Two-microphone Transfer Function Research Articles

Impedance measurements on reflective surfaces: effect of nodes

The ISO 13472-2 standardizes the in-situ measurement of normal-incidence acoustic absorption of road pavements. The standard describes the use of the two-microphone method with an impedance tube specially configured for sealing against pavement surfaces. The method is of interest to the tire-pavement noise and vehicle pass-by noise communities. For roads constructed of dense graded asphalt, for example, pass-by noise test tracks conforming to ISO 10844, the pavement is a highly reflective surface. Under this measurement condition, the acoustic field within the tube develops nodes at one or the other microphone positions at characteristic frequencies. These nodes present as sharp peaks and valleys in the two-microphone transfer function. Such narrow bandwidth features are susceptible to measurement error, in particular, signal processing leakage as evidenced by significant drops in the coherence function at the frequencies of the nodes. In this paper we study the nature of the nodes and the effect they have on the two-microphone transfer function and coherence function. We further investigate how different formulations of the transfer function affect the overall 1/3rd octave band accuracy. The effect of the number of averages on both coherence and leakage error is assessed. The potential to solve this leakage effect by switching microphones to other positions is evaluated in relation to a procedure with two fixed microphones. It was concluded that although shifting microphone positions does have advantages in terms of preventing leakage, the mandatory application of a reference measurement reduces the overall error. Finally, recommendations for improving the procedure in ISO 13472-2 are formulated.

Steam exploded wood cell walls reveals improved gas permeability and sound absorption capability

Steam explosion is a useful method to soften and dimensionally stabilizing wood. The effects of steam explosion at low pressure (6 bar) on wood cell walls are characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), capillary flow porometry and two-microphone transfer function. The average sound absorption coefficient (SAC) of treated Chinaberry samples on the cross-sectional surface exhibited significant improvement of 79% relative to control specimens over a wide frequency range of 250–6400 Hz. Whereas, a little increment SAC of 5% was observed in the case of Ginkgo wood. Color of steam-exploded woods became black due to chemical reactions in wood cell walls during the steam explosion. The air permeability, SAC and color change are correlated with the results of FTIR and XRD. These results suggest that steam explosion could be used to manufacture wood-based sound absorption board to control the acoustical housing environment.

Measurement of Sound Absorption Using a Single Fixed Microphone in a Circular Pulse-Tube

This study presents a method for measuring the normal incidence sound absorption coefficient of acoustical materials by separating the incident and first reflected waves in the time domain using a pulse-tube with only a single microphone whose position is fixed. Based on the pulse generation technique, the effect of the characteristics of tube termination can be eliminated, and the drive signal used for the measurement is obtained. A moveable piston is used to move the sample to a certain position in the tube, which renders the recorded incident and first reflected waves separated. As a validation of the proposed method, two different materials are investigated. Good agreement is found between the proposed method and both the well-established two-microphone transfer function (TMF) method and the verified pulse separation method.

Effect of characterization of porous metal fiber media on sound absorption coefficient

Porous metal fiber media (PMFM) is a kind of advanced structural and functional material, and it has attracted a wide spread attention owing to excellent sound absorption performance. The sound absorption property of PMFM is mainly influenced by the fiber diameter, the average pore size and thickness of PMFM. In the paper, three stainless steel fibers with the diameters (∅) of 8, 12 and 20 μm were used to make PMFM with the average pore sizes of 10, 20, 30 and 40 μm and the thicknesses of 1, 2 and 3 mm by air-laid and sintering processes. The sound absorption coefficients of PMFM were tested in impedance tube using two-microphone transfer-function method according to ISO 10534-2 and ASTM E1050-98 international standards at room temperature. The results show that when the frequency ranges from 50 Hz to 6,400 Hz in material with the average pore size of 20 μm and the thickness of 3 mm and the fiber diameter of ∅8 μm, the average sound absorption coefficient is the highest.

The Comparison of Sound Absorption of Kapok-Based Fiber Nonwoven Fabrics

As a natural fiber, kapok fiber has the high hollow degree that is very good for sound absorption. Four different kinds of kapok-based fiber nonwoven fabrics, made by kapok fiber mixing with hollow polyester, viscose fiber, PP fiber and cotton fiber respectively, were made and the sound absorption coefficients were measured in the frequency region of 100 - 6300 Hz by using a two-microphone transfer-function method. The comparisons of the sound absorption for four types of materials with similar thickness and densities with no air gap and with 1 cm, 3 cm air gap were made. The results indicate that the sound absorption of kapok/hollow polyester fiber nonwoven fabrics is superior to those of other three ones and kapok/hollow polyester fiber nonwoven fabrics can be used for sound-absorbing materials in engineering.