Room Measurement

If you want to do-it-yourself, here's what you'll need.

Equipment

The basic equipment is a measurement microphone, an audio interface, and a computer.

Microphone

It's important to use a measurement microphone, not just a vocal mic. Measurement microphones are designed for a very flat frequency response, and omnidirectional response. Nearly all measurement microphones are condenser mics, and require "phantom power" (voltage supplied by the recording equipment, to power the microphone's internal electronics).

We suggest one of the following:

  • Behringer ECM8000: inexpensive and good quality;
  • Earthworks M30: exceptional quality but more expensive.

    You'll also need a microphone stand, and a reasonable length microphone cable (with XLR plugs at each end).

    Audio Interface

    The audio interface provides phantom power to the microphone, amplifies the microphone signal, and converts the signal to a digital input for the computer to record. Since the mic needs phantom power, it's not possible to use a regular computer soundcard. The audio interface should include "line-in" as well as microphone inputs, and must be able to record stereo at 44.1KHz (not just 48KHz). Some suggestions for USB interfaces include:

    Some PCs have FireWire interfaces as well as USB. A wide range of Firewire audio interfaces are available, in addition to the USB interfaces listed above. A third option is to use a high-quality computer sound-card and a phantom-power microphone preamp; I prefer a self-contained USB interface because it's easily connected to a laptop computer.

    Instead of a PC-based system, it's also possible to use a standalone digital recorder such as the M-Audio MicroTrack or Marantz PMD660, with phantom power microphone input, line input, and stereo 44.1kHz WAV recording capability. If you have one of these, then you record directly to the device, instead of to the PC-based recording software; otherwise, the steps are approximately the same.

    Recording Software

    Use Audacity.

    In Edit - Preferences, check that the "uncompressed export format" is "WAV (Microsoft 32 bit float)".

    The measurement process

    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 one minute.

    Set up the measurement microphone at your preferred listening position. Connect the microphone output to the audio interface's mic Left input, and 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. (It's easiest to do this channel separation using the analog outputs, even if you will usually be playing Squeezebox through its digital output).

    Download the sweep files here. Unzip, and copy all the FLAC files into your SlimServer's music directory (for example, in a folder named "Sweeps").

    WARNING: Playing these test tones 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.

    Connect the USB interface to the computer. Open Audacity and start recording.

    Browse to the music folder, and find the "Pink Noise" file. Play. While this is playing, adjust the Squeezebox playback 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, stop recording and close this file (there's no need to save it).

    Start recording, and play "Stereo Sweep". 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 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, File - Export; save as "Left.wav".

    Switch off phantom power. Connect the microphone to the Right channel, connect the Squeezebox Left analog output to the audio interface's Left line-in, connect the Squeezebox Right analog output to your amplifier. Switch phantom power back on, check the recording levels with the pink noise track, and repeat the sweep recording process. Save as "right.wav".

    Once you have some measurements, the noisy stuff is done. Next, you process these recordings to create an "impulse response", and from there create a set of correction filters.

    • inguz audio - room correction - measurement copyright © 2006. all rights reserved.