Introduction
When using a digital sound level meter or noise meter, two common time weighting settings are FAST and SLOW. These are not different decibel units and do not alter the sound itself. Instead, they determine how quickly the instrument responds to and displays changes in sound pressure level over time.
Sound in real-world environments is rarely constant. Machine start-up, passing vehicles, operating equipment, and human activity can all cause sound levels to fluctuate. If an instrument responds very quickly to every change, the displayed value may fluctuate considerably. A slower response smooths these variations and provides a more stable indication.
Understanding the difference between FAST and SLOW is therefore essential for using a sound level meter correctly and interpreting measurement results properly.
Key Points
● FAST and SLOW are time weightings that primarily differ in how quickly they respond to changes in sound level.
● FAST has a nominal time constant of approximately 125 ms and follows sound level changes relatively quickly.
● SLOW has a nominal time constant of approximately 1 s and provides a smoother indication.
● FAST is suitable for observing rapidly varying noise, while SLOW is useful for relatively steady noise or when the displayed value fluctuates significantly.
● FAST/SLOW time weighting and A/C frequency weighting are different concepts and can be used together, for example dB(A) FAST.
● For standardized measurements, the time weighting should be selected according to the applicable standard, test procedure, or measurement objective rather than simply according to which setting produces the most stable reading.
What Is Time Weighting?
Sound levels in real environments normally change continuously. Even when a sound source operates steadily, the measured sound pressure level may fluctuate over very short periods.
To provide standardized dynamic response characteristics, sound level meters process time-varying signals according to defined response characteristics. This is generally referred to as time weighting.
In practical terms, time weighting determines:
● How quickly the instrument responds when the sound level suddenly increases;
● How quickly the displayed value decreases when the sound level falls;
● How responsive or stable the indication appears when the sound fluctuates continuously.
FAST and SLOW are two of the most commonly used time weightings in sound level measurement.
What Is FAST Time Weighting?
FAST is a fast time weighting with a nominal time constant of approximately 125 ms (0.125 s).
Because it responds relatively quickly, the instrument can follow changes in sound level more closely. In environments where noise fluctuates noticeably, the displayed value in FAST mode will generally change more actively than in SLOW mode.
For example, when measuring machinery noise, FAST can make short-term variations in operating sound levels easier to observe.
The main characteristics of FAST include:
● Relatively fast response to changes in sound level;
● Better visibility of variations in sound over time;
● More frequent fluctuations in the displayed value;
● Common use for general environmental noise, machinery noise, and measurements where dynamic sound level changes need to be observed.
FAST does not mean that the instrument has a higher overall measurement speed, sampling rate, or accuracy. It specifically describes the defined time-weighted response characteristic.
What Is SLOW Time Weighting?
SLOW is a slower time weighting with a nominal time constant of approximately 1 s.
Compared with FAST, SLOW responds more gradually to short-term sound level changes. It therefore smooths rapid fluctuations more strongly and usually provides a more stable displayed value.
For example, if the sound level of a source fluctuates continuously and the FAST indication is difficult to read, switching to SLOW can make the overall sound level easier to observe.
The main characteristics of SLOW include:
● Slower response to short-term changes in sound level;
● Generally more stable displayed readings;
● Easier observation of relatively steady sound;
● Reduced influence of rapid sound level fluctuations on manual readings.
However, SLOW should not simply be interpreted as a “one-second average.” It has a defined time response characteristic and is not equivalent to a conventional arithmetic average.
Key Differences Between FAST and SLOW
| Parameter | FAST | SLOW |
|---|---|---|
| Time weighting | Fast | Slow |
| Nominal time constant | Approx. 125 ms | Approx. 1 s |
| Response to sound level changes | Faster | Slower |
| Display fluctuations | Generally more noticeable | Generally smoother |
| Observation of short-term variations | More suitable | Variations may be smoothed |
| Stable visual reading | Less stable | Easier to observe |
| Typical applications | Dynamic noise, general sound level measurement | Relatively steady noise, manual observation |
In practical terms, the difference can be summarized as follows: FAST follows sound level changes more closely, while SLOW provides a smoother indication.
However, the setting should not be selected solely according to personal preference. Where measurements must comply with a standard, regulation, or defined test procedure, the specified time weighting should be used.
Why Can FAST and SLOW Produce Different Readings for the Same Sound?
If the measured sound is highly stable, the readings obtained with FAST and SLOW may eventually be very similar.
Real-world sound, however, usually varies continuously. When the sound level rises or falls rapidly, FAST follows the change more quickly, while SLOW responds more gradually. As a result, the displayed values at a particular moment may differ.
For example, if a sound level suddenly increases:
● FAST will indicate the increase relatively quickly;
● SLOW will rise more gradually.
A similar difference occurs when the sound level suddenly decreases.
Different instantaneous readings in FAST and SLOW therefore do not necessarily indicate a measurement error. They result from the different time response characteristics of the two settings.
How Should You Choose Between FAST and SLOW?
For general sound level measurements where changes in sound need to be observed relatively quickly, FAST is a common choice. Typical applications include environmental noise surveys, machinery inspections, and general on-site noise measurements.
If continuous fluctuations make the display difficult to read, or if the objective is to observe a relatively steady sound level, SLOW can provide a more stable indication.
The following principles can be used as a general guide:
● To observe relatively rapid sound level changes: consider FAST;
● To obtain a smoother display for easier visual observation: consider SLOW;
● For standardized testing: follow the requirements of the applicable standard or test procedure;
● When comparing measurements from different instruments or at different times: use the same time weighting whenever possible.
When measurement data are recorded or compared, the time weighting used should also be documented. Otherwise, results obtained under different measurement conditions may not be directly comparable.
What Is the Difference Between FAST/SLOW and A/C Weighting?
FAST/SLOW and A/C weighting describe different aspects of sound measurement.
A-weighting and C-weighting are frequency weightings. They determine how the instrument weights different frequency components of the sound signal. FAST and SLOW are time weightings and determine how the instrument responds to changes in sound over time.
The two settings can therefore be used together.
Examples include:
● dB(A) FAST: A-frequency weighting with FAST time weighting;
● dB(A) SLOW: A-frequency weighting with SLOW time weighting;
● dB(C) FAST: C-frequency weighting with FAST time weighting.
For professional sound level measurements, documenting both the frequency weighting and time weighting provides a more complete description of the measurement conditions.
Do FAST and SLOW Affect Measurement Accuracy?
FAST and SLOW do not represent different instrument accuracy classes.
They change how the sound level meter responds to a time-varying signal rather than simply increasing or decreasing the basic measurement accuracy of the instrument.
For a stable sound source, readings obtained with the two time weightings may be very similar. For rapidly varying sound, however, the instantaneous displayed values can differ considerably.
FAST should therefore not be considered inherently “more accurate” than SLOW, nor should SLOW be considered more reliable simply because the display appears more stable. The appropriate time weighting depends on the measurement objective and the applicable measurement procedure.
Important Considerations When Using FAST and SLOW
● Check the selected time weighting before starting a formal measurement.
● Use the same time weighting when comparing multiple sets of measurement data.
● When recording results, document both the frequency weighting and time weighting where appropriate, for example dB(A) FAST.
● Do not interpret SLOW simply as an average sound level, and do not interpret FAST as a peak measurement.
● FAST is not equivalent to Peak response when measuring rapidly changing or impulsive sounds.
● Where measurements must comply with regulations, industry standards, or equipment test procedures, use the time weighting specified by the applicable requirements.
FAQ
Which is more commonly used, FAST or SLOW?
Both settings have practical applications. FAST is commonly used for general on-site sound level measurements, while SLOW is useful when a smoother indication is required for easier visual observation. For formal testing, the applicable standard or test procedure should determine the setting.
Does 125 ms for FAST mean the meter takes one measurement every 125 ms?
No. The 125 ms value refers to the nominal time constant of FAST time weighting. It is not the same as the instrument's sampling interval or display refresh rate.
Does SLOW calculate the average of one second of data?
Not exactly. SLOW has a nominal time constant of approximately 1 s and follows a defined time response characteristic. It is not simply a one-second arithmetic average.
Why does the display appear more stable in SLOW mode?
Because SLOW responds more gradually to rapid sound level changes, short-term fluctuations are smoothed and the displayed value usually appears more stable.
Can FAST be used to measure instantaneous peak noise?
FAST responds relatively quickly to sound level changes, but it is still a time weighting and is not equivalent to Peak measurement. For impulsive or transient peak sound measurements, use an instrument and measurement mode with the appropriate peak measurement capability according to the test requirements.
Can A-weighting and FAST be used at the same time?
Yes. A-weighting is a frequency weighting, while FAST is a time weighting. They can be used together, as in dB(A) FAST.
Conclusion
FAST and SLOW are two common time weightings used in sound level meters to define their dynamic response characteristics. FAST has a nominal time constant of approximately 125 ms and responds relatively quickly to changes in sound level, while SLOW has a nominal time constant of approximately 1 s and provides greater smoothing of short-term fluctuations.
In practical measurements, FAST is more suitable for observing dynamic changes in sound, while SLOW makes relatively steady sound levels easier to observe. Neither setting is inherently more accurate; the appropriate choice depends on the measurement objective and the requirements of the applicable standard or test procedure.
FAST/SLOW time weighting should also be distinguished from A/C frequency weighting. Recording the frequency weighting, time weighting, and other relevant measurement conditions helps ensure that sound level measurement results remain comparable and traceable.
