The HD551 series is an ear-closed earphone with 102dB high efficiency sensitivity, which makes it easy to listen to music, effortless, and free from outside interference. The series features three different tones, the HD551 features deep bass, The HD551F is like the VIP seat in the front row, and the HD551B is like a depth of field and space in the concert hall. These three headphones are not only unique but also deliver excellent balance and crystal-clear details.