Not for nothing is the singing under the shower so popular: the voice sounds better in the bathroom than in the living room, because it has more reverberation, as one knows it from singers to recordings. The radio in the bathroom, on the other hand, sounds dreadful: the message speaker is badly understood, because the words are more blurred than they can be clearly distinguished.
Optimize room acoustics
So you quickly notice the great influence the room acoustics has on the sound. Raumakustik-Spezialist Thomas Fast formulates this way: "The plant is the drumstick and the space is the drum." Because sound is reflected or absorbed by objects. The sound of our bathingradio not only goes directly to the listener but also comes across the detour of the wall. There is therefore a mixture of direct and diffuse sound in the listening area.
Drone avoid
The diffuse-sound portion is very large in the bath because the hard, smooth surface of the tiles reflects sound well. This strong diffuse sound causes the hally sound. In order to optimize the room acoustics, one must therefore understand how objects and the space as a whole react to sound.
Sound tuning for everyone
We have already learned: If sound hits a sufficiently large object, it is reflected or absorbed to some extent. The following applies: angle of incidence equal to the angle of failure - just as a billiard ball bounces on a band. If sound hits straight on a wall, it is just thrown back.
Absorber Categories
For this reason, the sound coming back and forth is superimposed when a loudspeaker beams straight onto a wall. This causes discoloration in the sound image. The stronger these reflections are and the faster they arrive at the listener, the more clearly they are perceptible. Therefore, the goal of space-acoustic optimization is to master these reflections.
Practice: Home cinema - planning and room selection
But it is not only that objects sound reflect or absorb differently strongly. The degree of absorption of an object also depends on the frequency. Take, for example, an ordinary carpet: it absorbs no sound waves up to 500 hertz. From about 2,000 hertz it absorbs the sound by 80 per cent. The carpet is thus a pure height absorber.
As a result, a nonlinear frequency response is generated almost in any untreated room at the listening position. With a low set-up, low-frequency resonances are often very pronounced. Medium frequencies float quite a long time in the room if you do not mind. High frequencies, on the other hand, are underrepresented, since they lose more energy due to friction with the air. Ergo: There are different problems in different frequency ranges.
Anyone who has ever heard of a booming subwoofer will accuse someone of the fairy tale when he speaks of an "unbalanced sound". The woofer eavesdrops the rest of the action. The reason for this are resonances of space, often also referred to as modes which occur almost everywhere in low frequencies. They are caused by the superposition of sound waves.
If the distance of parallel walls is half the wavelength, or a whole number of them, the sound waves may oscillate. The booming is born. The background: sound waves have pressure maxima and minima. If pressure maxima of direct and reflected sound superimpose, the levels add up with correspondingly higher pressure. Particularly vulnerable are rooms with parallel walls.
The drone frequencies can be calculated quite simply: 170 divided by the distance of the opposite walls. If the wall distance is about three meters, the drone frequency is 57 hertz, at five meters it is 34 hertz. The larger the room, the smaller is the drone frequency. In giant rooms the problem can be neglected.
Please note: The pressure maxima and minima are location-dependent. This means that changing the listening position or repositioning the subwoofer can improve the sound.
Another phenomenon that can be found in rooms is flatterechoes. In small home cinemas, however, the problem occurs rather rarely. In order to make an echo, it must arrive at least 50 milliseconds after the direct sound. This corresponds to a path difference of 17 meters. For this, the sound has to be thrown back and forth several times in ordinary rooms. This creates only parallel, very bare walls.
Natural absorbers
Interview: Thomas Fast about sound optimization
In order to deal with all these acoustic problems, there is professional help: such as Thomas Fast from Fastaudio. He uses his professional measuring equipment, analyzes the problems that arise, and finally recommends measures for improving room acoustics. 1,000 euro including travel costs must be expected for the consultation approximately. For this purpose, a specialist is created that has already optimized more than 800 rooms. Also some HiFi and home cinema dealers help with the Klangtuning.
Conclusion
You can also take care of the optimization yourself. It is not helpful, however, to do it simply by listening, and a measurement is indispensable. If you want to make it simple, use an app on your smartphone.
For more accurate data, the computer is recommended. The corresponding high-precision software is available free of charge: The American high-end manufacturer Audionet offers the Carma program on www.audionet.de. The Room EQ Wizard is somewhat more complicated, but it is equipped with even more features. Registered users of the online community Hometheatershack can download the program for free.
In addition to computers and software, at least one measuring micro is needed, which is already for 50 euros, such as the Behringer ECM 8000. An external sound card also helps, because some integrated audio solution in laptops does not have a completely linear frequency response. The RaumRechenService of our sister magazine stereoplay works without measuring equipment.
The reverberation time is especially important for the assessment of the acoustics of a room. It is the time that elapses until the sound pressure of a sound event decreases by a certain value. If the level drops by 60 dB, one speaks of the reverberation time RT60. It is expressed as a function of frequency. In the home cinema, it should be between 0.3 and 0.5 seconds. In larger rooms, however, larger values are desirable.
If the reverberation time is too long, as in the bath, a faded sound is produced, and language is difficult to comprehend, as the whole picture suffers. If the reverberation time is too short, the sound is unnaturally dry and sterile. In such environments, most people feel uncomfortable. In modern apartments with parquet flooring, few furniture and hardly curtains is to be expected with too long reverberation times, especially at low frequencies. Absorbents are needed to compensate for this.
However, you do not have to buy special absorbers to optimize your home cinema acoustically. A carpet on the floor reduces the reflections of the heights tremendously. Heavy curtains can also improve the sound. A shelf for discs can again have a positive effect as a sound diffuser.
Practice: Sound tuning in the video room
More specifically, special absorbers can be used, since the manufacturers precisely specify in which frequency range they absorb as strongly. The rate of absorption is expressed either as a percentage or as a value between 0 and 1. 1 or 100 percent means that the sound is completely absorbed by a material. A value of 0 indicates that the entire sound is reflected.
It does not have to be the expensive special solution. Also, everyday home furnishings, walls and ceilings can have a positive effect on the spatial sound.
Despite all these theoretical considerations: To optimize the room acoustics, you have to try out different absorbers. The RaumRechenService provides a first clue: Afterwards, the absorbers have to be installed in the room, measured and maybe shifted again. The positioning of the loudspeakers and the computational accuracy of surround receivers - which we are dealing with in the next part of the home cinema series - are then ensured by the sound reduction. This is to be read in the video 5/2013, which will be released on April 5
Porous absorbers (fiber absorbers) are most frequently encountered. They are primarily foams, mineral, sheep or cotton, cellulose fibers or textiles. They are mostly used as center and height absorbers. The lower limit frequency is determined by the layer thickness. For example, a 1 cm thick carpet is effective only from 1 kHz, while the 4 cm thick nubmere foam is effective from 250 Hz upwards. In order to dampen deep frequencies, porous edge absorbers are also used. Thick absorbers in corners and edges not only disturb less, but basses also have sound pressure maxima. This is the most effective way to remove energy. For the minimization of booming bass, plate absorbers have also established themselves. They are designed to be slimmer than edge absorbers and are therefore often found on walls. However, plate absorbers are relatively narrow-band effective. They operate according to the mass-spring principle: The sound energy is absorbed by a plate with a certain mass and converted into mechanical energy. They usually work most effectively in room corners behind the auditorium, as the room resonances are concentrated there. Another category of absorbers are Helmholtz resonators. This is a closed box with a certain volume and an opening with a defined size. A Helmholtz resonator also operates according to the mass-spring principle, whereby the elasticity and the inert mass of the air volume are decisive. Since Helmholtz resonators are extremely fre- quency-selective, they are often custom-made. In addition to absorbers, diffusers are a proba- ble sound optimizer. Since the high-frequency range already often has ideal reverberation times, no attenuation is desirable here. But walls next to the listening area can cause loud and disturbing reflections (so-called early reflections). In order to minimize them, diffusers help in the place where the sound is beamed towards the listener. This can be the disk or bookcase, but also an effective special diffuser.
Carpet: This is a height absorber. Which carpet is used is less important. Parquet: This noble flooring does not dampen higher frequencies. For this, parquet bass absorbs, since it can be stimulated to vibrations - although the absorption rate is only about 15 percent. Linoleum soils do not absorb, on the other hand, plasterboard: these walls act as plate absorbers, which deprive energy from low frequencies. Depending on the wall, frequencies between 60 and 100 hertz are absorbed to about 30 percent. In the heights, plasterboard is highly reflective, such as stone, concrete, lime sand, brick, tiles or glass. Plaster: This wall cladding looks similar to stone. However, there are special acoustic plasters with an open-pore surface. Depending on the layer thickness, they absorb frequencies from 250 to 500 hertz to 50 to 80 per cent. Seating furniture: surfaces of wood and metal hardly absorb, leather somewhat more, fabric already relatively strong. Whereby a couch because of the larger volume dampens more than a chair. Especially effective as a deep absorber is a couch in the corners. Beds, shelves and shelves: shelves are acoustically much better than a naked wall. With discs, discs or books in the shelves, they have an irregular surface which acts as a diffuser. Depending on the design, cabinets can act as plate absorbers for low frequencies with their large areas (doors and walls). Curtains: They absorb different degrees of strength. The larger the distance to the wall, the lower the frequencies can be attenuated: at a distance of 20 cm to about 125 hertz. The thicker and heavier the fabric, the greater the absorbency (up to 80 percent).
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