Hacker News new | past | comments | ask | show | jobs | submit login

Getting 2.4GHz through the human body is hard. The human body is mostly water, and bluetooth is, after all, the same frequency of your microwave oven, which uses 2.4GHz because it's in a band of frequencies from 1-20GHz that water absorbs.

When you're indoors, you don't need to get bluetooth through your body. You're getting reflections off of nearby walls and ceilings which allow your bluetooth devices to communicate across your body, but without going through your body.

When you're outdoors, you no longer enjoy the benefit of reflected RF, and the design of the phone and the headset antennas needs to be very good, so the RF can make it through your body.

It is a hard problem, but a lot of headset manufacturers do achieve it. I'm surprised you still haven't found one that's acceptable. Of course, it's body dependent. Petite women will have less issues then large men, as the RF just has less water to travel through with them.

Today's bluetooth is limited to 4dBm max transmit power for class 2. Bluetooth 5 will be 20dBm, which is a lot more power. This is, actually, the same power that class 1 bluetooth devices now have, so I'm unsure why they brag about the higher power of bluetooth 5, but to be sure, most bluetooth devices now are not class 1.

The higher power will make even bad antenna engineers be able to get bluetooth through your body, but more importantly, you'll enjoy larger range when you are at the gym or in your home. Also, if you worry about RF effects on the human body, your worry can increase now as well!

I work in adult toy design/consulting, and radio/antenna design is actually a HUGE problem there. Trying to figure out how to build antennas for mobile-connected wireless adult toys (versus the cheaper < 1ghz radio toys that just come with a remote) that can work in a myriad of situations is a big problem that's delayed some products, killed others. Any glitches on those products usually guarantees online comment/review anger and that it'll never get used again.

Sounds like an interesting job, how did you get into it?

I started https://metafetish.com (NSFW, and when I started it was https://slashdong.org) in 2004, as a hobbyist kinda blog about the engineering behind adult toys. Almost 13 years later now and I'm still at it. Things just built up over the years to where my name kinda became synonymous with certain topics. Consulting has always been a smallish side-gig, because the industry is not a great place to make cash, but damn if it ain't constantly weird and interesting.

It's actually a really easy problem to solve with a cord. I have this exact same problem with bluetooth. I have to put my phone in my sweatshirt pocket or in a higher pocket in my backpack or it cuts out.

But thanks for the explanation. This is good to know. I'll plan my walk to and from work through alleys instead of open parks. Because thinking back, it always works in alleys.

This was a magnificent explanation, and precisely the kind of comment I hope for when reading HN. Thank you for taking the time to share.

If I were you, I'd try to forget it as soon as I can, becouse it's wrong. 2.4 GHz is not in the water absortion band (I'd show you a nice graph, but I can't find one that goes to such low frequencies), a microwave would work as nicely in 1 GHz or 5 GHz, what matters is the H-O bonds, that are esentially electric dipoles and are excited when high-ish frequency and power hits them. You probably have noticed that fat heats much faster than wattr in the microwave oven, that is becouse is full of OH bonds, just as the GP is full of shit[1].

This is one of those moments when I wonder if most of the things I have learned in HN are bullshit, I'm usually struck with awe at the sapience of the HN hivemind too, but in times like this...


I don't think your comment is very constructive, but more importantly what the OP actually said is not wrong.

If you re-read my comment, you'll note that I only said that 2.4GHZ was in the 1-20 GHz band that is easily absorbed by water.

Engineers juggle multiple constraints, cost, size, power, and efficacy, and 2.4 GHZ choice was the result of the size of the magnetron that would be cost effective and physically fit into the oven. But it did, of course, have to be within the range of frequencies best absorbed by water, which 2.4GHz is, and which is why it's difficult to get bluetooth through your body.

Why don't you look at this link. I think it will prove your understanding is incomplete, and that water does have significant absorption in the microwave region of the spectrum.


I read the title, clicked the link and thought to myself, "alright HN, teach me something about Bluetooth"

The parent comment is exactly the kind of comment I expected and hoped for!

Engineer's solution: drink less water when you want to use your wireless headphones (also, for a few days previous)

Also, become more similar to a petite woman.

> Also, become more similar to a petite woman.

This is the most diplomatic way to say "lose some weight" I have ever heard.

Use an umbrella.

2.4 GHz is also extremely noisy in any urban area. If you ever get access to a real spectrum analyzer capable of 2350 to 2500 MHz, it's amazing.

What will the effect on battery life be?

The higher data rates are hoped to at least partially compensate for the higher power. You burst data at high power, but for less time. Also, there are retransmits that occur now, if a packet is corrupted. Higher power should result in fewer retransmits, which will help with battery drain. Battery life is hoped to be better, even with the higher power.

And on the body?

There are regulatory limits on how much RF a device can send into the body. These are called SAR limits [1] and they are not changing. I was joking about the body effects. The legal limit is not changing. But devices that used to be further below the limit, will now be closer to it.

[1] https://en.wikipedia.org/wiki/Specific_absorption_rate

Applications are open for YC Winter 2022

Guidelines | FAQ | Lists | API | Security | Legal | Apply to YC | Contact