In the professional audio sector, headphones are used in live situations by disc jockeys with a DJ mixer, and sound engineers for monitoring signal sources. In radio studios, DJs use a pair of headphones when talking to the microphone while the speakers are turned off to eliminate acoustic feedback while monitoring their own voice. In studio recordings, musicians and singers use headphones to play or sing along to a backing track or band. In military applications, audio signals of many varieties are monitored using headphones.

Released in 2017, the Bose QuietComfort 35 (Series I) are essentially the exact same headphones as the Series II. They have the same design, feel, sound quality and noise-canceling skills. The difference is that the Series I don’t have Google Assistant built-in and a dedicated button on the left ear cup to activate it. If you don’t care about talking to a virtual assistant while wearing your headphones, which allows you to play/pause music or skip tracks via a verbal command, then Series I or Series II shouldn’t matter to you. The catch is that the Series I is more difficult to find online and they aren’t usually that much cheaper than the Series II.
The rule that I use is that the bigger the size of the headphone, the bigger the need for amplification. Of course factors like driver sensitivity and impedance will matter, but the general rule of thumb is, use a dedicated headphone amplifier for a full size headphone. Even a portable amplifier can be enough, depending on the type of the headphones.
Earbuds have small speakers that rest on the ear canal of the wearer. These are usually much less expensive than on-ear and over-ear headphones and reproduce the sound well. However, outside sounds, such as traffic or power tools, can get past the earbuds and interfere with your enjoyment. Earbuds also don't stay in place very well if the wearer is moving much, such as exercising. These also are lower fidelity than on-ear and over-ear headphones.
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