Compare like for like
Before/after comparison must keep equipment status, load, measurement points and method identical, and record environment and background sound; state any change in conditions.
Which conditions must match
Equipment combination and operating state, load, speed or operating phase, measurement positions and measurement method. If any of these differs, the before/after difference may come from the change in conditions rather than from the treatment.
How to do it
Before treatment, fix and record the measurement positions and note the environment and background sound. After treatment, return to the same positions under the same operating condition and measure again. Where needed, measure panel vibration and the sound at the target position together.
If conditions change
State what changed, such as a different load or a different environment or background sound, and do not present that comparison as a conclusion about the treatment. An elevator car running in a test tower and in a residential building also operates under different conditions and should not be compared directly.
How to report the result
Noise reduction is established by project measurement, for the specific equipment or vessel type and duty. Record sample, appearance and on-site acceptance separately, and do not let one result stand in for another.