BIZOMAT MAKEOS · Engineering
Machine Noise Acceptance Test (Background Correction, Distance, Limit)
From my sound level readings with the machine running and stopped, what is the machine’s own noise, is the background low enough for a valid test, and does it meet the limit at the specified distance?
Acceptance check of a machine against a noise limit: the machine’s own level from the total reading and the background with it stopped (as the machinery noise tool, machine mode), the validity of the test from the difference between the readings, the level corrected to the distance in the specification and to the operator’s position by the inverse-square law (as the machinery noise tool, distance mode), and the comparison with the limit.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Reading with the machine running | dB | Sound level with the machine running, at the measurement point. |
| Background reading, machine stopped | dB | Sound level at the same point with the machine stopped. |
| Measurement distance | m | Distance from the machine to the measurement point. |
| Distance in the specification | m | Distance at which the specification states its limit. |
| Operator distance | m | Distance from the machine to where the operator works. |
| Specified limit | dB | Highest machine level allowed at the specification distance. |
| Least difference between readings for a valid test | dB | Least difference between running and background readings for the result to be trusted (your test standard). |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Reading with the machine running
- What it is
- The sound level measured at the measurement point with the machine running, which includes the background noise.
- Units and format
- Enter a number in dB: decibels. The form accepts 0.000001 dB to 200 dB. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 88 dB as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the same frequency weighting (such as A-weighting) and meter settings for both readings.
What this means: Background reading, machine stopped
- What it is
- The background sound level at the same point with the machine stopped but everything else running as normal.
- Units and format
- Enter a number in dB: decibels. The form accepts 0.000001 dB to 200 dB. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 80 dB as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The closer the background is to the running reading, the larger the correction and the less reliable the result.
- Get it right
- Take it at exactly the same position and weighting as the running reading; it must be lower.
What this means: Measurement distance
- What it is
- The distance from the machine to the microphone where both readings were taken.
- Units and format
- Enter a number in m: metres. The form accepts 0.1 m to 1000 m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Measure from the machine surface or reference point the specification uses.
What this means: Distance in the specification
- What it is
- The distance from the machine at which the purchase specification or standard states its noise limit.
- Units and format
- Enter a number in m: metres. The form accepts 0.1 m to 1000 m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The tool converts the measured level to this distance by point-source spreading.
What this means: Operator distance
- What it is
- The distance from the machine to where the operator normally stands or works, used to estimate the level they hear.
- Units and format
- Enter a number in m: metres. The form accepts 0.1 m to 1000 m. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 m as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Specified limit
- What it is
- The highest machine sound level allowed at the specification distance, from the purchase specification or contract.
- Units and format
- Enter a number in dB: decibels. The form accepts 0.000001 dB to 200 dB. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 90 dB as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Make sure the limit and the readings use the same weighting and measurement type.
What this means: Least difference between readings for a valid t…
- What it is
- The least difference between the running and background readings for the test to count as valid, from the test standard or specification you follow.
- Units and format
- Enter a number in dB: decibels. The form accepts 0 dB to 20 dB. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 dB as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Method and formulas
- Machine level L = 10 log(10^(T/10) − 10^(B/10)) (as the machinery noise tool, machine mode).
- Level at distance r = L − 20 log(r ÷ r₁) (as the machinery noise tool, distance mode).
- Valid if T − B is at least the least difference; meets the limit if the level at the specification distance is at or below it.
- Example: the machine alone is 87.251 dB; at 1 m it is 87.251 dB against 90 dB; the operator hears about 77.708 dB.
Limitations
- Steady noise, the same frequency weighting for both readings; tonal or impulsive noise needs more.
- Spreading as from a point source in open space; reflections in a room make far levels higher.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Accept a new machine against a noise specification
- Check a measurement made in a noisy shop
- Estimate the level at the operator position
Who it is for
- Safety officers
- Maintenance engineers
- Project engineers
What you get
- Machine level from the two readings
- Level at the specification distance and the operator
- Validity and limit check
Interpreting the result
- When the readings differ by less than about 3 dB the machine level cannot be separated reliably: reduce the background and repeat.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3455–3456. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Harmonics, hum and magnetostriction
In Distribution Transformers by the Numbers
What it covers: Why transformers hum at 100 Hz, how noise is specified and tested, how drive harmonics add winding loss and distort voltage, and how impedance trades fault level for distortion.
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