Fig. 3 Lagging applied to actual engine
Fig. 4 TL of conventional lagging
Fig. 5 TL of new lagging
Table1 Relation between R. E. and Amount of Treatment
A.T.: Amount of treatment,
R.E.: Reduction effect
P.L.: Total power level
TL = 1810g(mf) - 44 (2)
This expression is called the mass-law. Where m is the surface density (kg/m2) and f is the frequency (Hz). Comparing the measured result with the calculated one in Fig. 4, the calculation is larger in the ranges of the low frequency (<250Hz) and high frequency (>2000Hz), but smaller in the medium frequency range (500Hz - 2000Hz).
However, the calculation is nearly equal to the measured one. Thus 70% of the calculation value based on the mass-law (Eq.2) is recommended to use in case of no experimental data.
On the other hand, the measured value of TL based on the new method is very large comparing with 70% of calculated one. Especially its tendency is remarkable in case of large surface density. The difference between these two methods is the kind of material of the thin plate used as the outer plate. The conventional method uses zinc steel and the new method uses a damping steel.
The tendency of TL becomes gentle slope in frequency range 2000〜4000Hz, in the conventional method. The cause is a resonant transmit phenomenon. That lagging has 50mm thickness of absorbed material and generates the standing wave with half wave length in this space. The frequency can be calculated as follows.
f = c/λ = 340/(0.05 × 2) = 3400Hz (3)
It is noted that this phenomenon decreases the noise reduction effect.
On the other hand, the rate of TL's going up is not small in the new method. The reason is that the damping steel is used as a outer plate and the loss factor η is large(0.2〜0.4). As a result, the resonant transmit phenomenon is hard to occur.
Here, the conventional lagging was applied to the part as shown in Fig. 1. First, the lagging with the total surface density 33.0kg/m2 was applied to the charging air pipe and the air cooler housing as the countermeasure (1). The detail of this lagging is as follows.
Rock wool: 2.0kg/m2
Lead: 11.4kg/m2
Rock wool: 2.0kg/m2
Lead: 11.4kg/m2
Zinc steel: 6.2kg/m2
Next, the lagging with the total surface density 17.9kg/m2 was applied to the scavenging air receiver as the counter measure (2). The detail of this lagging is as follows.
Rock wool: 4.0kg/m2
Lead: 5.7kg/m2
Rock wool: 2.0kg/m2
Zinc steel: 6.2kg/m2
