Impedance tube

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. 

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. 

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

An interesting type of viscous absorber, developed by Meyer et.al. [21] for underwater operation, is shown in Fig.4. It is clear that in this case the longitudinal particle motion is converted to a transverse viscous flow. The peak performance (and frequency range) of this absorber are functions of the mass of the cover plate, and the viscosity of the liquid between the cover plate and the backing structure. Tests in a water-filled impedance tube have shown that a relatively broadband... 

As the design matures, the direct measurement of the acoustic properties becomes necessary. These properties include the longitudinal wave speed, the coefficient of attenuation and the acoustic impedance, which can be obtained from measurements of the reflection and transmission of sound by the material. Two acoustic techniques are available for these measurements, the impedance tube and the panel test. 

In practice, we can use the impedance tube to find the complex coefficient of reflection and then vary the two propagation constants in the theory to produce the same complex reflection coefficient. These variations are not easy to perform as the equation is transcendental however, there are computer programs available to do this (7). 

A short computer program was used to find the propagation constants from the impedance tube data using suggestions from (6-7). Data for a silicone rubber sample appears in the table below and a plot of the data is shown in Figure 5. 

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. 

The measurements were carried out using the Impedance tube... 

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.

Fig. 7.1 Experimental setup using the B K t) pe 4206 impedance tube kit as per ASTM E1050...

Fig. 7.2 Schematic of plane wave generation for acoustic measurement using 2-microphone impedance tube...

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. 

Fig. 7.4 Filling of silica aerogel granules in impedance tube...

The two-microphone impedance tube determines the ratio of absorption coefficient and complex reflection coefficient based on the transfer function between the two microphones. The integrated PULSE software that calculates the various data ensures that the sum of the two coefficients shall always conform to unity. The plots in Fig. 7.5b show that AG2 and AGS have the best absorption coefficients of 0.52 and 0.48, respectively, compared to the other sizes when measured at 15 mm thickness. The absorption coefficients lowered to 0.32 and 0.29, respectively, when the thickness is reduced to 10 mm as shown in Fig. 7.5a. Naturally it can be seen that mixed granules, AGMX, for which 70 % of the composition come from the two sizes mentioned, has an absorption coefficient that is close to both the 1.2 and 1.7 mm sizes. The porous ply shows negligible absorption similar to air. Table 7.2 lists the absorption coefficients abstracted from Fig. 7.5 at the center and maximum frequency. 

Smaller GSA-SDS specimens (29 mm diameter) comprising 1.7 mm aerogel granules are tested for acoustic performance in the small impedance tube for frequencies from 500 to 6400 Hz. The absorption coefficients and the TL in the higher frequency range are shown in Fig. 7.10a, b. 

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... 

Sung Soo, J., K. Yong Tae, L. Yong Bong, C. Seung II, and L. Jong Kyu. 2008. Measurement of sound transmission loss by using impedance tubes. Journal of the Korean Physical Society 53P) 596-600. 

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. 

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