Apparently you have no idea what flanking paths are when testing walls, I see a lot of people do sound testing incorrect. Nor do you understand sound propagation or sound absorption. You also don't seem understand thermodynamics and the concept of insulation. Building walls doesn't mean you understand the physics behind sound and thermal energy transfer. It just means you can swing a hammer. Be respectful of your elders, they can help you learn and become more successful in your career. If you had been more polite we could have simply had a phone conversation and you could have asked all of the questions you have.
the acoustic benefits of fiberglass over other materials are well documented. as an acoustics engineer i design sound-rated walls in large commercial buildings, hospitals, etc. i understand the differences of all materials that are used for sound absorption, perform acoustic testing of spaces, and also do acoustic measurements for solving noise problems in buildings. as a senior electrical engineer my projects are hospitals that cost hundreds of millions. controlling sound and noise in hospitals and schools is important and requires an understanding of acoustical applications.
if you would like to read reports associated with the acoustic properties of various types of fill, i'm happy to share them. some of the materials require a membership to the Acoustical Society of America, and some are available for reading by all. I have thousands of pages of test data of walls with various forms of acoustical fill and fiberglass batt outperforms every other fill material and makes a substantial difference vs. without. The National Research Council of Canada has hundreds of reports testing thousands of different wall configurations. I have to study these reports in order to design buildings and ensure good sound isolation. my forum reputation over the past 8 years has been built on providing a lot of good advice to thousands of people. But this topic doesn't really pull from my 20 years experience in car audio, but instead the last 7 years i've spent as an acoustics engineer in parallel with my electrical engineering degree and engineering experience - yes, i carry two job titles. i've given presentations to architects, engineers, and also at ASA conferences. That is why I can talk about this as an expert, because in the eyes of the construction industry, i am.
Even easier relevant information to reference is the Loudspeaker Design Cookbook by Vance Dickason. he tested the acoustical benefits of various types of speaker box fill materials and noted the VAST improvement of fiberglass batt over polyfill. I am happy to provide this book to you at no cost if you send me a PM. I am also happy to provide you access to several acoustical text book and handbooks - easily a grand worth of literature and information on acoustics.
In the LDC you will learn that fiberglass in an enclosure does not make the airspace "smaller" as you claim, but instead slows the propagation of sound and absorbs sound to make the speaker behave as if it is in a larger enclosure. The absorption of the rear wave is what makes FG in doors successful. the results are instant and obvious, and the results match expectations.
Fiberglass Batt (open and compressed) is the primary material use in sound absorption products because of it's superior performance. Look in concert halls, recording studios, recording booths, fibrous silencers for HVAC or generator exhaust systems, etc. and you see fiberglass panels and other forms of absorption using fiberglass. When moisture is a concern, we employ a PVC encased fiberglass batt for sound absorption - this is true for natatoriums (swimming pools) and in the air stream of HVAC silencers.
The uses in commercial industry is where I got the idea to use them in a car door. It's no different than other commercially available products for room acoustics, just on a smaller DIY scale. CMA sells PVC encased fiberglass banners and bags that we will put above suspended GWB clouds - to absorb sound that would otherwise be reverberant.
Sounds like you have no idea about the field of acoustics - which is ok. Forums are for learning. Here are some companies that make acoustical products using fiberglass, none of them use polyfill - that shyt is for pillows.
RPG Diffusor Systems, Inc.
SoundSense – acoustic consulting, noise reduction, and sound design
Kinetics Noise Control, Room Acoustics, Vibration Isolation, Seismic Restraint
Golterman & Sabo Acoustics | St Louis, Missouri
CMA - Creative Materials for Acoustics
I've seen what competitors are using for absorption in the doors - and in some cases it's laughable. i've seen foam that is painted for water resistance - which takes a poor performing material and makes it nearly useless. open cell foams are better than nothing, but still poor performers. i don't concern myself trying to copy what other people do. i analyze the problem and engineer a solution that is backed by science... it's what makes me a good engineer.
cheers.
