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What do "phase difference" and "phase shift" mean?

In a multichannel system, many loudspeakers emit sound at the same time. In most cases, sound waves are produced by the back and forth movement of a driver. This creates wavelike changes in air pressure: peaks and troughs which can be represented as follows: 

Phase difference (also called phase angle) refers to the relative position of a sound wave’s peaks and troughs compared to another sound wave. If we have two loudspeaker that create the greatest amount of air pressure at the same time (because their drivers more forward at the exact same time), the phase difference will be 0°. These two waves are said to be “in phase.”

If one loudspeaker generates the highest air pressure while the other produces the lowest air pressure levels, the phase difference is 180°, or exactly opposite. This means that the trough of one wave is in the same position as the peak of the other. The two sound waves are then said to be “out of phase” with each other.

This could for example occur when two loudspeakers play the same signal completely synchronously but in polar opposite directions. It can also occur if the driver in one loudspeaker simply begins to play a little later than the other (non-synchronously) or if one loudspeaker is positioned considerably closer to the listening position than the other. The latter will create the same effect as if the drivers play slightly out of sync since the sound waves will take longer to reach the listener. This is why it is always recommended that stereo speakers and the two front speakers of a surround system are always placed apart from each other at the same distance to the listening position like a triangle with equal sides.

What effect does phase differece have on a system’s playback?
If two loudspeakers play with no phase difference, the signal will be experience as louder sound. The opposite – signals that are 180 degrees out of phase – will lead to a complete cancellation of the sound as the low air pressure of one sound wave’s trough compensates with the high air pressure of the other sound wave’s peak. The sound waves depicted above are in phase opposition and so cancel each other out.

This principle of sound cancellation is used in noise-canceling headphones. In these headphones, a microphone receives outside sound and then adds the same signal with a 180° phase shift for total noise cancellation.

The partial or complete cancellation of sound waves by other sound waves is more pronounced in the lower frequencies. Special attention needs to be paid to the phenomenon when it comes to the cross-over frequency between the subwoofer and satellites. That's because in this range, both the subwoofer and satellites play the same low frequencies but are often located in dffierent parts of the room. This can lead to phase interference.

What can be done to prevent destructive phase interference?

All Teufel loudspeaker systems are developed so that the individual components play in phase with each other if the subwoofer and satellites are positioned at the same distance from the listening position.

If subwoofer is not placed at the same distance from the listening position as the satellites, the phase may have to be adjusted in order to prevent phase interference. To this end, Teufel subwoofers are equipped with a switch that allow for a 180° adjustment of phase. A controller also allows the phase to be variably adjusted between 0° and 180°.
Finding the right setting only requires a small amount of time and energy, but you will require the assistance of another person. While you sit in the listening position, your friend should adjust the phase from 0° to 180 °. You will be able to notice a change in the upper bass range. Teufel’s systems are designed so that the bass frequencies in the cutoff range overlap in such a way that they reinforce each other. You will therefore know when the loudspeakers are in phase because the bass will be the loudest when this is the case.

Many newer A/V receivers also offer software that adjusts all speaker playback for an ideal phase alignment.

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