32. Acoustic Room Treatment
Published: 23/10/2024
Author: Karsten Hein
Category: High Fidelity
HiFi audio systems usually find their place within a family's the living quarters. Some of them serve as entertainment centres in the living room; others form part of a home cinema setup. Audio systems may accompany an evening meal in the dining room or make working time more enjoyable in the study. Some are part of a bedroom suite or even complete a children’s room. Many households have only one large living room, which is used for everything from cooking to entertaining. In fact, very few households can afford the luxury—or have the sense or wisdom—to dedicate an entire room, or even an entire floor, to listening. And of those who do, very few homeowners have chosen or designed their homes solely for this purpose. In fact, HiFi is often the last thing on a homebuyer’s mind.
And yet, when Sabina and I toured the north of Germany in search of a home for our family, I stood for the longest time in that cathedral-like empty space under the roof of our (now) house, clapping my hands in disbelief at the unbroken silence that greeted me each time. "I want this room!" I thought. "We need this house for our family, for the language school, and certainly for the sake of my continued audio explorations. At the time, the 1980s glass wool insulation was dangling in large sheets from the ceiling beams, and the floor was a single layer of worn wooden planks with visible cracks between them.
eiaudio Moving to Marne in Summer 2023
Watch the video on YouTube
It was clear that this magnificent-sounding room could not be left in its original, unfinished state. The lack of insulation was causing significant temperature differences between day and night, and between summer and winter. And the wind was blowing through the shingles, bringing dust and insects into the house. My challenge was to tame this wild beast without breaking its delicate spirit. To do this, I had to find out what made the original room work so well for audio purposes, and ensure I maintained and built upon that. I started reading up on room acoustics and quickly realised that there were some established listening room standards that our space met quite well.
For one thing, the room had almost no parallel walls that would have accentuated first-, second-, and third-order resonances. Instead, the side walls were almost three metres high and sloped inwards at a 45° angle from a height of about one metre to the roof. The structure itself was supported by exposed wooden beams with many uneven tongue-and-groove boards between them. One of the few straight walls along the side was made of exposed brick. The central part of the roof had a slight dome shape, rising 30 cm in the middle. There was plenty of open space to arrange the loudspeakers according to their acoustic needs, and room for me to walk behind each HiFi rack when setting things up. This meant that I could access my setups from all sides if necessary and take care not to accidentally cross signal wires.
I first consulted an acoustics expert on the best flooring for the room but felt that his preferred combination of insulation and screed would be too heavy for the old building. So, instead of pouring concrete, I added a 30 mm layer of OSB, which we covered with carpet. It was important to me to maintain the structural integrity of our building. However, this meant that I had to find a way of damping the resonance that traveled straight down from the speakers into the floor. I gave this some thought and decided that 38 kg slabs of concrete—the kind used to lay a garden path—placed directly under each speaker would act in a similar way to screed. At the same time, the slabs allowed me to place the spikes of each loudspeaker directly onto the concrete for greater structural stability. As a result, the soundstage and clarity improved dramatically, and floor resonance was reduced by around 50%.
I understood that designing a room to handle acoustics effectively requires a balance of absorption, diffusion, and (sometimes) bass trapping to manage reflections, standing waves, and reverberation times. I therefore set about determining the most linear position for each loudspeaker and the corresponding listening positions using the room mode calculation sheets provided to me by Peter English. I then set up my UMIK-1 measuring microphone with REW software to analyse the output of my speakers and the reverberation times of the room over the frequency spectrum as perceived from the listening position. It is sometimes said that covering 30% of the wall surface with absorbing materials will give a relatively natural sound impression. In rooms where typical household remedies such as bookshelves cannot be placed, professional sound absorbers from specialist shops are an option.
In general, the absorbing materials should be placed where the sound waves from the speakers are reflected directly from the walls and ceiling towards the listening position. These locations can be identified by moving a mirror—held parallel to the wall—along these surfaces until the loudspeaker drivers are visible from the listening position. In the case of our studio, I used the open joist structure of the room to hang curtains in the corners and along both sides. Rather than leaving them open, I gathered them together with a rope to form large, soft columns. This already quietened the acoustics and made conversations in the room a much more pleasant experience.
Instead of the usual windows, our listening room has small openings covered with 30 x 30 cm Perspex panels. These were less of an acoustic problem, but we did have a 120 x 60 cm window to the adjacent control room, which caused some harsh reflections. I used blinds to effectively diffuse the sound reflections from the glass. However, when I sat down to listen to the result, I could still hear that I was in a large room. I suspected that the few straight walls were reflecting too much sound, causing echoes.
As we had just bought and renovated our entire building, I needed to come up with an inexpensive solution for suitable absorbers. I found this on Michael Wynne's YouTube channel "In the Mix", where he builds 1200 x 600 x 100 mm acoustic panels using wood, damping materials, and cloth. In my case, I used jute mats (density 45-50 kg/sqm) recycled from old bean and coffee bags to dampen the treble and midrange. Shredded jute is an inexpensive choice, but there are some drawbacks: the damping factor is acceptable, but certainly not on par with more professional solutions such as Basotect (compare: rock wool 50-200 kg/sqm). There was also an unpleasant smell of mouse droppings at first. As a child, I once visited my uncle at the Bavarian public radio station and, walking past an absorber wall, had the feeling of being physically sucked into a vacuum. My DIY absorbers certainly do not produce this kind of dramatic effect.
To add more sound diffusion to the room, I bought four large, bushy plants and placed them along each of the side walls. Due to the lack of direct sunlight, these plants had to be artificial. From an acoustic point of view, it makes no difference whether a plant is organically grown or made of plastic. Artificial plants have the added advantage that they do not require a heavy pot of soil and can therefore be more easily moved to different positions to address various acoustic issues. To further reduce room reflections at the listening position, I chose large armchairs with high, absorbent backrests. These serve to increase the amount of direct rather than reflected sound reaching the ears.
Since the presence of sound waves also excites the furniture in the room, especially if the floor is made of wood, I chose HiFi racks that are capable of absorbing vibrations. In these, each HiFi unit is positioned separately on a shelf, each of which rests on spikes. The hollow steel legs are then filled with stabilising quartz sand to absorb acoustic vibrations within the structure. HiFi units containing large transformers are separated from the sensitive drive units by a separate structure to ensure correct reading of the source material without external noise. The racks are then coupled to the floor by steel spike pads on 4 mm felt pads. Felt is a chaotic structure and prevents floor vibrations from affecting the drive units.
The combination of measures works very well in the room. Both the treble and midrange sound vibrant and natural without over-emphasising any frequency. Vocals sound hauntingly real with an ever-present goosebump potential. Instruments are free from the speakers and are reproduced in their correct dimensions and timbre. There is a superior order to the music. Bass is mostly neutral to dry, with lots of layers and nuances, unless it falls uncomfortably close to the exact resonance frequency of the floor. At the time of writing, this is the only flaw that I can detect in my listening setup. As with everything in life, it is a concession to our house renovation budget. It might be worth experimenting with bass traps in the future to see if they, too, might have a positive effect on performance.
< 31. Understanding Room Acoustics | 29. Room Mode Calculations >
