Making Music with Multiphysics

The capabilities of simulation software appear to be endless (not really, but you know what I mean…) when it comes to modeling different systems and things that may not have been simulated before. This can be especially true when considering how model-based systems engineering (MBSE) has advanced in the past few decades from single-purpose tools focused on one system or set of physics, like engines or general fluid flow simulation, to the current state of multi-physics simulation that encompasses mechanical, fluid flow, electrical, thermal, and other domains. While looking at different possibilities considered by manufacturers, one system came to the attention of a few of us at Gamma Technologies and took us down an unexpected, yet interesting and even amusing path. 

Making Noise to Hide Noise...Virtually 

In the late 2010’s, different automotive exhaust suppliers were developing active noise cancellation (ANC) systems that could further reduce the sound coming from the passive systems of mufflers and resonators that have been a standard component of engine for many decades. As a result of this investigation, Gamma Technologies took some time to examine what was done and determine if GT-SUITE could be used to simulate such a system and if not, determine what might need to be added to make it possible. 

Description of the System and Source of the Noise 

The ANC system is made of a volume that is nearly spherical attached to a pipe that connects to the outflow of a muffler near the end of an automotive exhaust system. In the volume are speakers that produce the artificial noise that cancels or modifies the noise coming from the muffler. Engineers at the exhaust system maker Eberspaecher wrote a paper describing the system in more detail in 2017 that was published by the Society of Automotive Engineers. (“Active Cancellation of Exhaust Noise over Broad RPM Range with Simultaneous Exhaust Sound Enhancement”; Riddle, Bemman, Frei, Wu & Padalkar; SAE Technical Paper 2017-01-1753; 2017; doi:10.4271/2017-01-1753.)  A simplified depiction of this system is shown in Figure 1 below. 

Figure 1: Active Noise Cancellation system concept

Modeling Exhaust System Acoustics and Active Noise Cancellation 

To model this system, engineers at GT considered the possibilities and the physics included among the parts. GT-SUITE has been used for a few decades to model the pulsations of exhaust systems. Suppliers like Eberspaecher, Forvia (formerly Faurecia), and Tenneco have been using simulation as part of their design process for much of that time. Included in the software is a virtual microphone that can calculate the sound at a distance from the exit pipe and store it in a sound file playable on typical computers and other devices, like phones, often in the WAV format.  At the same time, GT-SUITE includes multiple multi-physics libraries for electrical and mechanical systems that can be incorporated into the fluid dynamic model to simulate the entire system. There is a template in GT-SUITE that can convert a current signal into a magnetic and mechanical force that can move a mechanical mass, whose motion can be  limited by a spring and damper.  This is the principle of a speaker, used in sound systems, phones headphones, and so on. These parts are shown in the yellow part of the image below (Figure 2): 

Figure 2: Model of the Active Noise Cancellation in GT-SUITE

The mass of the speaker coil and cone is connected the volume of the ANC system by a connection in GT-SUITE made for this purpose, MechFlowConn. This connection translates the motion of the mass into changes in the volume, and which transmits the sound to the fluid system, shown in the blue region of the image. The sound transmits to the pipe and down to the outlet, which is connected to the microphone and sound file generator in the pink region. 

In practice, one might produce a signal from another source, such as a control system model made in Matlab/Simulink, especially to test the controls that would take feedback from the system and change the signal going to the speaker system.  However, as this was just a conceptual study, a realistic system was not available to GT to test it and so it was decided to generate signals within the control system library of GT-SUITE and listen to the results.  Tests with individual frequencies passed and GT was satisfied with the results. 

Engineers Having Fun 

By good fortune, in the past weeks, it was remembered that this study had been performed, and the question was asked, “Could we make a little melody using this method?” The answer was YES! 

In the spirit of the upcoming holidays, the main melody from Beethoven’s Ninth Symphony (fourth moment) was selected. And so we present to you an original ANC recording, and our own version of Ode to Joy, performed by GT-SUITE:

Learn More About our Acoustic Simulation Solutions 

If you are interested in learning more about GT-SUITE’s intake and exhaust acoustic simulation offerings, visit this webpage here. If you’d like to learn more about how GT-SUITE‘s capabilities, contact us!