Guenter Villnow
Guenter Villnow

This app note is part of a series on car audio sound. The top level app note and list of all parts is:


Independent of your chosen microphone technique is the choice of measurement approach. The measurement approach describes the way you get your microphone(s) data into your computer for analysis.

Closed loop [Top]

In a closed loop measurement, signal generation and analysis are done in a single step. If you have ever done a basic acoustic measurement with REW, FuzzMeasure or an Audio Precision analyzer, then you have done a closed loop measurement. The generated signal goes through the device under test and then straight into the analyzer:

If you are new to this topic, you will probably find this tutorial helpful: https://www.minidsp.com/applications/acoustic-measurements/acoustic-measurements.

Why does that matter? Because in automotive sound, closed loop measurements are not possible in some situations.

Open loop [Top]

In an open loop measurement, the signal generation, recording and analysis are done in separate steps. Time and space can separate the steps:

Open loop measurements are necessary when you cannot connect your computer output to your automotive head unit/sound system. Some of the currently available OEM infotainment systems only support CarPlay, USB and Bluetooth input (as of early 2022). Getting the desired test signal into these systems from a generator or computer can be tricky. In this case the measurement file can be put onto a USB drive and then played back on the infotainment system for recording. This recording is analyzed in your measuring application.

Note that not all audio measuring applications support this approach, or the approach may vary slightly (e.g., a trigger sound is used). The open loop approach can also come in handy when you want to check the playback quality of a certain source (e.g., for troubleshooting why the analog input sounds different to the digital source).

Open and closed loop measurements in REW [Top]

REW supports both closed loop and open loop measurements.

To make an open loop sweep measurement

  1. Create a measurement stimulus:

  2. Play back the measurement stimulus in the car.

  3. Record the measurement stimulus with a measurement mic in a DAW (e.g., reaper from https://www.reaper.fm) or audio editing application (e.g., ocenaudio from https://www.ocenaudio.com) or with a recording device.

  4. Import the recorded measurement into REW (via File – Import – Import sweep recordings).

To make an open loop noise measurement

    1. Create noise in REW or use Meyer Sound's m-noise (https://m-noise.org).

    2. Record the noise with an application on your computer or with a recording device.

    3. Drag and drop the recorded files into the RTA window in REW:

Smoothing and dB scale [Top]

Since REW's x and y axis scaling resolution can range from "microscopic" to "30,000 feet above ground", taking a closer look at the scale is a good idea before you start tuning.

In acoustic automotive measurements a 5 dB per division zoom is a good starting point for the y axis. For the frequency (x) axis I recommend using the "20 ... 20000 Hz" frequency zoom preset on the bottom right. Note that zoom presets can be changed in the REW settings (Preferences – View – Freq axis preset).

Measurement results from cars can give quite startling frequency responses. To make finding center frequencies for the EQ bands and decision making in general easier, I recommend having a standard resolution of 1/24th octave for the analysis (setup in Preferences – Analysis – Frequency Response Calculation).

In order to decide on the tuning frequencies, I read one measurement in different smoothing settings. If a peak sticks out in all smoothing settings, it needs to be addressed.

If you are new to tuning, I recommend starting with 1/6th octave smoothing as your standard resolution and slowly work your way up to 1/12th octave and then 1/24th octave as you get better at interpreting the results.

As a reference while doing analysis, I recommend that you always analyse your stimulus signal first (via drag and drop import into REW) and keep that result for comparison with your measured results. This is especially important when you are using an RTA based approach.

Closing [Top]

That concludes this part! Jump back to the top: