Mode-Compensator for Precise Measurement of the
Sound Pressure Frequency Response in Presence of Room Modes

Purpose of the Equipment

This equipment is foreseen for precise and high-resolution Room-Mode-Free Measurement of the Frequency Response of Loudspeakers in a standard laboratory environment.

It is intended to be used in combination with a measurement equipment as e.g., a computer with audio interface and an appropriate evaluation software.

ModeCompensator control unit

Introduction

The usual method of measuring the frequency response of the Sound pressure level (SPL) of a loudspeaker chassis (device under test, DUT) uses time gating. The time window (time gate) in which the sound pressure is recorded is set such, that the reflections of the sound from the walls, which arrive after their distance depending traveling time are excluded from the measurement.

The time domain measurement is then transformed into the frequency domain by an appropriate evaluation software in order to get the frequency response of the SPL. Because of the transformation the time window determines the achievable frequency resolution of the measurement. Time window and Frequency resolution are reciprocal parameters.

In most laboratories the distance from the DUT to the next hard boundaries (walls. ceiling, floor) is below 2 meters. As the sound travels 343 Meters per Second, the first reflection from the wall arrives after about 11 Milliseconds. A corresponding time window set to 10 Milliseconds results in a frequency resolution of 100 Hz.

It becomes clear from the above realistic example that frequency measurements below 1 kHz are not possible with acceptable frequency resolution and accuracy.

The AudioChiemgau ModeCompensator enables the measurement of the SPL with very high frequency resolution and accuracy down to 10 Hz.

Its basic principle eliminates the reflections and modes (room resonances) resulting from hard boundaries from the time domain measurement. For this purpose, two microphones are used to distinguish the direct sound from the DUT from the unwanted room reflections and especially resonances (modes).

Such the time window of the evaluation software can be set according to the desired frequency resolution of the SPL frequency measurement. For low frequency measurements a time window of 10 Seconds is used in order to get 0.1 Hz frequency resolution down to 10 Hz (frequency limit of the MEMS microphones).

Theory of Operation

AudioChiemgau offers a simple technology solving the discussed measurement problem:

The loudspeaker is measured in the near field (few cm between membrane and measurement microphone) and simultaneously the room modes are measured with a second microphone (mode microphone) in some close distance.

Both microphones record the direct sound pressure from the chassis and the room modes according to the 1/distance (inverse proportional) law.

The SPLs of the direct sound are significantly different for both microphones, as the relative distances from the source (loudspeaker membrane) are significantly different.

The SPLs of the modes and the noise floor in the room are practically equal, as the sound wave traveled already some distance before arriving at the microphones.

The Mode-Compensator uses this physical relationship and derives in real time a mode free SPL signal from the loudspeaker under test.

More information on this subject can be found in the chapter background information

Package Content and Price

The ModeCompensator is available from AudioChiemgau.
The selling price of

€ 550.00 (€ 654,50 incl. 19% VAT)

per unit comprises the following equipment:

  • ModeCompensator control unit
  • Two calibrated MEMS measurement microphones with cables (1.5 m)
  • Wall-plug power supply (100 VAC … 240 VAC 50/60 Hz)
  • Operation Instructions (english)
  • Transportation and protection case (250 mm x 150 mm x 320 mm)
ModeCompensator transportation case

The price for ModeCompensator, purchased from AudioChiemgau (Germany) and delivered to a destination located in the European Union (EU), includes VAT and duties.

Necessary supporting Equipment

The following equipment is required for frequency response and harmonic distortion measurements:

  • Personal Computer
    Examples: Apple Mac Book, iMac or Windows based Computer
  • External or high quality internal audio interface
    Examples: USB audio interface Focusrite Scarlett 2i2 or Clarett 2pre USB
  • Evaluation software
    Examples: FuzzMeasure (Mac OS) or EQ Wizard (Windows, MacOS and Linux)

There are many more commercial evaluation software alternatives for Apple or Windows computers on the market.

Trademarks: Microsoft and Windows, as well as Apple and Focusrite are registered trademarks. All other trademarks are property of their respective holders.

Example for A SPL frequency Response

The measurement shows the SPL frequency responses of a chassis with 0.2 dB SPL accuracy and 0.1 Hz frequency resolution measured in a standard laboratory.
Sweep time 10 Seconds and time gate 10 Seconds corresponding to a frequency resolution of 0.1 Hz.

SPL frequency response

Red is the measured SPL of the “direct” microphone in the near field of the membrane.
This is what you usually measure in a standard laboratory.

Violet is the measured SPL of the “mode” microphone in close distance to the chassis.

Blue is the mode free SPL frequency response derived from the above two measurements using the ModeCompensator.

Remarks:

  • The visible low-end frequency response is resulting from the loudspeaker under test
  • The high frequency deviations from a straight line above 400 Hz are due to the beginning of the beaming of the chassis.
  • It is recommended to use the ModeCompensator for frequencies below 1 kHz, as the distance between the two microphones leads to an increasing phase shift between the two microphone signals at higher frequencies, which reduces the accuracy of the mode compensation. Above 1 kHz appropriate short time gating can be used in a standard laboratory environment.
  • The above Frequency plot is generated with a Clarett 2pre and FUZZMeasure.

Calibrated frequency response of the Microphones

The SPL frequency response of a Bruel&Kjaer reference microphone (red) is recorded in comparison to the two MEMS microphone channels (blue and green). Deviations are typically below 0.2 dB between 10 Hz and 1 kHz. The loudspeaker used for the calibration has a lower cut-off frequency of 15 Hz (-3 dB), which is visible in the below chart.

Calibrated frequency response of the MEMS-microphones (example)

Block diagram and I/O Specification

The following figure shows a block diagram and the input/output specification of the ModeCompensator

Block diagram of the ModeCompensator