Near field measurement
Characteristics: | ||
|---|---|---|
Extrapolated far-field SPL response | TRF, CAL | |
Low frequency measurements without anechoic condition | ||
Splicing near-field/ far-field measurement | TRF, CAL | |
Merging multiple sound sources | TRF, CAL | |
Distortion measurements at high SNR |
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.
- The figure above illustrates different kinds of methods developed for acoustical near-field measurements.
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 Scanning Vibrometer (SCN) can be used for scanning the near field of the transducer by using a microphone or velocity sensor. | |
Script for merging Near- and Far-field measurement (CAL) | An object template comprises two TRF measurements, and a dedicated script running in a CAL module generates the wide band amplitude response. |
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. |
- Application Note AN 38 describes extrapolation of the far field response from multiple near field measurements at vented-box loudspeaker system. The figure above shows the SPL response measured in the near field of the driver and at the port. Adding the complex transfer functions and considering the radiating surfaces of the driver and port gives the far field response in the figure below:
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
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 |
Standards:
- IEC Standard IEC 60268-5 Sound System Equipment, Part 5: Loudspeakers
- AES56-2008 AES standard on acoustics – Sound source modeling – Loudspeaker polar radiation measurement
- AES2-1984 AES Recommended practice Specification of Loudspeaker Components Used in Professional Audio and Sound Reinforcement
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.