First post. I also feel this is a situation where hiring a professional is the right choice for most people. I am in the early stages of planning a dedicated minimal compromise listening room for when I retire. Although I am by nature a DIY type person and have read extensively on the topic, I just don’t think I would be successful in creating a listening room with the performance I’m looking for without endless trial and error and the associated headache and added expense.
I thought I would share some of the information I have come across in the hope that it may be useful for others. This is by no means comprehensive and is somewhat stream of thought. Input is welcome.
Paying attention to the ratio of the L x W x H of the room pays dividends. This is certainly true in new construction, but it may also be the case that moving a wall or dropping a ceiling slightly may be of benefit in existing rooms. The general idea is that having room dimensions that do not share common factors will even out modal peaks and will decrease the amount of room treatment required. It follows that a cube is the worst possible room. The golden ratio is recommended by some (and isn’t necessarily bad) but isn’t special compared with many other options. I found the link below helpful but there are lots of resources around if one looks.
Having nonparallel side walls has some additional theoretical advantages in further decreasing modal peaks by increasing the average number of bounces (as well as eliminating slap echo); however, deviating from rectangular construction is problematic in modeling the room with software and obtaining predictable results. For instance, the room ratios in the above link only hold for rectangular (or near rectangular) rooms; and it is my understanding that this is also the case for other commonly used acoustics software. In any case, the above can easily be planned for and managed with room treatment. Personally, I want things to be predictable and to get it right or near right the first time. Other choices are valid.
In thinking about an “ideal room” it is helpful to consider the size of low bass notes. A 20 hz soundwave is longer than 56 feet. Rooms don’t need to be that large but ideally should be larger than ½ wavelength for best response. If one designs for 25 or 35 Hz, the room can be smaller.
Consider the needs for room isolation and room treatment and understand the difference. To what extent do I need to keep sound in /out based on the location of my room? Do I live near an airport or in the country by myself with no neighbors? Is my room next to a bedroom or an indoor space where noise is generated? What is my goal for background noise level? All things questions pertain to room isolation. One can look up Noise Control (NC) ratings (NC-15, NC-25, etc). Each of our situations are unique but will dictate what options are required for construction.
If one googles sound transmission class (STC), one will find different wall construction methods and their ratings in terms of isolation.
During construction, generally paying attention to details like air sealing and not bypassing wall isolation in construction are important. Small errors can have large impacts on system performance. Construction methods are likely to be new for the framing/drywall crew etc.
Plan for adequate ventilation and how to mitigate the associated sound so that it can be used while listening. Someone on the team will need to understand sizing HVAC, etc. There are lots of resources for DIY options regarding noise-absorbing baffles on forums that cater to recording studios.
Many diffusers that you can buy are not that effective. This isn’t because they don’t work, but because they only work over a limited range of frequencies and are otherwise reflective. This is related to physics and shipping and construction costs. Low frequencies require depth AND panel size, which is expensive to construct and ship, and high frequencies require nested solutions and more complicated construction. Some diffusers maintain phase relationships and others don’t. There are many different types. DIY is an option.
Here is a link for the “bible” for absorbers and diffusers (which can be purchased elsewhere).
Plan for lighting. LEDs on dimmers are great in some settings but are problematic in terms of noise and generally should be avoided. Consider having different lighting scenarios using groups of LEDs of different strengths, that can be switched on/off and combined to create the desired results. Alternatively, some music studios will still use incandescent bulbs and a Variac (an autotransformer that can continuously vary the supplied voltage). This is ideal in terms of flexibility and noise but is less efficient energy-wise. I plan to do a combination of all of the above. Please note that a Variac cannot be used with LEDs (as they require a specific voltage).
Have a plan for electrical in terms of clean power and wiring to limit ground loops.
Here are a couple of papers I found interesting. I read many others but can’t remember them off the top of my head.
As a final thought, personally, I will avoid selecting someone to design my room that sells their own acoustics products. There are many ways to accomplish the end goal, but I want someone who is making decisions based on performance and ideally in a cost-conscious manner that fits with my aesthetic priorities. Activated carbon, for instance, is tested and effective and has some performance advantages over other absorption options. However, the same end result can be obtained just by using more fiberglass or rockwool, etc. in a thoughtful manner and at a much lower overall cost. Anyway.
Best wishes to you all on your audio journeys,