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Speaker phase check

When connecting speakers to the stereo amplifier, we must be careful so that the polarity is in-phase. Otherwise, the outputs of the two speakers are subtracted each other, rather than be added together. This condition is referred to as the speakers being out of phase. If the instrument does not sound in the middle between the speakers even when the volumes of left and right speakers are equal, you should check the phase of speakers. To check it, let's measure sound with the Realtime Analyzer.

Microphone mixer SONY MX-50
USB I/F Roland UA-5
OS Microsoft Windows2000 Professional
Measurement software Yoshimasa Electronic DSSF3


Stereo amplifier audio-technica AT-SA55
Speaker BOSE 101VM
Microphone SONY C-355

After the equipments are ready, start Realtime analyzer, and open the signal generator and the oscilloscope.

In the signal generator, two tones with different frequencies can be generated. In the signal generator, open "Tone" tab. Check the operation of the signal generator before a measurement.

See the figure above. Sinusoidal waves of 800 Hz and 1600 Hz are generated from left and right channels. The inputs from two microphones are displayed in real time by the oscilloscope.

Next, let's see the phase relationship of two speakers.

This is the correct connection.
Two 1kHz tones are output from two speakers. As you see, two waveforms are in-phase.
In this case, the polarity of the left channel speaker is reversed.
You can see that the two waveforms are out of phase. In such a condition, the sound from two channel will be subtracted each other.

Usually, the oscilloscope is used as described above for checking the phase of speakers. But you can check it similarly using the "correlation meter". 

Open the "Correlation" from the Realtime analyzer. In the figure below, pink noise is generated monaurally (same signal is fed into two channels in phase). Cross-correlation function has a positive peak at the middle point. 

When the pink noise is generated with inverse phase, the peak of the correlation function becomes negative. By using this phenomenon, the phase of the speaker system can be checked.

Correlation function was measured with the out of phase condition of the speaker.

Although the pink noise is output monaurally, the correlation function has a negative peak. This is because the speakers are out of phase.


In phase and Out of phase
Two 1kHz tones with in phase and out of phase (phases of left and right channels are altered by 180 degree) are prepared. Be sure to hear these sounds with a stereo speaker or headphones. (To play .wav files, software like Windows Media Player is needed.)

In phase (inphase.wav / 187KB)
Out of phase (outphase.wav / 188KB)

The in phase sound is heard in the middle between the speakers, but the out of phase sound is heard as dull on one side and timbre is slightly changed.


When you purchase a notebook PC for sound measurement, it is recommended to choose a model with a stereo external input terminal. If you use a model with monaural input terminal, you have to prepare an external sound interface (for example, we use a USB audio interface).

Two microphones are needed for the measurement described in this page. If you use one point stereo microphone, sounds from two channels are mixed (cross talk), and it is difficult to measure sounds from two speakers correctly.

Two microphones should be placed at the same distance from the speakers to check the phase. Small difference has a large effect on the phase of sound. For example, the wavelength of the 1kHz sinusoidal is about 34 cm. When the distance is differed as 17 cm, the waveform is reversed even if the speaker is connected correctly.


1) Autocorrelation function
The autocorrelation function is calculated as the Fourier transform of the power spectrum. Usually, it is used to detect a signal buried in noise. Recent research in psychological and physiological acoustics revealed that there is a autocorrelation- like mechanism in the human auditory pathway for the signal processing. One of the important features of the autocorrelation is its power to detect a periodical component in the signal. For example, pitch, pitch strength, and formant frequency of sound is easily extracted.
2) Cross-correlation function
This is the function to express the similarity of signals arrived at two channels. The direction of the sound source is calculated from the time at which the correlation between two signals become the maximum. By constructing the spatial filter based on this information, it is possible to separate the several signals in the field.