# Acoustic short circuit

Acoustic short circuit is the reduction of the sound radiation from vibrating surfaces ( loudspeaker membrane ) through direct pressure equalization between areas vibrating out of phase. If the sound absorption by a vibrating membrane ( microphone ) is reduced by this effect, one speaks of an acoustic short circuit.

A first type of short circuit is the mutual partial cancellation of the sound waves radiated from both sides of the membrane . During operation, due to the vibrations of the loudspeaker membrane, sound waves are generated in front of and behind the membrane. The resulting pressure fluctuations are almost exactly in phase opposition in front of and behind the membrane (i.e. reversed polarity), which is much more true for low frequencies.

To put it clearly: the same amount of air that is pushed away at the front is sucked in at the back. This amount of air actually only has to "run around the corner".

If a loudspeaker is operated without a large baffle or housing , the sound waves emitted from both sides of the membrane can overlap in such a way that pressure and negative pressure almost cancel each other out. As a result, the sound pressure drops sharply. This acoustic short-circuit is more pronounced the smaller the dipole membrane is compared to the wavelength : that is, in the case of low tones.

Avoidance of the short circuit by a simple baffle

The simplest remedy is a baffle that forces the sound waves to take a detour so that a direct short circuit can no longer occur. The diameter of the baffle must correspond to the order of magnitude of the lowest wavelength of the sound to be reproduced.

This natural resonance of the membrane can be present at 1000 Hz, depending on its stiffness and the diameter

Another type of acoustic short circuit cannot be prevented by a sufficiently extended baffle: it is the short circuit between neighboring flat pieces on the same side of the membrane, which - at sufficiently high frequencies - vibrate out of phase. It takes a few microseconds for the drive pulse from the moving coil to propagate to the edge. It can therefore happen that the outer area of ​​the membrane is still moving backwards while the central area is already moving forward again. In this way, partial oscillations are formed , which are particularly pronounced at some frequencies. In this case, the air does not have to "run around the corner", but only a few centimeters further. Then hardly any sound is emitted and there is a drop in efficiency at this frequency.

In principle, the acoustic short circuit can occur not only with loudspeakers without a partition, but also with pressure gradient microphones . Here he is responsible for the weak absorption of the low notes. A long wavelength, i.e. a deep tone, hits the front and back of the microphone membrane with almost the same pressure, which therefore hardly moves and therefore only weakly records low tones.

 Loudspeaker floating freely in the room, without housing, without partition. The membrane swings back and creates air pressure. A negative pressure is created in the membrane cone. Air flows into the cone and balances the pressure, sound pressure is reduced. The acoustic short circuit can be prevented by a closed housing.

## literature

• Berndt Stark: Loudspeaker manual. 7th edition, Richard Pflaum Verlag GmbH & Co.KG, Munich, 1999, ISBN 3-7905-0807-1
• Heinz Sahm: HIFI loudspeakers. 2nd edition, Franzis Verlag GmbH, Munich, 1982, ISBN 3-7723-6522-1