The most common type of headphone jack size you'll likely see is a 3.5-millimeter jack, but there are other types of headphone jacks on some devices. Some use a smaller 2.5-millimeter jack, and some use another type of connection entirely such as a USB connection. Other audio jack types you may encounter include MIDI cables for digital musical instruments, RCA jacks for various audio equipment, and optical and coaxial cables. Make sure you plug the right jacks into the right ports, or you could cause physical or electrical damage.
3.5-Millimeter Headphone Jack Size
The most common sort of headphone jack you'll encounter is one of the most common audio jack types overall: the 3.5-millimeter jack. It's used to connect ordinary headphones and earbuds to smartphones, computers and other devices such as portable compact disc and cassette players. If you see headphones for sale in a store, particularly a general purpose business such as a convenience store or pharmacy, it's usually safe to assume they have a 3.5-millimeter jack, although it's worth double-checking before you make a purchase.
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The 3.5-millimeter jack should gently snap into the headphone port, sometimes labeled headphone or with a picture of a set of headphones, on a device. Double-check before plugging in the headphones that you have the correct port and don't force the jack if it appears not to pop into place. Remember that some devices might have a different type of headphone port, possibly even a proprietary connector.
Before you plug in the headphones, lower the volume on your device or remove them from your ears so that you don't hurt your ears if the volume is turned up loud. Some devices warn you if you turn up the volume to a potentially hazardous level, but many don't.
Some cellphone headsets that plug into a 3.5-millimeter port contain a microphone you can use to make calls or otherwise record audio. In some cases, they may only have output through one speaker rather than two, so don't be surprised if such headsets don't let you listen to music or other material in stereo. They often have buttons to let you control a call and, sometimes, the playback of music from the headphones.
2.5-Millimeter Headphone Jack Size
Some headphones and headsets for phones in particular use a 2.5-millimeter jack rather than a 3.5-millimeter jack. As the name suggests, these are slightly smaller than the 3.5-millimeter jack.
Recent phones have mostly switched to the standard 3.5-millimeter size, but if you have a phone that accepts the smaller-size headphone jack, you need to find compatible headphones. As with 3.5-millimeter jack headsets, devices using these jacks can support microphone and speaker audio or both.
If you have a set of headphones with a 2.5-millimeter jack and a device with a 3.5-millimeter port or vice versa, you can find adapters to convert between the two jack and port sizes. A 2.5-millimeter jack won't fit into a 3.5-millimeter port on its own.
Understanding USB Headsets
Some headphones and microphone-speaker headsets connect to computers using a standard USB connection, usually built into the headsets. They are most commonly used for making calls and video conferencing so that you don't have to run multiple cables to the microphone and speaker ports on the computer to connect the headset. Simply plug the jack into a standard USB port on your computer and verify that the computer detects the headset.
Many of these headsets include volume controls and other controls to adjust call settings from the headphones rather than using the keyboard and mouse on your computer.
Because most phones and other devices that play music and audio don't have standard USB ports, USB headsets are mostly used with desktop and laptop computers.
Some recent Apple devices, including recent iPhones, use the so-called Lightning connector to connect headphones rather than a traditional headphone jack. You can buy an adapter to connect 3.5-millimeter jack headphones with equipment that has only a Lightning port if you don't want to buy new headphones for the device.
Headphones Without Any Jacks
Many modern headphones don't have any jacks at all. That's because they connect wirelessly to devices like smartphones and computers using the Bluetooth wireless protocol.
Wireless headphones can be handy for walking, working or exercising, since you don't have to worry about getting tangled in cords or getting them caught on something. The downside is that they're usually powered by batteries separate from your main device, so you must remember to keep them charged or they won't work.
Bluetooth headphones and headsets usually need to be paired with devices before they can be used. Exactly how to do this varies from device to device, so read the instructions for your computer or phone operating system and for your headphones to know how to connect them.
Some recent phones have eliminated traditional headphone jacks, leaving Bluetooth as the only way to connect headphones to them.
Using Headsets With Landline Phones
Some headsets are designed for use with a traditional landline phone, such as for making calls in an office. These can connect to the phone with a variety of jacks and cables, so check to see what your phone accepts before purchasing a headset.
Some office phones accept a 2.5-millimeter jack, while others require a traditional headset jack that looks like a small version of a landline phone cord's jack. There are slight variations on this type of connector with different capabilities, so make sure you get a headset with one that's compatible with your phone and has the features you need, such as the ability to answer calls from the headset.
Some use another type of cable called a direct connect cable, and some must be plugged into an amplifier before being connected to the phone to get audio levels to the desired point.
If you work in an office setting, consult the IT department or whoever manages the phone system before connecting a headset.
Other Audio Jack Types
Other cables are used to connect audio devices, although they're not commonly used with headphones. RCA jacks, developed by the namesake company to connect record players to stereo systems, are still commonly used in home and professional audio. They're not usually found on consumer-grade computers or phones.
MIDI cables are used to connect digital instruments such as electronic keyboards to computers and other devices. Some newer instruments use USB cables instead.
You can also find audio transmitted over more esoteric devices, including some that resemble traditional phone lines and even optical cables. If you have a device and aren't sure how to connect it with the rest of your equipment, read the manual to determine the types of compatible cables and jacks. Look for adapters that can bridge the gap between your devices if you need to do so.