You are here: NI Home > Support > Product Reference > Manuals > Sound and Vibration Measurement Suite 6.0 Help

You typically perform system calibration with a dedicated calibrator, such as a pistonphone for microphones or a hand-held shaker for accelerometers. Use the SVL Calibrate Microphone (DAQmx) VI if you are calibrating a microphone. Use the SVL Calibrate Accelerometer (DAQmx) VI if you are calibrating an accelerometer. These VIs are similar to the general-purpose SVL Calibrate Sensor (DAQmx) VI, but these VIs provide the advantage of having default values commonly found for pistonphones or hand-held shakers. All the Calibration (DAQmx) VIs use the characteristics of a calibrator, such as reference calibration value and frequency, to perform the calibration.

Propagation Delay Calibration

The NI Sound and Vibration Measurement Suite provides VIs for calibrating the propagation delay of a measurement system. National Instruments dynamic signal acquisition (DSA) devices like the NI PXI-4461 can acquire and generate signals on the same device. The input and output channels have analog and digital circuitry, such as anti-aliasing and anti-imaging filters, that introduce a certain delay to the signal. The propagation delay is the number of samples from the time a sample is first written to the output channel to the time that sample is digitized on the input channel, assuming there is no delay from the output channel to the input channel. This delay varies by DSA device.

You can determine the propagation delay of a DSA device in two ways. You can refer to the documentation for the DSA device to find the propagation delay specifications, also referred to as group delay. You also can measure the propagation delay in samples with the SVL Measure Propagation Delay (DAQmx) VI. The SVL Measure Propagation Delay (DAQmx) VI enables you to measure the delay introduced in the input and output circuitry for a specific device at the desired sampling rate. Connect the DSA device output channel directly to the input channel, as displayed in the following illustration, to measure the device propagation delay.

Note  Do not put a device under test (DUT) in the signal path when measuring the propagation delay for the DAQ device.

For Multifunction I/O devices from NI, you expect to measure a one-sample propagation delay because of the time required for the signal to traverse the signal path between the D/A converter (DAC) on the analog output channel and the A/D converter (ADC) on the analog input channel. The following illustration shows the time domain data for the propagation delay measurement of an NI PCI-6052E.

For DSA devices, or for any other device that has onboard filtering on either the input, output, or both channels, you expect to measure a propagation delay consistent with the sum of the delays specified for the onboard filters on the input and output channels. The following illustration shows the delay of a smooth pulse generated and acquired by an NI PXI-4461 with a 204.8 kHz sampling rate.

Not all DSA devices have a constant propagation delay across the entire range of supported sampling rates. For example, the NI PXI-4461 propagation delay is dependent on the output update rate. The following front panel shows the total propagation delay versus sampling rate relationship for the NI PXI-4461 from output to input as a function of the sampling rate.

In the previous front panels, the propagation delay can vary significantly with different sampling rates and devices. To ensure measurement accuracy in the I/O applications, determine and account for the propagation delay of the DAQ device at the same sampling rate used in the application.

You must remove the effects of the delay that the data acquisition system causes for two reasons. First, a delay between the generated output signal and the acquired input on the device always exists, even when the output and input channels are hardware synchronized. Second, the anti-imaging and anti-aliasing filters of the device introduce additional delays. You must account for this delay to perform accurate dynamic measurements. Use the device propagation delay [samples] input in the examples located in the labview\examples\Sound and Vibration\Audio Measurements directory to remove the delay that the DAQ device causes.

The anti-imaging and anti-aliasing filters have a lowpass filter effect on the data. This effect results in a transient response at sharp transitions in the data. These transitions are common at the start and stop of a generation, at a change in frequency (swept sine), and when the amplitude changes (amplitude sweep). The swept-sine analysis and audio measurements examples in the Sound and Vibration Measurement Suite account for this transient behavior in the device response to achieve the highest degree of accuracy.

The propagation delay of the DUT is also an important specification in some applications. For example, the propagation delay for the DUT is a required input when performing audio measurements and when measuring the frequency response using swept sine. If the DUT and the propagation medium can successfully pass the pulse signal that the SVL Measure Propagation Delay VIs use without excessive attenuation, then this measurement also applies when measuring the propagation delay of the DUT and the propagation medium. The following illustration shows the wiring diagram for this configuration.

The DUT propagation delay is the delay of the entire system minus the device delay. You must measure the device delay without the DUT connected.

The propagation delay for an analog DUT is a constant time delay rather than a delay of samples. Use the following equation to convert the measured delay in samples to the equivalent delay in seconds:

delay [s] = delay [samples] / sampling rate [Hz]

Note  The Swept Sine VIs expect the DUT propagation delay measurement in seconds and use the equation to convert the delay in seconds to samples.