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 $13,000 per kilogram), depending on the quantity and its supplier.
As of November 2009 price for Ammonium Perrhenate (the most commonly traded form of Rhenium) is US$ 3500-4000/kg Rhenium content. see www.metal-pages.com Metal powder and pellets can be produced from APR at additional expense of US$ 200-300/kg. As of Tuesday, April 10, 2008, the mid spot price for rhenium was $10,700 per kilogram or $344.01 per troy ounce. Making it the eight most expensive precious metal following Osmium (at $380 per troy ounce). 2012 Update: Price of rhenium in APR (ammonium perrhenate) is around US$3000./kg Re. During 2011 we have seen a slight increase from slightly below 3000 to 3500 level and Q2/2012 falling back to around $3000./kg.