Listening Room: Achieving Good Acoustics with Absorbers, Resonators, and Diffusers

[Matze]

My Goals
In my Hifi listening room, I aim for a targeted room optimization to create an optimal listening experience. The focus is on establishing a clearly defined stereo image to precisely determine the positions of sound sources. A broad soundstage with clearly audible 3D layering is important to me for achieving the most authentic listening experience possible. Additionally, room optimization aims to enhance the fundamental clarity of vocals and instruments.

I see this project as a process through which I can learn more about acoustics while simultaneously creating a good listening space. The gradual improvement of acoustics takes precedence. Ultimately, I strive for a listening experience that corresponds to the feeling of being live on-site. Initially, I aim to achieve a good frequency response close to my house curve (modeled after Harman’s preferred listening curve) without relying on digital aids. The reverberation time should also be as uniform as possible across the frequency spectrum and within a good range; I'm currently targeting between 300 and 400ms. I have already built sound absorbers and am now considering creating and implementing Helmholtz resonators and/or diffusers.

My Room
I have a small listening room with high ceilings, which I also use as a home office. The dimensions are 376cm wide, 359cm deep, and 310cm high.

My Budget
As a DIY enthusiast, I have the ability and desire to do a lot myself. Although my hobby doesn't generate income, I'm willing to invest time and, if necessary, money to achieve the best results. Particularly in a rental apartment where structural changes are limited, I aim for optimal outcomes. I'm ready to allocate financial resources, but there is certainly a limit – especially when we're talking about the value of a small car.

What I have done so far
After much listening, I fell in love with speakers featuring Airmotion Transformers in the high-frequency range. They effortlessly reproduce many instruments with clarity, and voices come across even better. Purchasing them ready-made was too expensive for me at that time. Building them myself was far more budget-friendly. That's how I acquired my good floor-standing speakers years ago. And so began my journey into the world of Hifi...

Initially, I ruthlessly sorted out all poorly recorded tracks. On a mediocre system with basic speakers, you might not notice, but now, everything became audible. Then, I had to replace the amplifier and opted for a tube amplifier with KT88 tubes. It was a huge leap forward... BUT: The most significant change after the speakers was the acoustic treatment of the listening room. Just as the speakers perform, the room essentially becomes part of the performance.

I have built a total of 6 thick absorbers, placing two on each side and also on the ceiling at the 1st and 2nd reflection (or 1st reflection of the opposite speaker). Suddenly, one could hear things that were not audible before. Background crackles are clearly recognized as snaps; there's no need to strain to pinpoint instruments and much more. Simply brilliant. Pure goosebumps.

My Problems
I positioned and aligned the speakers using REW and microphone measurements. It became apparent that the absorbers work very well in this setup, ensuring an optimal frequency response in the high-frequency range. However, I've noticed that achieving a slight improvement in the positioning of speakers and absorbers comes at the expense of another part of the frequency response. Overall, I recognize some room modes and peaks, as well as nulls, at the listening position. Still, I am not quite sure how to address them to make progress. Any guidance on how to proceed would be appreciated.

Acoustic measurement
I followed the instructions provided in this forum and attempted to conduct a compliant measurement. However, as I have both a USB DAC as a sound card and the REW-preferred calibrated USB microphone, minidsp UMIK-1, points 2 and 3 of the calibration guide from REW in this forum are not applicable. Perhaps I have misunderstood something or made a mistake in my thinking. In that case, I would appreciate any advice. The rest of the measurement followed the provided instructions.

[gullfo]

376cm wide, 359cm deep, and 310cm high

right off the bat, you have a nearly cube room (1:1.11:1.15 ratio) which means some of your low end will have specific reinforcement modes. one option is looking into membrane traps to target those and also adjust your cloud absorption with some hard backed treatments to shift the modes slightly with those "wave guides".

realistically you probably need at least 25% coverage of the walls and ceiling to get the RT down on the low end and on top of that (since this is listening not critical decision making) some diffusive or diffraction (polycylindrical panels can do LF and scattering).

[Matze]

My 6 absorbers are relatively large, 2.40m x 0.60m. Currently, they cover 14.6% of the surfaces without considering the floor.

So I thought, instead of continuing with absorbers, to move towards Helmholtz resonators. But what do I tune them to? Is a null worse than a peak (the latter can be easily corrected with a parametric EQ)?

[gullfo]

using some membrane absorbers in the correct locations will likely be easy. limp mass traps using MLV can be efficient and can be wider band. HH resonators can be tricky to build to get them operating at the correct frequency and Q.
ultimately - nulls are manifested by corresponding peaks. so attenuation of the peak frequencies at the boundary will typically reduce the depth of the null. so centered front-back, side-side panels as well as across corners can do significant attenuation. dressing them with cloth and decorative panels (not touching the membrane) can balance the overall response and add scattering.

[Matze]

Thank you, gullfo, for the tip with the membrane absorber. I initially built one membrane absorber tuned to 128Hz as a test and placed it in the corresponding location in the room. With this acoustic measure, I could gain around 6dB. The approach with membrane absorbers for this frequency range works very well.

For symmetry, the construction of a second membrane absorber will now follow.

[Soundman2020]

Source of the post instead of continuing with absorbers, to move towards Helmholtz resonators. But what do I tune them to?

Helmholtz resonators are notoriously hard to tune. It can be done, of course, but it isn't easy to get right. Small dimension deviations can tune it far enough off from where it should be, as to make it ineffective. As Glenn pointed out, membrane traps are generally easier to do.
Absorbers are also an option, and have the advantage that they don't need tuning! As long as they are deep enough, then can have a substantial effect on damping even very low frequencies, and they are broad-band by nature. But "deep enough" for typical modal frequencies in studios, implies many, many cm of depth. Maybe even a meter or more. Most home studios can't afford to lose that much depth on the walls and still be accessible for humans! So membranes are good for low frequency stuff, absorption for mids, but being careful to compensate for the loss of highs.
Finally, once you have a good amount of absorption and other treatment in place, getting most of the issues under control, you can usually use digital tuning to help iron out some of the remaining stubborn problems.

- Stuart -