Headphones let a single user listen to an audio source privately, in contrast to a loudspeaker, which emits sound into the open air for anyone nearby to hear. Headphones are also known as earspeakers, earphones or, colloquially, cans. Circumaural and supra-aural headphones use a band over the top of the head to hold the speakers in place. The other type, known as earbuds or earpieces consist of individual units that plug into the user's ear canal. In the context of telecommunication, a headset is a combination of headphone and microphone.
Osmium is usually sold as a minimum 99.9% pure powder. Like other precious metals, it is measured by troy weight and by grams. Its price in 2012 was about $400 per troy ounce (or about $13,000 per kilogram), depending on the quantity and its supplier.
The price of rhodium rose to average $9,493 in June 2008. By December, the price of rhodium averaged $1,050, down 89% from its June peak. The price has remained range bound in last couple of years at around US$1000 per ounce. Annual rhodium production of Rhodium is extremely limited. Only about 25 tons of rhodium are mined each year.