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Transfer Function Measurement (TRF)

The TRF module measures two signals simultaneously and determines the magnitude and phase of the linear transfer function as well as the harmonic distortion at the same time. The stimulus is a logarithmic sweep with adjustable spectrum, bandwidth and crest factor. Window techniques (gating) applied to the impulse response allow a separation of the direct sound from early reflections, diffuse field and nonlinear artifacts. Finally, post-processing provides the time delay (zero delay plane), minimal phase, group delay and several frequency-time transformations (e.g. cumulative spectral decay and sonograph). A new measurement technique is provided for detecting low energy, impulsive disturbances, such as clicks and Rub & Buzz effects. It reveals distortion of a much lower level and with a considerably higher temporal resolution (“instantaneous distortion”) than the traditional Fourier analysis , which shows mean values only. In addition to the common 2D representation (distortion vs. frequency), the distortion can be visualized in a 3D plot. Here, the distortion is mapped vs. frequency and vs. one of the measured signals (voice coil displacement, sound pressure, etc.). This is quite useful to localize and identify the cause of the disturbance.

Transfer Function Measurement
  • Combines linear and nonlinear measurements
  • Measures linear transfer function and harmonic distortions simultaneously
  • Fast two channel data acquisition (< 43,6 kHz)  with noise floor monitoring
  • Highly adjustable stimulus (bandwidth, spectrum, crest factor)
  • Provides impulse response and energy-time curve (ETC)
  • Quasi-anechoic measurement due to windowing of impulse response
  • Calculates time delay, minimal phase and group delay
  • Provides cumulative spectral decay (CSD) and sonograph o
  • Overlay of up to 20 result curves
  • Detection of low energy, impulsive disturbances (clicks, Rub & Buzz, etc.)
  • 3D representation reveals location and physical cause of Rub and Buzz
  • Applicable to speakers and electronics
  • Separation from ambient noise


TRF Standard

is the universal (every-day) tool for electrical, mechanical and acoustical measurements.


provides the distortion analysis to localize loudspeaker defects, such as hard limiting, wire beat , voice coil rubbing, buzzing and loose particles.

Transfer Function Measurement

The figure above illustrates the measurement of the equivalent input distortion using the TRF module. The harmonic distortion measured at different points ri with i=1, ..4 in the sound pressure field (right diagram) are transformed to the input of the loudspeaker system using the inverse transfer function H(f,ri)-1. This transformation shows that the harmonic distortion has a common source close to the input of the loudspeaker system, and the linear system response performs only a post-shaping of the distortion component. The measurement of the equivalent input distortion is a powerful concept to identify the physical cause of the distortion, to reduce redundancy of the measured data and to simplify the interpretation of the results. 

Templates coming with the TRF module:

TRF Crest Harmonics (x,f)

Crest factor harmonic distortion versus displacement to find Rub & Buzz and other loudspeaker defects

TRF cumulative decay

Cumulative spectral decay

TRF Elect. Impedance (Sp 1)

Electrical impedance using the standard current sensor 1

TRF Elect. Impedance (Sp 2)

Electrical impedance using the sensitive current sensor 2

TRF Equiv. Input Harm. (SPL)

Equivalent harmonic input distortion calculated by inverse filtering of the measured distortion

TRF H(f)= X/voltage

Transfer function H(f)= displacement(f) / voltage(f)

TRF Harmonics current (Sp1)

Harmonics of the current signal using standard sensor 1

TRF mic calibration for IN1

Calibration of the microphone at IN1 using a pistonphone

TRF Peak harmonics, time domain

Peak value of higher-order harmonics in time domain for Rub & Buzz analysis

TRF rubb+buzz w/o Golden Unit

Rub & Buzz detection without "Golden Unit" according Application Note AN 22

TRF rubb+buzz with Golden Unit

Rub & Buzz detection with "Golden Unit" according Application Note AN 23

TRF Scanning Cone Vibration

Manual scanning of cone vibration using a laser sensor with high cut-off frequency (>15 kHz)

TRF sensitivity (Mic 2)

Calibration of the microphone at IN2 using a pistonphone

TRF SPL + harmonics

Standard measurement for fundamental component (SPL) and harmonic distortion

TRF SPL + waterfall

Sound pressure level and cumulative decay spectrum

TRF true acoustical phase

Total phase without time delay


Germany: 102009033614 P10214407
USA: 12/819,455 7,221,167
China: 201010228820.8 03108708.6

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