EXPLORE THE POSSIBILITIES
The subjective world of e-acoustics
How Trek Bicycle put sound quality on the e-bike metric map.
By Jenn Schlegel
It is the perfect autumn day. The leaves are just starting to turn. Summer is still in the air as the sun burns through the green-yellow canopy. The chirp of the chickadees. The crunch of pine needles on an empty trail. You’re off for a ride to remember. Without a care in the world…Just you, Mother Nature and your new electric bike – a Trek Fuel EXe.
Being a born-and-bred Wisconsinite, we’ve had Trek bikes in the family garage since the early days of the company in the 1980s. I have fond memories of tooling around the University Wisconsin-Madison on a much beloved black Trek – almost like a precursor to their now famous Electra Townie®. However, let’s put my nostalgic love of the brand aside and look ahead to the world of e-bikes.
One thing that I'm pretty sure about is that the sound of an e-bike is not 100% silent.
This statement comes from my eclectic experiences with first-generation EVs, e-vacuums and robotic lawnmowers. It is true – you don’t hear the dBs of a combustion engine or the hum of a small motor anymore – but whatever you electrify, you tend to hear an intriguing selection of rattles, hums, whirls, whistles, clicks and the occasional buzz.
Experts are quick to point out that quite a few of these sounds always existed – it is just the fact that you hear them better without the decibels of a combustion engine covering them up. Others admit that quite a few of these new sounds come from electric drivetrains and e-motors. But argue what you want, electrification and electric motors do bring their own soundscape to the bigger picture.
Electrification does not magically equal silence
Let’s turn back the clock to the late 1990s and the early days of modern EVs. Acoustics specialists and NVH guys were scratching their heads about what to do when you don’t hear the growl of a combustion engine anymore. Do you replace it with artificial sound? Were all those strange sounds driving customers away? Were they genuinely annoying? Or was it just a small group complaining? And more importantly, what about pedestrian safety?
Fast-forward two decades and the EV sector seems to have acoustics under its thumb. EVs today are required to have a PWS or pedestrian warning system or AVAS (acoustic vehicle warning systems). In most cases, this is an external soundscape to signal to pedestrians that an EV is moving in the area.
Of course, certain automotive brands have embraced the world of EV acoustics to their advantage. The booming EV market features everything from exclusive interior soundscapes from award-winning composers to luxury soundscapes to enhance that “silent” EV driving experience to individually-tuned soundscapes to match your personal preferences and driving style. It seems that today, the EV community has its head around the very subjective world of e-acoustics and, some might argue, have taken a serious slice of profit pie as well.
All that being said, it is still early days for e-bikes and acoustics which is where we start our story.
Now, purists will claim that an e-bike comes with a variety of inherent mechanical sounds – pleasant or unpleasant. And, well, it is just a bike – so why bother with e-acoustics at all? This wasn’t the thinking at Trek, the award-winning bike company from Waterloo, Wisconsin (USA) with innovation embedded in their DNA.
Trek Bicycle is one of those small-town dream-big global success stories. Started in 1976 in a red barn, a typical sight in any small rural Wisconsin town, Trek is synonymous with high-quality, high-tech innovation and top-biking performances – no matter if you are riding a Trek Electra cruiser to the beach or happen to be Mads Pedersen with your own custom-built Tour de France Madone SLR.
Putting sound quality on the e-bike metric map
Knowing it was a hot topic in the industry, the R&D team at Trek decided to put sound quality on the e-bike metric map with the launch of its new award-winning Fuel EXe in July 2022.
“As Trek’s acoustics testing and analysis expert, which was not only a new role for me but a new type of role for the bike industry, we aimed to take Trek’s acoustics competency to the next level while applying new test and analysis methods to the Fuel EXe development cycle,” explains Paul Harder, principal research and development (R&D) engineer within the Trek Performance Research Group at Trek. “It was quite the endeavor with a lot of firsts happening at the same time.”
The R&D engineers in Trek’s Performance Research Group tend to stay on the experimentation and simulation side of things, testing new prototype ideas and conducting experiments to seek out new knowledge in bike physics and human performance. This quest for new knowledge landed the team in the sound quality realm.
“Over the years, the industry has made a lot of progress to make e-bike motor and battery systems smaller, lighter, more integrated and more ‘natural’ to pedal,” says Harder. “But the inherent noise of electric motor and gearbox systems often remained a common downside. With the Fuel EXe, we had a unique opportunity to improve that last piece of the puzzle and make an e-bike that really has no downsides.”
Inspired by EVs
The team looked to the electric vehicle (EV) industry for inspiration. They realized that, like the early days of automotive EVs, the world of e-bike acoustics presented opportunities. The Trek team took acoustics full circle when investigating the options for the Fuel EXe in terms of the sound experience and unpleasant sounds. This included looking at industry-first acoustic metrics for the e-bike sector, like tonality and the articulation index.
“Since I almost always ride with other people, any noise that distracts from the ability to converse bothers me. This relates to the articulation index,” adds Harder. “Using tonality as our key metric for unpleasantness came from quite a bit of exploration and research into the many sound quality metrics that are commonly applied to electric motors.”
Integrating sound science with advanced engineering
Harder and the MTB engineering team at Trek knew the acoustic experience for the Fuel EXe depended heavily on the e-motor. The team was working with TQ’s HPR120 motor, which is a powerful but fairly loud motor. The team was looking for a smaller, quieter version of the motor. They found that TQ’s harmonic pin ring technology with minimum moving parts and unique gear meshing could play a role in making a new, quieter motor.
“We have an exclusive partnership with our e-motor OEM, TQ, so that means we can effectively collaborate when creating specs and testing our e-motors,” says Harder. This collaboration resulted in the HPR50 e-motor that’s in the Fuel EXe and Domane+”.
Making a great first impression
After the usual back and forth between Trek and TQ, the prototypes with the HPR50 were ready to hit the test trails. The Trek test riders thought the prototypes with the HPR50 e-motor sounded really good. This was great news, but Harder knew they would have more work to do.
“Our team had developed solid acoustics test and analysis capabilities, but we still needed to figure out how to quantify what ‘sounded really good’ meant in the context of an e-bike,” explains Harder. “We had acoustics as a key goal for the new TQ motor and we used the analysis to quantify that, understand it and track it as we decided between the many prototype iterations throughout the development process.”
From subjective opinions to objective measurements
The team started by measuring loudness and found out that Fuel EXe was nearly two times quieter than other e-bikes but the test riders subjectively thought this understated how much better it sounded.
“At this point, we realized just how important sound quality is to the e-bike experience and how important tonality as a key metric is for quantifying the pleasantness of electric motor noise,” says Harder. “We knew we had to get a little bit smarter about how we quantified these sound descriptions. So, we moved to the sound quality aspect, which has a ton of options. We looked closely at the EV acoustic space where we knew that tonality was a best practice.”
Executing the test on the trail and the sound studio
After the initial test rides, the team needed more concrete acoustic data to work with. They headed to the sound studio at the Trek factory headquarters in Waterloo, Wisconsin, which is where Trek’s private mountain bike trails are located, to run more exploratory acoustic analyses. This included sound power and sound quality metrics such as loudness, tonality and articulation index using Simcenter testing solutions, like the portable, Simcenter™ SCADAS™ XS hardware and Simcenter™ Testlab™ Neo software, the next-generation software platform for multidisciplinary test-based performance engineering.
"Using Simcenter SCADAS XS to perform field testing is pretty easy because it fits in a bike jersey pocket, integrates seamlessly with the binaural headset and can be controlled wirelessly with the Simcenter Testlab Scope App on a tablet outside,” adds Harder. “During the trail tests, we also used a GPS and action camera to visualize the data in Simcenter Testlab Neo.”
The lab test setup included the Simcenter SCADAS XS, professional microphones, an acoustically-isolated stationary trainer set to 300W total resistance and, of course, the Trek Fuel EXe adjusted to maximum assist mode. The Trek team used Simcenter Testlab Neo according to the ECMA-74:2019 standard to calculate tonality.
“After researching various sound quality metrics, it became clear that tonality was the most representative of what our riders were hearing,” says Harder. “Thanks to using Simcenter Testlab, we could easily explore other relevant metrics like prominence ratio and articulation index. Nothing is worse than coding an algorithm by hand for a couple of days only to realize it’s not a useful metric for this test.”
Harder and team provided feedback throughout the prototyping process and ended up creating an advanced set of methods to diagnose and improve e-bike motor acoustics. As a final step, they verified the field and lab results on the final production motor for two days in an anechoic chamber. The final results proved that using Simcenter helped them design the Fuel EXe to sound five times more pleasant and nearly two times quieter than other popular e-mountain bikes.
The future of e-bike acoustics
As the HPR50 and Fuel EXe rolled into production, the Trek Performance Research Group realized that they were working on cutting-edge e-bike acoustics.
“Quantifying human sound perception is extremely complicated”, says Harder. “Using Simcenter SCADAS XS and Simcenter Testlab Neo software let me focus less on how to get correct answers and more on what those answers were teaching me. The ability to quickly test different metrics and interactively visualize, filter and playback data led to much quicker and more confident insights than we couldn’t have gotten otherwise.” Harder says, “The really exciting thing is that we can now leverage these tools and acoustic knowledge in even earlier stages of future e-motor and e-bike R&D projects.”
Thanks to Paul Harder and the R&D team at Trek, you will be able to enjoy a whole new realm of forest sounds (and silence) on your morning trail rides for years to come.