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The Sound Level VIs provide the following time-averaging modes:

• Linear, or equivalent continuous sound level (L_eq)
• Peak hold
• Exponential

Linear Averaging

You compute the L_eq by integrating the square of the signal over a fixed-time interval and dividing by the time interval. When you select linear averaging, the Sound Level VIs return a single value. The value returned represents the continuous decibel level that would have produced the same sound energy in the same time T as the actual noise history. To obtain intermediate results, you must split a long time record into several smaller records. Linear averaging is represented by the following equation:

where

P₀ is the reference pressure of 20 µPa for acoustics.

Single-Shot Linear Averaging

The following block diagram illustrates an example of linear averaging on a single-shot acquisition.

The single-shot acquisition is configured with the following parameters:

• Sampling frequency f_s = 51,200 samples per second (S/s).
• Buffer size = 51,200 samples, which is one second of data at the specified sampling frequency.

The following front panel displays the resulting L_eq measurement and the instantaneous sound pressure level.

Measuring L_eq over a Longer Time Period

You might need to compute the L_eq over a longer period of time. For example, you might need to measure the L_eq over one hour in the entire audio range of 20 Hz–20 kHz. To measure the L_eq in this case, select a sampling frequency that enables you to perform measurements up to 20 kHz. According to the Shannon Sampling Theorem, the minimum sampling frequency is 40 kS/s, twice the 20 kHz maximum frequency of interest. Depending on the hardware you use, you might have additional considerations, such as anti-aliasing, when selecting the sampling frequency. Traditionally when working with data acquisition devices with aliasing protection, a sampling frequency of 51.2 kS/s is used to perform measurements up to 20 kHz.

For one hour of data with a sampling frequency of 51.2 kS/s, you need to acquire more than 184 million samples, as calculated by the following equation:

The memory required to accumulate such a large number of samples might prohibit you from accumulating the number of samples required for the analysis. An alternate method is to process small chunks of data, keep track of the intermediate results, and integrate the measurement over time. You can use the NI Sound and Vibration Measurement Suite to perform this alternate method of L_eq measurement. The following block diagram shows a VI designed to compute the L_eq for a one-hour period.

The VI in the previous block diagram performs an L_eq over one second and repeats the operation 3,600 times using a For Loop. The last result that the SVL Leq Sound Level VI returns is the L_eq over the one-hour period.

In order for the SVL Leq Sound Level VI to accumulate the intermediate results, set the restart averaging control to FALSE or leave the control unwired. You can make the intermediate results available by using the auto-indexing capability of the For Loop.

Instead of performing an L_eq over one second and repeating the operation 3,600 times, you can perform the measurement over two seconds and repeat it 1,800 times, or perform the measurement over four seconds and repeat it 900 times, and so forth.

Restart Averaging and Advanced Concepts

Some applications require you to perform L_eq measurements continuously using a specific integration time. For example, you might need to measure a reverberation time that requires the application to return an L_eq measurement every 50 ms for 10 seconds. You can use a For Loop and its auto-indexing capabilities to perform a continuous L_eq measurement, as illustrated in the following block diagram.

Because you divide the 10 s measurement period into 50 ms blocks, you need 200 iterations (10 s/50 ms) of the For Loop. To continuously measure the L_eq, set the restart averaging control to TRUE. When the restart averaging control is set to TRUE, the SVL Leq Sound Level VI does not accumulate intermediate results but restarts the averaging process with each iteration of the For Loop. The following front panel shows the results of performing a reverberation time measurement for 10 s.

Performing a Running L_eq

Use the SVL Running Leq Sound Level VI to perform a running L_eq over the integration time. The result returned by the SVL Running Leq Sound Level VI is the L_eq computed over the last N seconds. A new running L_eq value is returned each time you call the SVL Running Leq Sound Level VI.

Exponential Averaging

Exponential averaging is a continuous averaging process that weighs current and past data differently. The amount of weight given to past data as compared to current data depends on the exponential time constant. In exponential averaging, the averaging process continues indefinitely.

The exponential averaging mode supports the following time constants:

• Slow—uses a time constant of 1,000 ms. Slow averaging is useful for tracking the sound pressure levels of signals with sound pressure levels that vary slowly.
• Fast—uses a time constant of 125 ms. Fast averaging is useful for tracking the sound pressure of signals with sound pressure levels that vary quickly.
• Impulse—uses a time constant of 35 ms if the signal is rising and 1,500 ms if the signal is falling. Impulse averaging is useful for tracking sudden increases in the sound pressure level and recording the increases so that you have a record of the changes.
• Custom—enables you to specify a time constant suitable for your particular application.

Use the following sound level measurement VIs for exponential time averaging:

• SVL Exp Avg Sound Level VI
• SVL Decimated Exp Avg Sound Level VI

The following block diagram illustrates a VI that performs an exponential averaged measurement using the SVL Exp Avg Sound Level VI with a Slow time constant.

The VI in the previous block diagram uses the restart averaging control to reset the averaging.

Peak Hold

In peak-hold averaging, the largest measured sound pressure level value of all previous values is computed and returned until a new value exceeds the current maximum. The new value becomes the new maximum value and is the value returned until a new value exceeds it. Peak hold actually is not a true form of averaging because successive measurements are not mathematically averaged. However, as with other averaging processes, peak-hold averaging combines the results of several measurements into one final measurement. As with exponential averaging, the averaging process continues indefinitely. The formula for peak averaging is defined by the following equation.

y[k] = max(y[k‑1], x[k])

where

x[k] is the new measurement.

y[k] is the new average.

y[k – 1] is the previous average.