Method for balancing a 4 Subwoofer configuration via LFE on an AV Receiver

Peta · January 9, 2013

As the tag says ... this might go under s section called esoterica ... but I thought I might share

The problem

AV receiver that has four outputs for LFE (subwoofers) and one control that sets the output for all of them. How then to balance a mixed set of subwoofers that are distributed across the room for "best" performance?

One sub is a 15" passive infinite baffle monster (Aslan design) with its own 500W monoblock amp. The other three are self powered with about 200W amps built in. A Krix Volcanix 10" is a bass reflex and the other two are VAF SW4 models also bass reflex. The subs (apart from the Krix) are via SNA classifieds. The AV Receiver/preamp is a Marantz AV7005 and it has 2xRCA outputs for LFE and 2xXLR as well. Belcanto is the monoblock amp (500W is correct because the sub it is driving is mostly 4 Ohm).

Why such a crazy configuration? Partly an experiment to see if "perfect bass" can be obtained with minimal nodes in the room which is largeish at 8m x 5m x 4m (sloping cathedral like ceiling). Room "treatments include book cases (8 sq m), carpeting in the floor reflection zone, acoustic foam built into picture frames and the irregular shape of the room.

Method

First task is to set them all up with the wiring etc correct so that they all turn on and sound is coming out of them all. Set phase adjustments to 0 degrees. Choose a configuration where the subs are in an array at the front inside the main speakers or distribute them around the room as symetrically as possible using corners or middle of walls. I set mine up as an array at the front. Set the LFE band pass frequency to 80 Hz and set the active subs to maximum bandwidth (the LFE bass management on the AV preamp acts as the crossover and removing further crossover interaction is a good idea).

With a sound pressure meter and using the inbuilt tone generator for LFE turn off all but the passive sub at the power and measure the SPL at around 80dB at 1m from the speaker cone, adjusting the volume control to achieve this. Measure each of the other subs in isolation (others turned off) and adjust the volume on them (they are active) to match the passive sub. Try to keep the distance accurate with a tape measure to get this right. Overall SPL in the middle of the room was 82 dB for me. In each of my expected listening positions the readings for my final configuration were 81, 80, 84, 82, 79. That seems a good result because I could not notice a difference between the locations when I moved around to them with the meter. In my first configurations there were differences of up to 12 dB. Hold the meter between you and the subs as far as possible - it can make -5dB difference by standing in the wrong place. Move around the meter a bit and take the maximum reading.

Now the hard bit is to make adjustments to the phase on the active subs one at a time to get maximum sound pressure with them all on at your preferred listening position. That was the easy bit. Now move around the room and see if there are any noticeable anomalies and try to correct them with repositioning of the subs. Small changes can make a distinct difference. Adjust the phase on moved subs to again get the loudness to a maximum at your preferred listening position. It took half a day of experimentation to get this approximately right! If you are keen you can check SPL at multiple parts of the room - ignore locations where nobody will be listening because they don't matter.

You now have the subs balanced against each other and they will work together with a master control for LFE to bring the overall bass balance to where you need it.

That done, run the (Audessy in mine) setup built into the AV preamp and use that as the starting point to get the channel balance right. For me this is a 5.4 configuration but controlled as a 5.1 at the preamp. I used five points in the room where I expect people to listen to music or films and these are located around the middle to 2/3 towards the rear of the room.

Now do some listening tests to determine the best balance for the bass from several sources with a broad range of mid and low bass. I find that Goyte, Gorillaz and the Bach 1st Cello Concerto are a good reference set. I adjusted the LFE/Subwoofer volume up and down in 3dB increments at first and then narrowed it to a final adjustment of 1dB for no rational reason but it seemed right. It was 4dB less than the Audessy had chosen for me. Movie content like explosions matter mostly from a volume handling and overload perspective and I did not use this content to check the sound quality - see conclusion.

Now comes the harder part. Trying for the perfect balance. I moved the subs around to corners and and swapped sides then put one either side of my main speakers, repeating the above. Each time I thought the changes were sounding better and perhaps they were. I kept a record of the settings for each of the locations I thought sounded best to date. There were a lot of locations that sounded bad or had very uneven sound levels at the five listening locations. I think the most even and pleasing sound (incidentally the most efficient as well by 3dB) was with the subs in all four corners. For practical reasons I did not use that configuration.

Final configuration

Having gone through all that I made some choices. Firstly, I could not live with the speakers in the corners. The stereo image from my main speakers suffered at my "listening chair" and that is more important than tweaking the bass. Secondly, running long cables to the subs across a slate floor is not a good thing - putting 20m of cable into a wall to get them out of th eway seems like too much trouble for the small benefit. Finally, the compromises in living space were not so good. I went with a "balanced configuration of the active subs on the left and the passive on the right.

The active subs are rated at about 200W each and have a cutout protection for overload. The 15" passive is rated at 1000W and has 500W of amplification. I reduced each of the active subs by 1dB and increased the passive by 3dB. I then checked the balance and overall sound impression - I was still happy with it. Probably because I just made a change, I thought it sounded a little better overall. This is how I have left it. A bonus is that I can manage the cables and power much more easily with a noticeable reduction in hum on very quiet pieces compared to having long line level cables (admittedly these were not the highest quality and out of spec for RCA connections anyway).

Conclusion

Seems like a science report hey?

The method described above uses a rigorous approach to measuring sound pressure as its starting point for quickly determining a good location for the speakers and getting into the subjective part with some confidence that things are at least stable and without major nodes in places you do not want them.

So what does it sound like? Tight, powerful and very dynamic are the best words I can use. Almost anywhere in the room you get very good sound. The windows shake and books can dislodge form the bookshelves. Brickwork can be felt to vibrate in adjacent rooms. At volumes which are dangerous for more than a couple of seconds, the sound is intense. Movie sound effects are startling. Quiet passages have the subtlety one hopes for with almost undetectable hum. My noisy neighbours moved! Mission accomplished.