Introduction
Motors, fans, pumps, compressors, gearboxes, and other mechanical equipment generate sound during operation. Under normal operating conditions, their noise levels and acoustic characteristics are generally relatively stable. When bearings wear, components loosen, friction increases, impellers become damaged, or operating loads change, the sound produced by the equipment may also change.
A sound level meter can measure the sound pressure level around machinery. It can help maintenance personnel understand operating noise, compare sound levels between machines or operating conditions, identify changes over time, and provide measurement data for workplace noise assessment.
However, machinery noise measurements are easily influenced by distance, microphone position, equipment load, reflections, and other sound sources. For this reason, obtaining a single decibel reading is usually not enough. Establishing consistent and repeatable measurement conditions is more important.
Key Points
● Keep the measurement position, distance, microphone orientation, and machine operating condition as consistent as possible.
● A-weighting is commonly used for general machinery and workplace noise measurements, while FAST or SLOW time weighting should be selected according to the acoustic characteristics.
● Measurement points should not be changed arbitrarily if results are intended for comparison.
● Minimize interference from other machines, personnel, airflow, and reflecting surfaces.
● For condition trending, measurements taken repeatedly under the same conditions are generally more useful than a single reading.
● A sound level meter can identify changes in overall sound level, but a decibel reading alone cannot determine the exact mechanical fault.
Why Measure Machinery Noise?
Noise generated by machinery is not only part of the workplace acoustic environment; it can also serve as a supplementary indicator of equipment operating condition.
Common measurement purposes include:
● Determining the normal operating noise level of equipment;
● Comparing noise levels between different machines or models;
● Comparing equipment noise before and after maintenance;
● Tracking long-term changes in machine noise;
● Identifying major noise sources within a production area;
● Detecting machines with noticeably abnormal sound;
● Providing basic data for workplace noise assessment.
For example, if a motor operates under a similar load for a long period but the sound level measured at a fixed point gradually increases, further inspection of the bearings, mounting structure, load, or other mechanical components may be appropriate.
An abnormal sound level should nevertheless be treated as an indication for further investigation rather than proof of a specific fault.
What Should Be Checked Before Measurement?
Before taking measurements, confirm that both the sound level meter and the machinery are in suitable conditions for testing.
Check the sound level meter
Make sure the instrument has sufficient battery power, the microphone is clean and undamaged, and the measurement range, frequency weighting, and time weighting are correctly configured. Where higher measurement reliability is required, perform the appropriate calibration or instrument check according to the applicable measurement procedure.
Confirm the machine operating condition
Record the current operating condition, such as:
● No-load or loaded operation;
● Rotational speed;
● Operating mode;
● Production condition;
● Whether auxiliary equipment is running.
Machinery noise may vary significantly with speed and load. Measurements taken under different operating conditions should therefore not be compared without considering these differences.
Check the surrounding environment
Identify other significant noise sources, such as nearby machinery, ventilation systems, forklifts, conversations, alarms, or compressed-air discharge.
Where possible, conduct measurements during a period when background noise is relatively stable.
Which Frequency Weighting Should Be Used?
Sound level meters commonly provide frequency weighting and time weighting settings. For comparable results, the selected settings should remain consistent throughout the measurement program.
A-weighting
A-weighted sound levels are normally expressed in dB(A). A-weighting adjusts the frequency response to approximate the sensitivity of human hearing.
It is commonly used for general machinery noise and workplace noise measurements.
C-weighting
C-weighting applies less attenuation to low-frequency sound than A-weighting. It can therefore be useful as supplementary information when machinery produces significant low-frequency noise.
If results from different dates or operating states are being compared, do not switch arbitrarily between A-weighting and C-weighting.
When Should FAST or SLOW Be Used?
Time weighting determines how quickly the displayed sound level responds to changes in sound.
FAST
FAST responds relatively quickly and is useful for observing more rapid fluctuations.
Typical applications include:
● Intermittent machine operation;
● Periodic impacts;
● Equipment start-up;
● Machinery with rapidly changing sound levels.
FAST makes short-term variations easier to observe.
SLOW
SLOW smooths short-term fluctuations more strongly and usually produces a more stable display.
For continuously operating motors, fans, or pumps with relatively steady sound, SLOW can make the displayed value easier to observe and record.
Neither FAST nor SLOW is inherently “more accurate.” The correct choice depends on the measurement objective. For trend comparisons, the same time weighting should be used each time.
Where Should the Sound Level Meter Be Positioned?
Measurement position is one of the most important factors affecting machinery noise readings.
Different parts of a machine can produce significantly different sound levels. For example, the drive end of a motor, non-drive end, fan side, pump housing, and discharge area may each have different acoustic characteristics.
Measurement points should therefore be selected according to the purpose of the test.
For long-term condition comparisons, one or more fixed measurement points may be established, for example:
● Motor drive end;
● Motor non-drive end;
● Fan housing;
● Pump body;
● Gearbox;
● Compressor or major noise-generating area.
Once a measurement point has been defined, subsequent measurements should be taken from the same position whenever possible.
Changing the position from one inspection to another can produce different readings even when the machine itself has not changed.
What Measurement Distance Should Be Used?
Machinery noise levels vary with distance, so measurement distance should always be treated as part of the test condition.
There is no single distance that is suitable for every machine. The appropriate distance depends on:
● Equipment size;
● Measurement purpose;
● Available space;
● Test procedure;
● Safety requirements;
● Internal measurement specifications.
For routine trend monitoring, a company may define a fixed measurement point and distance for each machine. Future measurements can then be taken under the same conditions.
For comparative measurements, consistency of distance is generally more important than selecting an arbitrary “standard” distance.
How Should the Sound Level Meter Be Held?
Avoid allowing the operator's body, hands, or other objects to significantly obstruct or reflect sound reaching the microphone.
In general:
● Do not cover or obstruct the microphone;
● Avoid positioning your body directly between the sound source and microphone;
● Orient the microphone according to the instrument manufacturer's measurement requirements;
● Keep the instrument as stable as possible;
● Do not continuously change distance or orientation while reading the display.
For longer measurements, a tripod or fixed support can be useful. This improves positional stability and reduces reflections caused by the operator.
How to Measure Machinery Noise
Once the equipment and instrument are prepared, measurements can be performed using a consistent procedure.
Confirm the operating condition
Operate the machine at the condition to be evaluated and allow it to reach a reasonably stable operating state. Record relevant information such as load, rotational speed, or operating mode.
Define the measurement point
Choose a measurement point according to the machine structure and test objective. Record both the position and distance.
Configure the sound level meter
For a typical machinery noise measurement, settings may include:
● A-weighting;
● FAST or SLOW time weighting;
● An appropriate measurement range.
If maximum sound level is relevant, check whether the instrument provides MAX, MAX HOLD, or a similar function.
Keep the instrument stable
Position the microphone at the defined measurement point and avoid unnecessary movement.
Observe the reading
Machinery noise often fluctuates. Do not record the first instantaneous value without observing the normal range of variation.
Record the results
A useful measurement record should include at least:
● Equipment name or ID;
● Date and time;
● Measurement point;
● Measurement distance;
● A- or C-frequency weighting;
● FAST or SLOW time weighting;
● Machine operating condition;
● Measured sound level;
● Maximum or average value, if supported;
● Notes on any unusual sound.
This information makes future comparisons significantly more meaningful.
Why Are Multiple Measurements Recommended?
Machinery noise is rarely completely constant.
Changes in load, product flow, airflow, cyclic mechanical motion, and other operating conditions may cause sound levels to fluctuate.
For important equipment, several measurements under the same conditions can provide a better indication of the actual operating noise.
For example:
● 76.2 dB(A);
● 76.8 dB(A);
● 76.5 dB(A).
These readings provide more context than a single instantaneous measurement.
For equipment with significant fluctuations, a sound level meter with equivalent continuous sound level or other statistical measurement functions may also be used where appropriate.
How Can Abnormal Machinery Noise Be Identified?
For equipment condition monitoring, a single decibel value has limited meaning by itself.
A more useful approach is to establish baseline data while the machine is known to be operating normally.
The baseline should be measured under consistent conditions, including:
● Fixed measurement point;
● Fixed distance;
● Similar operating load;
● Same frequency weighting;
● Same time weighting;
● Similar surrounding environment.
Measurements can then be repeated periodically.
If a machine that normally measures approximately 75 dB(A) under the same conditions gradually increases to 78 dB(A), 80 dB(A), or higher, further inspection may be justified.
It is also important to remember that the decibel scale is logarithmic, so changes in dB should not be interpreted in the same way as ordinary linear numerical changes.
Which Mechanical Problems May Be Associated with Noise Changes?
Many changes in machine condition can affect sound, but an ordinary sound level meter cannot identify the exact fault.
Possible causes associated with changes in noise include:
● Bearing wear;
● Inadequate lubrication;
● Loose components;
● Increased mechanical friction;
● Fan or impeller problems;
● Coupling or transmission issues;
● Loose mounting structures;
● Increased imbalance or vibration;
● Changes in gear meshing;
● Motor load changes;
● Changes in airflow, exhaust, or fluid conditions.
If a significant increase in noise is detected, the result should be evaluated together with vibration, temperature, current, rotational speed, and mechanical inspection data.
A sound level meter is useful for detecting that the acoustic condition has changed, but it is not a dedicated machinery fault diagnostic instrument.
How Can the Main Machinery Noise Source Be Located?
For larger machines, measurements can be taken at several areas while maintaining appropriate safety distances.
Typical locations include:
● Motor;
● Bearing area;
● Gearbox;
● Fan section;
● Pump body;
● Exhaust or discharge point;
● Transmission mechanism;
● Different areas of the machine enclosure.
Comparing sound levels between these locations can help identify the region producing the strongest overall noise.
However, if the objective is to identify specific frequency components or diagnose complex acoustic sources, a basic sound level meter may not be sufficient. Frequency spectrum analysis, vibration analysis, or other specialized measurement techniques may be required.
Does Background Noise Affect Machinery Measurements?
Yes.
A sound level meter measures the total sound reaching the microphone. It does not automatically distinguish the target machine from other sound sources.
If the target machine produces approximately 75 dB(A), while background noise from nearby equipment is close to the same level, it becomes difficult to attribute the measured result entirely to the target machine.
Where possible:
● Switch off unnecessary equipment;
● Avoid measurements while forklifts or other mobile sources are passing;
● Avoid periods of heavy personnel activity;
● Measure when background noise is relatively stable;
● Record the background level with the target machine stopped when useful.
When the difference between machinery noise and background noise is small, the result should be interpreted with caution.
Do Walls and Machine Surfaces Affect the Measurement?
Yes.
Sound reflects from walls, floors, ceilings, machine enclosures, and other hard surfaces.
A measurement made near a wall, in a corner, or within a confined area may differ significantly from one taken in a more open location.
For comparison between machines or between different measurement dates, the surrounding measurement conditions should therefore remain as consistent as possible.
If the original measurement was taken at a fixed position in front of the machine, future trend measurements should not be moved to a wall or to the opposite side of the equipment.
How Does Airflow Affect Machinery Noise Measurement?
Strong airflow across a microphone can generate additional wind-induced noise.
Common airflow sources around machinery include:
● Industrial fans;
● Ventilation systems;
● HVAC outlets;
● Compressed air;
● Equipment cooling fans;
● Outdoor wind.
Where significant airflow is present, use an appropriate microphone windscreen according to the instrument requirements and avoid placing the microphone directly in a high-velocity air stream where possible.
A windscreen reduces airflow-induced interference; it does not remove the actual acoustic noise generated by the equipment.
How to Establish a Machinery Noise Monitoring Record
For critical equipment requiring long-term maintenance, a standardized machinery noise record can be useful.
Typical information includes:
● Equipment ID;
● Measurement date;
● Operator;
● Measurement point ID;
● Measurement distance;
● Machine load;
● Speed or operating mode;
● dB(A) reading;
● Maximum sound level;
● Measurement duration;
● Background conditions;
● Presence of unusual sound.
For example, the motor drive-end position could be designated M1 and the non-drive-end position M2.
Each inspection can then repeat measurements at M1 and M2 under comparable operating conditions. Over time, this creates a useful noise trend.
For preventive maintenance, such trend data are generally more informative than occasional measurements taken at arbitrary locations.
Common Machinery Noise Measurement Mistakes
Changing the measurement distance
Different distances can produce different sound levels and reduce comparability.
Recording only one instantaneous value
Machinery noise may fluctuate continuously, so one instantaneous reading may not represent normal operation.
Ignoring machine operating condition
No-load, partial-load, and full-load conditions may produce different noise levels.
Ignoring nearby machinery
Sound from other equipment may contribute to the measured level.
Frequently switching between FAST and SLOW
Measurements taken with different time weightings should not be treated as directly comparable trend data.
Diagnosing faults from the decibel value alone
An increase in noise does not automatically indicate a specific mechanical defect.
Changing the measurement point each time
Noise distribution varies around machinery, so randomly changing measurement positions reduces the value of long-term trend data.
FAQ
Can a sound level meter be used to measure motor noise?
Yes. A sound level meter can measure sound pressure levels around a motor and can be used to compare operating noise over time. For meaningful trend monitoring, keep the position, distance, and motor operating load consistent.
Should machinery noise be measured in dB(A) or dB(C)?
dB(A) is commonly used for general machinery and workplace noise measurements. dB(C) can provide supplementary information where significant low-frequency noise is of interest. Comparative measurements should use the same frequency weighting.
Should FAST or SLOW be used for machinery noise?
FAST is useful for rapidly changing sound, while SLOW provides a more stable display for relatively steady machinery noise. Select the setting according to the measurement objective and keep it consistent for trend comparisons.
Is it better to measure as close to the machine as possible?
No. The measurement distance should be selected according to the test objective, safety requirements, and site conditions. Consistent distance is more important when comparing repeated measurements.
Does a sudden increase in machinery noise mean the machine has failed?
Not necessarily. Noise can change because of load, speed, nearby sound sources, or environmental conditions. A persistent change under comparable conditions should be investigated together with other maintenance data.
Can a sound level meter determine whether a bearing is damaged?
A basic sound level meter can identify changes in overall sound level, but it normally cannot diagnose bearing damage from the decibel value alone. Vibration analysis, spectrum analysis, temperature measurement, and other inspection methods may also be required.
Why does the same machine produce different readings each time?
Differences may result from measurement position, distance, machine load, background noise, time weighting, airflow, and acoustic reflections. Standardized measurement conditions are therefore essential.
Why measure the background noise when the machine is stopped?
The measurement provides an indication of how much surrounding sound contributes to the reading. If the background level is close to the operating measurement, the result should be interpreted carefully.
Conclusion
Measuring machinery noise with a sound level meter is straightforward, but obtaining useful data depends on consistent measurement conditions.
Select the appropriate frequency and time weighting, use fixed measurement positions and distances, and record machine load, speed, and environmental conditions. Background noise, airflow, and reflected sound should also be considered.
For maintenance applications, a single decibel reading only represents the acoustic condition at one moment. Repeated measurements under comparable conditions are much more useful for identifying long-term changes.
If a significant increase in sound level or an unusual sound is detected, use it as an indication for further inspection and evaluate it together with vibration, temperature, electrical parameters, and mechanical condition.
