Hybrids’ silence brings big acoustic challenges

  • 13-Jul-2009 10:12 EDT
Bruel & Kjaer’s Pulse tool helps engineers analyze the subtle sounds that are audible in a hybrid’s quiet environment.

Though consumers often comment on the silence of hybrid vehicles, noise and vibration specialists hear a whole new set of sounds. The whine of electric motors and the whir of fans are heard loudly when they are not masked by the purr of a gasoline engine.

High-pitched whines and whirs do not generally get the same positive response as the engine’s deep rumble, raising a new set of challenges for acoustic engineers. Attendees at the SAE Noise and Vibration Conference in St. Charles, IL, highlighted a number of challenges and solutions that come with the industry’s shift to electric power.

“Sound quality is a whole new ball game with hybrids,” said Anthony Fleszar, Manager of Engineering Services for LMS North America. “In the absence of engine noise, a lot of things that weren’t significant before are now very apparent.”

Ambient noise from auxiliary products such as the alternator and cooling fans for the engine and battery packs are now audible in the cabin, as are fans used to control interior temperature. Though the electrical components in the powertrain are quieter than gasoline engines, they still factor into the analysis of noise. “In hybrids, the two main issues are whine and relay switching,” said Fraser Henderson, Pulse Product Manager at Bruel & Kjaer. “Whine is never really attractive.”

In conventional vehicles, the gasoline engine has long been criticized as noisy and praised as pleasing. In hybrids, these engine sounds bring new challenges. Drivers are accustomed to hearing the engine rev when they step on the throttle. But engines now start up at seemingly random times, altering perceptions that have been expected for decades.

“When you step on the gas, electric motors are virtually silent; there’s no sensation of power,” Fleszar said. “The gasoline engine now kicks in with no correlation to what the driver is doing with the gas pedal.”

When engines start up arbitrarily, many of their pleasing acoustic attributes disappear. One key is to mask the start-up sounds so they are not jarring.

“You need to carefully control how the engine ramps up,” said Kiran Govindswamy, Chief Engineer for Noise, Vibration, and Harshness at FEV. “You can see an increase of up to 10 dB when it kicks on.”

All these new acoustic concerns come at a time when carmakers and suppliers don’t have much manpower to dedicate to a new task. That’s prompting acoustic equipment and service providers to focus on simplifying audio tests.

Many of these tests now require specialists and so-called golden-ear technicians. The makers of test equipment are striving to develop equipment that is easier to set up. They are also trying to remove custom components so tests are less expensive and more repeatable.

“In the past, engineers usually built a microphone array for each vehicle,” said Brian Wright, Sales Vice President for SenSound. “We’ve got a microphone array that can be reused.”

Setup is simpler since SenSound uses a wireless digitizer that measures the distance between the microphone and the surface of the element under test. That frees engineers from the time-consuming task of measuring the position of each microphone, he noted.

Mainstream tool suppliers are expanding their offerings in a market that has been dominated by specialists. “We introduced a Sound and Vibration Assistant tool kit for LabVIEW last year. It steps engineers through setup and analysis,” said Kurt Veggeberg, Business Development Manager for National Instruments’ (NI) Sound and Vibration group.

NI also rolled out a USB module that lets engineers gather sensor input using a laptop computer. For tests that are difficult to connect using wired components, the instrument supplier also provides a Wi-Fi version, Veggeberg added.

Though there is a push to standardize tests, observers note that sound is subjective, so it is not easy to automate. "Sound qualities are often reduced to a metric problem, but metrics aren’t a good representation of the customer’s perception of sound,” said Alun Crewe, Vice President of Strategic Marketing at Bruel & Kjaer.

Bruel & Kjaer unveiled its Pulse toolset at the conference. This post-processing tool helps engineers analyze sound and devise solutions.

Much of the low-hanging fruit has been plucked, forcing engineers to focus in on minute contributors to the overall noise level. That often means going inside the electric motors, looking at specific elements such as gears.

“The preferred solution is to attack whine at its source,” said Fleszar of LMS North America. “We’re increasing the fidelity of our gear models, looking at things like surface finish instead of just the tooth profile."

When gears mesh together, noise often arises when the teeth merge. There are several different points of contact as these teeth engage and disengage, creating noise by what is known as transmission error.

“You want to minimize transmission error, which is adjusted through microgeometry,” said Kurt Sheridan, Senior Applications Engineer for Romax Technology Inc. “We’re talking adjustments that are often only a couple 10-thousandths of an inch in modification.”

Enclosures and mountings are also getting close attention. “You have to look at how noise gets from the source and absorb as much energy as you can,” Fleszar said.

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