EDIT: nope, not the default. From the data sheet, last sentence:
"To prevent brute-force attacks on the password, the maximum allowed number of negative password verification attempts can be set using AUTHLIM. This mechanism is disabled by setting AUTHLIM to a value of 000b, which is also the initial state of NTAG21x."
So Philips went out of their way to secure that toothbrush head. That's reassuring.
It gave as an example clothes dryers. The way most home clothes dryers working back then was you put the clothes in, you turn a dial on a timer to the number of minutes you want the dryer to run, and you press start.
The mechanical timers were very reliable. There hadn't been any substantial improvement in their design in decades because there really wasn't anything to improve. There had been improvement in the materials used, and in the cost, but fundamentally mechanical timers was a solved problem.
If the mechanical timer ever broke the repairperson would have replacements in their van. Even if they didn't have the specific one for your dryer it didn't matter because they all worked pretty much the same. They could just put in another one. Maybe the mounting holes wouldn't be in the right place, but they could easily improvise some way to mount it in your dryer.
The book went on to say that somewhere there is an engineer designing a new clothes dryer, and instead of a mechanical timer that engineer is putting in a digital timer. It has a microprocessor, 7 segment LED digit displays for the time, some buttons for interacting with it (such as setting the time and correcting mistakes), and a power supply. And let's not forget that it has software.
That digital timer has no advantage to the user over a mechanical timer. But it has disadvantages. The interface will be worse. It will cost more. It won't be more reliable and possibly will be less reliable, and if it does need repair the repairperson probable won't have the parts. If they have another brand's digital timer on hand they probably won't be able to adapt it to your dryer.
So why is that engineer designing the new dryer with a digital timer?
Because mechanical timers are boring. Digital electronics was at the cutting edge of consumer engineering then, and so by using a digital timer the engineer got to play with exciting new technology.
Additionally, I would be very surprised if the digital solution is not cheaper to make. Maybe not when first originally introduced, but nowadays it very likely is.
You're right that repair-ability is hurt in some ways... but the industry has moved to compensate. You can buy boards and replace them. They aren't inherently hard to service, because the form factor doesn't really have limitations.
As a homeowner, I wish someone (anyone!) still sold reliable analog appliances that just did their job simply and made repair parts and schematics reasonably available.
In comparison, the mechanical timer is physically moving. A clockspring, or some sort of mechanism that physically sets the time remaining. Depending on how it's built, vibrations are a harder problem to solve. Not impossible, obviously, but it certainly adds cost.
Also, for most appliances we deal with today... they usually ARE simple to work on. Simple switches and mechanical contrivances. Parts are typically readily available... even PCBs, although possibly not at great pricing. There's certain appliances where you are basically screwed (fridges come to mind...), but that is mainly in my view because the typical failing part is the compressor. Nobody is rebuilding a compressor themselves.
Source: The episode of 'The Secret Life of Machines' on fridges. Search it on YouTube.
On the compressors, there was a vast swath of Samsung and LG fridges that had known defects on compressors causing them to fail. Right now, the ice machines are probably most problematic. If you own a Samsung fridge with an ice maker you know what I mean.
A few years ago I was renting a house that came with a Samsung fridge that provided chilled water / ice. My kids loved the chilled water.
However, our usage of it caused the paint to start bubbling below the dispenser, and the owners of the rental wanted me to replace the door at a cost of ~$800 USD(!).
I argued that we were using the fridge as designed, so we weren't liable, instead they should discuss what looked to me like an obvious design flaw with Samsung.
They disagreed, so we ended up in court. My defence was about 12 pages printed from an appliance review site of people specifically complaining about this paint bubbling.
Easiest win ever, but seriously, how do you put a device that works with water into a fridge and fail to ensure it can't leak under the paintwork?
Granted, none of my large appliances are younger than 10 years, but I think I could build new ones (expensively) for the all the parts and schematics available, even wiring diagrams.
"Repairability" is becoming slightly nonsense because even as someone who is a programmer, who has done electronics at a hobbyist level myself, I'm not going to be able to fix a lot of stuff purely because you have to become an expert on it, the time investment is too high. As systems get more complex (to the overall benefit of all of us) the value of repairing something yourself vs getting an expert to do it, changes.
I think right to repair is good though, but purely meaning that companies to not intentionally attempt to thwart the repair of their devices and that parts/manuals are available where needed. Even so, this doesn't mean that every phone repair place is going to debug some sub-circuit inside some small part of the newest iPhone - they'll just identify the overall broken module and replace the entire board/module.
People make similar claims about cars, but old cars broke down all the time and new ones are basically appliances that “just work” without the driver needing to know anything. Similar for computers to smart phones (though obviously both digital in that transition)
Eventually digital became cheap, and enabled new features like dryers that had various sensors that could be used to optimize drying, but that was several years down the road.
The supply chain issues are hurting the servicing part, repair feasibility, and manufacturing part.
> That digital timer has no advantage to the user over a mechanical timer. But it has disadvantages.
The mechanical timer is known to degrade over time, which is why the repair person has spares in their van. Does the digital timer really have no advantages? Will it ever fail and need to be replaced? How much more does it cost?
Yes, engineers are tempted to use shiny tools all of the time. Evaluating whether or not the tool is right for the job is _hard_. But it feels wrong to say that novelty is the only motivation behind upgrading tools?
The digital timer was made by an engineer, too, who designed it to be a more reliable replacement for faulty mechanical timers. It has both advantages and disadvantages compared to mechanical timers, which is why the engineer made it in the first place?
Surely the expected lifetime of a digital timer is shorter than that of a mechanical timer.
In practice, cost engineering is going to mean neither is completely reliable, but it should be cheaper to make an electronic timer reliable enough. Especially today, where the cost of a functioning mechanical timer is probably an order of magnitude more than an equivalent electronic timer.
There are so many different mechanical things that can break, jam, get gummed up...
Going into the realm of unlikely scenarios, electronics are more susceptible to EMPs.
But also, some slop in the timing accuracy is just fine. The user doesn't really know how long precisely the drier needs to run to dry their clothes. They just know that if they set this timer to 45 then the clothes come out dry.
The only things that broke are plastic door hinges.
The worst engineers I've worked with are ones who, in their first week, fall behind on their onboarding plan because the company's compiler needs a rewrite.
They probably knew it was dumb but implementing it was easier than getting around all the organizational permissions to make an exception.
(I'm just kidding. I hope...)
The other day I was trying to buy a pair of bathroom scales and it took me far too long to find one that just, you know, weighed things without also demanding I connect it to the Wi-Fi and download a smartphone app. How is this an improvement?
First, I've owned several wireless connected bathroom scales from high-end to low-end brands. All of them work out of the box without needing or demanding you connect it to Wi-Fi and download an app. Put in batteries, step on scale, and weight is displayed. If you want to use an app, that feature is available, but definitely not mandatory, and I've never seen the device prompt to install an app. One had a removable sticker that advertised the app with a QR download code.
Second, it took me less than 5 seconds to find a non-connected bathroom scale, if you care about that. Maybe I'll assume good faith here, you may be shopping at a super high-end retail shop, or an electronics store that also sells appliances, or an online megaretailer that knows you're into tech and is recommending you smart devices? But when I search for "bathroom scale" on amazon, target, walmart, or home depot, the first result is a basic digital scale that has no connected features.
Same when I look at what is in stock in stores where I live in San Francisco. If there is a place where retailers would think they only need to stock connected bathroom scales, that would be the place. Nope, plenty of non-connected scales that only need a single button CR battery.
I never claimed that it's impossible to buy a non-connected set of bathroom scales; all I said was that it took me "far too long" - by which I mean I wasted maybe five to ten minutes deciding which bathroom scales to buy because the first few ones I looked at turned out to be overengineered piles of Wifi/smartphone-ready bullshit that I didn't want. Looking on Amazon UK now, I see several "non-connected" scales in the search results, so I'm not sure why I found it so difficult last time, but my memories are what they are.
This is, of course, as trivial of a first-world problem as they possibly come, but it felt vaguely relevant to the current discussion, and it's part of a general trend in consumer electronics that I constantly see people on HN complaining about. Sorry you found it so offensive.
First site I went to, first search term I tried.
Perhaps the commenter wasn't shopping online, took a while to find such a scale in the stores, and so wasn't lying.
Maybe if you go to an electronics retailer that also sells appliances like Best Buy, that's all you can find. But I live in a very techy area (San Francisco) and my local Target has plenty of non-connected digital scales for sale. For $10 more, you can get the connected version.
Now, saying that they're wrong is much more supportable and doesn't require you to engage in the tricky business of trying to read someone's mind.
What I believe is that we shouldn't be afraid of calling out people who casually spout easily disproven bullshit to make their argument. Saying you can only call out someone for being wrong doesn't go far enough. OP was arguing the position that there is too much connected tech, so much that in the future we may not even be able to buy a non-connected toothbrush in the future. I'm also IoT-skeptical, as many HNers are. It's a popular position that we don't want a world where IoT is mandatory. But as evidence, they gave an anecdote about shopping for a bathroom scale. I tried to be intellectually charitable to that position and assume good faith, but it didn't hold up without devolving into absurdity.
That's why I didn't say their anecdote was a lie. I can entertain the logical possibility that someone who sincerely does not want to buy a connected household appliance can go shopping for one and have difficulty finding one to buy. But it just doesn't play out, especially for someone who is on a specialist tech forum like HN and has clearly skeptical views on IoT. I said you have to work really hard to put yourself in a position where you can go shopping for such a scale, but face such difficulty that it took "far too long" to find one that is not "demanding I connect it to the Wi-Fi and download a smartphone app."
If you wanted to make a video of yourself not being able to buy a dumb scale, you could go to an tech-heavy electronics retailer that also sells appliances (like Best Buy, Microcenter) or a boutique high-tech gadgets store (like Sharper Image or Brookstone) and only find connected bathroom scales. I just checked what is in stock in a San Francisco Best Buy and the only scale is an IoT connected on. But that would be twisting the truth, because who in their right mind would check only Best Buy in order to buy a dumb scale. it takes a couple minutes on major retailers websites (Target, Walmart, Home Depot) to show that even if you limit to items in stock in tech-heavy places like San Francisco (if there is a place where retailers might assume their customers don't want dumb scales, it's SF), the first and usually cheapest options are dumb scales.
So let's Occam's Razor this. What is more likely? An IoT-skeptical HN poster actually went out to buy a non-connected bathroom scale and genuinely struggled to do so? Or an IoT-skeptical HN poster had to click or sort through a few different options and actually read product descriptions, then exaggerated this anecdote (or totally fabricated it) to advance their position?
But does that even matter? If they aren't a liar, then they are at best intellectually dishonest, and at worst intellectually incompetent. We don't have access to their mind and so can never know which of these three they are. No matter which of these three they are, any of them is a reason to invalidate their argument and call out their anecdote.
"Intellectually dishonest" is just a polite way to say "liar".
There's no need to go after someone personally when it's sufficient to simply point out that their statement was wrong. Why it was wrong isn't really important.
The other option is for me to assume that the guy is unintelligent and incapable - which do you think that they’d prefer to be characterised as?
A better explanation of your position (as I perceive it) might be: I think OP is lying for rhetorical points. The alternative explanations just seem too unlikely to me, and Occam's Razor screams a high likelihood of lying.
When making weight for a sport. Any more time than necessary spent dehydrating is energy and recovery you're not going to have for the event.
A couple of years ago I wanted to upgrade my bathroom scale to whatever the best one was and I found that there are no really good scales out there.
I have a "smart" scale from Polar that fortunately could be used as a regular dumb scale. I think Polar is really great in this regard. Their watches and other equipment can be used without syncing or connecting to their cloud, but the cloud does provide you extra value if you want to use it.
It claims 0.1 lb accuracy, and seems pretty repeatable, but I haven't tested it for accuracy. These digital scales have multiple sensors (this one has 4), so it makes sense that individual sensors may give different readings as you shift your weight around - not sure if there's an exact science to combining the 4 readings into the single one that is displayed.
0.1kg is the weight of 100ml of water, which is about 3.3oz
I'm not sure you can even buy "just works" dumb mechanical scales any more.
You can, they're just more expensive (relatively speaking) because they cost more to make.
... so yeah, we're doomed
This type of deviant behavior came to a screeching halt when Crest released their breath-activated toothbrush defense system (BATDS) in 2028. BATDS enabled devices deliver a non-lethal shock to the perpetrator, rendering them unconscious. While BATDS was immediately deemed illegal in most countries due to disputed claims that it caused significant brain damage throughout a large control group, it remains popular in single family households in the US.
The absolute personification of Chaotic Neutral
Fear the Toothbrush Terrorist!
I think a better write up would have front loaded that aspect.
Even titling it "How I locked myself out of my smart brush" or similar. If he wanted to be creative it could have had a Film Noir start but even in a technical write up you should start with interesting aspects.
But alternatively, since the head has an NFC tag, could you use it for stuff like a partnership with Marriott (open your hotel door with your toothbrush, so much convenience) or with transit companies to charge your monthly transit pass?
Possibilities of an NFC-enabled toothbrush head are infinite. The future truly is fascinating.
My wife and I had a lot of fun that night! Turns out the "personal massagers" work a lot better that way.
The difference between using a tongue brush and an electric toothbrush (vs just manual brushing with and without a tongue brush) was so stark that I've never for a moment felt like £40 for a toothbrush with an annual cost of £11.20 for toothbrush heads has ever not been worth it.
The tag is used to change the cleaning mode of the toothbrush automatically, to match the type of head you inserted. This makes very easy to change heads durring the same session.
It is also used to register how long you used that head. A warning is shown when the head should be replaced. After a few warnings you will no longer get them, just the led to replace the head remains on. You can continue to brush your teeth without any problems. What I've found is that the warning comes at the right time, you really feel a decrease in cleaning efficiency around that time.
You can use heads without the cip and they work. You just have to select the proper mode from the handle manually. Or not.
The early versions have a defect where when you push to insert the head, you also push to open the handle. With time, water will get in and the toothbrush will stop working. Not sure about the latest versions.
Still better than a manual brush even in that state.
This should be relatively easy to verify. One could take a new brush head and forward its counter to the limit, directly comparing it to a new unmodified brush.
It would be super easy to reveal as well. A family member with the same toothbrush, your head finishes first. Motor slows down, pitch goes down. Compare the two. Replace the old head, now they're the same. Scummy practice revealed. Scandal.
That said. I'm not totally sure on the mechanism that all electric toothbrushes use.
It's much harder to detect subtle amplitude changes.
Turned it into a Go library: https://github.com/raqbit/goralb
It's better to think about sustainability.
I had an Oral B IO electric toothbrush. The retail price is nuts and the brushes are expensive and can't really be recycled. Imagine millions of these out there slowly rotting.
I gave up on the IO and bought this one instead. Simple design and battery lasts longer too.
If your immune system doesn't react so aggressively to plaque then yes manual tooth brushes are cheaper and you have many sustainable options in this space, i.e. toothbrushes with a wood handle etc.
have to use your hands?? that's like a baby's toy!
That's why I bought this toothbrush to support them.
But if everyone thinks that this startup is not going to make it then yes they'll probably won't exist in a few years time.
I love my sonicare. The only thing I would change is the 2 minute shutoff. I have all of my wisdom teeth and never had braces, so I need more time for a good job, but the actual cleaning performance is great. I literally had a hygenist say "Your home care is excellent".
I don't know if it needs this much tech, but if people will buy it, they're gonna make it.
(...and people have come up with a "modchip" to bypass that restriction already: https://www.eevblog.com/forum/reviews/dymo-550-thermal-print... )
All my tries to guess to one-way function for generating the passwords failed.
In case anyone else wants to try having a go at this (without inspecting the firmware): ignoring the first and last two bytes of the UID, we see that 79 is farther from EC and D7 in a similar way that FF is far from 61 and 67, and EC and D7 look closer together too. I wonder if they used "real" crypto or just a simple XOR/shift/add/sub cipher.
There's more info about the device itself here: https://device.report/philips-oral-healthcare/hx68
(Unfortunately they've requested the schematics/block diagram/functional description to be kept "permanently" confidential, and the inside photos are difficult for me to make out the part numbers on the MCU and other components.)
 I wish those who have been leaking secrets about our government would've gone after stuff like this instead of things like the NSA...
Now, there might be instances where a business executive argues in favor of DRM or ensuring that certain coupons are limited to specific regions. In such cases, its sometimes suggested as a requirement to verify if the app is running in a simulated environment or is rooted. However, I can assure you that if you lock some kind of value behind this check and then rely solely on the operating system to provide this level of security, there will eventually be clever hackers who find ways to bypass the protection. The same principle applies to business-to-business apps that demand extensive control. In such situations, you need to rely on other software solutions or provide dedicated hardware. It's important to refrain from attempting to take ownership of my device, considering it's already under the control of Apple or Google anyway... /sarc. If you require stronger guarantees, I suggest reaching out to them.
Why is this kind of thing legal? For how many politicians and activist groups claim to care about the environment, why hasn't anyone introduced a bill to ban intentionally turning useful equipment into waste? Any legitimate security needs would be fulfilled just as well by doing a full wipe and factory reset instead.
For starters, my experience says that, unlike an HP printer, your toothbrush still works just fine if you ignore anything that tells you to replace the head.
 At least as fine as a toothbrush with a worn-out head is going to work.
And what does a "factory reset" accomplish? The hacker trying to get company IP (or whatever the password is protecting) gets three more attempts at it after the reset?
Finally, and I'm not saying this makes it okay, but e-fuses are common as dirt these days. I don't know that you're going to get that toothpaste back in the tube.
Aren't switches to temporarily bypass emissions controls in cars illegal, despite being a feature customers ask for?
> What laws do you want written?
I want all e-fuses to be banned, as well as any other means for manufacturers to permanently reduce, restrict, or remove functionality from products after they've been sold.
> How "secure" am I allowed to make my product before the Feds come a-knockin'?
If the one you're trying to make it "secure" against is the product's owner, then I'd say "not at all" would be a fine answer.
> And what does a "factory reset" accomplish? The hacker trying to get company IP (or whatever the password is protecting) gets three more attempts at it after the reset?
The point is that the factory reset would delete the company IP.
> Finally, and I'm not saying this makes it okay, but e-fuses are common as dirt these days. I don't know that you're going to get that toothpaste back in the tube.
Wasn't asbestos also as common as dirt before it was banned?
So you'd propose that we bans switches? You're saying that a microcontroller should never have a certain feature because you don't like how it has been implemented by a single company. Then you propose that nobody should have this feature ever because someone once used it to turn on a light reminding you that it's time to change your toothbrush head (and then let you brush your teeth normally with no further interruption).
I don't think your analogy holds up, nor have you thought through what you propose.
It's not the implementation. I don't want hardware to ever be able to permanently make itself less useful, no matter how it's done or what it's being used for.
One thing, I can think of, are hardware-based security devices that disable themselves after recognizing break-in attempts.
I am in opposition to this stance. What you're (rightfully) concerned about is when companies do bad things with these mechanisms. But the mechanisms themselves not only have no ethical/moral problems, but are also really useful for all sorts of things that entirely unobjectionable.
In any case, that sortof doesn't matter. Even if there was no legitimate use for them, that doesn't in and of itself mean they should be illegal. You should at the least demonstrate that their existence is causing great societal harm.
Why isn't just making sure the expected private key didn't get wiped a good enough way of making sure it's not modified?
> You should at the least demonstrate that their existence is causing great societal harm.
Okay, how about that it destroys the secondhand CPU market? Once you use an AMD CPU in a Lenovo computer, it blows e-fuses to keep you from ever using it in any other brand of computer: https://news.ycombinator.com/item?id=29958247
What's to stop someone from extracting and restoring the private key?
> Okay, how about that it destroys the secondhand CPU market?
Sure, then how about addressing that issue rather than proposing to outlaw an entire mechanism entirely? We have a lot of things that can be misused, but (generally) only in extreme cases do we outlaw the tech itself. More usually, we have laws targeting the misuse of the tech.
Isn't the whole point of these chips that you can't extract the private key, so that if it gets wiped, it's definitely gone forever?
> Sure, then how about addressing that issue rather than proposing to outlaw an entire mechanism entirely? We have a lot of things that can be misused, but (generally) only in extreme cases do we outlaw the tech itself. More usually, we have laws targeting the misuse of the tech.
But this particular technology doesn't seem to have any legitimate uses.
I don't think these chips include TPM. But if that's the case, wouldn't you object to that on the same grounds that you object to efuses?
> But this particular technology doesn't seem to have any legitimate uses.
Myself and at least one other commenter has mentioned a few legitimate uses.
No, because it would let you generate a new key instead of remaining keyless forever.
Only if they're used that way. They don't take control away from the actual owners of the devices if its the owners who put them in there, for instance. Again, I think you're conflating the existence of a mechanism with the abuse of the mechanism. If you were just railing against the abusive uses, I'd be behind you 100%.
I just don't see why we should outlaw a common and useful mechanism entirely, rather than outlaw certain uses of that mechanism.
I do love the feeling of a fresh sonicare brush.
You, a clever toothbrush-hacking genius: haha, the head is new again!
This is neat, and I find the process of reverse engineering the Sonicare toothbrush fascinating, especially sniffing the NFC communication, but please change your toothbrush head every three months.
Not having to keep track of brush head changes, awesome! One less thing to worry about, as the device is smart enough to tell me when it's time for a new one.
You don't need anything to tell you when it's time to change your toothbrush. You can tell by feeling it, or in the extreme, by just looking at it. There's no need to track anything.
Stops it from beeping at you when your allotted product lifetime is up though.
The main feature this seems to be used for is to put the body into "whitening" mode if you use a whitening brush.
Anyhow, blocking unofficial heads is just an OTA firmware update away.
Although the points don't go to Facebook - you get "rewards" for them. Eg:
> Pair one or more brushes to our iOS or Android app, then you’re all set. quip’s Bluetooth® Smart Motor will automatically store your routine, no phone needed! To check your brushing stats and the points you’ve earned, tap the app.
> Earn bonuses for never missing a beat! Redeem points for rewards you’ll love
Also, how would you know? I have an IOT product that can delta OTA, so only the bytes of the firmware that change are sent to the device, I can do firmware updates that are crazy fast. ESP if it’s just something like turning a feature on.
people skip brushing their teeth for all sorts of reasons. (yes you do. stop lying. your dentist doesn’t believe you, either.)
people go out of their way to not skip coffee.
Where it gets more interesting, though, is actually not at Philips but at shops that sell replacement heads. Sale assistants do go out of their way to actually lie to their customers and tell them that a single toothbrush head will last, like, many-many-many months. And when asked at the next shop visit about why the toothbrush started yelling three months after replacing it, they will blink their eyes and literally inform the customer of «having never heard before about it from any other customers». The situation happens on a regular basis, and the only recourse that works with such people is brushing the pesky flies aside and politely ignoring them. Since not every customer can or does that, the ink business of electric toothbrush replacement heads prospers.
We need to stop putting firmware in things that don't need firmware. Not everything needs a chip or intelligence. The rampant abuse of this kind of thing leads to shitty products and an uptick in electronic waste.
It really makes a difference:
- The connection to my phone helps coach me in making sure I'm brushing my teeth properly. Bad habits sink in easily, and my various dentists all point out that my teeth and gums are much, much healthier. Given that I am cursed with some bad oral genetics, it's "money well spent" for me.
- The counter helps remind me when to change my toothbrush head. I used to be much lazier about changing it; again; because bad habits easily sink in.
Could someone figure out how to do this without an NFC chip in the toothbrush? I hope so! The toothbrush heads cost much more than traditional "dumb" manual toothbrushes. I don't want to have to spend big bucks just to have good personal hygiene.
When things change gradually, I tend to ignore the change.
To put it in a different context, I've had the fadeout brush heads for years, but I had to get in the habit of changing my brush head when I went in for a dental cleaning because otherwise, I'd just keep using it forever.
Putting firmware in shit that doesn't need firmware is itself a problem, even if it's free.
"But how do you know when it's time to change the brush?"
Well, how about when it starts getting soft?
 I'm either easy on toothbrush heads, or Philips is lying, because when the indicator says "buy a new head" it still has plenty of life IMO. Bristles straight and tall, just like a new one, but no blue left being the only difference between that and new. So I ignore it and get a new one when the bristles go a little wonky.
You might be interested in this YouTube video from Applied Science with electron micrographs of 'new' and 'worn' toothbrush bristles - there is a very marked invisible change that happens: https://www.youtube.com/watch?v=cwN983PnJoA
Could be. My wife presses so hard, I hear the motor bog down. "JFC, honey, let up a bit", to no avail. She's constantly replacing her heads. I literally can't remember the last time I popped a new one on mine. I could easily believe it's been six months (and, yeah, it's about due).
a dental hygienist told me to hold it with just the thumb and forefinger.
I can't quite manage that, but if you look at how drummers hold their sticks, it's never in a fist. Their arms wouldn't last through one show like that.
So if you at least take your little finger off it, the amount of pressure goes down.
Watch Roy Mayorga.
Look at Ginger Baker & Elvin Jones, from 0:56 on.
Or Roy Haynes:
toothbrushes come with documentation???
No more, no less.
One is not enough.
Three is too much.
Four is right out.
if you brush for 2 minutes but don't get enough passes over each square mm of surface area, you have done a bad job of brushing
instead of watching the clock, imagine each of those surface areas and make sure you hit them
plus gums, tongue, roof of mouth, etc.
They used to provide color rings to attach to the head, in order to differencing.