Bass Behavior in the StudioEMPIRICAL TALES ON THE LOW DOWN 6/01/2006 8:00 AM Eastern
Bass…the final frontier. I don't know any subject that is less understood or creates more questions and anxiety for engineers in control rooms small and large. We are talking about the desire for coherent, solid bass, free from peaks or cancellations — tight bass, not the kind that fills the room by swimming around.
I rarely have clients call up and complain that their big room problem is with high frequencies. That's because frequencies above 400 Hz are relatively simple to deal with; a little absorption, some diffusion, and it's all set. Identifying the reflection source is also relatively straightforward; it's just like billiards. At high frequencies sound and light behave very similarly, and simple geometry goes a long way.
Engineers have long struggled with getting low frequencies under control. It's a struggle because there are many factors that contribute to the problem: wall angles, construction style (stiff or flexible), room size, speaker phase (not polarity reversals, but group delay), speaker placement, listener placement, and the primary focus of this article, gear placement. By gear I mean not only the racks of stuff scattered throughout the room but also the junk piled up in the corner, and even the console itself.
A primary issue of bass is predictability. I have seen control rooms that measured great when they were empty. At the early stages of tuning those rooms, when we did initial measurements, I had high hopes and was very excited about how well-behaved the bass was. But when I came back to the room after the console and producer's desk had been installed, the measurement didn't look as good (Fig. 1). I formed a theory that bass behaves a lot like water; it's simplistic, but it has helped me to visualize what is going on in a room. Picture a calm pond with a rock sticking up out of the surface. If you throw a small stone into the pond, the waves will travel out from the point of impact. When they hit the rock some of the water will travel around the rock, but some will reflect back and cause interference patterns. These interference patterns exhibit themselves as peaks and dips in the bass response. I'm not a Ph.D., nor do I play one on TV; my theory was just a way of visualizing a reason for my measurements. But the information I am going to impart here is based not on theory, but rather on solid empirical findings. This article offers real-world measurements taken with Meyer SIM 2 and SIM 3 analyzers, which point out real issues that need to be addressed in control room design and setup.
THE ELEPHANT IN THE ROOM
If all of this gear is making it harder for us to get a decent bass response in a room, it makes sense to try to minimize the gear, or position it in a way that minimizes its impact. One of the outstanding issues here is the size of some mixing consoles. Over the years producer/engineers have demanded more and more channels. I have seen consoles expand until people had to cut holes in the sidewalls to accommodate the console's front corners, where accessing the front of the console meant climbing over the console itself. Not only is an Über console a formidable barrier to the flow of bass from the front of a room to the rear, it creates a huge surface for first-order reflections directed right in your face. Many engineers have also complained about the amount of low end that rolls around in the front of the room, trapped by the console's front, especially those consoles that have no airspace beneath them.
I had the opportunity to see a console's impact on a room's bass response at Firehouse Recording in Pasadena, Calif., when the staff decided to replace their luxury liner mixing desk (at right) with a new control surface-style console that was completely open below and one-third the width of the previous board (in opening photo, opposite page). Figure 2 shows the before and after responses. As you can see, there were some real improvements in the bass response once there was a more open path through the mix position. Some problems remain but a lot of the missing energy has been filled in. And without the big desk in the room, there was also a dramatic visual effect, a feeling of openness in the room.
Perhaps you can't replace that monster in your control room, since many engineers must have a hard fader for every track. In this scenario, if there is room in front of the console I'll try to build a bass trap. For some of the lower frequencies this setup requires more real estate than most studios have available. In recent years, development of more space-efficient options, such as tuned membrane absorbers, makes the trap concept a bit more realistic. Figure 3 shows the response of a large room that had a pair of woofers in the soffit as part of the mains, and a pair of woofers built into the front wall on the floor in front of the console. The console had a little air space beneath it, but the issue was complicated by the fact that the mixing desk was on a raised platform — the woofers on the floor were firing directly into the support platform. Figure 3 compares the room as it existed in the original setup and after we built a bass trap in front of the console: The destructive high Q resonance has been eliminated, and the overall mid-bass level above 100 Hz has evened out.
Outboard gear racks also affect bass response. The industry has settled into a practice of putting a producer's desk three to four feet back behind the mix position. Typically, the desk is about three feet high and as wide as the console. I don't need to tell you what a formidable barrier the gear racks are, especially when the desk is piled high with even more outboard gear. I often see a nasty cancellation at the mix position due to the reflection off of these babies. To minimize this problem, I like the concept of putting the racks as far off to the side as possible. This configuration creates a U-shaped work area and is ergonomically quite handy. If the outboard gear has to be in the back, try distributing the gear so that it is in two separate producer's desks and move the desks to each rear corner so that there is some space between them. This will allow more bass to flow to the back of the room. Also, angle the racks to stop the direct reflections from hitting the mix position. Some of my clients have even put the racks on wheels so they can be placed in a more benign position when they're not in use.
If you've read any of my articles over the years, you know what a nut for symmetry I am. The placement of outboard gear needs to follow the rules of symmetry as well. If you pile up a bunch of gear on one side of the studio and not the other, you are going to wind up with inconsistent bass response from the left and right speakers. This discrepancy will prevent you from doing an acoustical fix for the room that applies equally to both speakers. I'm not talking about a huge pile of gear, either. As I said earlier, just one rock in a pond can cause some type of interference. I had a very educational experience early on in my room-tuning career that really opened up my eyes. I was measuring a very large control room that had been designed and built with precise symmetry. When I measured the room I saw different responses in the left and right speakers but simply could not figure out why the speakers looked different in analysis. As I looked around the room I finally realized that the only difference between the left and right sides of the room was a small rack against the wall on the right side. I had initially believed this small rack to be insignificant in such a large space, but after I pulled the rack out, the measurements for the two sides matched quite well. In fact, I did not have to pull the rack out of the room, but only two feet away from the wall. The culprit frequency wanted to travel down that wall, and the rack was preventing it from following its natural flow (Fig. 4).
This example points out that something considered insignificant can have a significant impact on a speaker's performance. This rule even applies to the pile of stuff you have lying around the room. I recently set up a mid-field system with a pair of subs for a client of mine. He had a collection of old, unused outboard gear, boxes, keyboards and guitar cases on the floor up against the front wall in the center and on the right side of the room. This was a fairly large control room, and he had a fairly significant garbage pile. Because he said he didn't really have any place to store it, I told him just to leave it. I didn't really think it would be much of a problem as it wasn't piled high; it just took up a lot of floor space. Because the room had good symmetry, I first found a place where the left subwoofer looked as balanced as possible. I then set the right sub up using the same distances from the wall so that the subs were both set up the same. When I measured the right sub, it had a big hole at 100 Hz. The only thing that was different was the junk on the right side, so we moved it out. I guess I shouldn't have been surprised but those few pieces made all the difference in the right sub's performance.
Once again, when it comes to bass, you can't make any assumptions. I'm a firm believer that you need to make measurements if you want to get your room set up properly. Measuring by ear is just too difficult, especially when you're trying to nail down the source of a problem that is inconsistent between left and right sides. I had a client whose room did not have symmetrical construction. The left side of the room was just a typical interior-wall house construction, and the right side was a beefier outside-wall construction with some extra layers for isolation from the neighbors. As with the client mentioned above, this studio owner had limited storage options, so he piled up about ten acoustic guitar cases from the right front corner of the room, stretching along the right wall to the console. What we found was pretty amazing. The guitar cases actually acted like some kind of bass trap. Normally I would assume that they would suck out some useful information, like the example above, but in this case (no pun intended), they made the right speaker look more like the left speaker (Fig. 5).
I want to leave you with the idea that you shouldn't take anything for granted when it comes to bass. As always, symmetry is very important, and as in life, try to avoid as many obstacles as possible if you want to flow like water.
Bob Hodas tunes studios around the world; from Sony in Tokyo to Abbey Road in London and all parts in between. His Website is woefully behind, but it can be found at www.bobhodas.com. Bob misses his good buddy Stephen St.Croix.