how to add dB (decibels)

[Admin]

It took me awhile to figure this out, so figured it'd be useful for someone if I posted this.

I was trying to figure out how to add decibel values, such as 5dB and -25dB. Decibels operate on the log scale with an arbitrary reference zero point so they can't just be added through simple addition.

let a=5 and b=-25

the equation is 10*log(10^a+10^b)

I'm summing rf signal attenuation caused by multiple successive mediums. These pages helped me:
http://answers.yahoo.com/question/index?qid=20080825204608AAHiwIJ

http://www.answerbag.com/q_view/11521

http://books.google.com/books?id=3T5ayFJadjwC&pg=RA3-PA234&lpg=RA3-PA234&dq=how+to+add+positive+and+negative+db&source=bl&ots=CrYaXO2z_V&sig=HZ03dTuYpcFR16x2wXhwyK6YIb8&hl=en&ei=V_dMSp2qE5yqtgfCw62zBA&sa=X&oi=book_result&ct=result&resnum=10