31. Understanding Room Acoustics
11/04/2024
Author: Karsten Hein
Category: High Fidelity
Often, even before we listen to a classical instrument, we can imagine how it sounds just from looking at it. This might seem like a bold statement, but think about it: A cello, for instance, is a large wooden instrument with long thick strings, whereas a saxophone is a brass instrument on which air is blown past a reed into a tube that culminates in a horn. Is it not true that we would imagine the cello to sound warm and wooden and the saxophone to sound cooler and more tinny? And would we not expect the larger cello to play low frequency tunes and the smaller and horned sax to play higher notes and very loud?
The sound of classical instruments is mostly determined by their size, shape, and inner dimensions, because these features serve to create rooms and chambers in which the instrument’s sound is shaped and amplified. The source of the sound (strings or reed) is mostly separate from the resonating chamber itself. In the case of the cello, its strings are made to vibrate and resonate over a well-conceived box behind. In the case of the saxophone, pressurised air passing over a reed sets it in motion, with the sound being augmented by valves and tubes and further amplified through the horn's mouth.
The exact match of the strings with the body of the cello, or of the reed with the size and design of the saxophone, finally determine how each instrument sounds. Great instruments get this balance just right, and no one would imagine that inserting a louder reed or getting more power to the strings would greatly improve the sound. And most of us would not think of changing the position of the reed or strings either. Instead, it is simply understood that the source of the sound and the corresponding chamber need to be perfectly matched for the instrument to deliver the optimal sound. In fact, it is the precise matching that gives each instrument its own distinct tonal colour and character.
When first entering the realm of High Fidelity we might be led to believe that larger loudspeakers or or more potent amplifiers will solve the issue of lacking musical presence or improve bass performance in a room. However, the instrument analogy shows that proper calculations and correct loudspeaker positioning within a room are just as important. More power, speed, and accuracy will in a first step serve to accentuate flaws in the setup, which is often the reason behind expensive High End systems sounding quite unbalanced in their owner's homes.
The more classical instruments we have heard and the more materials we have touched and played with, the better we become at predicting an instrument’s sound even before we actually listen to it. Experience, attention, and imagination play an important role when it comes to listening. Children, for instance, are often said to have superior ears, especially when it comes to detecting high frequency sounds. And that is probably true, however, the physical ear mechanics only play a very small role in respect to critical listening skills. This is how children are far more likely involved in traffic accidents than adults. Although they may hear a vehicle approaching, their cognitive skills are not yet fully developed, so that the bending of sound waves from the speed of movement and the increasing volume of an approaching vehicle will mean very little to them. The ability of the brain to filter and pay attention to specific sounds in order to derive meaning from them only evolves with listening experience.
In that same way, I must confess that room acoustics meant very little to me, until I first owned a room that I could personally build from scratch and thereby experience the steps of transformation first hand that were necessary to make it sound great. It was not until we moved to Marne, a small town at the North Sea coast of Germany, in summer 2023, that I had a dedicated listening room for experimentation. Located under the roof of the building, the wide open space was initially an attic with visible beams and bare insulation that was still dangling from the ceiling ready to fall off. I remember clapping my hands in this room and being thrilled by the fact that there was no echo to be heard. At that stage, I was standing in “silent” room, perfect for listening adventures.
Original Room Presentation Video
On the other hand, the attic was also dusty and still poorly insulated. Bugs and spiders had invaded every nook and cranny, and I could hear the wind blowing through small gaps under the shingles. This was no place to set up our Martin Logan electrostatic speakers. High voltage panels are dust magnets, and turntables to not benefit from dust and wind either. To turn this space into a studio for audio auditions would require some effort. We began work by insulating the floor and adding an extra layer of OSB (Oriented Strand Board). And we decided to preserve the original tongue and groove boards on the side walls and only added an extra layer of insulation to the ceiling, which we then covered with one layer of plaster board.
At this stage of development, clapping my hands in the room led to quite a different result: The OSB floor and the tongue and groove walls still gave off a warm and pleasant air, but they also reflected a lot of sound energy instead of absorbing it. The slightly domed ceiling gave the room an even more confined and boxy impression with an audible reverberation despite its generous height of three meters. The listening room was 7.80 x 9.20 meters in size, and so the first, second, and third order resonance frequencies were relatively low, starting at 18 Hz, however, there was plenty of time for the higher frequencies to bounce off the smooth ceiling and walls and reflect back to the listening position with obvious delay.
On the positive side, there were lots of open wooden beams to deflect some of the reverberation energy and three of the side walls were sloping from about 95cm height. The tongue and groove wood cladding also provided some natural deflection, especially of the higher frequencies. And the one straight wall in the room had a large recessed portion leading towards a narrow storage space in the back. On the negative side, the room was relatively square, which might serve to further accentuate specific room modes. And, most obviously, there still were no furnishings and fabrics in place to absorb some of the reverberation energy.
The most pressing first step, therefore, was to lay down a carpet that would cover the complete floor. Carpets are great at absorbing acoustic energy and can be used in combination with rugs to provide an even greater absorption effect. Once the carpet was in place, we brought in the first loudspeakers and set them up as described in the chapter on Room Mode Calculations. In keeping with our instrument analogy, it really does matter where the sound generator is positioned and what building materials are found in the room. After all, the listening spot is to be provided with a relatively linear sound over all frequencies that is similar to the experience of the sound engineer when listening to the recording via a decent pair of stereo headphones in the studio. For this to be the case, both the speakers and the listening position need to be in correct relation to each other and to the room.
Obviously, our journey of exploration is not finished here. The completed room with carpet still sounded far worse than the unfinished room that I originally fell in love with. Be sure to read the chapter on room mode calculations for loudspeaker positioning and the chapter on acoustic treatments to learn about some of the easy fixes to sound issues.
As always, let me know how you feel about this chapter of the journey. Reflecting on your own experiences, how have room acoustics affected your enjoyment or perception of sound? Share your stories in the comments below — I'm eager to hear about the unique soundscapes you've encountered!
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