Designing the Signature Sound of Car Exhaust

We’ve all heard the distinctive rumble of a performance car. But have you ever thought about how this is achieved? This week we spoke with Pat Hodgins, Chief Engineer of Callaway about how they design that signature sound into their cars.
 
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Allison: Hi, Welcome to the Product Design Show, IÆm Allison Toepperwein. WeÆre all familiar with automotive exhaust systems.

Vince: But exhaust systems do more than just channel fumes and tune performance, Sometimes they provide the "voice" thatÆs the signature sound of an automobile. IÆm Vince Penman.

Vince: Callaway Cars creates retrofitted Corvettes, Camaros and Sport Trucks, all with a signature Callaway sound. We spoke with Pat Hodgins, Chief Engineer at Callaway, about how they design their exhaust systems and what it means to for a car to have that "signature Callaway sound".

Allison: CallawayÆs signature sound represents an interesting design challenge because it requires both rigorous engineering, and a accurately tuned ear, something Pat described as a "black art".

Vince: Engineers at Callaway first examine the baseline volume parameters of each car by securing para-solid models from SEMA, the Specialty Equipment Marketing Association.

Allison: The models from SEMA give the engineers at Callaway the nominal tolerances for each component of the exhaust system. The Callaway engineers then use Creo/Parametric to import the para-solids and begin modifying the system to deliver greater performance and the right sound.

Vince: Creating the desired sound lies in the design of the packaging and the acoustics of the exhaust system. Acoustics in an exhaust system are generated within the exhaust canister and are, primarily, defined by the canister volume, itÆs cross-section, materials and internal components.

Allison: Sound energy generated by the engine cycle resonates through the exhaust where itÆs frequencies are amplified by the architecture of the muffler.

Vince: Like a choir, every frequency created by the engine cycle, has to be in harmony, and needs to have a deep, rich, sophisticated tone... much like this.

Allison: Because the engine firing order is set by other design parameters it may not be the perfect arrangement for generating a particular sound. The firing cycles of the left and right banks arenÆt uniformly alternating, which can create discordant engine frequencies. Attenuating those frequencies is key in attaining a harmonized sound

Vince: To attenuate undesirable frequencies CallawayÆs engineers use a combined approach that includes reflecting and absorbing the sound energy introduced into the exhaust system.

Allison: By modifying the volume and shape of the exhaust canister, sound can be reflected against itÆs walls, changing the wave frequency of the engine sound.

Vince: Additionally, material selection for the exhaust canister can greatly improve the quality of exhaust sound, particularly in the mid to high frequency range.

Allison: Selecting a material that is porous will allow sound passing through the exhaust canister to be dissipated as heat energy, eliminating many of the undesirable higher pitched frequencies.

Vince: With the engine frequencies tuned, engineers examine the decibel output for the tip in and tip out engine response.

Allison: Letting people know youÆre there when you hit the gas is important, but finding the right balance of loudness and sophistication gives a car character.

Vince: In the end, isolating the correct exhaust sound puts one of the finishing touches on an already, beautifully designed car, and is a testament to the importance of details in design.

Allison: You can see Creo Parametric in action, the same software that Callaway used to design their exhaust system, at PTC.com/go/creoparametric.

Vince: If you like this show, please give it a like on Facebook, subscribe on Youtube or give us a rating on iTunes.