Listening Room Dimensions... I'm building one and need your help


Sounds simple but I'm having a hard time coming up with most 'ideal' dimensions for this dual -purpose room that I'm building this year (hopefully).  



Purpose #1:
listening room for 2-ch hi-fi 
I have BIG speakers- several pairs 
*ideal is approx 25'x40'x12'
*want to sit 6-10 people in prime listening area 

Purpose #2: (other side of the room)
band rehearsal area (loud guitars, drums, keys, PA system, lots of 
*ideal is...bigger the better 

- I've read really good threads about general guidelines especially from Doctor Fine- hope he jumps in...
- I know the Cardas, and other rule ratios. But this room is a combination room so I'm not sure how much that changes anything 
- I know I'll probably have to use quite a bit of sound treatment 


My first inclination before research was 30' x 50' x 11'. 

What dimensions would you recommend and why??



128x128ramoneo14
As an exercise consider a contemporary performance Hall and note the 
design features.
Expanding from front to rear and overhead dimensions are expansive.
Avoid parallel surfaces and plan ahead for treatment.

Early on in a project like this is when the most far-reaching decisions are made, and it is when your knowledge and experience are the least. So this is arguably when you will benefit the most from the services of a professional. I am a reasonably educated amateur (been building speakers for a variety of applications for over forty years), yet what a real professional acoustician can do leaves me in the dust.

I highly recommend Jeff Hedback of Hedback Designed Acoustics, he is a multi-award-winning studio designer and also does superb home audio room designs and acoustic treatments, with budget and decor very much in mind. Jeff has extensive expertise in what you are trying to accomplish at BOTH ends of your room, AND he is still affordable. Bring him in as early as you can in the process and it will be the best investment you could possibly make.

Just to give you a taste, here is an article Jeff wrote for GIK Acoustics. It does not delve into room design itself but that is likewise an area where he has world-class expertise:
https://www.gikacoustics.com/diffusion-by-jeff-hedback/

Duke

What Duke said.

Other than that, realize that no matter how good the room design a lot of that band rehearsal stuff you're talking about putting in there is gonna be nothing but bad for the stereo. The sound a drum makes is from the skin moving, which is supposed to be from being hit but trust me its gonna move just as well when excited by the air playing music on the stereo. So you are gonna hear that drum when playing music whether you want to or not. Which same goes for the cymbals, guitar, cowbell, all of it. Which is fine if you are fine with it. I sure would not be. Especially not if I spent big on professional acoustic design only to discover later the pro totally asleep at the wheel missing what some schlub on a website could see coming a mile away.
The ratios still apply regardless of how big you want to make the space.  Since this isn’t going to be dedicated, which you’re alluding to it being multipurpose, work the ratios the best you can based on what you want to do with it.  Most people who strive for a “great sounding room” rarely get it. They end up over-damped and dead sounding or there’s not enough damping and things are too reflective with more modern hard surfaces.  Odd walls and dimensions can help break things up and with furniture, rugs, carpet, wall hangings, there’s a lot that can help.  This is an area that can easily be overthought.  
https://www.stereophile.com/content/listening-room-forgotten-factor-page-2

https://audiophilereview.com/room-acoustics/ideal-room-size-dimensions-and-ratios-for-audiophile-listening.html

https://www.psaudio.com/pauls-posts/critical-room-dimensions/


Another "what @audiokinesis said" OR, if you do it alone, here are the ideal height - width / length relations to minimize modes. If H is for height and X & Y the two walls of a rectangular room, then H=1 
to X=1.9 to Y=1.4/1.3 is best
X=1.5, Y=2.1 / 2.2 is second best