From: peter_frost@hp.com
Date: Mon, 20 Mar 2000 13:12:06 -0500
Message-Id: <H00013ab0f465ff1@MHS>
Subject: noise
To: lancair.list@olsusa.com
Mime-Version: 1.0

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Anyone wanting to measure noise levels in their cockpit and has (or can borrow) 
a laptop computer can download a free audio spectrum analyzer from the web at:

http://www.mnsinc.com/rshorne/gram.html

It samples the mic input using the laptop's sound card and shows sound volume 
(db) at frequency (hz). It can record to a file or show noise reatime. Prop 
noise show up as a spike around 100hz depending on rpm and number of blades, 
wind noise is spread across a broad spectrum. Using a unidirectional mic 
instead of the omnidirectional builtin might help you to identify noise 
penetration locations. Needless to say, have someone else fly the plane if you 
want to hunt noise in flight.

Passive noise reduction materials work best on higher frequency sound. They 
work by absorbing sound in batts of fiberglass or open cell foam, and can be 
used to minimize transmission or reflection of sound. I haven't tried it, but I 
would think that the hard bulkhead in a LNC2 directly behind the seats would 
reflect most of the sound that hits it and might benefit from some treatment. 
Thicker is better. (NASA uses 48 in. of what is essetially fiberglass behind 
perforated steel in the acoustic section of their 40x80ft wind tunnel at Ames)

Active noise cancelling headsets are your best bet for low frequencies (ie. 
prop noise). These headsets usually come with a chart which shows noise 
reduction in db by frequency. Prop noise will come through your windshield. 
This is a bad place for foam even if it worked on low frequencies, which it 
doesn't.

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