Introduction and Background
Sound level meters are indispensable tools in acoustics for measuring and analyzing the level of noise. In addition to frequency weighting, which takes into account the different sensitivity of the human ear to different frequencies, time weighting also plays an important role. It describes how quickly a sound level meter reacts to changes in the sound pressure level. This article takes a closer look at the different time weightings (fast, slow, impulse), how they work and their historical development.
What is a time evaluation?
Time weighting simulates the reaction of the human ear to temporal changes in the sound level. It determines how quickly a sound level meter reacts to changes in the sound level input
The first sound level meters were mechanical pointer instruments. Damping the "wriggling" pointer was essential in order to be able to read meaningful values at all.
The different time weightings in a sound level meter
- Fast: This setting is used most frequently. Fast uses a time constant of 125ms. However, short-term impulses are suppressed and can be better detected with the Impulse setting.
- Slow: In contrast to Fast, Slow reacts more slowly to level changes. This setting is suitable for measuring relatively constant sound levels, such as those found in production halls or offices.
- EQ: Energy Equivalent Averaging. This averaging corresponds to the effective value (RMS). The sound levels are squared and then arithmetically averaged. This evaluation averages the sound level over a longer period of time, e.g. 10s or hours.
- Impulse: The impulse evaluation is specially designed for measuring impulsive noises. With the impulse filter, it should be noted that the display initially follows an impulse quickly and then decays slowly. The fast and slow filters, on the other hand, are symmetrical
How does time weighting work in a sound level meter?
The time evaluation is carried out by filters in the signal path. These filters have different time constants that determine how quickly the measurement signal reacts to changes.
The time weighting is implemented using so-called time weighting filters. These filters are electronic circuits that modify the temporal progression of the sound signal. This creates an averaging effect that smooths the display of the sound level meter.
Typical implementations for time evaluation filters:
- RC elements: Simple RC elements (resistor and capacitor) are often used as time weighting filters. The time constant of the filter can be adjusted by varying the values of the resistor and capacitor. This technology is now only found in very simple sound level meters or "museum pieces". However, this was state of the art until the 1980s.
- Digital filters: Modern sound level meters often use digital filters. These offer greater flexibility and accuracy when implementing time evaluation. These digital implementations are not dependent on the environment (temperature, humidity) and do not age. Energy-equivalent averaging is very difficult to implement in analog form.
Time constants
The time constant τ (Tau) of a filter indicates how quickly a filter reacts to a sudden change in the input signal. It is given in seconds. After the time τ, the output signal of the filter has reached about 63% of the final value.
The time constants for the different time weightings are standardized:
- Fast: τ ≈ 125 ms
- Slow: τ ≈ 1 s
- Pulses: τ ≈ 35 ms
Historical Development
The development of time weighting is closely linked to the history of sound measurement. In the early days of sound measurement, various analogue RC filters were used behind the rectifier in order to be able to read the sound level on a pointer instrument. Later, more complex filters were developed to implement the various time weightings. Energy equivalent averaging is very difficult to implement in analogue form, so this important measurement only became established with the introduction of microprocessors in sound level meters. The standardisation of time weightings took place gradually and is now laid down in international standards such as IEC 61672.
