Page 760 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Exterior noise: Assessment and control C HAPTER 22.1

Intake filter box with conical snorkel
35
Conical snorkel

Parallel pipe snorkel
30
25
20
Attenuation (dB) 15

10

0 5
–5
–10
0 100 200 300 400 500 600 700 800 900 1000
Frequency (Hz)
Fig. 22.1-32 Effect of adopting a conical snorkel. Calculated using APINEX software.

The frequencies at which the system attenuation is 22.1.3.13.3 Helmholtz resonators
improved through the action of the sidebranches may be These feature a 1–3-litre volume placed as a sidebranch to
calculated for the zero flow case from:
the snorkel. Connection to the snorkel is by a tuned length
4xf of pipe. These devices provide useful low-frequency
¼ n n ¼ 1; 3; 5; 7; 9; . (22.1.79) attenuation with little extra static pressure loss. The
c
effect of adding of a Helmholtz resonator, tuned to
where x is the sidebranch length and f is the resonant around 90 Hz to the snorkel (as illustrated in Fig. 22.1-34)
frequency. is shown in Fig. 22.1-35.
In practical cases, the attenuation shown in Fig. 22.1- The Helmholtz resonator is a reliable (and often the
33 would be limited to around 40 dB by the effects of only practical) way of increasing intake system attenua-
flow noise. tion at frequencies below 120 Hz.

Intake filter box with 120-mm protrusion of a parallel-pipe snorkel into the box
80
Protruding snorkel
Non-protruding snorkel
70
60
50
Attenuation (dB) 40

30
20
10

0

–10
0 100 200 300 400 500 600 700 800 900 1000
Frequency (Hz)
Fig. 22.1-33 Effect of protruding the snorkel into the filter box. Calculated using APINEX software.

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