Jag känner att det behövs benas lite i definitioner på vad direktljud är innan man kan börja dissekera och fundera vidare över vad som bör ingå i direktljudet, speciellt inom ljudåtergivning med högtalare i rum av storleksordning 40 kubikmeter - 300 kubikmeter.
Jag väljer att saxa från en bok som heter: The Psychology of Music, Third Edition (Cognition and Perception).
Kapitlet är: The Listener and the Acoustic Environment
Författare är: R. A. Rasch and R. Plomp
R. A. Rasch and R. Plomp skrev:1. INTRODUCTION
If a sound source and a listener are situated in an open field without any sound-reflecting surfaces in the neighbourhood, the emitted sound will reach the ears of the listener only via the straight line that-connects source and listener. The sound image that the listener receives will roughly correspond to the sound emitted by the source.
However, that is not the usual situation in listening to music. Producing musical Sounds and listening to them is almost always done in rooms or halls-technically speaking, in enclosed spaces. These enclosed spaces have bounding surfaces (walls, floor, ceiling) that reflect the incident sound.
Because of these reflections the emitted Sound dots not only reach the ears of the listener via the straight line from source to listener, but also via numerous other paths.
The sound that reaches the listener without any reflection is called the direct sound; the sound that arrives after one or more reflections is called the indirect sound or reverberation (see Fig. 1).
The presence of an indirect sound field has a profound influence on the sound image that the listener receives. The subjective effects of the indirect sound field make up what is loosely called the acoustics of a room or hall.
- Fig1_Direct_indirect_sound.png (18.04 KiB) Visad 3434 gånger
R. A. Rasch and R. Plomp skrev:We will first briefly examine the physical aspects of indirect sound. Fig. 1 shows several paths by which the sound of a source can reach a listener. Since all sound paths work equally well in both directions, source and listener can always be interchanged. The differences between a situation with and one without indirect sound may be summarized in three points:
1. The indirect sound adds sound energy at the position of the listener, resulting in a higher intensity than there would be without indirect sound. The gain can be substantial and depends, of course, on the sound absorption (and reflection) of the boundaries. It can be up to 10 or 15 dB.
2. The indirect sound arrives later than the direct sound because its path is always longer. If the velocity of sound is approximately 340 m/sec, it can be stated that every additional meter in a sound path causes a delay of 3.4 msec. Roughly, the time delays of indirect sound can he up to 100 msec per reflection. If the indirect sound includes some strong single late reflections with delays of more than 50 msec, these are called echoes.
Usually, it is possible to distinguish some discretely traceable reflections from the walls and ceiling that arrive first after the direct sound and a mass of diffuse later reflections coming from all directions.
The corresponding subjective effects may be described as follows:
1. The increase in sound intensity is perceived as an increase in loudness.
2. The later arrival of indirect sound has the effect that the source seems to sound a little longer than it really does. The direct sound is followed by a "cloud" of indirect sound. This gives continuity to a stream of notes that may have small discontinuities, such as staccato -notes: But the indirect sound may also coincide with or even mask the direct sound of the succeeding notes, which may confuse the sound image to a lesser or greater extent. The temporal aspects of indirect sound correspond to the subjective attribute definition, the ability to distinguish and to recognize sounds.
3. The incidence of sound from all directions results in an impression of spaciousness. This is usually considered a positive quality, although it seems necessary that the position of the sound source should be recognizable in the sound field.
Det här är en början som kan preciseras betydligt, men ändock en början. Sedan bör man skilja på musiker som spelar och sjunger live kontra ljudåtergivning av inspelad musik. Men förstår man inte det första så förstår man inte det andra heller och speciellt inte då det rör sig om fantomprojicering av ljudobjekt via högtalare i små rum.
Med vänlig hälsning
Peter
VD Bremen Production AB + Ortho-Reality AB; Grundare av Ljudbutiken AB; Fd import av hifi; Konstruktör av LICENCE No1 D/A, Bremen No1 D/A, Forsell D/A, SMS FrameSound, Bremen 3D8 m.fl.