In an earlier article, I discussed how front speaker placement affects listener placement. We learned that the sweet spot is actually a sweet triangle that is created by the use of a center channel in a home theater. The triangles position in the room is a function of the separation of the left and right speakers. Moving the speakers closer together pulls the triangle more forward and vice versa. It would be nice if we only had to move the right and left speakers to optimize seat placement, but there’s more. The way that the room resonates from bass frequencies is also a factor. We call these resonances, modes. The modes created in your room strongly distort the low frequency output of your system. We deal with modes by using a combination of seat placement, subwoofer placement, and electronic equalization (EQ). We’ll discuss seats and modes today.
This is a basic discussion so I won’t jump into complex terminology and any math. One important premise is, that we will assume your room is mostly rectangular. Rooms with complex shapes i.e. “L’ shaped, splayed (non-parallel) walls, hexagon shaped rooms, etc. create resonances which are not easily mapped, so this article doesn’t address such rooms. It’s not that your room needs to be perfectly rectangular to follow the advice here. Actually an “L” shaped room behaves like two rectangular rooms overlapping each other. If you avoid placing seats where the two rectangles join, much of what we’ll discuss is still valid, but not perfectly. It’s just that the farther the room is from being rectangular, the more complex the patterns of the modes become. The mostly rectangular room shown here in Diagram 1 creates nice easily predicted modal patterns. Let’s see how this works.
As I mentioned, I’m going to keep this article away from complex terminology. So the full scientific explanation of what’s going on here might have to wait for a later article. Diagram 1 fast-forwards us past the scientific explanation of modes and shows a simple map of where and how the lowest modal frequencies affect where the best seats are.
Green Zones: The green areas represent good starting locations. There are two good things about green areas. First, the worst modes are moderate there; moderated peaking and no nulling. Modal nulls are areas where the mode frequency is cancelled becoming very quiet. Modal peaks are areas where mode frequencies are blasting loud, causing distortions such as boomy bass. The modes in green zones are moderated because their sound levels are less than a peak but greater than a null. The second good thing is that the average sound level of the worst mode frequencies in the green areas are similar. Simply put, seats in the green areas sound very similar to each other in the deep bass. Why is that good? Because any remedies for modal distortion (sub placement or EQ) will improve all green areas similarly. If we equalize a modal peak down, everyone in a green area benefits from the adjustment.
Yellow Zones: The yellow areas are where the two lowest frequency (and typically strongest) modes null and peak; one nulls and one peaks. They create the boundaries between the green areas and are found at the ¼ and ½ length and width divisions as shown in Diagram 1. We avoid these areas because these areas are highly dissimilar to green areas and exhibit more uneven bass response. In other words, a listener in a yellow zone hears a very different bass quality than a green zone listener. The basic premise in seat placement is: get everyone’s bass response as similar as possible. This way, when we start equalizing (by EQ or sub placement), everyone hears the improvement. The bottom line is: don’t sit in a yellow zone. So why is it yellow and not red? Answer; you can turn a yellow zone into a green one. For audiophiles scoffing at the yellow zone splashed right through the center sweet spot, there is a remedy.
Red Zones: The red areas are the absolute worst places to place a seat. In the proximity of the walls there are strong modal peaks. In these areas, listeners are subjected to strong and uneven bass. In addition, for a home theater, these seats are usually very close to surround speakers. Sitting too near any speaker causes the surround envelopment to collapse. Near-wall seat placement should be forbidden in a high performance room.
For the record, keeping this explanation simple is tough. I know many readers have more than a passing understanding of room modes. In the THX-HAA training we discuss the concept very thoroughly. For the sake of this article I’m keeping the theory to a minimum, but I invite everyone to the classes for the full explanation. The secret to our process for seat location is that we can create larger green zones so we can have more good seats and added flexibility in placing those seats. We do this using subwoofer placement enabled by turning on bass management in our AVR or processor.
Bass Management: Sometimes considered a dirty phrase to audiophiles, it is a tremendous game changer for small room acoustics. The primary advantage of bass management is the correction of the bottom two octaves of bass response by using a separate subwoofer. It also extends the headroom of the main amplifiers by “bi-amplification”; they are relieved of the chore of reproducing the bottom two octaves of sound. Bass management’s works because most humans cannot detect the location of a bass source below 120 Hz. Most home theaters use a crossover point of 80 Hz for the subwoofers which includes the bottom two octaves of bass. This means the subwoofer can be placed in many positions in the room without disrupting the sense of focus or imaging. This is something we routinely prove in HAA class. In addition, by redirecting the bottom two octaves of bass for all channels (see Diagram 2) to the sub or subs, we can make the bass response of the system independent of the sound track; i.e. no matter which channel the bass is sourced from, the response is good. Most systems enable bass management by switching the all the main speakers in the AVR speaker setup menu to “small”. Ok, I’m beginning to get off topic; bass management enables the sub to solve bass problems by playing all the bass in the bottom two octaves for all the speakers.
I have a caveat on this idea that we can’t hear where deep bass originates in a room. The fact is, that many can localize the sub; hear where it is located. The rest of the story is that the ability to localize a sub depends on how well calibrated it is and how good the sub is. A poorly calibrated sub has uneven response which makes it stick out like a sore thumb. A poor quality sub can create added vibrations or artifacts which can draw our attention to its location. The solution is to always calibrate the response of the sub and to always use high quality subs. Calibration means proper sub placement and proper EQ. If you do this it is highly unlikely that you would notice the sub playing, even if it’s right next to you.
Sub Position: This concept is very cool, but I am going to save the theory for another venue. The process is simple; place the sub in a yellow zone and magically, it turns to a green zone. Any questions? Of course, there are questions, allow me to explain. The creation of a room mode depends on how bass sound waves interact. The waves bounce back and forth between the walls and for those frequencies we call modes, the interaction creates a standing phase difference. Standing, as in it doesn’t move. Guess where the phase difference is always out-of-phase? You guessed it, the nulls… cancellations remember? Guess where they are always in phase? Yup, the peaks, meaning that these room locations are bombarded by waves adding to each other creating a very loud mode frequency at those locations. What about all the other bass frequencies? These simply don’t create standing phase differences thus they don’t create standing waves or, as we call them, modes. I skipped the part about how the wavelength of the mode frequency, conveniently, is some even multiple or fraction of the rooms various dimensions, but then, this is a non-technical article.
Inserting the sub in the null simply makes the source of the sound (the sub) add energy out of phase from the mode. By doing this, the mode is no longer stimulated. Diagram 3 shows how placing the sub in the center of the rooms width eliminates the central width yellow zone and widens the green zones. Our audiophile seat is saved!
With this simple movement of the subwoofer we now have much more room to place our seats, including a centrally located audiophile seat. Overlaying our sweet triangle (we discussed in an earlier article) allows us to find the very best locations for our seats. In Diagram 4 we can see that there’s enough room for 8 really good seats in the room now. All listeners should be able to hear focused imaging and moderated bass response with good seat to seat consistency. However, we won’t stop with moderated bass. We should equalize the subwoofer to smooth bass response even more. By avoiding the null locations for our seats, we now can deal with the remaining peaked bass by implementing parametric filters to smooth the response. With a little luck we should see that all the seats will then have excellent response.