In this application note, we will show you some basic acoustic analysis techniques that you can apply to measurements of your listening room made with the UMIK-1 and Room EQ Wizard (REW).
- Requirements and setup
- Frequency response measurements
- Reverberation time
- Time-frequency plots
- Further Reading
Requirements and setup [Top]
- A miniDSP UMIK-1.
- Room EQ Wizard (REW). Be sure to download the latest version from the Downloads Area for UMIK-1 support.
- Optionally, a microphone stand with boom arm (the UMIK-1 is supplied with a small stand that can be rested on a table or the back of a sofa.)
Please use one of the following app notes to get configured with the UMIK-1 and REW:
Frequency response measurements [Top]
If you have not already done so, start REW and click on the Measurement button. Check the sweep settings and click the Start Measuring button. REW will generate the sweep tone and capture the acoustic signal from the UMIK-1. The main REW window will now look something like this:
(If the Phase checkbutton at the lower right is checked, uncheck it. The Phase measurement is a more advanced concept that we won't cover in this app note.)
The main measurement plot shows a frequency measurement graph in red. At the left is a small "thumbnail" version of the measurement - each time you make a new measurement, a new thumbnail will appear here. At the top of the screen are buttons that access various tools and immediately above the measurement is a row of tabs that show different "views" of the measurement.
Let's do a few basic operations on this first measurement:
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Give the measurement a meaningful label. Click in the text window next to the thumbnail and type in a label such as "Left spkr in-room". (By default, the label is the date and time the measurement was taken.)
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Change the scale of the measurement. The default scale is typically too large in the vertical direction, so click on the Limits button (towards the top right) and set the dB scale like this:
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Change the color of the plot. Click on the "brush" icon next to the thumbnail and select a color.
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Add some smoothing to the graph. From the Graph menu, select "Apply 1/6 Octave Smoothing."
The result now looks like this:
Much better! You can click on the little camera icon to create an image that is better for viewing on the web and for sharing on forums. Here is the image generated using this feature:
Now we can examine in more detail what we have here. The scale along the bottom shows the frequency range, in this case 20 Hz to 20 kHz (20,000 Hz). At each frequency, the UMIK-1 has picked up a sound pressure level (SPL), which is the vertical height of the graph at that point. The scale at the left shows the SPL, which we have set to go from 50 to 90 dB. For example, at 100 Hz the SPL is just under 75 dB.
(The scale at the right of the graph is the phase values. Since we are not looking at phase in this app note, this can be ignored.)
Why is the graph so uneven? Shouldn't we have a flat frequency response? Mostly, it's because of the room. Room modes (resonances) and reflections in a room act to make a nice flat loudspeaker response into a roller-coaster.
Recall that we applied smoothing to the graph. An in-room measurement has a lot of reflections that create peaks and dips in the frequency response. Above 250 Hz or so, much of this is not relevant to what we hear, so for full-range measurements, 1/3 or 1/6th octave smoothing is generally used. For graphs limited to the low frequency range (up to say 300 Hz), use 1/24, 1/48 or no smoothing.
miniDSP offers products that can be used to equalize a system or apply full-range room correction, such as the nanoDIGI 2x8 (digital in-out, parametric EQ), miniDSP 10x10 HD (multichannel analog in-out, parametric EQ), and OpenDRC-DI and OpenDRC-AN (digital or analog in-out, FIR filtering and room correction). Note that equalization for room correction should generally make use of multiple measurements over the listening area, as measured response will change with microphone position.
Reverberation time [Top]
Sound generated in a room decays over time. You've probably been in a "live" hall where footsteps echo and a handclap can be heard to decay over seconds. In other rooms, sound decays very quickly. A convenient measure for this is the reverberation time, or the time that a transient sound takes to decay by 60 dB from its initial level. For home listening rooms, this measurement can be considered relevant above about 200 Hz.
In REW, click on the RT60 tab above the main plot. Deselect all plots except T30 and Topt. (For more information on the various plots, see the REW Help.) Set the frequency range from 100 Hz to 10,000 Hz, and the time range from 0.0 to 1.0 seconds. The resulting plot for the example measurement is:
Recommended values of reverberation time depend on the type of system (e.g. music or home theater), the size of the room, and the type of speaker. However, 300 ms to 500 ms (0.3 to 0.5 seconds) is a commonly recommended range. It can be seen that this room is within this range but towards the high end for a good part of the frequency band.
DSP equalization can't change reverberation time. Heavy drapes and carpets will reduce reverberation time, but if you need to go further, then dedicated acoustic treatment may be required.
Time-frequency plots [Top]
REW offers a set of plots that show how different frequencies decay in time. These are generally used in the "modal region" of the room's acoustic response, from say 20 to 300 Hz, where resonances in the room dominate the measured frequency response. A well-known plot of this type is the "waterfall" plot, or cumulative spectral decay:
This plot shows that there are slowly-decaying resonances around 26, 30, 57 and 74 Hz. With this type of graph, you may need to adjust some display parameters on the Controls and Limits popup windows to get the most useful and informative plot.
Resonances like this can be addressed with a range of techniques: by equalization (if they show up as peaks in the frequency response measurement), by moving and placement of speakers or subwoofers, by the use of multiple subwoofers, and by extensive bass trapping.
A plot that shows the same type of information in a different way is the spectrogram. You may find this plot easier to interpret in some cases — in particular, it can be useful to highlight resonances after equalization has been applied to address measured frequency response peaks.
With all of these plots, you can click on the camera button and save the plot as an image. This will make it easier to share your measurements with others and for "before and after" comparisons.
Further reading [Top]
See the following app notes for information on addressing room problems with miniDSP products:
The following links contain additional information on room acoustics and measurement that you may find helpful:
- Acoustic Measurement Standards for Stereo Listening Rooms
- Hard proof that equalization kills room modes
- Bass Integration Guide
