This is a quick guide to taking measurements with REW.
You will need:
- Calibrated measurement mic
- Mic phantom power/preamp
- Mic stand
- Cables and adaptors
Initially you will be prompted to perform SPL level calibration. This may be considered optional. You need an SPL meter and this calibration must be done each time you adjust anything that will affect the signal level. This includes your sound card volume control, the gain on your mic preamp, the volume control on your sound system, sub volume. If you change the level at any point, then the SPL calibration must be repeated. Chances are you will change the volume more than once in a measurement session. I normally skip this step and simply ensure that the level is reasonable. A high level may cause room rattles that interfere with the results. A low level may not be much louder than your noise floor and cause inaccurate results.
1. Go into Preferences under the Preferences Tab.
2. Sound card tab - select input and output devices (start with default).
3. In the middle of the window, you will see the following:
Click on calibrate and follow the instructions. This will calibrate your sound card.
4. Mic/mater tab - enter mic calibration file.
If using a calibrated mic from Cross Spectrum Labs, choose the calibartion at 90 degrees as you will be measuring with the mic pointing down.
What type of measurement?
There are a few different basic measurement techniques you might use for bass integration:
- Farfield (listening position)
- Ground plane outdoors
Ground plane measurements are a more suitable technique but tend to require a quiet outdoor space such as a vacant parking area. It's an inconvenient technique. The speaker in question is placed so that it sits on the ground to avoid "floor bounce." The mic is placed on the ground. A floorstander measured this way would ideally be placed on its side to avoid floor bounce.
The measurement of primary interest is the farfield taken from the listening position. The mic is placed where your ears would go. If you are taking a lot of measurements, you may speed up the process by putting the speaker close to that position and moving the mic to the various speaker positions. The result is similar and it's more convenient than moving the speakers.
Your first measurement
1. Click the measure icon and this dialogue box will appear:
Click on check levels to make sure the level is right. Choose a sensible volume firstly for the signal - one that you might call "moderately loud." Then adjust the volume of the mic preamp so that you have adequate headroom.
2. Choose the frequency range for the measurement. I choose a range from 15 Hz (start freq) to 400 Hz (end freq).
Click start measuring.
A measurement signal will run and you will see a response plot.
This plot is a subwoofer and it is a far field measurement. You can see a dominant peak based on a room mode at 44 Hz.
Now adjust the limits:
It is important to set these so that you can read the chart easily and not be misled. You should limit the frequency range so you can see the data clearly. You might set the upper limit as low as 200 Hz and as high as 400 Hz. The SPL range will also affect how the chart is perceived. Set the range too large and everything will look too flat. Set it to a narrow range and you will make everything look worse.
After you have taken a number of plots with different positions, you might like to view "all SPL." Select this tab above the measurement window and you can select which plots to overlay.
Here you can start to appreciate consistent trends. In particular you can see that the peak at 44 Hz is present in many of the other measurements.
Waterfall plots are one of the most useful for analysing bass performance. They show the frequency response over time.
You can see how over time, the 44 Hz peak decays at a slower rate.
To generate a waterfall, first select the appropriate measurement in the left column. Then choose the waterfall tab. The chart area will be empty, with a "generate" button in the bottom left hand corner. Click on generate and you will see a waterfall plot.
You can generate a decay plot in the same way. It shows the same data in 2D form.
The waterfall tends to show the overall trend in a more intuitive way, but a decay plot such as this is easier to read if you want to see the decay at certain time intervals. Both plots are useful for assessing bass trap effectiveness as well as testing your EQ settings. You can see if your EQ filters are causing problems.