I work on quite a few projects and I also have my own diffuser product which can be seen on the back walls and doors of the demo room I have been building.
My interest lies with small listening spaces and let me tell you, it is not a popular one. Besides the ones who are working on various scientific experiments, most acousticians deal with noise or voice intelligibility. A very limited number of them work with concert halls and Some work for recording studio designs. Studio design has two faces (recording and mixing) and two separate goals. The mixing room goals and applications are very similar to our listening room goals in a way that the goals are fixed and we know what to shoot for. For concert halls or recording spaces there are some broad rules but no set goals, as recording and creating a sound is an art form in itself with no single right answers.
Back to our listening spaces; Contrary to studio mixing spaces we have many more problems. A domestic listening room adds multiple complexities because of set room shapes, furnitures, variety of speakers that are not as easy to work with as studio monitors, not being able to make all needed treatment because of decor and lastly the clients tend to pick and choose. I am also sorry to say the audiophiles are a hard breed to work with. Sometimes it becomes frustrating.
The descending rt60 is right, like the frequency response curve, mostly even but with a slight rise at LF. It will be like that if the treatment is rightly done, it will follow the speakers response. If you have strong LF but straight rt60 that means you are dampening lf more than hf. So, a good fr and a good rt60 will actually look similar.
The rt60 I showed was flat because the response was flat. I was adjusting my speakers and room. I attach the corresponding fr response of that rt60 so you can see. This is not how I listen, I prefer a slight rise on LF but not as much as Harman target curve but closer to Toole's preferred response curve.
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