I have a sound isolation studio that is a room inside a room inside a room where the innermost room sits atop rubber mats made from ground truck tires. There are air gaps between the innermost room and the outer rooms, including the respective ceilings; but after starting to calibrate the audio to a flat equal-loudness curve running from 20-Hz to 20-kHz at 85 to 90 db SPL with a dBA weighting, I discovered there was a gnarly standing wave at approximately 70-Hz which made everything muddy and prevented any possibility of clarity.
I did a bit of research, including watching some YouTube videos by Ethan Winer; and discovered (a) that rolls of fiberglass insulation and (b) cubes of compressed cellulose insulation are excellent for handling low-frequency standing waves.
The strategy was to play "Billie Jean" (Michael Jackson) at 85 to 90 dB SPL and then to add rolls of fiberglass insulation and cubes of compressed cellulose insulation until everything sounded crisp and distinct, which for the sound isolation studio that is 6' wide by 7' tall and 12' long mapped to 9 rolls of fiberglass insulation and 3 cubes of compressed cellulose insulation.
The midrange and high frequencies were not a problem due to the combination of (a) using several layers of different thickness sheetrock, (b) using built-in Helmholtz resonators in the wall, and (c) having a carpet on the floor, as well as air gaps between the rooms and floating the innermost room.
I used a Behringer DEC 2496 Ultra Curve Pro and matching condenser microphone for the overall calibration and checked it with software and matching condenser microphone with ARC X (IK Multimedia), along with a NADY DSM-1X Digital SPL Meter, while wearing OSHA-approved ear protection like the ones used by airline workers on the tarmac when jet engines are running since the Kustom PA loudspeakers and subwoofers are vastly too powerful for what essentially is a walk-in closet, hence configuring everything require a bit of expertise and ear protection until it's calibrated and safe to use.
There are more expensive systems that probably are better, but these calibrating and measuring devices are good and are not so expensive.
Considering there already are curtains or other absorbing materials that should handle the mid-range and high-frequencies, I think adding some rolls of fiberglass insulation and cubes of compressed cellulose insulation will map to crisp and distinct audio in the studio--noting that if you are concerned about appearance, then you can cover the rolls and cubes of insulation or put them behind decorative cloth screens, so long as the cloth is porous, although for deep bass one can suggest accurately that porosity is not important, that is best understood by hearing deep bass from highly-amplified custom vehicle sound systems which among other things act as early-warning devices to announce the approach of massive deep bass and literally can shake the walls of your house, which is the way it works because deep bass waves are 16' or longer and travel through nearly everything, which is in contrast to mid-range and higher frequencies which easily are filtered by shrubbery and cloth curtains.
For reference, a Coca-Cola bottle is a Helmholtz resonator, and if you blow across the top of the bottle, you will hear a pitch, which is the resonating frequency. Curiously, Romans put wine bottles in the walls of their theaters and adjusted the room accountings by adding or removing sand depending on whether the performance was musical, theatrical, or a meeting.
You can control standing waves with PVC pipes that are closed at the bottom but open at the top, where the sizes can be determined visually by studying a cathedral pipe organ, where bigger maps to removing deep bass, but smaller maps to remove the respective mid-range and higher frequencies.
Nearly everything except glass and mirrors absorbs sound, which includes instruments acoustic guitars, violins, violas, cellos, contrabasses, as well as people, furniture, and so forth.
Instruments like acoustic guitar, non-solid body guitars, string, brass, woodwinds, drums, and so forth have Helmholtz resonator properties and behaviors; and the practical aspect is that since people absorb sound, the studio might sound differently depending on how many people are in the room as well as what the people are wearing, with heavy winter coats being more like a roll of fiberglass insulation and just a tank-top or short-sleeve cotton shirt not absorbing so much deep bass, although overall the human body absorbs a lot of deep bass.
I use (a) scientific measuring devices and (b) the "by ear" strategy combined with using "Billie Jean" (Michael Jackson) and songs from "Dark Side of the Moon" (Pink Floyd) as references, since they are mixed perfectly for studio-tuning purposes, where in particular "Billie Jean" is excellent for centering the stereo image of your studio monitor system, since the kick drum, snare drum, and bass are center (or "top-center" if you are listening with studio-quality headphones) and there are other instruments and voices that appear in distinct far-left and far-right locations in what I call the "Rainbow Panning Arc", which is something Quincy Jones (producer) and BruceSweden (audio-engineer) did and is something Quincy Jones did as early as the late1950's and then in 1963 when he produced Leslie Gore's first hit song ("It's My Party").
It doesn't matter whether you like the songs when you are adjusting the studio monitors and checking the acoustic behaviors of a studio or listening room, because what matters is that they are excellent reference songs and have distinct locations for the various sounds--instruments and voices.
[NOTE: I reverse the integer numbers for panning locations from the way it's numbered in Studio One; so far-left is "L.1", far-right is "R.1", and top-center is between "L.4" and "R.4". ]