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Introduction

Lighter vehicles are more fuel efficient than heavy ones but they transmit more low frequency noise compared to heavier vehicles. To make lighter vehicles acceptable to passengers and at the same time fuel efficient, the low frequency noise must be addressed. Conventionally accepted passive noise treatments (non-woven mats namely SM200L and SM300L) for that low-frequency noise add weight to the light structures, declining the environmental benefits.

Passive noise absorbers work best to perfectly shield from and discourage the high frequency sound waves (short wavelength) transmissions. Wall constructed of dense non-woven materials are likely to most effective in reduction of noise transmission.

Tortuosity is a parameter related to a fluid that fills a porous material mostly air, and shows the apertures complexity in a poro-elastic material. Tortuosity is calculated in the ratio in-between the average length of apertures in poro-elastic material as well as the material thickness. The more the complexity of the inner structure and the higher the tortuosity, same impact to the thicker material is anticipated.

Characteristic length has two types: viscous characteristic length (VCL) and thermal characteristic length (TCL). The characteristics are related to the apertures inside the porous material shapes. Each one of them becomes a parameter which has relation to the structure attributable to the fluid that filling the porous material which is air.

Using Biot theory it becomes possible to explain the acoustic behavior of porous elements. Similarly when a material sample get excited by acoustic waves the materials’ frames behave exactly as acoustically rigid over a wide frequencies range. In such a case the porous material may be replaced by on a macroscopic parameter by an equivalent fluid of an effective density as well as effective bulk modulus. The stationary frame situation may occur either because of elasticity modulus or high density of may result from particular test conditions. Flow resistivity was found to increase with declining fiber size and nonwoven porosity.

Statistical energy analysis is a way for predicting sound of resonance as well as vibration transmission in dynamic systems constituted of coupled cavities of acoustic and structural parts. The system vibrational behavior is defined in energy terms and the energy here is both the kinetic and potential energy of the modal resonances. To correct a vibration or sound problem with SEA, the systems get partitioned into components which are represented by one or more modal subsystems in turns.

Absorption of sound is a vital attribute of automotive interior items since it determines how effectively sound gets dissipated upon entrance to the interior, which interferes with the overall level of the sound. When sound wave hits the surface, some fraction of acoustic energy get absorbed and remaining energy is reflected. Sabine absorption coefficient of the surface becomes the ratio of energy absorbed to the incident energy averaged on all probable angles of incidence. Sabine area is given by the product of absorption coefficient and the actual surface area (Stephens’s 1966 pg.43) Absorption coefficient is determined by placement of a sample in a room that has reverberation, introduction of a source of sound, termination of the sound and measurement of the resultant sound field decay.

Impedance Tube Testing

Impedance tubes are equipment used in testing normal incidence absorption of sound in material samples in accordance to ASTM E1050.

An impedance tube usually has a loudspeaker mounted on one end and then a material sample placed on the other end. (Bruce Lindsay 1999, pg74) The loudspeaker will generate sound waves randomly which then propagate like plane waves in that tube and get reflected off the surface of the sample. (Wilson 2006 pg.112) This generates a standing-wave interference trend/pattern coming from backward-traveling and forward-travelling waves inside that tube. (Benade 1976, pg. 33) The pressure of the sound gets determined at two locations of microphone and eventually, the transfer function in-between both measurements get calculated. Using this data, it becomes possible to measure the complex coefficient of reflection, the absorption sound coefficient as well as the material’s normal acoustic impedance.

The frequency range usable is dependent on the tube’s diameter and the distance between the two microphones. Roush has capacity to provide measurements ranging from 100 Hz to 1,600 Hz and from 400 Hz to 6,300 Hz.

Alpha Cabin Testing

This is a one-third scale (8.6 MS) reverberation room used to determine the random incidence absorption of sound of parts and materials. (Theodor 1955, pg. 17) This test is very similar to ASTM C423 but in a much smaller room. An Alpha Cabin usually has approximate measurements of 1.2 m x 1.6 m x 1.8 m with non-parallel walls. The non-parallel walls compel the reflecting waves of sound that produce standing waves to do reflections from every wall in the room. (Paulapuro 2000, pg. 95-98) Therefore, a sample placed somewhere on the floor will interfere with the decay period of every of the mode.

Alpha Cabin equipment tests a sample, one third-octave band at a given time. The rate of sound field decay is determined and recorded at every stage of the five microphone positions. (Boyer 1991, pg. 5) The process gets repeated for every third octave band ranging from 400 Hz to 10,000 Hz. The coefficient of absorption is determined from the average of the five recorded decay rates for every third octave band.

Modeling non- woven to obtain non Acoustical Properties

An indirect method based on a 3-microphone impedance tube set-up may be applied may be applied to establish the non-acoustic properties of a sound absorbing porous material (non-woven mat). (Farina 200, pg.20) To start with, a 3-microphone impedance tube technique gets applied to determine some acoustic characteristics of the non-woven material. I.e. sound transmission loss, coefficient of sound absorption, effective bulk modulus and effective density referred here as an equivalent fluid. (Kenneth 1947, pg. 65) An indirect characterization then allows extraction of its non-acoustic qualities. I.e. tortuosity, Static airflow, viscous, resistivity as well as thermal characteristic lengths from the determined effective characteristics as well as material open porosity. (Dover 1894, pg. 2) This process is applied to four distinct sound absorbing materials (non- woven mats) and findings of the characterization get standardized against the existing inverse and direct methods. Predictions about the acoustic characteristic using a model of an equivalent fluid as well as the found non-acoustic qualities coincide with the impedance tube measurements.