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| Jul 15, 2009 2:31 pm |
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What I Learned about Atmospheric Absorption Loss from my Whistle Tests |
Richard Weisenberger
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I put the following on Facebook yesterday. I want to share it with this group:
I started questioning sound propagation following my whistle tests of July, 1982. I was getting measured readings of 85 dB at 1 mile and 75 dB at 2 miles from a 125 dB at 100 ft source, whereas sirens with a 10 dB higher rating at 100 feet were getting less than 70 dB at 10,000 feet. The readings at 1 mile and 2 miles are the only cases where I have ever observed the FEMA loss of 10 dB/doubling the distance-and it was occuring at a much greater distance than any siren of equal dB rating.
Since I also got a reading of 70 dB at 2.5 miles, I knew that it was falling off much faster than the inverse square law loss of 6 dB/doubling the distance, but much slower than the sirens, so I knew I was onto something. Even so I patented the 10 dB louder Type 2 inverted, horn loaded toroidal whistle to compete in terms of efficiency and output with the loudest sirens.
I did a lot of reading and learned about atmospheric absorption loss, which explained what was happening here and in everything else I had ever experienced regarding loud sounds from a distance, such as jets, thunder, fireworks, etc. It was already becoming a common practice among those in high level sound reinforcement to EQ the highs for large outdoor events to compensate for distance.
By the mid 90s I had access to the internet and started finding the answers to my questions. Before all of this I never really thought about why you never heard really high frequencies from a great distance. I just sort of thought that lower frequency sources must be louder or that objects such as leaves, grass and buildings must be absorbing sounds of a shorter wavelength. The objects turned out to be the air molecules themselves!
Private Reply to Richard Weisenberger (new win) |
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