Room modes

Spatial mode (from English room fashion, there from the Latin modus, plural: room modes ), is a technical term of acoustics. He describes stationary properties of standing waves with a natural frequency in enclosed spaces, wherein especially the effect on the hearing impression of the human of interest therein.

The shafts are exclusively acoustic sound waves in this context. The air in cavities can oscillate with different natural frequencies. A space mode is a space -filling mode shape of the air while she vibrates at a natural frequency. The vibration commutes between two opposing Auslenkungszuständen. So the room modes show where trained in space vibration nodes and antinodes at certain natural frequencies in the room.

For the period of observation the wave no longer travels through the space, but has fixed amplitude maxima and minima. The vibration nodes are the zeros of the amplitude, that is, at the point at which a node occurs, it is no deflection. In practice this means that, for example, for living rooms with hi-fi systems the sensation varies with the position of the person in the room. Depending on the room acoustic form especially in conventional housing Dimensions some living room modes in the low frequency range, which can be very disturbing. Of primary importance are those modes which are most trained. For rooms, there are six degrees of freedom for self-oscillations, resulting in a multi-dimensional composition of the possible resonant frequencies and the mode shapes. In principle, the maximum possible number of degrees of freedom is reduced again by the prevailing constraints. If one excludes the integer harmonics of each degree of freedom there is a natural oscillation. The degrees of freedom for modes in rooms can be limited to a good approximation on three calculations.

Excitation of eigenmodes

While small spaces have very discrete natural frequencies, all the modes overlap in large spaces like churches to a continuum - it gets worse on Hall. In rooms the room modes reflect how the sound of a room is discolored because have certain sounds particularly stand out and a non-uniform energy distribution within the space. If discrete resonant frequencies, these are more conspicuous than when multiple resonances are evenly distributed in the spectrum. A specific resonant frequency distribution is a physical property of the room, which is dependent on its dimensions. Only certain frequencies are excited. These resonance effects, both the increased level and the temporal continuity of the sound plays a role. The amplitude of an acoustic mode depends on the position in space. The degree of coloration is therefore different from place to place.

Schroeder frequency

Knowing the reverberation time of a room, it can be determined by the following formula, the Schroeder frequency, which is in most rooms around 300 Hz.

" Schroeder frequency, or simply Greater frequency. It can be determined with the Nachallzeit in seconds and the volume of space in a very easy to

Above the Schroeder frequency acoustic modes cause the space in homes without audible distortions playback, because the modes proceed in the form of dense reflections and reverberation each other. Below you can cause a perceptible coloration contrast. Since these relate to the particularly low notes, this is perceived as a drone, Booming, or one-note bass.

Calculating the axial, tangential and diagonal spatial modes

Primarily, three types of standing modes that occur in a typical ( cuboid ) listening room, is calculated. These are axial (longitudinal ), tangential and diagonal modes ( also obligue or oblique modes called ). Because the axial modes clearly dominate, they are very important.

" First order spatial mode occurs at a frequency whose half wave length corresponds to the distance between the two walls. [ ... ] The natural frequencies of the enclosed wall on the considered pair of one-dimensional space is calculated from

It is the speed of sound, the distance between the two walls and the order of spatial mode, which also corresponds to the same number of Schalldruckminima [...]. The considerations on two parallel walls reasoning can be applied to three-dimensional rectangular spaces. It additionally occur to those described as axially designated modes between two opposite pairs of walls also modes whose paths are moving in two and three dimensions of space. They are referred to in the two-dimensional case as tangential and three-dimensional case as oblique modes. The calculation of all the natural frequencies of a cuboid space can be done with the 1896, described by Lord Rayleigh formula:

Here again, the speed of sound, and the dimensions of the room, so the length, width and height, and the call and orders of the modes in the respective directions. [ ... ] The superposition of all modes of a room, the spatial sound pressure - particle velocity distribution and thus the three-dimensional field of complex sound field is composed impedances. Room modes are resonance- capable systems. " (Stefan Weinzierl )

Atomic number

The frequencies and mode shapes are ( number) named according to their atomic number, so:

If the composition of the natural frequencies of complex spaces such as the atomic number is specified more digits or separated by commas in parentheses.

Minimizing the effects of room resonances

A room with hard walls shows distinctive peaks at certain room resonance frequencies. Measures for sound absorption can be changed. Depending on the amount and position of the absorbent materials in a room these distinctive characteristics are reduced. There are now a variety of acoustic Absobermaterialien, certain frequency ranges which are preferably suitable for damping. Micro-perforated ceiling panels, specialty films with perforations and conventional absorber panels can be used or combined in order to optimize room acoustics for a given application.

"For hanger heights between 200 and 600 mm, as they occur frequently in practice, the maximum effect of this new acoustic ceiling is in the all-important frequency range 125-500 Hz, where at the present-day arid furniture with consistently reverberant surfaces, the sound absorption of the ceiling is urgently needed. At frequencies between 500 and 2000 Hz, where the ceiling absorbs less strongly, ia is Sound absorption by carpets, curtains and even the people present. This results in a relatively balanced on the reverberation time frequency, and a lower sound level in the room. "

Even passive and active resonance absorbers are used. Is it possible the spatial geometry in the planning phase to change, so you can achieve favorable proportions. In combination with suitable Schalldämmungsmaßmahnen room acoustics for the scope can be further optimized. Also the type of wall construction has an impact on the room acoustics, lightweight construction usually leads to a reduced need for additional measures. However, carpet, or heavy drapes to change the acoustics in an area which should not necessarily be attenuated. In the deep space drone leading Raummmoden these have virtually no influence.

Sound systems

Some providers currently offer extensive room acoustics devices with precision microphones, and use consecutive digital filtering to implement the necessary compensation for room modes. Given the very high cost of these systems, there is a controversy about the relative value of the improvement in normal rooms. Optimal use requires a comprehensive collection of data on the site must be carried out automatically in the setting phase of the devices. The compensation and equalization on the frequency response of the sound system used is of very limited use. The equalization will only fit for a particular listening position and results that in fact other listening positions deteriorate. Another compromise is, if possible, to suppress the resonance frequencies of known space through the use of digital comb filters or notch filters at the sound of the room. A further possibility is to increase the number of woofer and amplifier channels and so through complex digital processing targeted at the affected resonance frequencies in opposite phase, and taking into account the signal propagation times across the room, to control and thus wipe out reflections on the part. Where the site of the speakers in the room is of particular importance. To succeed in part or in larger rooms and neglecting the spatial modes in tangential and diagonal orientation to improve the playback.