Listening Room Test

[Michael TubeTrap]

We've been working with a couple projects lately that illustrated the difference between between perceived sound quality by humans versus the measured sound quality by machines.

Everyone here knows what to trust first, right? With audiophiles, our own ears get priority. Usually someone notices their system just doesn't sound as good as it should, no matter what they do. After trying lots of sensible things (room layout, cables, better power conditioning, preamp upgrades, etc) testing comes into the picture. RTA's are cheap now, built into processors and laptops, so its easy to measure the static frequency response of a listening room. A bump or dip at a narrow frequency range will get the blame for the poor sound. And now the quest is on.

This is how TubeTrap founder Art Noxon followed the listening room test adventure. This is from a recent article he wrote.

“High end audio started for me in 1984, having just invented the TubeTrap. In the beginning I spent a lot of time trying to figure out why audiophiles got so excited about putting a pair of TubeTraps in the front corners of their listening room. Yes. I was pleased and proud, but being the hard core engineer that I am, I needed to measure, to quantify, what caused their thrill. I tried all the standard tests known then, which are the same ones known today; sine sweep, pink noise, RT-60s, narrow spectrum analysis, TEF waterfalls, and even Q changes of resonant modes.

What did I get? The best was about 1 dB adjustment in anything, usually less. Certainly not enough to warrant the pleasure those trapped corners gave the audiophile crowd. I published my findings (failure to find) in the AES and didn’t know what to do after that. In the mean time we had gotten so bored with reverb chamber testing of TubeTraps we tried to speed the process up and were getting very illuminating results using rapid short tone bursts. Not only in the chamber but in real rooms we were documenting changes upwards of 10 to 15 dB for rooms with and without TubeTraps in the corners.

Coincidentally at this time, 1986, I was going to the SynAudCon meetings. Victor Peutz, from the Netherlands, showed up and presented the new/next wave of audio performance testing: Intelligibility. I couldn’t believe my ears. I realized right then and confirmed with him that our fast tone burst testing was actually a tonal intelligibility test. Much more has come to light since that first peek into the wonderful world of audio playback intelligibility, which turns out to be what really matters the most in hifi playback.”

The result of this insight was our MATT Test (Music Articulation Test Tones). It mimics the attack rate of modern music, and is based on a standardized test, the B&K RASTI test.

You can download the MATT Test from the ASC website. Included is a description of the MATT test, a training demo and a down-loadable MP3. This test is also on TRACK 19 of THE STEREOPHILE TEST CD 2 and many audiophiles have that reference CD. Do your own A/B test. Listen to the signal first on headphones and then listen to your audio system try to play it.

When you send a recording in for analysis we can show you graphically which frequency ranges have good articulation and which don't. But the thing we like best about this test is that any audiophile can get an immediate result without the need for fancy hardware or added costs. Change your speaker's position and hear if the MATT improved.

I would love to hear how this test has worked in your listening rooms. Next time, how this test applies to imaging.

Michael Adams, ASC

[jdandy]

Michael.......Thank you for the MATT test information. I have the Stereophile Tesy CD2, so I play to go through the process and see what happens.

[Dj_AmTraX]

Thank you Michael. I have to play around this weekend.

[Michael TubeTrap]

Dan & Mike, an alternative to the headphones is to use the volume control of your preamp. Most systems sound great at low volumes, but fall apart when you get to ___ volume (you know what it is). The MATT Test will do the same thing.

[chessman]

Michael,

I have a 6 foot column of ASC bass traps in each corner (stacked 3' by 11"), a 6 foot column (stacked 3' by 14") in the center of the back wall, and a couple of others in a kind of haphazard placement. Will the MATT test help me "tune" these, i.e. decide how to rotate them to reflect/absorb?

[Michael TubeTrap]

Randy, you are spot-on. The MATT Test became an incredibly useful tool for setting up room acoustics. While the others tests Art had been using (and still are used today) showed only a minor change from TubeTraps being set up in a room, the MATT Test shows tremendous levels of feedback.

The test tone sweep runs from 28Hz to 780Hz, the entire bass range, but only a small part of the treble or image range. When we display the recorded tones in our software, we can see the frequencies that have good articulation and those that don't. When there is poor articulation that means there is energy storage happening.

Now here is our secret: we take the "problem" frequency, calculate the wavelength, and go looking for that dimension in the room. Sometimes it's easy, like the length, width, or height of the room. Sometimes it's more exotic, like the distance the speaker is from the nearest wall. Sometimes it's really exotic, like when it's the wall itself shuddering.

To answer a specific question you asked, can this help you to decide how to rotate the Tubes for diffusion/absorption, the answer is "just barely". Since the upper limit is 780Hz, and the diffusion just barely starts at 400Hz, the test doesn't give us much useful data. But treble control is easy, treble energy travels in pretty straight lines, and can be mapped out on paper pretty reliably. By contrast, bass pressure waves slosh all around like bath water in a tub, and bass energy is powerful enough to mask delicate treble sounds. So, our practice has been to calm the stormy bass water first, then trim up the treble.

One way you can use the MATT Test informally is to play it, and note where the pulses slur together. Make changes to your acoustics layout, then play it again. You should be able to hear improvements in the tracking, just by ear. Play some music and see if the songs have improved as well.

And as always, if you want to contact me with a room drawing or some photos, we can give our advice on where we would place the Tubes you have now.

[chessman]

Originally Posted by Michael TubeTrap

By contrast, bass pressure waves slosh all around like bath water in a tub, and bass energy is powerful enough to mask delicate treble sounds.

I think you just explained something that has mystified me up until now. My first foray into tube traps was just the front two corners. I had expected better bass (and got it), but was shocked to have a much better mid-range. I decided the bloated bass harmonics had been masking some of the mid-range. But when I added the rest of the tube traps, the system got too bright. I got back to great by rotating the front columns to absorb at about half, instead of the full reflection of the first set-up.

But I have been racking my brain to figure out why I had the problem in the first place. Based on what you said, it must be that I still had mid-range smearing after the first two columns and just did not know it. When I tamed the rest of the bass, the mid-range did not get louder - I just perceived it more clearly.

The other weird thing is that my mains seemed to suddenly go lower - an obvious impossibility. I think when my "one note" bass (24' long room, 40H "boom", what a shock) got tamed enough to not have such sustained decay time, it stopped masking the mid-bass. Again, not louder, I could just hear it.

If I have this wrong, please let me know.

[Michael TubeTrap]

Randy, that was a very good read-back of what was probably happening. Un-masking is a lot of what we do, especially in the bass region.

If you think about how we hear, bass is hard to hear all by itself. The part of the sound spectrum that we hear as sharp, clear, even jagged outlines is all midrange and treble. When you just hear bass, it sounds in-distinct, rumbly. Add to that the sound of reverberant bass, bass pressure waves that have reflected off of a couple walls and have lost their sharp pressure-high/pressure-low clarity, and you have something that is very hard to hear as a distinct sound, but still has a loudness to it. Now have it occur in time with the music, so when the music dies away so does the noise, and what you are left with is a hard to hear noise, that hides what you want, and doesn't seem to be there when the music stops playing.

The bass energy we do battle with can be likened to someone following you around, tapping you on the shoulder. You know they are behind you, but you can never turn around fast enough to ever see them.

The particular noise that the MATT Test uncovered and other tests did not was the noise that collected at the front of the room, right around the speakers. In physical space, it's the 1/3 of the room where the speakers live. In temporal space (time) it falls into the attack phase of musical notes. We have found that the more we control the front of the room, and reduce off-axis sound, the higher the proportion of direct sound you hear. That is probably why your speakers started playing lower/deeper in the bass. That 40Hz went away, and probably other frequencies too. Think speaker to near wall, speaker to front wall, speaker to ceiling above it, all those dimensions trap sound and let it keep booming long after the original note is gone.

[chessman]

Hmmm, should the rear corner columns go the front of the room? Say at a right angle to the speakers? Or I could leave them alone and move the half round column (not previously mentioned) from its mid-wall location and put it at the left main first reflection point.

[Michael TubeTrap]

Randy,
The best way to solve this problem is by seeing some photos of the room. Give is panorama-type shots, such as from each corner of the room looking out. Then give me a list of what Tubes you have. We'll recommend what should go where.

Thanks.