Friday, July 17, 2020

Road noise and what we can do about it

I'm really noise sensitive.  Perhaps you are, too.  I did some research reading and am collating it here.

(My local main street of Artesia Boulevard is painfully loud.  My ears ring after I've walked more than a couple of blocks along it.)

First off, I found a recently published book that has an entire chapter about Traffic Noise By Sanja Grubesa and Mia Suhanek. BTW, I just learned about IntechOpen, an open access science book publisher. This is so much better than the for-profit model. I'm looking at you, Elsevier Press.

Brubesa and Suhanek wrote a concise and excellent chapter. I'm excerpting a little bit but I hope you click on the link and read it yourself.
Road traffic noise depends on the following three factors:
  • Type of road vehicles.
  • Friction between the vehicle wheels and the road surface.
  • Driving style and driver behavior.
What types of vehicles?  In general, bigger vehicles produce more noise.

Electric vehicles don't produce any engine noise. Some hybrids run in all electric mode at low speeds (below 10 mph for our family's car.)  In congested city driving, electric and hybrid cars reduce traffic noise relative to normal ICE, Internal Combustion Engine, ones.

Vehicles also produce wind noise from the turbulent air that they produce as they travel.  Blocky vehicles create more turbulence.  Aerodynamic ones produce less.  Many economy cars and older minivans are wedges on wheels, delivering better fuel economy and less noise than trucks and SUVs.

The fashion for making minivans "cooler" by shaping their front ends to be more vertical and blocky like SUVs is so bad for noise, fuel economy and public safety for everyone outside the vehicle.  SMDH

Tire noise is a major noise generator.  Tread matters.  If you ride a bike, listen to the difference between smooth and knobby tires.  Car tires are similar, only the car cabin insulates you from having to hear the noise of your tires.  Everyone outside your car notices and is cursing you.

In general, the larger the tire-street contact area, the bigger the noise.  So you may buy the bigger tires for more cabin comfort/smoothness, but you are creating more friction and making more noise.

The heavier the vehicle, the greater the noise.  So EVs have silent engines, but may have greater tire noise.  To have decent driving range, their batteries are heavier than carrying an engine and a tank of gasoline.

The faster the driving, the greater the wind noise.  That's why Noise Increases with Vehicle Speed.

At 30 mph, a typical auto may produce 62 decibels, about the noise of a human conversation. At 54 mph, the same auto would produce 72 decibels. Decibels are a logarithmic scale so an increase of 10 decibels is a doubling of noise.

California had an 85% rule.  The fastest 15% of drivers determines the new speed limit.  Over time, the speed limits on urban arterials climbed from 35 to 40 to 45.  This has been a disaster for road safety for people outside of cars.  It's also made cities noisier.

What can you do about road noise?
  • Reduce the number of cars.  Pedestrians and bicyclists are mostly silent.
  • Lower speed limits and enforce them.
  • Reduce vehicle sizes with sticks and carrots.  People are using cars much too big for their daily needs. (They can rent larger vehicles as needed.)
  • Increase the distance between noise generators and people.  
  • This and the next 2 figures are from this Traffic Noise Factsheet
  • Replace a car traffic lane with a protected bicycle lane and wider sidewalk. The bike lane protector is a low wall that protects the bicyclists from swerving automobiles and creates a noise shadow. A waist-high wall can make the stores & restaurants along arterials much quieter

Trees don't do anything to reduce noise, but they provide much-needed shade to counter the urban heat island effect.

Speaking of urban heat islands, ICEs produce a lot of heat.  Burn anything and it produces heat. Reducing the amount of ICEs on the streets, and making engines smaller would reduce urban heat, noise and air pollution and climate change.


Thursday, July 16, 2020

More face mask science

I'm emotionally exhausted by the pandemic and all the crappy news.  I've posted links to useful Covid-19 information when I come across them on microblogging platforms like IG and Twitter.  It's just easier than blogging.  But, there is also value to putting it on a blog, with more context and explanations.

I've written about masks in The mask/no mask dilemma back on March 18, when some US government messaging was that masks were not effective for the lay public. A lot of people have written about that bad messaging and I'm not going to belabor it here.  Professor Zeynep Tufecki does it as well as anyone.

Anyway, I also posted some cool visual proof of the efficacy of masks in This is why we wear masks. Those are Schlieren images and show breath vapor, not the motion of aerosols and droplets, which will stay even closer than the images shown.

Almost any type of cloth face covering will catch droplets. Most multi-layered ones will catch the majority of the aerosols as well. Masks hold your air closer to your face than without masks.

As Dr Lucy Jones says, "don't share your air."

In my first post, I (and many readers) didn't know what they meant by "Cotton Mix" but were impressed by its filtration efficacy.

After reading this subsequent article, I learned that it was Cotton/Poly blend. Polyester, and any fabric that has static cling, has electrostatic properties that attract and hold particles. That's why your furnace filter is made of polyester fabric. In fact, some people were buying furnace filters and cutting them up to insert inside their DIY masks.

Read Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks. I excerpted some figures and a table from the paper. Even though it is published in a scientific journal, it's written so they lay public can understand it. It is freely available and I hope you download and read it. It's good.
We have carried out these studies for several common fabrics including cotton, silk, chiffon, flannel, various synthetics, and their combinations. Although the filtration efficiencies for various fabrics when a single layer was used ranged from 5 to 80% and 5 to 95% for particle sizes of <300 and="" nm="">300 nm, respectively, the efficiencies improved when multiple layers were used and when using a specific combination of different fabrics. 
This figure shows that filters can trap particles by either mechanical filtration (tight weave) or by electrostatic attraction.

Cotton, the most widely used material for cloth masks performs better at higher weave densities (i.e., thread count) and can make a significant difference in filtration efficiencies.
Notice that Cotton Quilt fabric (medium weave 120 threads per inch) performs better overall than the 600 threads per inch Cotton except at larger particle sizes?  Looser weave Cotton (cheaper quilting cotton at 80 threads per inch) does a lousy job.
Filtration efficiencies of the hybrids (such as cotton–silk, cotton–chiffon, cotton–flannel) was >80% (for particles <300 and="" nm="">90% (for particles >300 nm). We speculate that the enhanced performance of the hybrids is likely due to the combined effect of mechanical and electrostatic-based filtration.
The hybrids do a better job than pure Cotton. Some even outperform surgical masks, which were never designed to catch smaller particles.  (They were designed to catch larger droplets exhaled by health care workers to protect the patients.)

Our studies also imply that gaps (as caused by an improper fit of the mask) can result in over a 60% decrease in the filtration efficiency, implying the need for future cloth mask design studies to take into account issues of “fit” and leakage, while allowing the exhaled air to vent efficiently.
The biggest factor is fit. Isn't that the #1 reason why we sew?  We can customize the fit.

We also sew for improved comfort and face masks are no exception.  If the mask fits and is comfortable, we can wear them for as long as we need.

Pay attention to the pressure drop.  It doesn't matter if the weave and fit are tight, but it's too hot and uncomfortable to wear.  I know that I can wear a real surgical mask while riding my bike or for a long-haul airplane flight.  So I want a combo that has a similar pressure drop of 2.5.  I'll sew something with a good enough filtration efficiency, with the same comfort rating.

Overall, we find that combinations of various commonly available fabrics used in cloth masks can potentially provide significant protection against the transmission of aerosol particles.
Notice that, in every case, more layers offers more protection. 

If you read up on Brownian motion, you learn that smaller particles move faster and encounter more obstacles than larger particles.  So your fabric or filter material doesn't have to be tight enough to mechanically trap the smallest aerosols.  A filter can alter the particle trajectory so that it comes at a slant instead of perpendicular to the filter.  That makes the particle more likely to be caught by subsequent layers. 

That's the magic behind multiple layers.

Knit fabric structure kind of has those layer properties, which is why just one layer of t-shirt jersey can be effective.  Aerosol expert, Professor Linsey Marr, Tweeted that she wears a knit gaiter-style mask single layer when running outside and then doubles it up (by rolling the top over her nose and mouth) if anyone is nearby.

I've come to the realization that I need a wardrobe of masks for different situations.  I'll make some knit gaiter ones for outdoor exercise.  I'll make some fitted woven ones for my essential trips to indoor locales.

It's time to bust out all my cotton/poly or cotton/silk mixes for fitted masks and some high-performance poly knits for gaiters.

I wear a mask to protect you. You wear a mask to protect me. We can sew our own and make it a fashion statement.

Finally, read Masks offer much more protection against coronavirus than many think. Even if the mask is not 100% efficient at filtering out aerosols and droplets containing Coronavirus, it reduces the dose of exposure. So, even if we do get sick, the illness is milder.

"Perfect is the enemy of good," means that waiting for perfection to act is a bad strategy.  We can always strive to do better, learn more.  But we already know enough and have the tools to reduce our collective risk by a huge amount.  Let's all be good enough and defeat this pandemic.