(excerpt from “Optimizing ASC TubeTraps” by J. Peter Moncrieff)

We’ve written before about the amazing power and sensitivity of Tube Traps in sonically affecting all those sonic as­pects that benefit from proper perimeter reflection control. We can often hear the difference if one TubeTrap column is moved a fraction of an inch, or rotated a fraction of an inch.

But many of you are probably looking with trepidation at all the TubeTraps involved in emulating Figure 3 for your room. The budget requirements are not inconsequential. But think of all the money invested in your library of music record­ings and in your stereo system. What’s it worth to hear all this investment at its proper potential for a change? It might also seem strange to be surrounded by a veritable forest of Tube Traps. But our listening room has the comforting feel of an open Greek temple, with the columns around the perimeter.

Still, you might be doubting that all those TubeTrap col­umns are really necessary. So we conducted a measurement ex­periment, to prove to you that every one of those Tube Trap columns is necessary, to achieving adequate perimeter reflection control for the room. The proper scientific method for proving this thesis is to fully treat a room, and then remove just one Tube Trap column. This should make a very small difference, but just enough of a difference to test whether every one of those Tube Trap columns is truly necessary.

So that’s the experiment we performed. We removed just one 9 inch TubeTrap column used only for perimeter reflection control (the one shown with stripes) from the complete room setup shown in Figure 3. We set up the measuring microphone along the listening path, between the wall reflection point shown and the listener. It’s an omnidirectional microphone, so it would be measuring the total room behavior, as heard along this path.

When the one TubeTrap column is removed, there is one 5.25 foot gap between TubeTraps, with all other gaps being 2.25 feet as recommended; when it is replaced, this 5.25 foot gap is eliminated, and is replaced by two of the recommended 2.25 foot gaps. Would there be a measurable difference in the room from the elimination of just one TubeTrap column from the total set of 33 columns we had installed for total perimeter control?

lf there was to be a difference, it would probably show up in that dreaded mud factor region, somewhere between 200 hz and 400 hz. That’s because we would be allowing one unpro­tected gap in the room’s entire perimeter, a gap of 5.25 feet to be precise (6 feet minus the 9 inch diameter of the columns themselves). That gap would have the capability of sustaining a coherent reflection packet of two half wavelengths, or three, four, etc. Two half wavelengths within 5.25 feet calculate out as equivalent to 210 hz. Thus, by exposing an unprotected gap of 5.25 feet, we would expect to see some room troubles some­where above 210 hz, troubles which would be cured by control­ling that gap and narrowing it to the recommended 2.25 feet. If, that is, every single Tube Trap column (with 2.25 foot gaps) were truly important to adequate perimeter reflection control in this room.

Incidentally, because this is a huge room, the effects of re­moving one column out of 33 should be much smaller upon the huge room than the effects upon a smaller room of remov­ing one column out of say 20. So the results we prove here will be even more relevant to smaller rooms.

The standard time honored test for echoes, smearing, and resonances is the tone burst test. ASC makes available a con­venient cassette, which contains a range of tone bursts span­ning the relevant frequencies of 20 hz to 755 hz (technically, the tape contains a sine wave that slowly slides in frequency and then is chopped into 8 tone bursts per second). The perfect tone burst response should look like a solid rectangular block of full amplitude sine waves, followed by a period of silence (of the same duration as the rectangular block, on this tape), followed by another rectangular block of full amplitude sine waves, etc. Figure 15 shows a pretty good looking example. Each picture show 4 tone bursts, with 4 supposedly silent peri­ods. Note that each of the 4 tone bursts in a picture will be slightly different, since each represents a slightly different fre­quency from the sliding sine wave tone.

Incidentally, don’t pay attention to the irregularities you’ll see in what should ideally be a flat top and bottom of each tone burst’s rectangular block; they are mostly random artifacts of measurement in a reverberant room. Also, you should naturally expect to see some reduction in the amplitude of every main tone burst when we add the absorbent TubeTrap in the reflec­tive path.

What then do you look for in this tone burst measure­ment? The most important thing to examine is how well the room falls to silence after each rectangular block of tone burst. The tone burst itself mimics a musical transient or any piece of musical information. After that musical transient or piece of information is over, the room should not continue with too much energy as smearing echoes. If it does, these smearing echoes of the room will form a sea of mud, the dreaded mud factor.

This sea of mud will obscure the end of the musical tran­sient (and the subtle after resonances that tell you about the texture, timbre, and material of the musical instrument); it will obscure the next transient or piece of musical information; and it will generally blur all the distinctions among various musical transients and pieces of information, as the music vainly struggles to rise above the sea of mud. The music’s temporal coherence and even basic clarity will be lost in this sea of mud. You’ll want to turn up the music’s volume for more clarity, but this won’t help, since the sea of mud will also rise. In addi­tion, if the smearing echoes are from a coherent reflection pack­et (as is the case here), it will degrade stereo imaging and cause tonal colorations.

You’ll see this sonic description of a sea of mud vividly illustrated in some of the measurements. In some cases, the sea of mud rises so high relative to the music (represented by the rectangular tone burst) that it will be difficult to even see where each tone burst supposedly stops and restarts (for example the 3rd and 4th bursts of Figure 10).

Sometimes, as the sea of mud between rectangular tone bursts rises, you’ll also see the amplitude of the rectangular tone burst itself diminish. This might be caused by cancella­tions occurring, where the undesirable packet of coherent re­flected energy interferes with the direct sound from the speaker.

Sometimes the main tone burst is so diminished, or the sea of mud after the tone burst rises so high, that again you won’t be able to see where each tone burst itself is supposed to begin and end (for example Figure 16). That is true room gar­bage, playing just as loudly as the original music signal. The sea of mud has risen to swamp the music. When you see this happening to our lab room from just the removal of one Tube Trap column out of 33, you’ll realize graphically just how im­portant each and every TubeTrap column is for adequate perim­eter reflection control, for giving you clear music instead of mud. ASC calls this tone burst tape the Music Articulation Test Tape (MATT), with good reason. You can use this tape yourself, even without our fancy lab test equipment; all you need are a set of headphones and your ears. Borrow this tape from your ASC dealer, and simply listen to the test tone bursts first through headphones and then through your speakers. Through the headphones you will hear clear bursts of tones. Through your speakers you will hear smearing echoes after each burst, getting much worse at some frequencies. What you are hearing is the sea of mud contributed by your room. Your room is doing that to all your music. And you can fix it with TubeTraps, using them for full perimeter reflection control.

Now on to the measurements. As expected, removing the single TubeTrap column caused more problems at some fre­quencies than at others. Here is a selection of the most illustra­tive examples. We predicted that, if the perimeter reflection control thesis were correct, we should see the first problems somewhere above 210 hz. Sure enough, the first major differ­ence cropped up around 250 hz (all frequencies are approximate, because of the sweeping nature of the test tone).

Figure 7 shows the room’s performance around 250 hz with all TubeTrap columns in place; Figure 8 shows exactly the same signal, measured at the same point, with just the one TubeTrap column removed out of 33. Figure 8, without the single TubeTrap column, shows a lot of smearing echo energy lingering after each tone burst stops, with only a brief moment of relative quiet before the next tone burst begins. The tail of smearing mud is so large in its beginning that you cannot even tell where the original tone burst itself stops. With the control­ling TubeTrap column restored to its place, Figure 7 shows that the smearing echo is reduced, and there is a much longer, better quiet period before the next tone burst begins. Of course, it’s still not perfect (you can see two small single slaps of en­ergy left in the time domain after the tone burst), but there’s an easily visible difference.

The fact that some imperfect echo smear is seen even with the TubeTrap in place simply means that our room is still not perfect. The important point is that there is a clear difference, a clear degradation, when just one column of a complete 33 col­umn perimeter reflection control setup is removed.

Figures 9 and 10 show a difference evident around 300 hz. In Figure IO (without the TubeTrap column) you can see a large lingering echo smear. For the 1st and 2nd bursts, the smear begins after a small gap of quiet following each tone burst. For the 3rd and 4th bursts, the smear is congruent with the tone burst (so you can’t even see or hear where the burst or musical transient is supposed to stop), and the smear itself is in the shape of a decaying triangle. Figure 9 (with the TubeTrap column) shows a smaller, better controlled version of this echo smear.

Figures 11 and 12 show a difference found around 315 hz. Figure 12 (without Trap) shows a big, blotchy echo smear; for the 4th burst, the echo smear is again congruent, and has that decaying triangle shape. Figure 11 (with Trap) shows a smaller echo smear, which is also more even and well behaved (more like the random incoherent reverberation background noise that is actually desirable).

Figures 13 and 14 show a difference found around 420 hz, at the upper edge of the mud factor region. Figure 14 (without) shows a fat decaying triangle of echo smear, while Figure 13, though still not perfect, shows a much smaller decaying trian­gle of this echo smear.

Figures 15 and 16 show a difference around 600 hz, which may be a harmonic of the trouble seen at 300 hz. Figure 15 (with Trap) shows a pretty decent signal, while Figure 16 dra­matically shows total degeneration without that one TubeTrap column. The main tone burst signal collapses down to noise level in the middle of itself, probably due to some path length cancellations. It’s hard to tell which is the inter-burst smear and which is the mid-burst collapse.

Figures 17 and 18 show a difference around 630 hz, which may be a harmonic of the trouble seen at 315 hz. Figure 17 (with Trap) shows pretty good silence in between tone bursts, with the visible residual noise perhaps representing just the de­sirable random incoherent room reverb. Figure 18 (without Trap) shows a larger amplitude of echo smear, in the form of a triangular decaying tail.

These measurements provide clear, indeed dramatic visual proof of the sonic importance of even one TubeTrap column to total perimeter reflection control in your room. They show the mud factor already rising, in the predicted warmth and lower midrange regions, when just I out of 33 perimeter control col­umns is removed. They show the importance of placing a TubeTrap column every 3 feet or so, as recommended, instead of wider spacing. We rest our case.