| Materials Testing:
ISO-17497-1 (Continued)
We built another reverberation room (temporary) of the same volume as the old testing facility. The equipment and samples were then reinstalled and the testing was done again. The answers came back close to the original data. The difference was the size of the reverberation room! The difference was that larger rooms were more diffuse than the smaller reverberation room. This pointed to an error in the current standard, the assumption of diffusivity in the smaller rooms. A truly diffuse room has an energy field that has a fully "mixed" energy field both in energy magnitude and energy phase. The idea of comparing phase differences to remove energy from the energy field cannot be accomplished when you cannot have differences in phase responses from different impulse reponses in a fully "mixed" phase field. You can detect differences in a free field but NOT in a truly diffuse field. This means the "standard" for measuring scattering cannot operate the way it was described and can only operate in a non diffuse field. As a full scale measureing standard it is not a useable way to measure this parameter. SO what to do? How to provide a useable parameter to work inside of simulation programs is the problem. The solution was to look into the past and adapt what we already know. ASTM started looking, with guidence from Ron, into what is the function of "diffusion" panels and structures. Ron thought that diffusers perform exactly the same task as speakers do. They take energy from a source and radiate this energy into the room with a directivity function. Speakers provide their own energy in using a voice coil to convert electrical energy to acoustic energy and using the cone or horn to "direct" the energy. Diffusers do the same thing using reflected energy. How do we use this information in the simulation programs? we provide "directivity balloons" at each 1/3 octave frequency from 100Hz to 10KHz and each 5 degree resolution around the front of the speaker, for in wall speakers. Why can we not provide the same "reflection balloon" for various reflection devices? Enter the development of a new standard!
Developing a new standard at ASTM on Surface Reflectivity:
Ron had measured speakers for many years using the AES-156 standard. This standard developed a directivity balloon of the sound radiation in 5 degree increments using a matrix of a 360 degree by 360 degree balloon matrix. A complimentary section of the standard allowed a 180 degree by 360 degree measurement of a in-wall mounting of a speaker. This results in a hemisphere of the front facing half of a directivity balloon being shown only. This allowed the use of speakers in simulation programs for acousticians, such as EASE, Odeon and CATT Acoustics. Ron thought this might be a great way of including reflectors and diffusers in the simulation programs, as well. The question was how to provide a signal to be measured from these reflectors! Speakers have a voice coil to convert an electrical signal to acoustic energy by moving a diaphragm. This makes the air compress and decompress to move the eardrum. Reflectors do not have self contained "voice coils". They are intended to "reflect" energy already present in the "room".In ISO-17497-2 a source was proposed outboard of the measring system so that the direct sound can be "windowed" out of the Impulse response. This results in the mesurement of 1st order reflections from the "diffuser" or DUT. These windowed IP's can than be used to construct attenuation balloons similar to speaker balloons that can be used just like speakers in the simulations programs. There are numerous papaers and presentations to ASA at this location that can be used to understand more detail of the process
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