It's not just the low volume - it's the airtight seal. A very low frequency sound wave is a slow sequence of low- and high-pressures. With an airtight seal, it'

It's not just the low volume - it's the airtight seal. A very low frequency sound wave is a slow sequence of low- and high-pressures. With an airtight seal, it's much easier to sustain pressure differences from 1 atmosphere from a long time. With no seal, the high or low pressure leaks away from the region near the ear. This is also why open-ear headphones tend to have lower bass response than closed-ear headphones. For example, compare these frequency response curves: http://graphs.headphone.com/index.php?graphID%5B0%5D=853&graphID%5B1%5D=4323&graphID%5B2%5D=&graphID%5B3%5D=&scale=30&graphType=0&buttonSelection=Update+Graph http://graphs.headphone.com/index.php?graphID%5B0%5D=853&gra... The open-ear Sennheiser HD650 has around 10-12 db lower bass response than the closed-ear Fostex. Even more dramatic, look at the dynamic response to a 50hz square wave - the Sennheiser's amplitude drops completely to zero before the next jump of the square wave, while the Fostex manages to maintain about 1/3 of the original air pressure in the headphone cup by the end. http://graphs.headphone.com/index.php?graphID%5B0%5D=853&graphID%5B1%5D=4323&graphID%5B2%5D=&graphID%5B3%5D=&scale=30&graphType=3&buttonSelection=Update+Graph http://graphs.headphone.com/index.php?graphID%5B0%5D=853&gra...