Free Field Technologies

An automobile’s exhaust system is becoming more and more critical to its success in the marketplace. Most important, the sound produced by the vehicle serves to a considerable degree as the signature of the brand. For example, an auto enthusiast can recognize the approach of a Bentley or Ferrari with his or her eyes closed. Purchasers of lower-priced vehicles may not be quite so finicky but they still expect to hear a certain sound when they start up the engine. Meanwhile, automotive original equipment manufacturers (OEM) are being forced by government regulations to reduce the levels of noise emitted from the tailpipe. Automakers are also hoping to reduce the back pressure of exhaust systems in order to achieve improvements in fuel economy.

It’s becoming increasingly difficult to meet these often conflicting goals using conventional passive exhaust system technology which relies upon the use of perforated tubes and chambers to filter out acoustic waves. Automotive original equipment manufacturers (OEMs) are looking at active exhaust systems as a way to address these issues. Active exhaust systems use a loudspeaker driven by a microprocessor to cancel out unwanted sound generated by the engine as well as to produce more desirable sounds. A key advantage of active exhaust systems is that they can be controlled by software to adjust the output of the loudspeaker to deliver just the right sound under a wide range of different operating conditions.

“Actran has enabled Tenneco to develop a process for electroacoustic simulation of an active exhaust system including the loudspeaker and housing that correlates very well with physical experiments,” said Nicolas Driot, Senior Core Science Engineer for Tenneco. “We are now using simulation to develop our next generation active exhaust system. Simulation will make it possible to evaluate the performance of many alternative design concepts in a minimal amount of time without the expense of building physical prototypes. This should make it possible to improve the performance of the exhaust system beyond what can be achieved with the traditional process where only a few different design alternatives can normally be evaluated. Simulation will also make it possible to bring new products to market faster.”

• Actran results match very closely with physical test measurements both when modeling the loudspeaker alone and then when simulating the loudspeaker integrated it in a complete exhaust system.
• Simulation allows evaluating the performance of many alternative design concepts for active noise cancellation in a minimal amount of time without the expense of building physical prototypes.

Noise regulations in a key Latin American country were revised to a lower level, forcing CNH and other construction equipment manufacturers to reduce the noise footprint of their products. The new specification uses a formula to determine the allowable sound pressure level based on the power of the machine. In this case, the new standard required a noise reduction of about 6 dB(A) relative to the existing design. The noise measurements are based on the ISO 6393 standard. Six microphones are arranged in a hemisphere at specified locations around the equipment and the sound pressure level is measured while the equipment is operated under high idle conditions. CNH engineers ran physical tests in an effort to better understand the noise contributors. Among any things, they turned the fan on and off, turned the hydraulic pump on and off, and masked the sound coming from the exhaust to better understand the relative importance of the noise contributors. The analysis showed that the fan was the number one contributor.

Simulating the acoustic performance of alternative approaches to noise remediation helped us meet the tighter noise specification in about three months.

Panos Tamamidis, Global Manager at CNH Industrial

• Quicker understanding of noise contributors
• Simulations proved more comprehensive than physical test
• Met noise specifications faster
• Able to ensure acoustic performance of new designs