传递损失之所以常被用来评价消声器的消声性能，主要是因为只要有消声器的几何参数，传递损失就可以获得，它不受声源的阻抗和辐射阻抗的影响，是消声器本身固有的一种特性。传递损失测试平台和消声器模型建立好之后，就可以对任何一个声学元件进行传递损失的测试通过延长的阻抗管。待测试的声学元件通过两端很短的直管连上白噪声发生器和消声末端。不一定非要加上额外的两根管子，但用在这里经过实践证明有很好大效果。 试验中经常用这两个额外的管子来布置麦克风。 这两个外加管子一定要和消声器的进口和出口有相同的直径，否则会有额外的压力损失。待测的声学元件可以是简单的组件也可以是复杂的排气系统。对本文用到的消声器进行了传递损失计算，发现改消声器有很宽的消声频带。
穿孔管在低频范围内有很好的消声效果。国外的Sullivan 和 Crocker已推导出穿孔管的穿孔率在5－10％范围内会起到共振腔的效果。所以在尾管处增加了一段穿孔率为5.29％的穿孔管。计算的结果显示在200Hz和500Hz处噪声降低2-6dB左右。从传递损失、插入损失的计算结果都是在500Hz左右的中低频范围内起到了很好的效果。同时排气背压与原机相比基本相同，说明这样的改进没有对原发动机的性能有很大的影响。
The exhaust noise was one of the most important noise sources of the automobile. Muffler was used to control the exhaust noise. And it was the effective method to reduce the noise.
In today’s competitive world market, it is important for a company to shorten product development cycle time in order to be successful in the target markets in which they compete. Engineers can no longer rely on just the “build and test” method to design products because sometimes it simply takes too long and is too expensive. Along with the improvement of the computer application level in recent years, computer aided engineering tools are often used by engineers to evaluate different designs quickly before building prototypes. With the computer simulation of exhaust mufflers and engines, the influence of mufflers on engine performance as well as exhaust noise can be predicted. In this paper, we discuss the acoustic performance of the exhaust muffler in transmission loss, Insertion loss, and tailpipe noise.
Transmission loss and insertion loss are the most frequently used acoustic performance criteria of automotive exhaust mufflers. Transmission loss of a muffler is usually determined and analyzed computationally and experimentally in the development stage of an exhaust system, while insertion loss is the final acoustic performance indicator of the system.
Transmission loss (TL) is one of the most frequently used criteria of muffler performance because it can be predicted very easily from the known physical parameters of a muffler and don’t have effect on it for source impedance and the radiation impedance, TL is a property of the muffler only. Transmission Loss Test and muffler model was built; transmission loss of an acoustic element is measured by using an extended impedance tube setup. The test piece is connected to the acoustic driver and an anechoic termination through two straight pipes. It is not absolutely necessary to have extra pipe lengths added to the muffler, but is used here to demonstrate good practice. Experiments often use these extra lengths for placement of the four microphones. These extra lengths must have the same diameter of the connecting pipes to prevent additional pressure loss. The test acoustic element could be a simple component or a complex exhaust system. The TL is calculated for the muffler, the muffler exhibits a broad attenuation band.
The engine model must be built before the predicted of insertion loss.
The model is a basic single-cylinder engine, consist of environment, intake port, intake pipe, injector, intake valve, cylinder, engine, exhaust valve, exhaust port, exhaust pipe, entering the data for these object, then connect all the components together from left to right in the general flow direction. Once the engine model is fully built, the exhaust muffler is attached to the engine. A microphone is located at 12 inches away from the tailpipe. During the test, the engine runs from 4500 to 8000 r/min under the wide-open throttle condition. It is seen that the predicted and measured tailpipe noise match very well in low engine speed. With the speed increased, the prediction differs from the measured result. This is due to that at low r/min, flow speed is slow, the air flow noise dominates tailpipe noise. At high r/min, flow speed is high, the friction noise dominates tailpipe noise. But GT-POWER is 1-D software， it can not predicted the friction noise.
Insertion loss (IL) is defined to be the difference in the acoustic power at one point in space with and without the muffler inserted between that point and source. For comparison and calculated insertion loss, the muffler is replaced with one straight pipe with the same length as the corresponding components. The predicted and measured tailpipe noises match very well for the straight pipe. At higher frequencies, the prediction differs from the measured result due to the simulation software is a one-dimensional simulation software. We make a comparison in tailpipe noise between straight pipe and muffler, it is seen that there is a sharp peak in low frequent. From the predicted IL, it is seen that there are two peaks at 4500r/min, one is 35.5dB at 397Hz, and the other is 38.6dB at 793.7Hz. There is a valley at 500 Hz that match very well with the measured which has a peak at 500 Hz for the pass noise. So the attenuation of low frequency noise about 500 Hz is the main objectives when designing the new muffler.
Perforated pipe has good acoustic performance at low frequent. The work of Sullivan and Crocker suggests porosity in the range of 5 to 10% to achieve the expansion chamber. So we add a perforate tube in tailpipe, take a 5.29% porosity. The predicted result has a good effect in low frequent. At 200 Hz, the tailpipe noise attenuates 2-6dB. At 500Hz, the tailpipe noise attenuate 1.5dB.From the other acoustic performance, transmission loss, insertion loss have the same results. The back pressure is almost same with the old muffler, so adding perforated tube doesn’t change the engine performance.