Impedance tube method

Test Methods. Two basic types of test methods are commonly used to measure sound-absorption in test laboratories the reverberation room method and the impedance tube method. 

Test Methodfor Impedance and Absorption of Acoustical Materials by the Impedance Tube Method, ASTM C384-90a, ASTM, Philadelphia, Pa., 1990. 

ASTM C 384 is another method used for acoustical materials. This method is an impedance-tube method. In the test a flexible-foam specimen is placed in a cavity over a vacuum chamber. A specified constant-air-pressure differential is then created. The air-flow value is the rate of flow of air required to maintain this pressure differential. The test is carried out 1) with air flow parallel to foam rise, and 2) with air flow perpendicular to foam rise. Air-flow values are proportional to porosity in flexible foams. 

Two-component IPN foams consisting of polyurethane and epoxy were prepared by the one-shot, free-rise method. The effects of PU/E ratio on the sound absorption and mechanical energy attenuation characteristics were determined with varying levels of different fillers and plasticizers. The formulations (Table IX) were based on the best elastomer results. An average of over 90X absorption was obtained at high frequencies by the Impedance tube method. However, this average drops dramatically at low frequencies. This reduction may be seen in Figs. 3 and 4 for 90/10 and 70/30 IPN foams vlth 20X... 

In order to evaluate the acoustic properties of the various formulated composites, the sound absorption coefficients were determined by the impedance tube method according to ASTM E 1050-12. A soxmd absorption coefficients and impedance measurements instruments (Figure 16.3) made by Walen Audio Technologies, Maharashtra, India was adopted. The sample diameter was 100 mm, and each value represented the average of six samples. The soxmd absorption coefficients were measured in six frequencies 100, 200,400, 800,1600 and 3200 Hz. 

Figure 16.3 Schematic diagram of the impedance tube method.

Subsequently, several research teams became interested in studying the damping characteristics of IPN s. Wong and Williams characterized the tan d vs temperature behavior of polyurethane-epoxy semi-IPN s, noting one very broad peak ranging from 273 to 363 K for the 50/50 composition. Klempner and co-workers also studied the polyurethane/epoxy system, but prepared foams by a one-shot method, producing SIN s. Using an impedance tube method of measurement, enhanced sound absorption was found. 

Impedance Tube Test Methods. There are two impedance tube test methods ASTM C384-90a (3) and ASTM E1050-90 (4). Test method C384-90a makes use of a tube with a test specimen at one end, a loudspeaker at the other, and a probe microphone that can be moved inside the tube. Sound emitted from the loudspeaker propagates down the tube and is reflected back by the specimen. A standing wave pattern develops inside the tube. 

One advantage of the impedance tube test methods is the small (usually <10 cm (4 ia.) dia) size of the test samples. For these tests sound impinges on the test sample only at normal iacidence to the surface, and the sound-absorption coefficients derived ia this manner are vaUd only at this angle. 

A fragment of a tube with the wall 0.5 mm thick was cut for conductivity measurements. Large opposite surfaces had silver electrodes, which were fused at 700 °C for nearly 18 hours. Measurements were made using the two-probe impedance spectroscopy method on a lm6 instrument (Zahner-Elektrik) in air at frequencies from 10 to 8-10 Hz over the temperature interval of 220 to 700 °C with steps of 20-50 degrees. 

One of the drawbacks of the two-microphone transfer function method is that the absorption coefficient determined may not be a true representation of the material s characteristic. In the case of a porous material, such as silica aerogels, the reflected wave from the rigid wall could contribute to a rise in the absorbed energy by the material. To account for this uncertainty, the four-microphone impedance tube setup is usually used to determine the transmission loss (TL) and absorption coefficient (Feng 2013). In the absence of additional microphones downstream of the specimen, a sound meter could be used instead to measure the TL of the specimen under test. However, the sound meter picks up discrete transmitted signals at periodic interval, which could result in a mismatch with the generated signals from the source. 

F. 7.7 Evaluation of TL for AG2 and AG3 via proposed inferential method using 2-microphone impedance tube... 

The second aspect is the proposed novel Inferential Transmission Loss method (InTLM) in determining TL using 2-microphone impedance tube. The approach is a modification to the usual transfer method that infers transmission coefficient with and without the rigid wall. The calculated results showed high accuracy from —0.2 to -3.2 dB compared with sound meter measurements. Thus, the InTLM can be applied for 100 mm diameter specimens which use the large tube in estimating the TL without the need to use 4-microphone impedance tube... 

Vigran, T.E. 2012. Normal incidence sound transmission loss in impedance tube—measurement and prediction methods using perforated plates. Applied Acoustics 73(4) 454-459. 

Sound absorbers are usually employed for modilying the sound reverberation in a room, suppressing undesired sound reflections from remote walls (echoes), and reducing the acoustical energy density and, hence, the sound pressure level in noisy rooms. There are two standard methods of measuring absorption coefficients. One is for measuring normal incidence absorption coefficients in an impedance tube and the other is for measuring random incidence absorption coefficients in a reverberation room (Bies and Hansen, 2009). 

There are many acoustical methods proposed for measuring flow resistivity (Delany and Bazley, 1971 Smith and Parott, 1983). A method that uses a standard impedance tube directly to measure the static flow resistivity without any additional requirements to tube modification or sensor location change is described in ISO Standard, 10534-2 (1998) and by Tao et al. (2015). In the method, the specific acoustic impedance on the front surface of the test specimen is measured first by using the traditional transfer function method with the test specimen being placed against and with a known interval to the rigid termination, and then the characteristic impedance, the propagation constant, and the static flow resistivity are calculated based on the obtained impedance transfer functions. 

The transmission coefficient is the fraction of incident energy transmitted through a partition or a textile layer when a sound wave is incident upon it, and the transmission loss is the logarithm of the reciprocal of the transmission coefficient. Two types of transmission loss are commonly measured with a layer of textile material. The random incidence transmission loss is measured in a reverberation room using the reverberant room method, while the normal incidence sound transmission loss is measured in an impedance tube using the transfer matrix method. 

Fig. 5.6 Setup for normal incidence absorption coefficient measurements within an impedance tube based on the standing wave method.

McGrory, M., Cirac, D.C., Gaussen, O., Cahrera, D., 2012. Sound absorption coefficient measurement Re-examining the relationship between impedance tube and reverberant room method. In Proceedings of Acoustics 2012—Fremantle, Australia. 

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