Typical e-bike motors transfer motor power from the motor to the crank through a network of gears, shafts, belts and/or pulleys. Lots of fast-moving parts meshing together means lots of opportunities for high-pitched unpleasant noise.
In contrast, the TQ HPR50’s harmonic pin ring motor has just a single gear interface where load is quietly shared across many of teeth at all times.
We ride with all of our senses, and the way your bike sounds has a surprising impact on your ride experience. To date, electric bike assistance has come with an acoustical cost that we have all learned to live with — e-bike noise.
That is, until TQ HPR50.
In this article, we’ll take you on a scientific journey in bicycle acoustics. It’s a whole new category of bicycle performance that probably matters to you a lot more than you think.
In the end, we’ll show that the TQ HPR50 sounds 5x more pleasant and 1.8x quieter than other popular e-MTBs. In fact, a TQ motor sounds much closer to a traditional unassisted bike, bringing you back to the way riding was meant to be.
Sceptical? Before we get into the details, let’s listen to some quick samples of a TQ motor compared to two popular e-MTBs out on the trail. Finally an e-bike that’s easy on the ears!
Your amazing ears
Hearing is perhaps your most powerful sense, and using microphones to match your hearing entails some pretty extreme science.
Your ears can detect sound pressure amplitudes ranging from 20 to 100,000,000+ micro pascals – an absolutely massive range. That’s like having a single ruler that can measure anything from the thickness of a sheet of paper up to the height of a 100-storey building! To accommodate this huge range, we typically talk about sound in the logarithmic scale of decibels (dB).
Your ears can also detect sound frequencies from 20 to 20,000 Hz – another huge range. The single pressure wave that reaches your ear contains a combination of all these frequencies from all the sound sources around you. Your ear’s spiral-shaped cochlea separates this combined pressure wave back into the individual frequencies and encodes them as nerve signals. Your ears are truly powerful and fascinating sound sensors!
Fascinating sound sensorsPsychoacoustics
The nerve signals from your ears are then interpreted by the acoustic analysis supercomputer that is your brain. Just imagine the nearly-miraculous processing power and precision required to separate and pinpoint the three-dimensional location of multiple sound sources in real-time (called sound localisation). Your brain further analyses the patterns of all these sounds and then assigns meanings, emotions and associations to them.
Psychoacoustics is the study of how your ear-brain system senses and interprets sound. And various psychoacoustic metrics have been developed to convert raw microphone data into how you perceive sounds in terms of both quantity (loudness) and quality.
Perceived loudness
Your hearing sensitivity varies greatly across the frequency range. For example, a 75 dB sound wave at 1,000 Hz sounds much louder than a 75 dB sound wave at 100 Hz. A common way to account for this varying sensitivity is to apply an A-weighting curve to convert decibels (dB) to A-weighted decibels (dBA). dB defines the physical magnitude of the sound wave, whereas dBA approximates the perceived loudness of that same wave.
Since the development of the single A-weighting curve, scientists have mapped out a more complete series of “equal loudness contours” that more fully captures the intricacies of your ears. In this graph, any two points along a given line sound equally loud, and each curve is roughly twice as loud as the curve below it. If your ears worked in the same way as microphones, all of these curves would just be equally spaced horizontal lines.
This chart also introduces the loudness metric sones, which achieves the same purpose as dBA but is more sophisticated and intuitive. Sones scales directly with perceived loudness (2x loudness = 2x sones) whereas dBA is fairly unintuitive (2x loudness = add 10 dB).
Sound quality
Often it is a sound’s quality, rather than its loudness, that dictates your brain’s distinction between good sounds and bad sounds. For example, the high-pitched sound of a mosquito is relatively quiet, but it is also quite unpleasant and cuts through the background to grab your attention. Engineers describe this particular type of sound as 'tonal', but our brain can interpret many other sound patterns into categories like rattle, squeal, creak, shrill, rumble and many more.
Many of these interpretations can be quantified from microphone data using sound-quality metrics, such as tonality, sharpness, roughness, prominence ratio, fluctuation strength and articulation index. These metrics can predict your enjoyment using a product, along with your impression of its build quality and performance.
An interesting example in sound quality is the significant engineering effort put into the sound of your car door closing. This sound is secondary to the car’s core function, but it greatly shapes your initial impression of the car’s sturdiness and reliability.
E-Bike psychoacoustics
So how does this all relate to bikes?! During several years of pioneering bicycle psychoacoustics, Trek Performance Research has discovered that your bike’s sound quality – often even more than its loudness – has a big impact on the enjoyment of your ride. For e-MTBs, we have focused on two sound-quality metrics – Tonality and Articulation Index.
Tonality
Electric motors tend to make high-pitched tones which can be perceived as especially unpleasant. Just like the example of the mosquito, the whine of an e-bike motor cuts through the background to grab your attention.
Tonality (specifically Tonality HMS) is a modern sound quality metric that uses a series of advanced algorithms to accurately model human perception of these kinds of unpleasant tones. We believe that tonality is a key new metric for what riders experience on an e-bike.
The Tonality HMS calculation uses a complex sequence of 14 algorithms to model your ear-brain system's perception of annoying tones (from the ECMA-74:2019 standard)
Articulation index
A big part of riding with friends and family is the ability to chat, teach each other new skills and guide each other through new trails. But various sounds on the trail – including your e-bike motor – can block your ability to hear what others are saying. Articulation index is a sound-quality metric that predicts the proportion of speech that is audible and is a good indicator of how sounds may detract from the group ride experience.
Learn about articulation indexTools and knowledge
Psychoacoustics is a challenging new science in cycling, but it makes a big difference to the ride experience. Trek’s development of bicycle psychoacoustic tools and knowledge reflects its commitment to improving the ride experience through science. When added to our well-established vibration capabilities and expertise, Trek Engineering can now measure, understand and design for everything you feel and hear on the bike.
In the lab with the Madone IsoSpeedTQ HPR50 psychoacoustics
The TQ HPR50 revolutionises how quiet and pleasant an e-bike can sound – a fact that we can illustrate using advanced psychoacoustic testing and analysis techniques that were developed throughout the bike’s prototyping phase. The culmination of this testing was a final production bike shoot-out in the most controlled sound environment possible – an anaechoic chamber.
In the anaechoic chamber, we compared the TQ HPR50 to a traditional unassisted bike, a popular light-assist e-MTB and a popular high-powered e-MTB across a wide range of conditions on a customised sound-isolated trainer fixture. Over the course of two days, we collected 225 million data points using 21 microphones and a cadence sensor that allowed us to relate sound frequency to motor speed.
In these tests, we analysed the bikes’ tonality, loudness, sound power and articulation index over a cadence range of 40–100 rpm, at total power of 300 W, and in the two highest motor assist levels. All charts are based on a B&K 4966-H-041 microphone located 1 m laterally from the bike and 1.7 m vertically from the floor (at head height, circled blue.)
“The HPR50 equipped bike sounds 4 to 5x more pleasant than other popular e-MTBs”
Not a big fan of graphs? Listen for yourself from inside the anechoic chamber!
Take a listenSolid lines are the highest assist mode and dashed lines are the second-highest assist mode. Tonality calculated to the ECMA-74:2019 standard.
TQ HPR50 loudness
While the tonality best relates to your e-bike ride experience, we didn’t forget about loudness. The graphs below show perceived loudness in both dBA and sones. Depending on which combination of bikes and metrics we consider, the HPR50 is 1.5–1.8x quieter than the other e-bikes and most comparable to unassisted bikes.
TQ HPR50 sound power
Loudness is a key acoustic metric, but it depends on the microphone’s chosen distance direction from the sound source. Our microphone’s location was chosen to represent the sound at the rider’s or riding partner’s ears, since that’s the location that matters most.
But we additionally took the next step of measuring sound power, which uses a hemispherical array of 12 microphones to quantify the total sound energy emitted from the bike in all directions. In other words, sound power represents how the bikes compare to listeners at any location around the bike.
As we see in the graph, sound power ranks very similarly to loudness near the rider’s head. This validates both our loudness results and this chosen location for our single-microphone metrics.
TQ HPR50 articulation index
As previously discussed, the sound of your bike can interfere with your ability to talk to others while riding. Articulation Index predicts the amount of speech cues that can be heard over a given noise. Again, the HPR50 is much closer to a traditional bike than to other e-bikes and doesn’t get in the way of your conversations out on the trail.
Field tests
While this article focuses on tests from the highly controlled anaechoic chamber, we also took our acoustic test equipment to the trail for validation. The trail results agreed, with the HPR50 measuring 3-5x lower in tonality and 1.5-1.8x lower in loudness than the other e-MTBs.
Colour map analysis
A very powerful analysis technique is to map loudness as colours across a scale of both cadence and sound frequency. In the colour maps below, each diagonal line represents a tone whose frequency (pitch) increases with cadence – these are the tones that cut through the background and grab your attention as unpleasant.
Each of these diagonal lines correspond to a physical spinning component within the motor, whose gear ratio and tooth/magnet count relates to the slope of the line. Clearly, the traditional e-bike transmissions have a lot of moving parts which create a lot of tones, whereas the Fuel EXe’s harmonic pin ring gear creates just a single, much quieter tone.