Acoustic Treatment – here’s what we’re trying to fix

Thursday, February 19th, 2009

Room EQ Wizard is a wonderful free tool for evaluating the acoustic conditions in a room. And Sketchup is a wonderful free tool for modeling a room. I used both of these to evaluate my recording space, and to develop a baseline for comparison as I add treatment. (more…)

 

Room EQ Wizard – free room evaluation software

Friday, February 13th, 2009

During the learning process that was my first CD I found Ethan Winer and learned a bit about his ideas for improving the acoustics of small rooms. Since then I’ve learned about the Acoustics forum at StudioTips.com, another great source of information on small room acoustics.

Even though it’s not strictly necessary, it’s interesting to use some kind of acoustic measuring tool to evaluate the room and gauge the results of treatment. I recently learned of a free program that works with a PC or Mac and their audio systems to measure and display room response. It’s called Room EQ Wizard and it’s available for download at the Home Theater Shack. The Shack is a discussion forum focusing on home theater (well, duh), but Room EQ Wizard, or REW as they call it, is just as useful for recording spaces as it is for home theater.

The home theater users seem to have systems that are a bit more complex than my simple recording rig. I don’t have a subwoofer and accompanying crossover, nor do I have an equalizer in my playback chain. So my connections were simpler than those illustrated in the REW help pages.

I bought one extra piece of equipment for this project, a Sound Pressure Level (SPL) meter. These are available from Radio Shack and other sources. The Radio Shack unit I bought is accurate enough to measure a baseline volume setting for all my readings. It’s possible to use the SPL meter as the measurement mic, but it is not accurate above about 3Khz, fine for tuning subwoofers but not much help for a full range monitor system. The Behringer ECM8000 is widely recommended for an inexpensive measurement mic, but I have a couple of DPA 4061 miniature omni mics in my collection, so I chose that for my measurement mic.

The first step in using REW is a soundcard calibration procedure. This procedure compensates for nonlinearities in the soundcard and also ensures that the basic system is working, with REW finding the soundcard inputs and outputs as needed.

I use a LynxTwo-C soundcard and REW found it easily. Here’s the Settings page where the soundcard is configured and measured.

settings-soundcard

The soundcard calibration starts by creating a loopback connection, that is, the output of the audio interface is connected to the input. This is easy with a recording oriented system, where balanced line in and line out connectors should be available. On the Lynx breakout cable the XLRs just clicked into place. On other systems a male TRS to male TRS may be needed. Don’t forget to turn off your power amp or mute your speakers – now how would I know to remind you about that??

I’ve created a YouTube account for videos related to the blog. You might stop by http://www.youtube.com/user/homebrewedmusic if you’re in the neighborhood. This video goes through the steps to run the calibration measurement.

After saving the soundcard calibration file we need to adjust the levels so we’re getting a good signal to noise ratio but avoiding clipping. First we restore the loopback connection to our normal hookup and turn on our power amp or unmute our speakers.

This video demonstrates the steps:

Since we’re moving these input and output volumes all the time we’ll probably need to run this level adjustment routine before taking measurements.

With the levels set we’re ready to measure our room/speaker response. Well, actually we’re also including the mic preamp and power amp, but those are probably quite linear, especially compared to our room and speakers.

Here’s a screen video of the measurement process.

And here’s the resulting graph. Looks pretty ragged to me, with huge swings between 40 hz and 200 hz, and lots of comb filtering in the higher frequencies. I suspect that this is the normal response of a medium small room. In our next entry we’ll see what we can accomplish with our current batch of broadband absorbers.

 




About the Blog

    Howdy, my name is Fran Guidry and this is my Homebrewed Music blog.

    I play Hawaiian slack key guitar and recorded my solo acoustic CD at home. Most of the recording information I find on the internet seems focused on bands, drums, multitracking, and so on but my main focus is recording solo acoustic guitar. Lately I’ve been enjoying video recording along with audio, so that shows up in the blog as well.

    I’m also a guitar nut. I love big ones and little ones, handmades and factory guitars, cheap ones and expensive ones. So I’ll be sharing the fun of exploring guitars as well, along with the challenges of amplifying acoustic guitars for live performance.

    Welcome!

Philosophy

    My recording philosophy is pragmatic, skeptical, not super critical. After all, the performance is by far the most important component of a track, and every aspect of any recording is a matter of taste.

    But I do like to know “about stuff.” Back in hifi days I learned about double blind testing. I learned that we humans can easily hear differences that don’t really exist. The more I’ve learned about our human auditory system, the more I’m skeptical of what people say they hear, especially if they claim that a particular microphone or preamp or cable has some magical property.

    I’ve only been recording since 2001, and when I started I found the usual places on the internet. I sought advice and accepted it, thought I would improve my recordings by using more expensive equipment. It didn’t work.

    Two things that did seem to lead to better recordings were experience and room treatment. Getting an appealing sound is the combination of many small details, and learning those details only comes from experience. Amd the sound of the recording space is obviously a big factor.

    I’ve only recorded seriously using digital technology, but I remember trying to record rehearsals and gigs back in analog days. I don’t have any nostalgia for analog recording and playback systems at all. I think even low end digital systems can capture marvelous recordings. So when I look at gear, I look for good specs: low noise, broad flat frequency response, wide dynamic range, low distortion. I’m not interested in colorful components, mics and preamps with a sound, I want the sound to be the sound of my guitar.

    But the last word is that I’m just learning and I hope you find something useful in my posts.