Very interesting video about how Apple Airpods headphones work (in the comments). You can read about it, or you can just like this post and go check out the original video in the comments. It has pictures!
Battery. 6 hours of operation, but the capacity is only 2% of the iPhone battery capacity. “Dead zones” in the battery, which lead to reduced operating time, can occur due to sudden temperature changes or even just dropping the headphones on the floor. There is a very dense “layered cake” made from a couple dozen layers of anode-cathode. Batteries of fake AirPods or cheap analogs are much worse. Physics: Poor packaging means less active material and fewer lithium ions moving with each cycle => reduced energy density and increased internal resistance => more energy is lost as heat => battery wears out faster.
Antenna. It is located in the stem because the human head significantly dampens the signal. But there is little space in the stem. Metal strip antenna, size 2 mm by 10 microns(!). That’s thinner than human hair. At such size, it cannot maintain shape on its own. In other consumer electronics, antennas can be etched on the printed circuit board, but this limits them to two dimensions. For the AirPod stem, there isn’t enough space. Therefore, Apple uses a clever solution. They embedded the antenna in the surface of a molded plastic cylindrical part. There, clever conductive plastic is used, with added metal. A laser engraves the exact shape of the antenna in the form of small channels with a rough surface. Then, this groove is subjected to electroplating, first with copper, then covered with gold to protect against corrosion. As a result, a durable conductive track is formed, which matches the 3D geometry of the molded part, which would be impossible to create using traditional machining methods. The plastic not only structurally supports the antenna. Other components are attached to it, such as the cable wrapping around the stem to connect the antenna to the Bluetooth chip, the pressure sensor in the stem.
Microphone. In AirPods, not electret microphones, but MEMS: a microelectronic” version of the condenser type. Well, actually, this is not only Apple – any modern TWS headphones, unless they are the cheapest ones. That is to say, modern microphones are made using the same technology as types – photolithography, layer by layer, only in this case it’s a mechanical device, with calculated cavities and flexible layers. Separately interesting is how they make the cavities – they make holes through which etching solution penetrates inside and dissolves the sacrificial layers of silicon dioxide.
Because of such microscopic size, there are several microphones. But why more than one microphone is needed? At the bottom of the AirPods, you will see a small mesh that allows air to enter the second microphone. When you talk, your voice reaches both microphones, but not at the same time. With a difference of only a few millimeters, the chip can detect a delay of six microseconds between when your voice reaches each microphone. This is enough to determine where the sound is coming from and focus on it. Since it precisely knows the distance the microphones are from one another, the chip can compare each signal and amplify your voice during calls.
The third microphone is for noise cancellation. It is located right in front of the speaker, inside your ear.
The microphones consume about 130 mA, which would quickly drain the battery if they were always active. That’s why they are only turned on when you make a call or use noise cancellation. But AirPods are always waiting for a Siri request. How is this possible without constantly active microphones? Here’s a clever solution. Inside the part that is in your ear, there is a small sensor—an accelerometer. It’s the same type of sensor used in phones to determine orientation. But here it serves a different purpose. Instead of measuring orientation, it senses vibration. When you talk, your voice moves through your jawbone. And this vibration is detected by the accelerometer. This low-power consumption signal is enough to wake up the system and activate the microphones when it senses you want to activate Siri. Imagine that, eh?
The sound in AirPods is tuned not “by ear,” but based on a scientific model of the “ideal sound” (Harman curve), which describes the combination of frequencies most people find most pleasing. For this, there is a complicated system of calculated vents and meshes — to control the air flow, which prevents the occurrence of unpleasant “humming” or sharp sounds inside the ear canal. The larger the cells — more air passes through, smaller — less. Such is the mesh, visible as black things on the white earphone—I thought it was for beauty. No, this is exactly that mesh. But at the same time, some kind of moisture protection must be made, and here the mesh is porous. It is claimed that there is some sort of nano-coating that repels water.
Bluetooth. Why it is so immune to interference. Turns out, it uses frequency-hopping spread spectrum technology (Frequency Hopping). Bluetooth devices quickly switch between different channels many times a second and adapt accordingly.
