Home theater has put bass reproduction and subwoofers into the spotlight. Various techniques and tools have been used to manage bass and integrate it within the whole system while bass became a crucial aspect of a home theater system performance. If the bass is not good, most would then consider the system inadequate.
So just what makes good bass? Understandably, the first answer would be a good subwoofer. This is true, but this is only part of the solution. The reality is that THE ROOM rules to a great extent here and very often room-related issues can relegate a very good subwoofer to just a boomy cube. What can we do to get the best possible bass with a given sub in a given room? Before we look at our options, we need to understand what happens in real rooms.
Our home theaters have small rooms from the perspective of architectural acoustics. In a small residential room, we primarily focus on “modal resonance structure” generated by standing waves and its influence on sound quality. In small rooms, these modes occur in the bass frequency range and present a serious problem.
A room is a volume of air, confined within relatively hard boundaries, that has its own resonance structure which depends on the room’s size and boundary conditions (floor, walls, ceiling structure and treatments). Fig.1 shows simplified picture of room’s cross section and few modes that are energized by a subwoofer placed in a front corner.
The graph in Fig.2 below shows actual room measurements at typical three listening positions on a couch with one subwoofer near the corner. The positions are only 24” apart. Curve colors correspond to seating position colors. Not only does the resulting bass response look bad, but what’s important the bass would be quite different for each listener.
Sound or look familiar? How often have we struggled with subwoofer placement only to find that we are getting lots of inarticulate boom near the walls and nothing in the middle of the room or where the listeners are actually positioned? Some probably give up, concluding that the room is just plain bad and nothing can be done.
The fact is that the room is very important part of the system. No matter how good your subwoofer is, if you do not take time to investigate how the room is affecting its performance and how to remedy the problems, you may end up not getting what you and/or your customer have expected, based on the subwoofer’s specs alone.
What are the possible solutions? Most would say that first step is to find the best location for the subwoofer. Optimizing its position indeed may improve the bass, but unfortunately one sub does not allow effective control of multiple room modes, as demonstrated above. For example, by moving the subwoofer out of the corner (where it produces the most boomy bass with most modes energized) closer to the middle of the room, we would control the first mode, but at the same time we may actually increase the effect of the second mode that has magnitude maximum near this location. The result would still be unbalanced bass. I realize that some may actually put total volume (very often unnecessarily exaggerated) ahead of accurate balance and articulation and simply use corner location for subwoofer. I am sure however, those most home theater enthusiasts would much prefer to hear the sound that the people who created it wanted them to hear.
The other solution for good bass that probably comes to mind to many is room EQ. I would say that one has to be very careful using it, despite advertised “automatic” benefits. It is clear that at best, room EQ can make response smoother in one location. There is no guarantee, as we see in Fig.1, that it would make sound better for the next nearby listener.
There is another problem with electronic room EQ. The bass that we hear in the room is a combination of direct sound from the subwoofer and room resonance modes. When EQ is applied to reduce a peak that is created by a room mode, it actually does not affect that mode. The room will keep resonating on this mode. We simply reduce the original incoming signal so that the combined subwoofer+room signal stays more or less balanced. The problem here is that the room mode with its high Q and longer decay would mask and smear bass details. The more room modes we have, the less accurate would be the bass.
This is not all. Yet another potential problem exists when EQ is applied to fix response dips. Most of us know that this could lead to dynamic range reduction at best or amp clipping and even to a woofer failure. A response dip that is associated with room modal null is simply impossible to eliminate using EQ. As noted above, EQ does not affect room modes and if a room mode has a null in a particular spot it will stay a null regardless of how much power we put into a sub. The effect of signal level boost in this frequency range would be minimal. Yet we would stress the amp and the woofer leading to clipping without noticeable output increase. Bottom line:, do not try to fix response dips by EQ boost!
Another solution that I sometimes hear about from AV professionals is using acoustic absorbers – bass traps. There are two types: resonators and wide-band porous absorbers. The problem with them is that in the real bass range, below 100Hz, only resonating absorbers could be effective (unless you are willing to cover the walls with at least foot thick panels). Resonating absorbers could be expensive, bulky and they have to be precisely tuned to room modes using measurement equipment to be truly effective. Keep in mind that simple room mode calculation is almost useless, since our rooms are not chambers with rigid walls. You may need several precisely tuned resonating traps to control different room modes.
Over the past several years, the experience of helping installers to solve their bass problems, proved to me that the first and most effective solution for better bass that we should be considering is using multiple subwoofers. Harman Research Group have worked on this subject for many years  and they concluded that noticeably better results in terms of bass accuracy and consistency of response over several seats can be achieved using at least two , better four subwoofers. Using multiple subwoofers allows counteracting room mode generation in very effective way. By putting subwoofers in at least two or better 4 different locations it is possible to achieve rather minimal room mode excitation. The result is not only smoother response but also more consistent bass from seat to seat. Only having achieved the latter allows an installer to use room EQ to get further improvements. Fig.3 shows general recommended positions for two and four subwoofer installations. I would like to stress that each room is different and I would strongly advise to take an RTA measurement system and send some time optimizing the positions based on particular room response. The reality shows that this optimization can pay off greatly.
Fig.4 shows real-life example of gradual bass response improvement in a room using one, two and then 4 subwoofers. While there is no dramatic SPL increase when the second module was added, the room response became amazingly smoother. Adding yet another pair of subs on the left wall only slightly improved (already very good) response but at the same time we got almost 6 dB SPL increase. These are the details that installers have to look for.
Note the set up in the lower right corner of Fig3. The subs shown there are installed in the ceiling. Yes… in the ceiling! You can do this using an in-wall subwoofer.
Fig.5 below shows the measurement of such installation from a BG Radia’s customer in a typical 25 x 17 feet room. The customer used 4 BGX THX Ultra 2 certified subwoofer modules. Because the sub is truly vibration-cancelling, he was able to install modules in the ceiling without any concerns. It is apparent that neither front nor rear par of subs are able to generate satisfactory response, yet when they are combined they produce truly fascinating result!
And lastly, I would like to show how even a highly regarded “intelligent” room EQ system can make mistakes in automated mode. Fig.6 shows the room response of the above subwoofer system integrated with the main front speakers.
The room EQ was run repeatedly in automatic mode. It generated the resulting response shown in purple color. It could not determine the optimum delay and level setting for the subwoofers. Apparently it needed some intelligent help. After manual level and delay adjustments for the subwoofers the final room response curve is shown in green. These are remarkable results. This demonstrates that any room EQ system, even the most sophisticated, should be used carefully with understanding of what it is doing and what you would like to achieve. Do not rely blindly on it and always measure RTA room response for verification.
Good luck and successful projects, with great bass to all!