The instrument standard IEC 61043 specifies the requirements for phase matching in terms of the pressure-intensity residual index δ_pI0 [dB], which is the F_pI when the probe microphones are exposed to an identical pink noise sound field. The required δ_pI0 is defined based on a phase-match requirement for the whole analyser.

To obtain the δ_pI0, the Nor150 is delivered with the Nor1294 Phase Verification Coupler. This coupler ensures an identical sound field for both microphones, with exceptional low-frequency response.

Traditionally, the analyser and the probe is calibrated separately and as a total in a laboratory to ensure the requirement is fulfilled. This is still a requirement, but using the Nor150 and the Nor1294 together, both verification and a new correction algorithm is available to the user. A much better Dynamic Capability may then be obtained.

A phase correction algorithm has two main benefits:
1. A higher L_d allows highly reactive fields to be measured. This means that higher levels of extraneous noise may be separated from the noise source of interest.
2. A single spacer may be used for the whole frequency range of interest. The 12 mm spacer with phase correction applied can measure from 31.5 Hz and up to 6.3 kHz.

To obtain the δ_pI0, IEC 61043 specifies pink noise excitation for 4 minutes to ensure that the full spectrum bandwidth has been excited. The stochastic nature of the pink-noise signal does not allow for shorter averaging times.

In the Nor150, an internal signal generator instead provides a logarithmic sine sweep. The spectral energy for such a signal is equal to a pink-noise signal, and because of the deterministic nature, all frequencies are excited. Using only 10 seconds of sweep, a similar performance to the 4 minutes of pink-noise is obtained.

In addition, the low-frequency response allows for well-defined correction at low frequencies.

The phase correction algorithm uses two different techniques to obtain a smooth phase match.

First, a 20 Hz sine is produced in the Nor1294 by the Nor150. Simultaneously, the signal is recorded. The 20 Hz frequency is close to the cut-off frequency of around 5 Hz for the microphones and High-pass filter in the Nor150. The phase difference is measured, and a set of two all-pass filters are set up to correct the phase mismatch. The cut-off of the all-pass filters are centered around 5 Hz to approximate the nature of microphones and high-pass filter in the Nor150.

Now, a swept-sine is generated to record the δ_pI0 for all 1/3 octave bands. When the measurement is complete, coefficients are generated to individually correct each of the 1/3 octave band filters. The algorithm is based on subtracting the residual intensity for each band, effectivly correcting each bands frequency matching.

This principle will only give a good result if the phase mismatch does not vary greatly within the band. This is the reason for applying stage 1 first.

When the two correction stages are completed, a new swept-sine exitation is applied to generate a verification spectrum. This spectrum is stored, and may be found in the calibration menu for a probe under Residual History.

The verification spectrum is also used to generate a L_d for the instrument. The last obtained L_d is automatically applied to an intensity measurement to enable comparison with F_pI.

Intensity theory

Measurement surface theory

Probe theory

Probe Calibration

The Nor1290 probe

The Nor1294 coupler

Sound Intensity help index