The Sennheiser HD 518 are a good, low-budget pair of critical listening headphones. They deliver a decently balanced and open sound quality. They're comfortable and somewhat sturdy but are a bit bulky and tight on the head. However, they're not casual headphones and are only meant to be used in quiet, isolated environments.
The Sennheiser HD 518 have a similar design to the HD 598 but look less high-end and also feel tighter on the head. They're well padded, decently comfortable headphones, and the additional tension in the headband also makes them a bit more stable. However, they do not feel as comfortable as the HD 598. Also, like many of other open back headphones, they're not intended for use outdoors and are too bulky and cumbersome, to comfortably carry around on your person.
The Sennheiser HD 518 are not the most eye-catching or stylish open over-ear headphones. They have a similar aesthetic to the HD 598 but lack a few of the premium touches that the more expensive models of Sennheiser HD series provide. This makes them look a bit cheap at a glance, especially with the thick and plastic ear cup grills. On the other hand, the headband and ear cup padding do look and feel high-end for their price.
The Sennheiser HD 518, like the HD 598 and the Audio Technica ATH-M50x, are well padded comfortable headphones. The large oval ear cups fully encompass the ears and the cushioned fabric is soft and feels good on the skin. Unfortunately, they're a bit tight on the head. It's not as noticeable as some other over-ear headphones but it does make them a bit less comfortable than the HD 598.
Like the Beyerdynamic DT 770, the HD 518 are on the larger side for over-ear headphones. They're not designed to be very portable. They're bulky, and the ear cups do not lay flat to take less space. They also don't fold into a more compact format. They will fit into a backpack and some larger bags but are too cumbersome to carry around on your person. Sadly, they don't come with a case or pouch either.
Decent build quality. The Sennheiser HD 518 feel sturdy enough to withstand a few falls without damage. The ear cups are comparatively dense, and the headband has a thin metal frame that gives a bit more durability to the design. However, like the HD 598 the metal frame connects to the ear cups by a relatively thin swivel hinge that seems susceptible to heavy physical stress.
The Sennheiser HD 518 are decently stable headphones. However, they're not designed to be used while doing any physical activities. They will sway and quickly fall off your head but do maintain a stable and comfortable fit during casual listening sessions. Unfortunately, the cable is not detachable and will yank the headphones of your head if it gets hooked on something.
These headphones like all open-back models are not meant to isolate the listener from ambient noise. They're designed specifically to leak and have an open sound that immerses you in your environment. Therefore, they won't be usable in loud settings or if you want to keep what you're listening to private.
Poor isolation. These headphones have an open-back design and therefore don't isolate any sound below 2KHz. Above 2KHz, they perform poorly, achieving only 13dB of overall attenuation in the Treble Range.
Poor leakage. Due to their open-back design, the Sennheiser HD 518 leak a lot of sound throughout a broad range of frequencies. The significant portion of the leakage is spread between 300Hz and 20KHz which makes the leaked sound quite audible to the people around you.
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