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Measurement of the air conditioner noise 2
Periodical diagnosis by operation sound

Usually, low frequency sound is not generated from the fan of the air-conditioner. The frequency of the fan noise can be calculated by multiplying the number of feather by the number of rotations per second. This sound is designed so that it may not go into the low frequency region. However, low frequency sound may occur when the rotation is damped by surging or damage of the feather, or when the surge-flow vibrates a duct or a machine itself.

When the low frequency flow moves objects, sound and vibration occur from there and it becomes the origin of displeasure. If the velocity of the air jet is high, low frequency sound will occur under the blow-off mouth. Operating the air conditioner with a small amount of ventilation is important for reducing the noise level.

Maintenance of the air conditioner might be possible by investigating unusual sound. In this report, sound of the air-conditioner during operation was measured and investigated whether it would be normal.

The oscillation of VAIO at the time of a non-signal (SONY PCG-R505 R/DK)
It is oscillating at 5kHz, 10kHz, and 20kHz. This may be a problem of PC's sound card,  but I dare continue the measurement as it is.

1/3 octave analysis at the time of operation (FLAT)

Level meters "ALL" in the left of the bar graph indicate about 58 dB. Because the recording level is raised 15.5 dB to increase the S/N ratio, the actual sound pressure level is 42.5 dB.

The power spectrum (FLAT)
There is a peak at 370 Hz. The low frequency region is same as compared with the background noise.

The sound level meter showed 42.2 dBA (A-weighting) at the time of strong wind air conditioning.

1/3 Octave analysis at the time of operation (A-weighting)

Power spectrum (A-weighting)

The running ACF measurement

Sound file : aircon2.wav (44.1kHz / Stereo / 10sec / 1.68MB)

Download the sound file and load it on the Running ACF screen. Same analysis on this page can be done. 

Now the sound is analyzed by SA (Sound Analyzer). To analyze the wav file, load it on the Running ACF window of RA and save it. Then, start SA. 

Main window of the running ACF analysis in SA

Calculation condition for the running ACF analysis

Results of the running ACF analysis (A-weighting)
In SA, time change of the sound properties, such as the sound level, pitch and pitch strength, reverberation, can be investigated. For more information of the autocorrelation analysis, see this page. It seems that the operation sound of the air conditioner is stable. Any failures are not found in the analysis result. This is the first measurement of the air-conditioner operation sound with this system. If more data is obtained continuously, I can compare them. Periodical measurement like this is widely applicable to other products, like a refrigerator, laundry machine, and so.


We can see the strange spike-like waveform in the autocorrelation function (in the lower left of the picture above). This is the oscillation at 5kHz. According to a thorough check, VAIO was oscillating (5kHz, 10kHz, 20kHz) from the beginning of the measurement. Because I forgot to do this check before the measurement, the trouble of PC has not been noticed to the last. The acoustic measurement system enables us to find unexpected problems which a conventional sound level meter never discovers.

August 2002 by M.Sakurai