Common Validation Problems
Specific Issues:
| Provide data to 0.01dB. |
Applicant's results cannot be duplicated to tenths unless raw acoustic data as well as any adjustments or corrections are provided to hundredths. |
| Apply Microphone Free-Field Corrections properly. |
Due to nomenclature used by manufacturers, free-field incidence data are often provided as "corrections" to pressure response. In effect, these corrections are actually increments to response, and should be subtracted from acoustical data. |
| Do not apply frequency-based corrections to data before test for Masking. |
Allowable corrections are Calibrator pressure & drift, and system gain. |
| Do not apply corrections to Masked data. |
Use of Masked data is allowed when the masked SPL is lower than the extrapolated SPL, but corrections should not be applied to bad data. |
General Problem Areas:
| Improper Time Synchronization. |
Typical errors include absence of handling for Analyzer filter settling time and Recorder head delay between data track and cue track. |
| Improper Identification of Masking Criteria |
Part 36 criteria: Pre-Detection Level +3dB, Post-Detection Level +1dB for Transport Category Aircraft; Pre-Detection Level +5dB, Post-Detection Level +1dB for Helicopters. |
| Use of ICAO Procedures which conflict with Part 36 Procedures |
These include association of time to midpoint of two-second averaging period. (ICAO associates time 0.25 seconds later) |
| Calculation of Emission time |
Common errors include: Incorrect calculation for Speed of Sound; Errors in equation for calculating propagation time; and Lack of precision in iterative process (If used). |
| Errors in Tone-Correction |
Frequently the handling of the lowest or highest frequency bands is in error. |
| Identification of 10dB-down points. |
If Bandsharing is present, it must be included in the PNLTm for determination of 10dB-down value. Also, Part 36 specifies that the half-second record whose PNLT is closest to the value of PNLTm-10dB must be used. Note that the elimination of records whose PNLT is below 90dB is no longer allowed. |
Volpe Center
Acoustics Facility
26 July, 1996
