Reverb radius

The Hall radius or Hall radius  r _H in acoustics in a closed room is the distance from the sound source  Q at which the direct sound level  L _{D is} equal to the room sound level  L _R in the statistical sound field . The term Hall radius r _{H is} always used when it comes to a sound source with a spherical directional characteristic . In the more general case of a sound source with any directional characteristic, this distance is referred to as the Hall distance and is then different in size in different directions.

The " reverberant energy level " to be found in the literature ( direct energy level / reverberant energy level according to Reichardt ) is now referred to as the reverb measure .

introduction

If in an enclosed space, e.g. B. a church, a speaker speaks, then the language is reflected again and again on the floor, walls and ceiling ( reverberation ) until it has subsided after a few seconds. As a result, the whole room is filled with a sound field, i.e. H. at every point in space the amplitudes of all possible reflections add up , e.g. B. the third reflection of a sound that was spoken 20 milliseconds ago, maybe the tenth reflection of a sound 100 milliseconds old, etc.

This room sound field is quasi-static and roughly the same loud at every location in the church, as the reflections come from all sides. A distinction is made between a direct field ( free field ) and a diffuse field.

The direct sound from the speaker, i.e. the sound that reaches the listener directly without reflection, is louder the closer the listener is to the speaker. For the sound pressure decrease is considered here the 1 / r - law of distance ( distance law ) for linear sound field sizes . Very close to the speaker, the speaker is easy to understand and the reverberation is barely noticeable, further away the voice is more and more drowned out in the reverberation. In between there is a distance (a distance) at which the room sound R (early reflections and reverberation) is the same size as the direct sound D; this distance is the reverberation radius.

Acoustic consideration

Since the direct sound becomes weaker with increasing distance from the sound source (by about 6  dB when the distance is doubled), while the diffuse sound (the sum of reflections) remains roughly constant throughout the room, there is a distance from the sound source at which both components are equal are strong: the reverb radius.

The equations for the reverberation radius follow from the equality of the two sound field components ( Sabine formula ):

${\displaystyle r_{H}={\sqrt {\frac {V}{c_{S}\,T_{60}}}}={\sqrt {\frac {A}{24\ln 10}}}\approx {\sqrt {\frac {A}{50}}}}$

With

• the volume (volume) of the room in cubic meters ${\displaystyle V}$
• the reverberation time of the room in seconds ${\displaystyle T_{60}}$
• the equivalent absorption area ${\displaystyle A}$
• the speed of sound c _S

and for the reverb distance:

${\displaystyle r_{H}={\sqrt {\frac {\gamma V}{c_{S}\,T_{60}}}}\approx {\sqrt {\frac {\gamma A}{50}}}}$

With

• the degree of concentration of the sound source. ${\displaystyle \gamma }$

In the open air the volume is infinite and the reverberation time is zero ; thus the reverberation radius is infinite , i.e. H. There is no reverberation radius outdoors. A typical value for the reverberation radius in a reverberant room, e.g. B. in a church, depending on the size of the room volume, the reverberation time and the directivity of the microphone z. B. by 2 meters.

Since the sound pressure level  L _{D of} the direct sound is equal to the sound pressure level L _{R of} the diffuse field at a distance that corresponds to the Hall distance, the total sound pressure level  at this point is 3 dB higher than either of the two parts alone because of the incoherent addition of the two components.

Practical meaning

With the help of the reverberation radius, it is possible in practice to roughly calculate , without taking into account the frequency dependency , at which distance from the sound source the room sound exceeds the direct sound. This knowledge is useful for roughly determining the optimal location for the microphones to be used for recording ( miking ). This is why a motto in recording technology is : microphones always have to be within the reverberation radius. Using the example of the church, this means: the closer a microphone is to the direct field of the sound source, the louder the recorded level. If, however, in the above example, the microphone is more than 2 m away from the sound source, the diffuse field signal predominates and the entire sound signal is picked up unclearly and blurred.

In addition, the directional characteristic of a microphone loses its effectiveness in an increasingly diffuse sound field . You can no longer "hear" the direct signal from the microphone signal beyond the reverberation radius when you turn it around its axis. Human binaural hearing , however, behaves differently : We can determine the direction of the sound source far outside the reverberation radius.

See also

• Localization (acoustics)
• Cocktail party effect

Web links

• Effective reverb radius and relative distance factor (PDF file; 71 kB)