My formal education in physics ended in high school - what a shame!

As an adult, I've had big interests in psychedelic mind-bending modern physics (Kip Thorne et al.) and early 20th century atom-splitting physics (Einstein, Oppenheimer, Bohrs, et al.). I mean, hello, sci fi!

In addition to big physics ideas, I'm also interested in the humble wave. Specifically, sound waves and radio waves. (Also rogue ocean waves and surfing memoirs.)

Recently, I was noticing that my Bluetooth headphones were stuttering whenever I went jogging. They did this near certain busy intersections, and were stutter-free when I was on trails or rural roads. This got me thinking: how does Bluetooth work? How do radio waves work? Clearly it was time for a PERSONAL WAVE WORKSHOP.

"Hearing" is tiny hairs in your ears being moved by sound waves. These are called stereocilia.

Some random stuff I learned about Bluetooth

Bluetooth was developed by Ericsson in the 1990s and was named after a 10th century Viking king, Harald Blåtand Gormsen, AKA King Harald Bluetooth, I am not making this up, the jokes write themselves, people.

Blåtand was known as a bringer together of people, helping them to communicate (get it...). The Bluetooth logo is made up of his initials, H and B, written in runes - AKA ye olde Germanic script.

One of the world's biggest big wave hot spots is Teahupo'o, Tahiti. Here's some mystical big wave surfing to enjoy.

Modern Bluetooth/Blåtand operates via radio waves in the 2.40 GHz - 2.485 GHz frequency band. It has a range of ~1m (Arduino widgets?) to ~10m (my phone and headphones) to ~100m (industrial strength). It sends data in discrete packets. To avoid crowding problems, these packets can be sent along 79 "Bluetooth channels". They hop around, looking for the least crowded channel.

Stepping back: What's a radio wave?

That was my next question. Okay, so WiFi and Bluetooth are technologies that send data on radio waves. But what is a radio wave? And how does information get encoded (in binary, I guess?) and sent along it? I literally had no idea. Here's what I learned:

Radio waves == electromagnetic waves, since they're made up of electric and magnetic signals. The signals are at right angles to each other. So it's like two waves, one electric and one magnetic, in a helix-y.

em waves source

These EM waves are the same you use in your microwave, from your WiFi router, from the cell phone tower, the radio antenna and the TV antenna. And Bluetooth!

They travel at the speed of light. You can measure the wavelength of an EM signal by dividing the frequency (e.g. 2.4 GHz for WiFi/Bluetooth) from the speed of light (299,792,458 m/s). Okay, I did this for WiFi/Bluetooth and it's 0.12m, or 12 cm. Google says I'm right! Scientific notation for "wavelength" is \(\lambda\).

There's a direct relationship between the wavelength and the size of the antennae - bigger (slower? SPEED OF LIGHT) frequencies, like AM radio stations, have much bigger wavelengths (see equation above - the denominator shrinks, ergo the wavelength expands) and thus require LITERALLY LARGE ANTENNAS. I guess that means that our cell phone antennas, which are tiny, pick up the (much faster?) EM waves of WiFi and phone signals.

You can jack up antennas to have "gain", which boosts the signal. This is done by manipulating the shape of the antenna. I think this might be related to what Crash Course was saying about constructive interference, where you amplify waves.

There are: - ground waves, that follow the shape of the Earth - line of sight waves, that get thrown off by the shape of the Earth (at the radio horizon, new band name?)

Short wavelengths, like Bluetooth (!), can be absorbed by rain droplets.

Random stuff I learned about waves in general

  • Frequency is the number of wave cycles that occur within one second. This is measured in hertz. So 2.4 GHz - the frequency band where Bluetooth and WiFi are - is 2,400,000,000 wave, or 2.4 billion, cycles in 1 second.
  • Waves carry information (through space and time!).
  • Most wave equations seem pretty simple!
  • Waves coming at each other in opposite directions can augment (constructive interference) or decrease (destructive interference) each other's energy. This is now noise-cancelling headphones work - they send out "inverse" sound waves! SO COOL!
  • Sound waves don't move perpendicular in motion (like a rope being oscillated) but in a compression-squeezing way (like a spring). WHAT. Is this why, when my phone is on speaker and I have it right up in my face, I can feel small bursts of air coming from the speakers as the person on the other side talks?
  • Sound perception is logarithmic, not linear, to actual loudness/intensity.
  • That you can use a method developed by a guy named Huygens to figure out where a wave is going: basically, every point on a wave generates a semi-circular (semi-spherical?) 'wavelet' - the line tangent to all the wavelets is where the wave will next be.

Colors are just light waves with different wave lengths. Red has the biggest distance between wave peaks - blue the shortest. There are "colors" beyond this, in wavelengths the human eye can't perceive. The unit measure of light wavelength is the nanometer, or 0.000000001 meters.

Conclusion: Some hypotheses for why my Bluetooth headphones are crapping out

Action plan time!

  1. Bluetooth's fault: Rain. There's a high correlation between me running in a dense city rain, and me running on a rural road with clear skies. Just by chance. So maybe I've misattributed the problem to city density, and not rain, and the fact that high-frequency EM waves can be absorbed by radio droplets.
    • Test: Go running in the city dense area when it's sunny. Do this enough times for statistical significance.
  2. Everyone else's fault: Busy channels. Bluetooth travels along 79 channels, one for each megahertz between 2.4 GHz and 2483.5 MHz. When I run in device-dense areas, especially near intersections, all 79 channels might be maxed out.
    • Test: Identify places where (I guess) the channels will be more or less crowded. Run there.
  3. My headphones's fault: Antenna design. Signals have a polarity which can be horizontal, vertical, or circular. Antenna get the best signal when they're rotated to the same axis as the polarity. If my cellphone emits, say, a horizontally-polarized signal, and my headphones are better positioned for a vertical one, I may get a worse signal.
    • Test: Wear headphones and place phone at different spots around them. e.g. Above the headphones, below them, rotated, etc.
  4. My fault: my running pouch. It sounded like high frequency signals, such as WiFi/Bluetooth, are pretty fragile and can get disrupted more easily than long, slow, low frequency signals. While some materials (e.g. cement, trees) are known to disrupt stuff, maybe my cloth and plastic running pouch is disturbing the cell phone-headphone link.
    • Test: Run with the phone in my hand.

So I have a bunch of potential experiments now, and - given the amount that I run - my sample size won't be big enough to detect changes for another, well, several months or so. But I have a plan! Which is awesome.



Radio waves: