoustics of Machines (DVM2016)

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1 Available online at ScienceDirect Procedia Engineering 176 (2017 ) Dynamics and Vibroacoustics of Machines (DVM2016) The effectiveness of the suppression of low frequency acoustic resonances with porous sound absorbing structures of multifunctional upholstery materials of car body interior Michael I. Fesina, Alexander V. Krasnov, Larisa N. Gorina * Togliatti State University, 14 Belorusskaya Str, Togliatti, Russian Federation Abstract The article presents the results of researches on the effectiveness of the technical method of suppression of low frequency acoustic resonances occurring in the air volumes of passenger and luggage areas of the car body by increasing the absorption degree of sound wave energy by multifunctional panel and volumetric elements of a car body interior. The research reviews a sliding shutter and integral carpeting of a luggage compartment, rear wheel arch trim, backrest upholstery and car door trim as the modified multi-functional elements of the car body interior. The results of experimental researches show that putting the modified multifunctional car body interior upholstery, characterized by high sound absorbing properties, into the studied prototype of a car allows to reach the effect of reduction the overall sound levels up to 9 dba on the mode of intensive acceleration movement and sound pressure levels, which are registered at a frequency of the second motor harmonic of the engine 2n / db The The Authors. Published by Elsevier by Elsevier Ltd. This Ltd. is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of organizing committee of the Dynamics and Vibroacoustics of Machines (DVM2016). Peer-review under responsibility of the organizing committee of the international conference on Dynamics and Vibroacoustics of Machines Keywords: passenger car; low frequency acoustic resonance; noise level; multifunctional interior parts of a car body. The predominant proportion of the acoustic energy, being localized in air volumes of passenger and luggage areas of cars of M1 and M1G categories, is formed by structural vibrations of enclosing car body panels and is determined by acoustic energy air transmission (by energy crossflow) through various vibro-acoustic communication links of a car body with units and systems. In the spaces of the passenger area and the luggage compartment low frequency acoustic resonances could appear, caused by frequent coincidence of car body panel mechanical vibrations with * Corresponding author. Tel.: address: kaw@ya.ru, michailfes@yandex.ru, gorina@tltsu.ru The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the organizing committee of the international conference on Dynamics and Vibroacoustics of Machines doi: /j.proeng

2 160 Michael I. Fesina et al. / Procedia Engineering 176 ( 2017 ) acoustic eigenmones of the air volumes of defined spaces [1...10]. Their effective removal is difficult and some of the reasons are the use of a typical car body noise reducing kit, containing a limited number of sound-absorbing and sound-insulating parts (upholstery, panels, linings) and the use of car body interior multifunctional elements, which have poor acoustic performance in low frequency region of the sound spectrum [3]. More productive technique is a significant reduction of the power vibration impact on rigid noise producing car body structures, which is transmitted to them from the sources of their dynamic excitation. This explains the use of advanced anti-vibration suspension of units and systems of a car, the additional use of rigid vibration-isolated bearing components (frames, subframes), and frequency tuned vibration damper [4]. However, improvement of car vibro-acoustic characteristics, connected with the above-mentioned structural changes, as a rule, requires significant material, financial and time expenses. The results of the research show that the reasonable technical method of suppression of low frequency acoustic resonances occurring in the interconnected air volume of the passenger area and the luggage compartment of a car body is increasing the absorption degree of sound wave energy by multifunctional panel and volumetric elements of the interior of a car body. The formed closed air volumes of luggage compartment and the passenger area of most mass-produced cars are limited by hard solid cover sound-reflecting walls of interior parts and glass units of a car body. This contributes to forming of prominent acoustic resonances of air volumes, where standing sound waves are formed. They are the result of coherent summation of the sound pressure amplitudes of direct and reflected sound waves. This summation is carried out without spatial transfer and dissipative sound energy absorption. Solid cover sound-reflecting walls of interior parts of the car body are not only passive sound-reflecting elements with a low (zero) sound absorption, but can also be active secondary acoustic radiating structures of vibration type, which are excited by solid and air vibro-acoustic energy transfer passages. The car body interior elements of this type are multifunctional lining of doors, sidewalls, wheel arches, piles, roof, floor carpeting, seat fillers and lining, sliding shutter or fixed luggage compartment shelf. It is reasonable to replace the solid cover structures of car body interior multifunctional elements with porous sound-absorbing structures. They are not only able to absorb effectively the energy of the incident sound waves, but also simultaneously exclude the direct generation of acoustic energy at a dynamic excitation due to intense vibration signal transmitted by hard passages, the incoming sound field or incident sound wave air paths. In particular, these factors are relevant to the vehicles using units and systems, characterized by high vibro-acoustic activity. The basic constructional materials of car body interior multifunctional elements, endowed with improved sound absorption characteristics are one-piece molded air-permeable fibrous and / or foamed open-cell structure of materials. Their acoustic characteristics are formed and predetermined by physical parameters of airflow resistance σ, Young's modulus Е, porosity ϕ, pore tortuosity τ, structural volume q and structural surface density q s, structural loss coefficient η, reverberant (diffusive) sound absorption coefficient α r, normal sound absorption coefficient α n, and transmission loss TL. Through the fact that the multifunctional car body interior elements are mostly represented by acoustic multilayer structures, their mutual surface interference should provide (keep) air-permeable and / or sound transparency properties as a part of the multilayer structure. For this purpose, we should use the sound transparent film, fibrous or powdered adhesive compounds (sticky adhesive, hot-melt adhesives), apply sound transparent transmission bearing and inner frame embedded (rod, perforated plated) and / or the outer facing elements (layers). Intentional additional increasing of low frequency sound energy absorption efficiency is achieved by constructive forming of enlarged air gaps between the opposing walls of the disposable panels a rigid sound reflecting car body metal panel and multi-layered structure of the wall of a multifunctional car body interior element, endowed with prominent acoustic function. As investigated and modified multifunctional car body interior elements, in particular, a sliding shutter of a luggage compartment, integral floor carpeting of a luggage compartment, rear wheel arch trim, seatback upholstery and rear door trim were considered (see. Fig. 1). In the initial state of the project development the sliding shutter was represented by flexible film polymer coagulating structure. Constructions of such type are used, in particular, in the production models of Nissan cars (Terrano, X-Trail, Murano, Pathfinder) and Toyota (Land Cruiser Prado, Land Cruiser 200, RAV4). According to the research, this structure was replaced with an solid cover mockup prototype of one-piece molded detail made of porous sound absorbing air-permeable structure (Pos. A), fixed to the car body in the form of a luggage compartment shelf. Material of a similar structure type (Pos. B) was used to make sound absorbing backrest upholstery, mounted on the rear (back) surface. This upholstery was also used as a part of a car body noise reducing kit (not applicable for standard package of studied car prototype).

3 Michael I. Fesina et al. / Procedia Engineering 176 ( 2017 ) B А C D Е A B C Luggage compartment shelf Seatback upholstery 2 1 Floor carpeting of a luggage compartment E Rear door trim D Rear wheel arch and sidewall trim Figure 1 The scheme of the positions and structural compositions of the studied modified multifunctional interior linings, trims and upholstery, used for low-frequency acoustic resonance suppression in the passenger and luggage compartments of a car body 1 - a layer of porous air-permeable fibrous sound absorbing material; 2 - facing layer of nonwoven air-permeable sound transparent material; 3 car body supporting metal panel; 4 - supporting layer of micro-perforated plastic EPDM; 5 - needle-punched nonwoven air-permeable pile fabric; 6 - perforated layer of dense sound-reflecting polymer material; 7 - an air gap between the back surface of the interior trim panel and the supporting panel of a car body Instead of dismantled serial integral carpet of a luggage compartment made of interfaced layers of an airpermeable resistant and sound reflecting foam-flecked polyethylene and needle-punched fabric, there is a multilayer integrated mat model mounted. Its structure is formed from layers of porous air blown sound absorbing material, the carrier layer of micro-perforated Ethylene Propylene Diene Monomer (EPDM) plastic and facing air-permeable layer of needle-punched fabric (pos. C). Serial wheel arches, side bars and back door trim provided with solid cover air-impermeable polymeric materials have been replaced by corresponding multi-layer air-permeable structures. In particular, arches and side bars trim is composed of a two-layer molded structure (pos. D), comprising a layer of porous air-permeable sound absorbing material and facing air-permeable sound transparent material. Serial back door trim has been modified (pos. E) by opened perforating of structure of the base layer wall of close sound reflecting material. This provided the delivery of sufficient sound transparency property to it with a «connection» to the air volume of the luggage compartment of a back door area (situated between the back surface of the upholstery and the metal door panel). The delivery of functional sound absorption properties to the prototype of modified back door trim was performed with the inclusion of the air-permeable porous fibrous sound absorbing material to the rear side of base layer wall. The above mentioned sound absorbing air-permeable structures were presented as fibrous and foam open-cell materials (thermoformed from natural fiber and synthetic fiber, foam open-cell polyurethan). Also, the facing sound transparent (decorative, protective) film and fibrous materials were used which are polyethylene terephthalate, polyamide, ethylene vinyl acetate, polyethylene, polypropylene, polyester. Formed structure of mentioned car body interior multifunctional elements prototypes were characterized by determined effective ranges of values of the physical parameters. According to the existing experience of usage of the effective acoustic materials, the impedance by airflow resistance is in the range of σ = N s m -3, Young's modulus Е = MPa, porosity ϕ = 0,75...0,98 conventional units, pore tortuosity τ = conventional units, structural bulk density is q = kg/m 3, surface structural density q s = kg/m 2, structural loss factor η = 0,1...0,2 conventional units, reverberant sound absorption coefficient α r = 0,50...0,99 conventional units, normal sound

4 162 Michael I. Fesina et al. / Procedia Engineering 176 ( 2017 ) absorption coefficient α n = 0,30...0,90 conventional units, transmission loss TL = db. Specific examples of the frequency characteristics of the acoustic parameters used in the material structures of the modified multifunctional interior upholstery of the body are shown in Figure 2. α r, conv. unit 1,0 0,8 α n, conv. unit 1,0 0,8 TL, db Floor carpeting of a luggage compartment Rear wheel arch and sidewall trim 0,6 0,6 60 0,4 0,4 40 Backseat upholstery 0,2 0,0 Floor carpeting of a luggage compartment Luggage compartment shelf 0,4 0,6 1,0 1,6 2,5 4,0 6,3 10,0 f, khz 0,2 0,0 Rear wheel arch and sidewall trim Rear door trim 0,1 0,3 0,6 1,3 2,5 5,0 10,0 f, khz ,1 0,3 0,6 1,3 2,5 5,0 10,0 f, khz Figure 2 Frequency properties of the acoustic parameters of α r, α n and TL of materials structures of modified multifunctional car interior upholsteries The effectiveness of the technical reception of the suppression of low frequency acoustic resonances occurring in the interconnected air volumes of a car body is illustrated by the results of experimental studies of a car prototype of M1G category, which is all-wheel drive layout equipped with inline 4-cylinder engine in the transient (acceleration) and steady (constant velocity) motion modes. Points of measurement of internal noise levels were located in the central longitudinal plane of the body of the car on the driver's ear level (checkpoint T1) and the passengers in the back seat (checkpoint T2). As follows from the results of experimental studies, the overall levels of internal noise (sound levels) of the studied car prototype of the initial state (up to the stage of the acoustic processing) measured on the driving mode with an intense acceleration on 3rd gear transmission (hereinafter - Gearbox), exceed the set target values provided in the specifications to the acoustic modification construction (see Fig. 3) on 4, db. The maximum of the overall sound levels at the same time are captured in the check point T2. Changes in the general sound levels on the driving mode with an intense acceleration of the car is characterized by a pronounced resonant increase of their values in a narrow range of the operational speed of the engine speed of min -1. This high-speed mode corresponds to the frequency change of the engine process 2n/60 as part of the width of 1/3 octave band with 125 Hz center, at which the sound level with average of 20 db exceeds the limits of the specification requirement and subjectively perceived as an unacceptable acoustic vehicle defect. To identify possible correlations of car body solid structure dynamic excitation and caused acoustic radiation, which is in particular produced by structural vibration of the rear floor panel (luggage compartment floor panel), with the acoustic field of air space formed by the areas of luggage and passenger compartments, the measurement of vibroaccelerations at two checkpoints (B1 and B2) of setting the accelerometers at the surface of the car body rear floor panel was made (see. Fig. 4). Registered ranges and dynamics of changes in resonant vibration acceleration level of a luggage compartment floor panel and the noise levels in the air space of the passenger area have a correlation in an identical speed range of the engine crankshaft rotation frequency min -1. This, in

5 Michael I. Fesina et al. / Procedia Engineering 176 ( 2017 ) particular, points to the luggage compartment floor panel as the primary potential dynamic agent of low frequency resonant sound radiation, generated in the luggage compartment and the passenger area of the car body. Checkpoint Т1 Checkpoint Т2 Requirement specification Requirement specification (overall levels) 1/3 octave levels in the 125 Hz frequency band) (overall levels) 1/3 octave levels in the 125 Hz frequency band) Figure 3 - Overall levels and sound levels recorded in the 1/3 octave band with 125 Hz center in the passenger compartment of the studied car prototype when driving with intensive acceleration on 3rd gear В1 Checkpoint B1 В2 Checkpoint B2 Overall levels Overall levels Levels in 1/3 octave frequence band with the center of 125 Hz Levels in 1/3 octave frequence band with the center of 125 Hz Figure 4 The vibration acceleration levels, registered at checkpoints B1 and B2 of the luggage compartment floor surface of a car body of the studied car prototype with a standard package at the moment of acceleration at the 3 gear

6 164 Michael I. Fesina et al. / Procedia Engineering 176 ( 2017 ) The car driving mode with constant speed of the engine crankshaft at the different transmission gear speeds (1, 2, 3, 4) was also studied. The constant speed corresponded to the formation of a resonant sound radiation, registered with tools and the hearing of expert analysts in their subjective perception. This made it possible to maintain constant dominant frequency of the dynamic resonant excitation of a car body structures, transmitted to them mainly by solid support connections of powertrain suspension and suspension of the exhaust system. Analysis of 1/3 octave spectra of the level of vibration acceleration panel of the luggage compartment (see. Fig. 5) and sound levels in the passenger compartment at the control point T2 (see. Fig. 6) standard (serial) package of a moving vehicle at the intensive acoustic resonance mode of 1 2, 3 and 4, transmission gear speeds with constant crankshaft speed (n = 3750 min -1 ) at 30, 55, 85 and 120 km/h indicates the dominant role of the frequency component of 1/3 octave frequency band with the center of 125 Hz. At the same time, there is a very significant energy vibrational contribution to the floor panel excitation of the luggage compartment with 1/3 octave frequency bands with the centers 200 and 250 Hz, which are multiples of the panel and which do not cause resonance of the acoustic response in the frequency spectrum of sound levels (airborne noise). This confirms the presence of the resonance vibroacoustic excitation on the frequency component of 1/3-octave frequency band with the center of 125 Hz. The frequency of the registered resonant excitation f R = 125 Hz of the considered air volume of interconnected air cavities of the passenger compartment and the luggage compartment which has a longitudinal overall dimension L 2,75 m is characterized by a length of a radiated sound wave λ R 2,75 m coinciding with the frequency of its longitudinal acoustic mode (waveform) f L. These conditions of coinciding of the f R excitation frequency with its own f L frequency contribute to the realization of the physical process of low frequency acoustic resonance f R frequency in the interconnected airspaces of the passenger area and the luggage compartment of the car body. Practical implementation of detuning (exception) of the frequency value coinciding process of dynamic excitation f R with the eigen frequency of the system f L - in this case is impossible for obvious structural reasons while maintaining the constant dimensions of the air cavity of the vehicle body L. It is also impossible to exclude the operation of the engine in the operating range near the resonance of speeding min -1 ). For these reasons, the damping of air acoustic resonance occurring in the interconnected cavities of the passenger compartment and the luggage compartment was used. Installation of modified multifunctional car interior upholstery on the studied car prototype which characterized by high sound absorbing properties, has allowed to achieve the reducing effects of the overall sound levels which reach 9 dba (see. Fig. 7) on the investigated intensive acceleration transient driving of a car. Sound levels recorded in the 1/3-octave band centered on 125 Hz (at a frequency of the second motor harmonic) have decreased by up to 12 db (see. Fig. 7). Noise reducing effects recorded in the passenger area when the vehicle is moving at a constant engine speed of 3750 min -1 at 1, 2, 3 and 4 gears of gearbox responsible for the formation of a powerful low frequency acoustic resonance (f R = 125 Hz), are dba - on general levels of sound and up to 1 - on the levels of the sound recorded in 1/3-octave band centered on 125 Hz (see Fig. 6). Thus, the studied use of the modified multifunctional car body interior elements, endowed with enhanced sound absorbing properties, can not only provide with improvement of acoustic comfort in the high-frequency region of the sound spectrum, but also very effectively influence the suppression of low frequency acoustic resonances detected in the passenger compartment of the car. The solution of the target allowed to eliminate the acoustic defect of a car and to improve its consumer appeal in respect of achievable acoustic comfort. It did not require any significant changes in geometric shapes and dimensions of the body interior parts. This type of technique is useful when designing and fine-tuning (optimization) of noise reducing kit of a car body. One of the important areas of focus can be a setting up the special acoustic characteristics (sound absorption, sound insulation, sound transparency) to the structures of multifunctional car body interior parts, ensuring effective reduction of sound levels at the low frequency acoustic resonances occurring in the interconnected air volumes of passenger compartment and luggage compartment of a car body. As the evaluation and normalizing acoustic characteristics, forming a subjective perception of occurring acoustic resonances, the time (speed) parameters of the dynamic changes can be used. In this capacity we can consider the rate of growth (L R-max -L R-str ) /c R and recession (L R-max -L R-end )/b R of a sound level, the width of the resonance peak a R, deviation of the sound level curve h R of the projected monotonically changing target line (idealized smooth change in sound level) when changing the engine operation speed (vehicle speed), as shown in the example of Fig. 7.

7 Michael I. Fesina et al. / Procedia Engineering 176 ( 2017 ) First gear - 30 km/h Second gear - 55 km/h Check point B1 Check point B1 Check point B2 Check point B2 Third gear - 85 km/h Fourth gear km/h Контрольная Check point B1 точка В1 Check point B1 Контрольная Check point B2 точка В2 Check point B2 Figure 5-1/3 octave specters of vibrational acceleration recorded at the surface of the luggage compartment floor of the studied car prototype in a regular (standard) configuration as it moves with constant speed of 1, 2, 3 and 4 transmission gears at engine speed of 3750 min -1

8 166 Michael I. Fesina et al. / Procedia Engineering 176 ( 2017 ) First gear - 30 km/h Second gear - 55 km/h A A The nominal (standard) equipment The multifunctional modified interior trim kit is installed The nominal (standard) equipment The multifunctional modified interior trim kit is installed Third gear - 85 km/h Fourth gear km/h A A The nominal (standard) equipment The multifunctional modified interior trim kit is installed The nominal (standard) equipment The multifunctional modified interior trim kit is installed Figure 6-1/3 octave spectra of the internal noise of the studied car prototype, registered in the check point T2 at a constant engine speed of 3750 min -1 at 1, 2, 3 and 4 transmission gears

9 Michael I. Fesina et al. / Procedia Engineering 176 ( 2017 ) Check point B1 Check point B2 L R-max Overall sound level, dba Штатная The nominal (серийная) (standard) комплектация equipment Overall sound level, dba h R L R-end L R-str c R b R a R Установлен The multifunctional комплект modified модифицированных interior trim многофункциональных kit is installed обивок интерьера The multifunctional modified interior trim kit is installed Sound level in 1/3 octave band 125 Hz, dba Check point B1 Sound level in 1/3 octave band 125 Hz, dba Check point B2 L R-str c R b R a R L R-max h R L R-end The multifunctional modified interior trim kit is installed The multifunctional modified interior trim kit is installed Figure 7 Overall sound levels and sound levels in 1/3 octave frequency band with the center of 125 Hz, registered in the passenger compartment of studied car prototype while moving with intensive acceleration at the 3 transmission gear

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