16.3.4 Addition of Side Branch Tube Silencers on the Same Axis to the Opening Here, we discuss about a silencer that has the tail pipe and grating of the same length as the tail pipe discussed in the previous section. The donut-shaped cross-section pipe fitted along the same axis between the tail pipe and grating as shown in Fig. 16.9 is open on one end and closed on the opposite end. This arrangement produces the same effect as a side branch silencer installed on the same axis as the tail-pipe opening. Figure 16.10 shows the tail pipe and grating expressed as an electrical equivalent circuit. The open end of the tail pipe expressed with a four-terminal network transfer matrix is terminated with radiation impedance at the open end and incident impedance of the side branch tube. At the resonance frequency of the length of the side branch tube (here, the depth of the grating), the impedance is smaller at the open end. Accordingly, we can expect a sound absorption effect. The measurement results are shown in Fig. 16.11. Centered on frequency of 1,200–1,400 Hz, sound attenuation was improved in the range of 1–2 kHz. Accordingly, the results show that we matched the previous predictions for the sound attenuation frequency. Through the addition of the grating, the decreasing of the peak frequency of 1,700 Hz contributed to sound attenuation for dog barking model. And we obtained sound attenuation of 1.2 dB in comparison with that achieved in the previous section. We achieved sound attenuation of 11.01 dB for the dog barking model, which exceeds our initial target of 10 dB. 0 2000 4000 6000 8000 40 50 60 70 80 90 Frequency [Hz] SPL (A-weighted) [dB] Non-woven 92.10 dB Non-woven + tail tube 90.19 dB Model of dogs barking 100.00 dB Fig. 16.8 Equalized attenuation with or without tail tube (with nonwoven fabric) Fig. 16.9 Partial outlook of surrounding tail tube with grating 158 S. Sakamoto et al.
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