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What I Learned about Atmospheric Absorption Loss from my Whistle TestsViews: 1113
Jul 15, 2009 2:31 pmWhat I Learned about Atmospheric Absorption Loss from my Whistle Tests#

Richard Weisenberger
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

Jul 18, 2009 10:16 pmre: What I Learned about Atmospheric Absorption Loss from my Whistle Tests#

Ken Hilving
So, are there optimal acoustical frequencies to minimize atmospheric absorption loss, and did you tune your whistle accordingly? Does relative humidity change the optimal frequency?

Private Reply to Ken Hilving

Jul 18, 2009 11:28 pmre: re: What I Learned about Atmospheric Absorption Loss from my Whistle Tests#

John Stephen Veitch

You seem to have made a critical breakthrough. Modern engineers assume that they know how to make a LOUD noise, and that it requires an electronic solution. Build a big boom box. Feed in a screaming high pitched noise. "Isn't is obvious?"

Trying to make them understand that a mechanical / physical solution has distinct advantages isn't an easy task. I've tried several times to tell people about your work with zero response.

This news might be a turning point.

John Stephen Veitch
Open Future Limited - http://www.openfuture.biz/
Innovation Network - http://veech-network.ryze.com/
Building an Open Future - http://openfuture-network.ryze.com/

Private Reply to John Stephen Veitch

Jul 28, 2009 2:34 pmre: re: What I Learned about Atmospheric Absorption Loss from my Whistle Tests#

Richard Weisenberger
Atmospheric absorption loss is inversely proportional to the frequency, thus the higher the frequency, the greater the loss. This loss is measured in x dB/1000 feet. Most current warning sirens average a loss of around 2 dB/1000 feet. This loss is in addition to the inverse square law loss of 6 dB per doubling the distance.

However, you also have to account for the Fletcher-Munson curves of equal loudness, since the ear is most sensitive to higher frequencies at lower sound pressure levels, so there is a practical limit to how low you can go and still have the required sound pressure level at the intended distance.

I have found that the best frequencies for long range sound propagation lie between 200 Hz-500 Hz instead of the higher frequencies with high upper harmonic content used by most warning sirens. My prototype produced a fundamental frequency of around 430 Hz with low harmonic content and thus lost a mere 0.8 dB/1000 feet.

This gave it a measured 70 db radius of 13,200 feet for an initial SPL of 125 dB at 100 feet rather than the typical 4500 feet of a siren of the same 100 foot rating. The frequency and tonal spectrum matters this much! My 10 dB louder production model rated at 135 dB at 100 feet, the same as an American Signal P-50 siren, will have a predicted 70 dB radius of up to 4 miles (the 60 dB radius of my prototype) rather than the 70 dB radius of 9000 feet of the comparably rated siren.

Private Reply to Richard Weisenberger

Jul 28, 2009 2:44 pmre: re: re: What I Learned about Atmospheric Absorption Loss from my Whistle Tests#

Richard Weisenberger
The problem is perceived rather than real. Whistles are not INHERENTLY inefficient. There has to be a paradigm shift in people's way of thinking.

Most people still equate whistles with inefficient, outdated technology. The Dynawhistle (patent 4686928) is the loudest and most efficient true whistle design ever.

A typical 4" diameter steam whistle uses around 150 boiler HP (150 PSI at 400 CFM) and produces about 110 dB at 100 feet and has a 70 dB radius of less than 1 mile. This is inefficient.

Private Reply to Richard Weisenberger

Aug 11, 2009 4:28 pmre: re: re: What I Learned about Atmospheric Absorption Loss from my Whistle Tests#

Richard Weisenberger
I now have a link to my Facebook group. The problem with current warning sirens is that they are not designed to be audible indoors from a very large distance. I have a better alternative in the form of my toroidal whistles. I invite you to join my group and tell your friends about it.

Private Reply to Richard Weisenberger

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