Room Acoustics

Describe the room where you are. Even music lovers and sound obsessed would probably begin with the visible. Spaces will always have their own sound but rarely penetrates our consciousness. Unless you’re being swept away when the music in the concert hall sweeps us or by the blaring bass of the stereo as you moved through space. Maybe you’ve enjoyed participating in a lively discussion in a public place as well as in confined meeting room? To inform yourself on the strain on your nerves to hear sound in certain spaces and by what laws spaces otherwise react to sound, please refer to our article on some key terms on the interesting theme of room acoustics.

Standing_wave_2

Spatial Modes

We begin with a phenomenon inherent to all areas which is at the same time is one of the starting points for the acoustic optimization, the so-called ‘Room Modes’.
Alternatively referred to as space, room resonance or resonant frequencies of the room, the slightly strange-sounding word can be used to understand it. Equivalent to the English term “Room Mode” or simply to the Latin word “mode”. This just means – depending on context – means “dimensions” or “size”, but also “limit”, “restriction” or just “type” and “fashion”. These translations are in fact very apt, because ‘room mode’ describes the acoustic character of an area that is largely defined by its proportions or its limits. If you want to largely decide with physical details, the natural frequencies of an area can perhaps be described as: pre-mastered to an empty room with music or as coming-and-going according to their sound waves with very different frequencies, including different wavelengths. These sound waves propagate and bounce off the walls for a well-known principle: the angle of incidence equals the angle of reflection. Depending on the geometry of the respective compartment there is then a set of sound waves of a wavelength particularly if they fit well in the space. As they are reflected from the walls, they lose much less energy and much more intense and longer affect the sound impression as such frequencies that do not belong to this group and abate significantly faster. So the real spectrum of the reproduced music is distorted. This group of waves is also called “standing waves”. For those who want to dive a little deeper into the matter had recommended the next paragraphs. You can also find a further link from the bold text below.

Standing waves then occur when an integer multiple (once, twice, three times, …) fits half the wavelength of a sound wave in a room. The boundary condition that is always on the wall and so-called pressure builds because the air particles cannot move there. While the standing wave makes its way through the room, so it is depending on the wavelength at various points to pressure antinodes (maxima) and pressure nodes (minima) on the walls but are in any case pressure nodes.

An example that helps immensely to clarify this. It is for the sake of simplicity that the expected to velocity of sound of 340 m/s:

A sine wave with a frequency of 40 Hz has a wavelength of 8.5 m. Between two parallel walls at a distance of half the wavelength – ie 4.25 meters – to each other, the 40 Hz sound wave would be a standing wave, while the first natural frequency or fundamental frequency of the room, as the half wavelength fits exactly once. From this fundamental frequency is the next natural frequencies between our two walls can be derived, for they all are always relative to the fundamental frequency:

Once half the wavelength of 40 Hz (4.25 m * 1) fits in the room.
Twice matches half the wavelength of 80 Hz (2 * 2.125 m = 4.25 m) in the room.
Three times adjusts the half wavelength of 120 Hz (3 * 1.4167 m = 4.25 m) in the room.

And so on …

The general formula is: natural frequency (n) = c /2 * n/a.

c is the speed of sound.
n is an integer (1, 2, 3, 4, …) and gives us information about which number it is natural frequency itself.
a is the distance between the two walls.

This would be done and hopefully somewhat understandable. The bad news is now that our example only applies to the one-dimensional space. For three-dimensional spaces (please be in touch with us if your listening room or two is four-dimensional!) you must be appropriately taken into account all dimensions, so that one has to do it in reality a complex interaction of the natural frequencies. The good news however is that we will not go into detail on it, instead it will be easier again. If you want to retrace the fashions of his room and get an overview of the pressure antinodes and nodes of the individual frequencies, which, at this point the ….. Room Mode calculator Hunecke of room acoustics code Raummodenrechner von Hunecke Raumakustik reccomended.

It is important once more to emphasize that the modes of a space can have different effects depending on the listening position. So it can at certain points in space to prolonged about accents — for example, the typical booming bass — come, while a few steps further away cancellations give the impression that this frequency range is substantially underrepresented. You can’t avoid spatial totally by room acoustic measures. Instead, attempts a good distribution of the natural frequencies is reached, so that they are distributed as widely as possible in the spectrum and not clench in certain frequency ranges. In addition modes can selectively reduce and the listening position can be optimized. Particularly problematic are just square rooms because parallel walls offer an excellent projection for sound waves. Here, then closes the circuit to the glass conference room mentioned above. The combination of material, shape and acoustic voices clutter leads with high probability to an unbearable rocking certain frequency ranges, in which several natural frequencies are close together. So a reasonable conversation is acoustically almost impossible.

At this point, it lends itself to another acoustic phenomenon to note: such spaces with reverberant walls and a square base are predestined to produce flutter echoes. The comings and goings of the sound waves can understand it very well. If you clap your hands you may hear a lot of short echoes can even sound tonal and stand for a while in the room, before they fade away eventually. We have arrived at another important criterion in the acoustic observation of a room.

Reverberation Time

The reverberation time is the time it takes for a sound event lost in a room 60dB SPL. From our consideration of the spatial modes we take with the knowledge that the size and geometry are crucial for how long and with what intensity reverberate the individual frequencies of a sound event. In addition, the acoustic properties of various materials or objects within the space may seem very significant impact on the reverberation time. This fact is exploited in room acoustic optimization advantage. How we are used to the acoustic reflections from our environment, although we perceive it rarely separated by a sound event is a stay in an anechoic chamber. As the name suggests, this is designed so that almost no reflections occur and the mere presence triggers a strange and oppressive feeling from.

Direct Sound and the Reverberant

Thus we have reached the end of our little tour on the topic key terms of room acoustics. We hope that our comments were both comprehensible and interesting. If you are interested in room acoustics in your own home, have a look at our partner
Finally, we want to shed light on the concepts of direct and reverberent sound. Direct sounds reach the listener without detours penetrating through an instrument or speakers directly to your ear, while the latter comes only after at least one reflection there. Take place, the more reflections from the perception by the ear, the more the frequency spectrum has been lost and a spatial localization of the sound event is no longer readily available. The area surrounding a source of sound in the sound pressure level of the direct sound is higher than that of the diffuse sound is called direct sound or free field. Are the ratios vice versa, this is called diffuse field. Where both levels are equal, there is the so-called critical distance.

Thus we have reached the end of our little tour on the topic key terms of room acoustics. We hope that our comments were both comprehensible and interesting. If you are interested in room acoustics in your own home, have a look at our partner Davidsound vorbei. Late July and early August, you have the oppotunity of getting a general impression of various acoustic elements in your living room or listening environment. For more information, see in our blog .

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