The SP540 features the natural fitting "D-profile" earcup design first featured on the ergonomically designed Koss Pro4AAA. The D-profile mimics the shape of the ear, creating an ideal seal for the most consistent sound reproduction and an incredibly natural fit for extended personal listening sessions. The full-sized, over-ear, SP540 features the new Koss PLX40 dynamic element, tuned specifically for personal listening. The D-profile, circumaural design of the SP540 is highlighted by a new contour seal with asymmetrical foam that gradually increases in density near the back and bottom of the cups to balance the pressure around the ear, resulting in a perfect acoustical seal and a balanced fit for comfort. The cushions are comprised of slow recovery memory foam, premium acoustical fabric and protein leather. The SP540 features outer earcups reinforced by anodized aluminum and insert-molded metal hinges, which provide strength in areas where other headphones often fail. The advanced headband design allows for both size and pressure adjustment. The SP540 headband contains soft memory foam wrapped in breathable fabric for extended comfort. The SP540 also has a detachable cord and folds flat into a slim travel case for easy portability. Specifications: Frequency range: 20-25,000 Hz, impedance: 35 Ohms, Sensitivity: 99dB SPL