A Look at the Room

Since we’re treating a room it makes sense to start by examining the room a bit further. After a little playing in Sketchup I came up with this:

Digital Duck Recording Studio in Sketchup

Believe me I feel lucky to have so much room to devote to recording. My space has no sound proofing features, and sits on a fairly busy street, with a playing field across the street, so serious recording has to wait until the late evening. Soundproofing is a difficult and expensive business, so I just live with the noise.

The triangle represents the listening position, with a pair of Dynaudio BM6p speakers a little over 5 feet in front of the window. Here’s a pic that shows the layout:

Digital Duck Studio with Bobby and Chuck providing inspiration

Gear

The computer sits in the next room, our guests get to listen to the noise so I don’t have to. My audio interface is a LynxTwo-C, with 6 inputs and 2 outs. The recording rack holds a John Hardy M-1 preamp along with a pair of M-Audio DMP-3 preamps.

I have a half-broken mic boom stand that I use for a headphone stand. Since I’ve embarked on room my room measurement endeavors I’ve pressed it into service to position the SPL meter and the measurement mic. Here’s a shot of the boom – the phones are Audio Technica ATH-M50s and they’re terrific.

Boom stand holding SPL meter and measurement mic

Just to give you an idea of the level of engineering that goes on around here, a closeup of the DPA 4061 and the Radio Shack SPL meter. Note the NASA grade paper clip and masking tape:

Measurement mic and SPL meter up close

Measuring with Room EQ Wizard

I was fooling around with Room EQ Wizard and decided to try moving the measurement mic around a bit to see how the results changed. One of the usual problems with an untreated room is a wide variation in the sound in different locations in the room. I set up REW and took three measurements, starting at the listening position then moving back a foot each time.

At the listening position

This first shot shows the full spectrum from 40 hz to 20 khz, but in an untreated room the reflections swamp the signal in the higher frequencies. The problems at the low frequencies seem pretty fierce, with big swings between 100 and 200 hz. These are the bass notes on the guitar, extending from 73.42 hz for the low D I usually use, up to the third string G at 196 hz. The huge swings in response will make some of these notes boom and some disappear.

Here’s the graph for the same measurement, but 1 foot further away from the speakers.

1 foot behind the listening position

This graph looks a bit better, but we still see over a 20 dB swing in the frequency response between 100 and 200 hz, and we’re seeing that different notes will be emphasized and muted.

The third graph shows the measurement from 2 feet behind the listening position.

2 feet behind the listening position

Once again the specific notes impacted by the frequency response variations has changed. The effect is easier to see when all the plots are displayed on a single chart:

REW graph 40 to 1000 hz, 3 measurements combined

Each of these measurements has at least one steep dip between 100 and 200 hz, and each one will mute a different musical note. And these effects were found by moving only 1 foot each time.

The Sound of the Room

Since my goal is to improve my recording space it seems reasonable to do some before and after comparisons of recordings, so I set up a Jecklin disk mic array with my Shure KSM141 mics switched to their omni setting. This arrangement of omni mics with a baffle between them is usually used for concert hall recording and other classical music sessions:

Jecklin disk mic array with a Shure KSM141 pair

This mic arrangement uses omnidirectional mics, so we’re getting the great extended bass of a fine omni mic but we’re also getting a lot of the room sound. I positioned the mic about 2 feet from the guitar which gives a wonderful stereo image and very natural tonality, but once again brings in a lot of the room sound. Here’s a picture of the recording arrangement:

Vamping for the microphones

Most of us recording at home usually use directional mics and place them much closer to the guitar, in an effort to reduce the impact of the room. But directional mics exhibit a rising bass response called proximity effect which increases as the mic gets closer to the sound source. If we can help our room sound better, we gain a lot of flexibility in the mic techniques we can use.

Here’s the brief clip I recorded:

or if your prefer, download clip 1

Then as an experiment I moved the mics and chair both about 2 feet, keeping the distance between them the same, and recorded again:

or if your prefer, download clip 2

I don’t put much stock in listening tests where we hear different performances, because it’s impossible to tell whether the sonic differences are due to the recording technique or the playing technique. In this case I can tell you that the difference in the two recorded clips is much greater than the difference in the two performances, and I think it’s due to the wildly varying frequency response in this untreated room.

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.

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.