Near Field Measurement
Characteristics: | KLIPPEL R&D System | KLIPPEL QC System |
|---|---|---|
| Extrapolated far-field SPL response | TRF, NFS, SCN-NF | |
| Low frequency measurements without anechoic condition | TRF, DIS, NFS, SCN-NF | SPL |
| Splicing near-field/ far-field measurement | TRF, NFS, SCN-NF | |
| Merging multiple sound sources | TRF, ISC, NFS, SCN-NF | |
| Distortion measurements at high SNR | TRF, DIS, NFS, SCN-NF | SPL |
The sound pressure is measured at one or multiple points in the near field of the sound source. At this position, the direct sound is much higher than the diffuse sound which is reflected from boundaries and ambient noise corrupting the measurement. This gives a high signal-to-noise ratio which is important for measurements of the fundamental response at low amplitudes and for nonlinear distortion measurements.
The fundamental component of the sound pressure magnitude response measured in the near field can be used for extrapolating the far field response. At very low frequencies, where the acoustical wavelength is much larger than the radiator, far field response is identical with the attenuated near field response considering the distance of the measurement points. To extrapolate the far field response at higher frequencies and to measure the full directional characteristics (sound power, polar plots), multiple measurements are required at a surface with sufficient spatial resolution, and more complex computation has to be applied (near-field holography).
The measurement of the nonlinear distortion generated by motor and suspension nonlinearities is not critical because the distortion is generated in the one-dimensional signal domain close to the terminals and can be transformed into equivalent input distortion which is independent on the acoustical sound propagation and microphone properties.
Near Field Scanner - Measurement Method
The Near-Field Scanner 3D (NFS) uses a moving microphone to scan the sound pressure in the near field of a compact sound source such as a loudspeaker system or a transducer mounted in a baffle. The device under test (< 500 kg) does not move during the scanning process. The reflections in the non-anechoic environment are then consistent and can be monitored with our novel analysis software, which uses acoustical holography and field separation techniques to extract the direct sound and to reduce room reflections.
Multi-pole Expansion
The sound field generated by the source is reconstructed by a weighted sum of spherical harmonics and Hankel functions which are solutions of the wave equation.
The weighting coefficients in this expansion represent the unique information found in the near-field scan while gaining a significant data reduction.
Result Data
Near Field Analysis
The wave expansion provides the sound pressure at any point outside the scanning surface which is required for assessing studio monitors, mobile phones and tablets and other personal audio devices where the near field scanner properties are important.
Far-Field Extrapolation
The near-field data, measured at high SNR, is the basis for predicting the direct sound at larger distances.
This avoids diffraction problems of classical far-field measurements (non-homogeneous media)
Near Field and Far Field Merging
KLIPPEL R&D SYSTEM (development)
Module | Comment |
|---|---|
TRF is optimal for performing near field measurements by using a sinusoidal sweep technique and calculating the transfer function between the real voltage at the loudspeaker terminals and the microphone signal and for performing distortion measurements. | |
The hardware and control software of the SCN can be used for scanning the near field of the transducer by using a microphone or velocity sensor. | |
| Near Field Scanner (NFS) | The NFS captures the entire sound field at any point in 3D space. The automatic measurement in the near field can be performed in a normal room (non-anechoic) and the software visualizes far field characteristics (Sound Pressure, Sound Power, Directivity Index, Directivity Balloon) as well as the near field characteristics (sound pressure distribution). |
| In-Situ Room Compensation (ISC) | The ISC module may be used to splice the near field and far field response measurements of a transducer. An dedicated object template containing all the necessary operations for measurement and post-processing is available in dB-Lab. |
KLIPPEL QC SYSTEM (end-of-line testing)
Module | Comment |
|---|---|
Measuring the sound pressure at one point in the near field of the transducer is the preferred way of performing end-of-line testing. | |
| The air leakage tracer is a small handy sensor using a microphone array revealing the direction or position of the noise source. This tool can also apply for near field measurements by using a particular triangulation technique. |
Templates of KLIPPEL products
Name of the Template | Application |
|---|---|
TRF SPL + waterfall | Sound pressure level and cumulative decay spectrum |
TRF sensitivity (Mic 2) | Calibration of the microphone at IN2 using a pistonphone |
TRF true acoustical phase | Total phase without time delay |
TRF 3rd oct. spectr. analyzer | Continuous loop measurement giving the spectrum of the signal acquired via IN1 integrated over 1/3 octave |
IEC 20.6 Mean SPL | Mean sound pressure level in a stated frequency band according IEC 60268-5 chapter 20.6 |
IEC 21.2 Frequency Range | Effective frequency range according IEC 60268-5 chapter 21.2 |
IEC 22.4 Mean Efficiency | Mean efficiency in a frequency band according IEC 60268-5 chapter 22.4 |
Response Smoothness | Frequency response smoothness |
SPL Merging Near / Farfield | Merges near-field response and far-field response according to Application Note AN 39 |
Application Notes
AN 24 Measuring Telecommunication Drivers, Microspeaker, Headphones
AN 26 Cone Vibration and Radiation Diagnostics
AN 38 Near-field Measurement with multiple Drivers and Port
AN 39 Merging Near and Far-field Measurements
AN 41 Measurement at defined terminal voltage
AN 69 Far Field Measurement using Microphone Arrays
Standards
Audio Engineering Society
AES2 Recommended practice Specification of Loudspeaker Components Used in Professional Audio and Sound Reinforcement
AES56 Standard on acoustics – Sound source modeling – Loudspeaker polar radiation measurement
International Electrotechnical Commission
IEC 60268-5 Sound System Equipment, Part 5: Loudspeakers
Other Related Tests
Typical Test Objects
Papers and Preprints
D. Keele, “Low Frequency Measurement of Loudspeakers by the Near-Field Sound Pressure Sampling Technique,” presented at the 45th Convention of the Audio Eng. Soc., May 1973, Preprint 909.
M. Malon, et al., “Comparison of Four Subwoofer Measurement Techniques,” J. of Audio Eng. Soc., Volume 55, Issue 12, pp. 1077-1091, December 2007.
W. Klippel, et al., “Distributed Mechanical Parameters of Loudspeakers Part 2: Diagnostics,” J. of Audio Eng. Soc. 57, No. 9, pp. 696-708 (2009 Sept.).
W. Klippel, et al., “Distributed Mechanical Parameters of Loudspeakers Part 1: Measurement,” J. of Audio Eng. Soc. 57, No. 9, pp. 500-511 (2009 Sept.).
C. Struck, et al., “Simulated Free Field Measurements,” J. of Audio Eng. Soc., Volume 42, Issue 6, pp. 467-482, June 1994.
Earl G. Williams: "Fourier Acoustics – Sound Radiation and Nearfield Acoustical Holography", 1999 Academic Press, ISNG 0-12-753960-3
G. Weinreich, E. B. Arnold: "Method for measuring acoustic radiation fields", J. Acoust. Soc. Am., 68 (2), 404–411, 1980
M. Melon, C. Langrenne, A. Garcia: "Measurement of subwoofers with the field separation method: comparison of p- p and p-v formulations", Proc. Acoustics 2012 Nantes, 3491-3496, 2012
C.-X. Bi, D.-Y. Hu, L. Xu and Y.-B. Zhang: "Recovery of the free field using the spherical wave superposition method", Acoustics 2012 Nantes, 1781-1786, 2012
Z. Wang, S. F. Wu: "Helmholtz equation-least-squares method for reconstructing the acoustic pressure field", J. Acoust. Soc. Am., 102 (4), 2020-2032, 1997
H. Lu, S. Wu, D. B. Keele: "High-Accuracy Full-Sphere Electro Acoustic Polar Measurements at High Frequencies using the HELS Method", Audio Eng. Soc. October 2006, Convention Paper 6881
D. B. Keele: Low Frequency Loudspeaker Assessment by Nearfield Sound-Pressure Measurement, J. of the Audio Eng. Soc., April 1974, Vol. 22, No. 3
C. Bellmann, W. Klippel, D. Knobloch: Holographic loudspeaker measurement based on near field scanning, DAGA 2015 - 41th Convention, DEGA e.V.