Bundling dimension

The directivity index (Engl. Directivity index DI ) makes for microphones a statement about the background noise suppression in dB , for. B. with directional microphones via noise suppression in the direct field ( free field ). The sound picked up by a directional microphone is compared with the sound that an ideal microphone with omnidirectional characteristics would pick up with the same transmission factor.

Calculation:

${\displaystyle {\begin{aligned}{\text{DI}}&=20\cdot \log {\sqrt {\gamma }}\\&=10\cdot \log \,{\gamma }\end{aligned}}}$

With

• the bundling factor ${\displaystyle {\sqrt {\gamma }}}$
• the degree of concentration , which indicates the ability of the sound to be bundled by loudspeakers , microphones and individual sound sources in the diffuse sound field . ${\displaystyle \gamma }$

Some values

Microphone bundling ability
Microphone characteristics	Degree of bundling γ	Bundling dimension
Sphere ( reference )	1,000	0 dB
Wide cardioid (-9.5)	2.077	3.17 dB
kidney	3,000	4.77 dB
eight	3,000	4.77 dB
Supercardioid	3.732	5.72 dB
Hypercardioid	4,000	6.02 dB

literature

• Heinz Sahm: HIFI loudspeakers. Basics of electrodynamic loudspeakers in an infinite baffle and in the housing. 2nd, improved edition. Franzis Verlag GmbH, Munich 1982, ISBN 3-7723-6522-1 .
• Thomas Görne: Sound engineering. Fachbuchverlag Leipzig in Carl Hanser Verlag, Munich et al. 2006, ISBN 3-446-40198-9 .

Web links

• Bundling dimension and degree of bundling of all microphone directional characteristics (PDF file; 100 kB)
• Theoretical microphone data on the directional characteristics (PDF file; 37 kB)
• Explanation of theoretical microphone data (PDF file; 33 kB)
• Difference between hypercardioid and supercardioid (PDF file; 118 kB)