The measurement process consists of playing a series of “sweeps” through the Squeezebox and your stereo system, while recording them directly onto a PC. Separate sweeps are recorded for the left and right channels. For best results, record several sweeps separately per channel (for example, 5 each for left and right) so you can choose the best one after processing. Each sweep lasts 45 seconds.
Warning
Playing test signals very loud will cause irreversible damage to your loudspeakers. Do NOT play the sweeps at maximum volume: they WILL likely destroy your tweeters. Use a moderate volume, and adjust the recording level appropriately.
Before You Begin
Queue up a playlist of several tracks of your favorite music.
Turn the volume down.
Before recording, make sure your Left and Right channels are the right way around. (If not, you’ll end up correcting the left speaker for the right channel’s response, and vice versa). With the plugin, select
then press Fast-Forward button on the remote control. In the left speaker you should hear an ascending sweep — from low to high — with Morse code (saying “Left”). In the right speaker you should hear a descending sweep with Morse code (”Right”). If the channels are reversed, fix it now!
Left Channel: Setup
Open Audacity, and select .
- On the “Audio I/O” tab, select the correct recording device, and select Channels: 2 (Stereo).
- On the “File Formats” tab, select Uncompressed Export Format: WAV (Microsoft 32 bit float).
Set up the measurement microphone at your preferred listening position. For best results, position the microphone at ear height in your best listening seat, pointed directly between the loudspeakers.
Connect the microphone output to the audio interface’s mic Left input, and then make sure that “phantom power” is switched on.
Connect the Right analog output of your Squeezebox to the Right line-input of the audio interface. Connect the Left analog output of your Squeezebox to the Left input of your amplifier. (If you will usually be playing Squeezebox through its digital output to a DAC, then make these connections at the analog output of your DAC).
Play some music. Check that you hear music from only the left loudspeaker. Turn the amplifier volume all the way down (or the Squeezebox volume, if your amplifier doesn’t have a volume control).
From the remote control, select
then press Fast-Forward button on the remote control. Turn the volume gradually back up: you should hear a rushing noise sound.
Connect the USB interface to the computer. Open Audacity and start recording.
While the pink noise is playing, adjust volume so that the left channel is moderately loud (but not unbearably so; somewhere around a normal listening level). Adjust the levels on the audio inputs so that each channel reaches around the -6dB mark (no higher) in Audacity. When this is done: press Pause on the Squeezebox, stop recording and close this file (there’s no need to save it).
Left channel: Recording
In Audacity, start recording.
From the remote control, select
then press Fast-Forward button on the remote control.
You should hear a low, moderately loud rumbling as the sweep begins, which quickly increases in frequency up to a piercing whistle. Remind yourself to warn the neighbors next time. Check that this sweep sound is only emanating from the left loudspeaker.
Watch the peak level in Audacity as the recording progresses; if the loudest levels are hitting the top and bottom of the range (clipping), then stop recording and start again (pressing Fast-Forward to restart the sweep) with slightly lower preamp gain. If there are any loud environmental noises while recording (for example, loud passing traffic), it’ll be best to stop and restart too. Repeat as necessary until you have a clean recording.
After recording, trim any spurious noise from the beginning and end of the recording by selecting those areas and pressing Delete, as in the screenshot below.
Finally, ; save as “Left.wav”.
Double-check by playing back the track in Audacity. Then close the track.
Right channel: Recording
Now you’re more than half way there. It gets easier with practice, and swapping channels is simple: simply connect the cable from Squeezebox to amplifier or receiver — in the picture above, connected from the left output of the SB to the left input of the amp — into the right channel input at the amplifier.
Don’t adjust the volume settings, or the audio input gain, between recording left and right channels.
Start recording in Audacity, and press “fast forward” on the Squeezebox remote to start the sweep again.
Trim any spurious noise from the beginning and end of the recording, then save as “right.wav”.
Processing
Once you have some sweep measurements, the noisy stuff is done. Next, you process these recordings to create an “impulse response” and a set of correction filters.
