NVH ENGINEERING & TEST SERVICES


MATERIAL TESTING

Acoustic Properties

Roush conducts test of acoustic materials to measure acoustic absorption, transmission loss, and insertion loss.

Acoustic Absorption Testing (Alpha Cabin, impedance tube, reverb room)

Sound absorption is an important property of automotive interior components because it measures how effectively sound is dissipated once it enters the interior, which affects the overall sound level.

When a sound wave strikes a surface, a fraction of the acoustic energy is absorbed and the remainder is reflected. The ratio of absorbed energy to incident energy averaged over all possible angles of incidence is the Sabine absorption coefficient (or simply the absorption coefficient) of the surface. The Sabine area is the absorption coefficient multiplied by the actual surface area. The absorption coefficient is measured by placing a sample in a reverberant room, introducing a sound source, terminating the sound source, and measuring the resulting sound field decay. To minimize laboratory-to-laboratory variation, standard reverberation rooms are used to make these measurements.

Alpha Cabin Testing

The Alpha Cabin is a one-third scale (8.6 ms) reverberation room that is used to measure the random incidence sound absorption of materials and parts. The test is similar to ASTM C423 but in a smaller room.

The Alpha Cabin has approximate dimensions of 1.2 m x 1.6 m x 1.8 m with nonparallel walls. Nonparallel walls force the reflecting sound waves that produce the modes (standing waves) to reflect from all the walls in the room. Thus, a sample placed on the floor will affect the decay time of all of the modes.

The Alpha Cabin equipment tests the sample, one third-octave band at a time. The sound field decay rate is measured and recorded at each of five microphone positions. This process is repeated for each third octave band from 400 Hz to 10,000 Hz. The absorption coefficient is calculated from the average of five measured decay rates for each third octave band.

Impedance Tube Testing

An impedance tube is used to test material samples for normal incidence sound absorption according to ASTM E1050, “Standard Test Method for Impedance and Absorption of Acoustical Materials Using A Tube, Two Microphones and A Digital Frequency Analysis System.”

In an impedance tube, a loudspeaker is mounted at one end and a material sample is placed at the other end. The loudspeaker generates random sound waves which propagate as plane waves in the tube and are reflected off of the sample surface. This leads to a standing-wave interference pattern resulting from forward- and backward-traveling waves inside the tube. The sound pressure is measured at two microphone locations and the transfer function between the two measurements is calculated. From this information, it is possible to determine the complex reflection coefficient, the sound absorption coefficient and the normal acoustic impedance of the material.

The usable frequency range depends on the diameter of the tube and the spacing between the microphone positions. Roush can provide measurements from 100 Hz to 1,600 Hz, as well as from 400 Hz to 6,300 Hz.

Transmission Loss Testing (J1400, reverb/anechoic room)

The acoustic transmission loss test apparatus measures the attenuation of sound through a material sample or part. The test is per SAE J1400 which is similar to ASTM E90. Sound transmission loss measures how effectively a material performs as a sound barrier. This is an important function for acoustic insulation materials and parts.

The acoustic transmission loss test apparatus consists of a reverberant source chamber with an aperture into a hemi-anechoic chamber in which the sample is mounted. The sound transmission loss of each sample is measured per SAE standard J1400 “Laboratory Measurement of the Airborne Sound Barrier Performance of Automotive Materials and Assemblies.” The J1400 standard refers to flat material testing; however, it is also possible to build fixtures out of automotive floors, dash panels, etc., and test entire components for sound transmission loss.

Inside the reverberant chamber, acoustic source noise is introduced. The sound field is then measured at several different locations. The average of these measurements is recorded as the source sound pressure. Microphones are then used to measure the transmitted sound field on the other side of the sample. The difference between the source sound pressure, obtained as described above, and the sound power through the sample is the transmission loss of the sample.

The SAE J1400 standard calls for the measurements to be corrected using measurements of a lead reference sample. The measurement range is third-octave bands from 250 Hz to 10,000 Hz.

Insertion Loss Testing (APAMAT II chamber)

APAMAT II Testing

The APAMAT II measures the noise radiation from an acoustic material treatment when subjected to a structural vibration input. In the APAMAT II, a sample is placed on a sheet of steel similar in thickness to an automobile body. Ball bearings impinge against the bottom of the steel sheet to provide random structural excitation. The sound pressure is measured in a reverberant chamber sealed to the top of the sample. Third octave band sound pressure measurements are made with and without the sample on the steel sheet, and the difference is the APAMAT II insertion loss for the sample. The results provide a combined measure of transmission loss, structural damping, and radiation efficiency and are useful for rank ordering sound package treatments.

For additional information, please contact 734-779-7400 or NVH@roushind.com

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