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.
Silver prices per ounce remained below $10 per ounce during the 1980s, 90s and much of the early 2000s. The silver price per ounce began to climb around 2006, and over the next few years went from under $10 per ounce to around $20 per ounce in 2008. As the financial crises of 2008/2009 got underway, silver prices per ounce began to climb rapidly, eventually rising to nearly $50 per ounce in 2011.