Critical distance

The reverberation radius or distance rH Hall 's acoustics in a confined space that distance from the sound source Q, in which the direct signal LD is equal to the space sound level LR in the statistical sound field. The term reverberation radius rh is always used when there is a sound source with a spherical directivity pattern. In the more general case of a sound source with arbitrary directional pattern, this distance is referred to as a Hall distance and is then different in size in different directions.

Introduction

When speaking in an enclosed space, such as a church, a speaker, then the language is reflected again and again to the floor, walls and ceiling until it subsided after a few seconds (see reverberation). Thus, the whole room is filled with a so-called " sound field ", that is, at every point in space add up the amplitudes of all possible reflections, such as the 3rd reflection of a sound that was spoken before 20 milliseconds, maybe the 10. reflection of 100 milliseconds old sound, etc. This space sound field is quasi-static and in any place of the Church about the same noisy as the reflections come from all sides. We distinguish direct field (free field) from the diffuse field. The direct sound from the speaker, so the sound reaches the listener directly without reflection is, the louder, the closer the listener at the speaker. For the sound pressure decrease here the law of distance ( distance law) for linear sound field sizes, this is the 1/r-Gesetz for the sound pressure. The street is the speaker is easy to understand and the reverb is barely perceptible. Further away that voice will sink more and more in the aftertaste. In between, there is a distance (a distance ), where the space R has sound (early reflections and reverberation ) same size as the direct sound D. This distance is the reverberation radius.

Acoustic viewing

Since in a corresponding to the Hall stand-off distance of the direct sound pressure level LD is equal to the diffuse-field sound pressure level LR, the total sound pressure level at this point due to the incoherent addition of the two components is 3 dB higher than each of the two components alone.

From the equality of the two sound field shares the defining equations for the Hall radius follow:

And the Hall distance:

Here, the directivity of the sound source, the volume ( capacity ) of the room in cubic meters, the reverberation time in seconds and the equivalent absorption area.

As the direct sound with increasing distance from the sound source is less ( by about 6 dB at a distance of doubling ), the diffuse sound (the sum of reflections ), however, remains approximately constant in the entire space, it is a distance from the sound source, in which both parts equally are strong. This distance is called reverberation radius rH and can be specified as a function of room volume and reverberation time on the Sabinesche approximation formula with

Hall radius rH in m Volume V in m3 Reverberation time T60 in s

Practical significance

Outdoors, the volume is infinite and the reverberation time zero; thus the reverberation radius is infinite, which means outdoors there is no reverberation radius. A typical value for the Hall radius in a reverberant space, such as a church, is, depending on the size of the room volume, reverberation time and the directivity of the microphone, for example, by 2 meters. That is the example of the Church: The closer a microphone in the direct field is at a sound source, the louder the recorded level. If the microphone, however, more than 2 meters away from the sound source, then outweighs the diffuse field signal and the entire sound signal is received unclear and blurry. That's why is called a mnemonic recording techniques: Microphones always have to stand within the reverberation radius.

Mnemonic: In an increasingly diffuse sound field, the directional characteristic of a microphone loses its effect.

One can choose from the microphone signal beyond the reverberation radius is no longer " hear " the direct signal when it rotates about its axis. Unlike the other hand, behaves the human binaural hearing. We can do much to determine the direction of the sound source outside the reverberation radius.

What is the reverberation radius use in practice? Using the reverberation radius can moderate impact, ie without taking into account the frequency dependence are calculated at what distance from the sound source of the sound space R the direct sound D exceeds. This knowledge is for the rough determination of the optimal site ( microphones ) are useful for the microphones used in the recording.

The to-find in the literature ' Hall Distance " = Direct energy level / reverberant energy level ( Reichardt ) is now indicated by Hallmaß.