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Low-frequency sound measurement

Low-frequency sound measurement

When requested to investigate a low-frequency sound, Nihon Onkyo Engineering will carefully interview those complaining about the sound and those who is suspected to be related to the generation of the sound, and examine the suspected causes of the low-frequency sound. Equipment used may include a sound level meter and vibration level meter in addition to a low-frequency sound level meter. With various possibilities in mind, we propose an analysis plan that excludes prejudice as much as possible.

Low-frequency sounds

Frequency range of low-frequency sounds

We may receive enquiries and complaints like, "Neighbors are complaining about low-frequency sounds that are expected to be caused by facility equipment.", "The windows and sliding doors begin shaking and rattling suddenly, like an earthquake is happening.", and "Strong pressure is felt even though no sounds seem to be heard." In these kinds of cases, low-frequency sounds may be their cause. The audible frequency range of sounds is said to be between 20 Hz and 20,000 Hz (20 kHz). Any sound below 100 Hz is considered as low-frequency sound, and sounds below 20 Hz (infrasound) are difficult to hear especially.

Evaluation of low-frequency sounds

Low-frequency sound evaluation curve

Low-frequency sounds are often the cause of feelings of discomfort and pressure in people and can cause fittings and other components of windows and doors to sway and rattle. In order to ascertain the extent of influence, frequency analysis may be performed using the 1/3 octave band.

Low-frequency sound evaluation (G-frequency weighting)

G-frequency weighting sound pressure level evaluation

In an effort to understand how low-frequency sounds can affect people, a low-frequency sound pressure level meter with a frequency weighting, called G-weighting, is sometimes used for measurement. G-weighting is like a low-frequency sound version of A- weighting used for audible sounds and includes human sensitivity correction targeting in the 1 to 80 Hz band.

Low-frequency sound measurement

Low-frequency sound level meter with wind screen

Outside measurements are easily affected by wind. As such, Nihon Onkyo Engineering uses a wind screen, but the effect is limited. In addition, because wind speed decreases closer to the ground, the microphone is sometimes placed closer to the ground for measurement.
The time waiting for low-frequency sound measurement is usually 1 second. However, when measuring the band below 5 Hz, the dynamic characteristic may be set to 10 seconds, in which case level fluctuations cannot be checked.
When the low-frequency sound is steady, the low-frequency sound level meter can be held by hand in order to measure the energy average value over a 10-second period. If the sound fluctuations are large or sudden, the sound should be recorded and analyzed later.

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