Study on the Monolithic Muffler Acoustic Field Performance of the Train Air-Conditioning - Advanced Research on Computer Education, Simulation and Modeling - page 143

3.2 The Evaluation Indexes of the Muffler Acoustics Performance

The sound deadening capacity usually is used to evaluate the quality of the muffler
acoustics performance, according to the different test method, the evaluation index of
the muffler acoustics performance include in transmission loss, insertion loss,
transmission sound pressure level differential and insertion sound pressure level
differential, this paper apply the transmission loss as the evaluation index.
The muffler transmission loss LTL is the difference of the sound power level of the
muffler inlet and export. It is in terms of the sound insulation of the components to
reflect the sound deadening capacity of the muffler with the transmission loss. The
formula is as follow:

⎛ W ⎞

⎜ ⎝ ⎠ (6)
where: L TL —The transmission loss ,dB,W 1 —The entrance sound power,W,W 2 —The
exit sound power,W,L W1 —The entrance sound power level,dB,L W2 —The exit sound
power level, dB. The transmission loss is an intrinsic feature of the muffler. It is little
subject to the impact of the sound source and the environmental. Because the sound
power levels is difficult to measure directly in the practical engineering tests. So
usually by measuring the average level of the cross-section before and after the
muffler, and then calculated as follows:

L=10lg = L − L

1

W

TL W W

1 2

2

L=L + 10lg S

(7)

w1 P 1 1

L=L 10l P + (8)
Where: L P1 —Average sound pressure level of the muffler entrance,dB, L P2 —Average
sound pressure level of the muffler exit,dB,S 1 —Cross-sectional area of the muffler
entrance,m 2 , S 2 —Cross-sectional area of the muffler exit,m 2 .

w2 2 2

3.3 The Analysis of Test Results and Simulation Results of the Sound Field

The test values and simulation values under the center frequency of the 1/3 octave are
just as Table 2.

Table 2. Test and simulation values of the transmission loss by A weighted

7HVW 7HVW YDOXH YDOXH 6LPXODWLRQ 6LPXODWLRQYDOXHV YDOXHV YDOXHV 'HYLDWLRQ 'HYLDWLRQ

,QOHW 2XWOHW

G% G% G%

G%

Table 2 shows that the test value of the muffler transmission loss by A-weighted is
28.1dB, and the deviation compared with simulation value is 5.24%. It indicates that the
simulation model is reasonable, and provides a good way for the muffler further research.
Figure 3 shows the size of the test value and simulation value of each frequency in 1/3
octave band, what can be seen from Figure 3 is as follows: (1) The attenuation frequency
of the monolithic resistive muffler is very wide and the attenuation effect is obvious,
especially for the middle and high frequency sound wave, but to low frequency sound