27. Subwoofer Placement
05/10/2022
Author: Karsten Hein
Category: High Fidelity
Subwoofers are dedicated loudspeakers designed to support the main speakers of a Hi-Fi system in playing back the frequencies found at the lower end of the spectrum. When it comes to the placing of subwoofers, it is important to keep in mind that they do react similarly to larger loudspeakers so that many of the principles relevant in placing them remain the same. I have sometimes read in dedicated forums and on specialist sites that subwoofers allow for some liberty in placement, as the source of low frequencies is more difficult to locate for the human ear. However, this only seems to apply to a smaller extent and to very specific applications, such as the LFE (Low Frequency Effects) channel found on some home cinema systems.
The idea that a subwoofer’s position should not matter much stems from the correct observation that subwoofers mostly cut in somewhere between 100 Hz and 50 Hz, depending on the cut-off frequency of the main speakers, where the sound waves are already considerably long, while our ears are only about 17.5cm apart to detect the subtle differences in timing. And while this does make some sense, I wanted to learn more deeply about the exact wave lengths and started doing some research. Here is a table of some wave lengths that are typically associated with subwoofers. They are calculated at room temperature at an estimated speed of sound of 343.2 meters per second. Changes in ambient temperature, air pressure, and moisture will effect the speed of sound and can therefore lead to slightly different results:
Subwoofer Frequencies & Wave Lengths
- 35 Hertz (Hz) = 9.8 meters (m)
- 50 Hertz (Hz) = 6.9 meters (m)
- 60 Hertz (Hz) = 5,7 meters (m)
- 80 Hertz (Hz) = 4,3 meters (m)
- 100 Hertz (Hz) = 3,4 meters (m)
- 150 Hertz (Hz) = 2,3 meters (m)
The table shows that the relevant sound waves for subwoofers are between two and ten meters long. One could therefore come to the conclusion that these are long enough for the subwoofer location to become negligible. And yet, in real-life listening, I found that the source of the pressure often remains perceptible, especially on front-firing subs. While placement may not matter so much for subwoofers supporting movie sound, audiophile two-channel setups do benefit from ultra-exact positioning and mindful integration with the other speakers in the room. Due to this circumstance, the necessity for a subwoofer to be added to a system should best be determined as the famous English comma rule suggests: If in doubt, leave it out.
In fact, I have not yet heard a single Hi-Fi stereo setup using a single subwoofer that worked smoothly across a whole range of song materials without either exaggerating or smothering certain aspects of the music or giving away the subwoofer location. The general idea is: If you can hear the subwoofer, if it draws the least bit of attention to itself, your job in placing and calibrating your setup is not yet done. What further hindered me from an early entry into the subject of subwoofers was my own scepticism regarding the use of DSPs (Digital Signal Processors). While I have not yet used a top-of-the-line DSP myself, I have had the privilege to listen to professionally calibrated DSPs in action on a number of occasions and found that, unless the listening space was the driver’s seat of a car, similarly good, if not better, results could have been obtained through a proper selection of components and a better placing of these on the rack and in the listening room.
In my own experiments regarding the placement of subwoofers, I therefore decided that I would work without DSP, equalisers, tone controls, and loudness to listen for differences in sound. The only tonal adjustments would be made through the built-in attenuators on the speakers. The subwoofer to be used for my tests was our actively powered ‘Dream 2 Sub’, made by the Chinese Dragon Audio, or DA, in 2012. I had originally bought this to accompany our Shure ‘701 Pro Master’ PA speakers, and this was also going to be the combination of my test. The Shures had been fitted with a treble attenuator to reign in the horns in smaller listening rooms, and the DA Dream 2 featured an input volume control, crossover frequency adjustment from 20 to 200 Hz (sadly, non-scaled and possibly also non-linear) as well as a 180° phase-correction switch. All other results had to be achieved by means of positioning the components alone. No active crossover was used in the process.
In the very beginning, while doing research on loudspeaker positioning, I had often come across the proportional value of one-fifth (1/5). In these articles and videos it was suggested that setting up the loudspeakers and listening position at approximately one fifths of the room’s depth was a worthwhile starting point to begin with the fine-tuning. It seems that, due to the variations of frequency lengths and the overlapping of multiple frequencies, there were less likely to be audible peeks or dips in the critical bass area. Usually, the closer the speakers and listening position were moved to the wall, the more bass frequencies would begin to dominate the music. On the other hand, moving the speakers and listening position further away from the wall would have the opposite effect. Peter Englisch did some excellent work on determining more exact loudspeaker and musical instrument placement for bands that I will present in a later report.
The side walls also played a role in the sound equation. Loudspeakers should not be positioned in close proximity of the side walls to lessen the amount of side-wall reflections. The distance to the side walls should also not be the same as the distance to the front wall. It is sometimes said that a 1/3 deviation would be a good place to start. In a rectangular room on which the front and side walls are of different lengths, you might be able to achieve good results with 1/5 distance from the side walls as starting point for your fine-tuning. If this is not the case, gradually moving your speakers closer toward the centre axis might do the trick. In my graphic illustration you can see that the room has been divided into five squares in each direction to indicate the approximate starting points of speaker positioning. The first components placed were the left and right loudspeakers (LL and LR).
The illustration also shows the couch positioned so that the listener’s head is approximately one fifth away from the back wall. The left and right loudspeaker (LL and LR) have been toed in at an angle of ten degrees (10°) towards the listening position. Since the Shure 701 Pro Master were horn-loaded speakers calibrated to a 60° radiation pattern, a small toe-in angle was a good starting point. I have indicated the driver’s position with a black line on that side of the speaker and pointed to the Shures’ horn effect with a broken black line. Given the circumstances in my real-world listening room, I ended up with a narrower placement of the speakers and also positioned them a bit closer to front wall. This was a compromise between using the space as a listening room as well as a living room for the whole family.
The 1/5-point does not apply to all speakers or rooms. The Snell A-Series, for instance, prefers to be placed in close proximity to room walls and also may not work so well when toed in. When positioning speakers we are mostly looking for the position where the speakers click in with the room. Given some time and experience, this position can be found. I did find, however, that the large diaphragm Shures benefited from the rule, and so did our Martin Logan SL3 and the Snell C-IV speakers tested here earlier. And although we are in the process of exploring the placement of subwoofers, the correct placement of our main speakers needs to happen first, because the two are connected. Unless you are setting your system up to enhance movie effects, where the loudest boom might still be your preference, loudspeakers and subwoofers need to form one acoustic unit.
In placing the subwoofer, I was looking for the most audiophile placement option, on the basis that the term ‘audiophile’ means balanced tonality, joy and agility, nuance and instrument separation, and all this embedded in believability and homogeneity. As we shall see, changing the subwoofer’s position had a profound effect on all these factors, often to the point of killing the merits of the two-channel setup altogether.
I began my explorations placing the subwoofer to the left side of the right loudspeaker (S/R). Since our Hi-Fi rack occupied the space in the centre of the front wall, I could not choose a more central position. A first listening test unpleasantly revealed the subwoofer’s position and smothered the sound. Toeing in the subwoofer slightly did not improve the situation much on its own, but when I started moving the subwoofer forwards and backwards, I could hear the resulting changes with each centimetre. When the sub finally locked in with the room, I was surprised that it was nearly flush with the main speakers. However, from the perspective of the listening position, it was even protruding forward.
Before making my drawing of the room, I would have accepted that placing the subwoofer flush with the loudspeakers might produce the best result, as this would result in the wave signal reaching the listener at the same time, but drawing the radius around the listener’s head quickly revealed that a similar listening distance would have required a more recessed subwoofer position. It seems that the error in my visual assessment had been caused by the low toe-in angle of the main speakers that had already proven itself for better stereo separation and tonality. Why then did the subwoofer sound in phase when being flush with the main speakers? It seems that this may have resulted from a phase delay caused by the subwoofer’s built in active crossover and that bringing the subwoofer closer to the listening position had made up for this delay.
When listening to music in flush position, I could hear that bass lines had an actual story to tell and that they were created by multiple instruments. There were many layered and contrasting bass lines instead of just one bass. I enjoyed this increase of tonal nuance and musical insight, but I could still feel the sub’s pressure and force coming from the right, which made me feel dissatisfied and restless. This may have stemmed from the Shure speakers being fed the complete signal and naturally fading out where their role as PA tops demanded instead of being cut off by the active crossover as well. But since I enjoyed the tonality and musicality of the setup, I simply would have preferred to see one such subwoofer underneath each Shure speaker, perfectly phase aligned with the main speaker drivers to balance the image and maintain bass nuance. If I had had a second subwoofer at hand, I think this would have been my favourite position.
As the main door to our real-life listening room was located on the left-hand side of the front wall, our HiFi setup had to leave the entry way clear and was centred towards the right wall. Due to this small imbalance I could not help but wonder if placing the subwoofer to the right side of the left speaker (S/L) might provide better room integration, but I found that this only led to dominant bass pressure on the left. Perhaps placing our single subwoofer right in front of us might have worked better, but if you consider the radius around the listening position, you will see that this leaves no room for a rear-ported subwoofer. In my further attempts, corner placement (S/C) produced a slow, non-musical, and bass-heavy sound. Looking at the radius around the listening position, and considering the length of the diagonal crossing, this was perhaps not too surprising. The overly boomy sound in this position was surely increased by the DA Dream 2 being rear-ported.
Closed cabinet down-firing subwoofers perhaps would have offered some advantages in placement flexibility, however, I once owned a closed cabinet down-firing Canton Karat-series subwoofer that somehow sounded misplaced wherever it was. I have heard it said that two down-firing subs placed near the sitting position could work well with movies, and this might be true for systems using DSPs, but for classic 2-channel setups without DSP such delays and phase shifts should prove problematic. In the placement of our front-firing rear-ported DA sub, I finally decided that a 90° angle position behind either of the main speakers worked best. In this arrangement, the subwoofer’s position was no longer obvious. I also set the bass volume to a level that would make the sub untraceable until it was turned off, when the bass notes went sorely missing. With the 90° angle, I sadly lost the exactness I had experiences with the front-firing subwoofer. Bass nuance and layering were far less pronounced and bass punch, too, could have been better.
I spoke to friends in the industry about my findings, and they suggested that rear-ported subwoofers were difficult to place, because they had a built-in delay of the rear-projected bass bouncing off the walls. They would have preferred a front-ported design. They also pointed to the mismatch between the single 30cm woofer adding the bass lines to two 38cm midrange drivers. This match would lead to unintended differences in running time, because the subwoofer would need to compensate with a much higher amplitude. They also pointed out that running both the tops and the subwoofer through the sub’s active crossover would have resulted in a cleaner crossover point. Both friends agreed that using two subwoofers of adequate size and configuration would have been beneficial.
In summary, I can report that subwoofers are loudspeakers and should be treated as such during placement. When working with full-spectrum main speakers and the subwoofer only cuts in below 40 Hz, one might not need to worry about exact placement so much, although the system would still benefit. But when paired with speakers such as the Shure 701 Pro Master that need an extra woofer to take over from 50-60 Hz, exact placement of the subwoofer will make a noticeable difference. Good results can more easily be accomplished using two subwoofers instead of one. This is especially true, for those looking for an audiophile stereo listening experience with well-timed and proportionate musicality.
Parameters for Consideration
- room width, depth, and height
- room furnishings and wall treatment
- setup distances and radiation axis
- phase and time coherence
- subwoofer ports and vents
- low cut and crossover frequencies
- diaphragm size and mass coherence
