Sound reproduction
The sound reproduction is usually playing a sound recording .
In principle, any sound recording can be played back, whether it is saved as a graph, as a record, tape or digitally (CD).
The sound reproduction can be done purely mechanically ( phonograph , gramophone and early record players ) or electrically ( loudspeakers ).
In a broader sense, z. For example, the imitation of animal voices is a kind of sound reproduction, but in the following only the narrower sense of technical sound reproduction is pursued.
Interaction with the playback room
The transducer interacts with the listening room in many ways, which is an insurmountable cause of sound changes (sound distortions). Even if the transducer worked ideally without any error, the sound waves reflected from the walls are superimposed in a complicated manner with the direct sound.
reflections
Assuming the sound transducer is an ideal spherical emitter , there is a uniform transmission in all directions for all frequencies. In an anechoic room, a uniform sound field is created at all locations and for all frequencies. Low-echo means: There are absorber wedges up to 17 meters long on the floor, ceiling and walls. Carpets, curtains and furniture do not make a room hypoechoic. Due to the lack of reflections , the sound pressure decreases with the distance from the sound source according to the 1 / r law .
If walls, floors and ceilings are completely reflective in the room, which is to be assumed for concrete walls at low frequencies, the acoustics change or deteriorate. For different frequencies, there are cancellations and amplifications at different locations . This leads to comb filter effects, certain tones overlap or cancel each other out at individual points in the room. Irregular behavior occurs. At two slightly different locations there are very different amplitudes at high frequencies.
With conventional loudspeakers in normal rooms, the diffuse field sound level of the reflections already exceeds the directly radiated sound component at a distance of about one meter from the speakers. This is why strong changes in sound occur in rooms. The group delivery times also differ quite significantly depending on the location.
The reflections are less stimulated and the disturbing effects are weaker due to the directional emission of the wave fronts. However, the sound in the lower frequency range cannot be bundled with conventional loudspeakers. Therefore, new problems then arise.
For a single point in space, the changes can be eliminated by inverse filtering. However, the problems a few centimeters away don't get better, but rather worse. Therefore, the sound field of a recording room cannot be reproduced correctly in a normal listening room.
It should be mentioned that these effects also occur with other sound sources, for example with a speaker or a musical instrument instead of the loudspeaker. These falsifications are always there and are part of everyday experience. It is no coincidence that the hearing is quite insensitive to such disturbances.
Illustration and deepening: (please click on the pictures for an explanation)
A spherical radiator in an anechoic room
A spherical radiator in a room where only the ceiling and left wall reflect the sound
A spherical radiator in a room where the ceiling, floor, left and right wall reflect the sound
Same situation, taking into account the second-order reflections
Narrow band resonances
Narrow-band resonances (e.g. from the housing, furniture, window panes ...) cause only minor errors in the direct sound frequency response and in the group delay, but audible changes when musical instruments oscillate.
The change is also called "roar" when it is clearly expressed.
Double space
The problems described above are exacerbated by the fact that the sound was already exposed to similar effects in the recording room.
The recordings generally “see” two spaces before they reach the ear:
- Recording room in the studio or concert hall
- Play room at home.
Even if both rooms have the same sound, the duality of space has a negative impact on the sound (for example, a self-recording that is recorded in the living room and then played back there).
The double spatiality could be avoided by recording in an anechoic room , but this would correspond to a waiver of a design element. In fact, audio recordings are often made in anechoic rooms, but not to avoid duplication, but to be able to flexibly impress ("fold") the acoustics of a certain room (mountains, church, hall) when mixing or mastering .
Location of sound source and receiver
Another limitation in sound reproduction is that the sound is recorded and played back with a finite number of transducers.
To achieve ideal reproduction, each individual sound source would have to be recorded directly on its own channel. These channels would then each have to be reproduced on their own sound transducer, which (if the original sound source is moving) would also have to be movable.
While recording the individual channels may be practicable, one (movable) sound transducer per sound source is not practicable, so that a compromise must always be made in terms of the spatial impression when reproducing sound.
With monophony there is only one channel and therefore no spatial impression. With stereophony , the spatial impression theoretically only works as long as the head is not moved, with surround sound it is already possible to turn the head and with 2 + 2 + 2 systems it is also possible to tilt the head.
Free movement in the sound room is not possible with the current state of the art. In practice, however, this is usually not necessary anyway, especially if the sound reproduction is part of a film playback, since the canvas or screen also restricts freedom of movement.