Adventures With Subwoofers

I have had consistent success in adding a sub (first the SVS SB1000, then a JLAudio e110) to my home office’s desktop system. There are 2 keys to success, in my experience:

1 - I use an external crossover with 24 dB/octave high- & low-pass filter slopes.

2 - I can run the speakers full range if I wish, but I find it’s absolutely critical to use a high-pass filter to cut off frequencies below the point where the sub will operate. Failure to do this creates a muddy, indistinct and bloated sound in the upper bass and lower midrange.

3 - Finally, I use sealed/acoustic suspension mains and sub. I find this design gives not only the most predictable, easy-to-manage bass overall, but also give bass that is qualitatively better than ported designs. This is ideal for music-only listenings. I might feel differently if I was using the sub in a home theater, but for music appreciation, sealed/acoustic suspension mains & sub give me the best results.

Just get an SVS sub and enjoy your system. Also try running your mains full range to keep the electronic xover distortion out of them.

Feim my Adventures with Subwoofers, here are some things that seem critical to solving integration.

Firstly, low end highlights smearing most. If your source is allowing smearing, focus on that otherwise the rest will be a battle.  For example, noise in you ethernet system is, in my experience, a damaging source of noise that causes smearing. Also subs power cables, interconnects and fuses all contribute.

Next, you need to be able to place subeoofers according to room acoustics, which means you need to be able to delay the main speaker output.  Adjusting phase might sound similar, but it's a convenient short cut.

Delay needs to account for not only distance but also the latency of the subwoofer - it's filters, amp, acoustic design, etc.  This is not to say subs are slow,just a tad late.  12ms is alot later than 4ms.  It's enough to cause smearing.

It's best to control the slopes of both the subs and the mains, so both HP and LP filters are needed.  The volume level of the sub can ruin the crossover blending 

24dB slope better than 12dB because it's over less frequency range.

Choose a crossover frequency that allows best tone,which may be influences by placement of the sub/s and their interaction with the room.

All this dialing in should be the foundation, the neutral/correct, for then choosing what you prefer.  EQ can help tune to suit preference if you prefer a less neutral ... suits you hearing (eg. change with age).

I hope this is helpful.

Dear jfrmusic, this needs to be approached scientifically if you hope to achieve proper integration of subs into your system. You have some really nice kit and it's worth doing what is necessary to hear what it's capable of.

The only way to do this without endless experimentation is to embrace measurement so please download REW (free) and buy a suitable mic. for $100 which will remove the guesswork. There has been some good advice here in certain repects but most of it is all over the place and confusing. Best is to go about things methodically.

Any sub like REL and its ilk are unsuitable for 1 reason: they do not have variable phase. It's also easier and better to use sealed subs which your D108 is and has a natural 12dB/octave roll-off so plug the ports on your speakers which force them to now behave as a sealed box (a good thing)  which decays at 12dB/octave and so match the D108.  I suggest at least 2 subs  which is neccessary to truly smooth out the performance of speakers, extra bass sources (subs) and the huge influence of your room's acoustics. This brings into the picture the importance of plugging the Harbeth's ports. You have effectively removed 2 additional bass sources that resonate at only 1 specific frequency which by definition can not be tuned and will complicate set up. Point: AI has misled you claiming a 12dB slope. Your REL  has a passive radiator which rolls off slightly steeper than a ported design which is 24dB/octve.

Your D108 partnered with a sealed sub like the great value SVS SB 1000 PRO with numerous controls will allow you to achieve the industry standard of +/- 3dB across the spectrum. Without the ability to see the frequency response continually change as you adjust the controls you are left guessing. REW will also display if you have sufficient absorption or perhaps too much. As I mentioned it's a scientific approach which has been well documented. It works. 

A further bonus is the ability to place the subs where you would like them to be which is a function of continuously variable phase. Ignore the advice in posts above to place them behind or in front or whatever. I do recommend starting with 1 sub in a corner, any corner, and the other in the middle of the opposite wall. 

I state with confidence that you will not have a smeared image nor loss of air just the opposite. By using measurement and subs and some absorption you will have effectively removed the damaging effects of your room's acoustics and will hear the acoustics of the event. The audiophools holy grail is it not? 

For years I have put my Subs on an Aurelex Sub Dude isolation platform.   Cheapest way to eliminate boomy bass.   That and a low X over that does not excite room modes.