My Home Theater: Equalizing Subs – Four at a Time

PB12-NSD Sub                                                            SVS SB2000 Sub

I finally got the room to sound as I wanted it to, or I thought I had, until immersive sound came along. There is no rest for the wicked!

This is the last post in this series of building my A/V Room. Or is it? I still have a 4K upgrade to do this year and maybe, immersive sound.

This post will explain, in some detail, the entire process of getting my room to perform as I wanted it to, once I had added the two additional subs. The entire process is quite lengthy, and a little technical, as there is no way around not going into some of the finer acoustic issues that raised their heads.

The majority of this post refers to how to equalize multiple dissimilar subs, with a few brief references to equalizing the satellites. This is because most commercial EQ systems do a good job of equalizing satellites, but do not seem to handle multiple subs too well, and certainly don’t handle a mix of different subs well at all.

So I will provide a quick introduction into the requirements of installing subs before delving into the ‘meat and potatoes’ of this post.

Sub Installation

Equalizing subs in a room – four at a time – can be very challenging. Aligning multiple subs in order to get their best combined frequency response in a room is always a little tricky. The process becomes even more complicated if the subs are not symmetrical about the main listening position (MLP), if they are different types such as sealed and vented, and/or different sizes.

Remember that what you are ultimately trying to do is get a reasonably flat LF response, the fastest room decay and a smooth transition from the subs to the satellites over the crossover region. All of this not just at the MLP, but for all the other seats in the listening/viewing area. Being a little selfish on this point, I always optimize the LF response for the MLP where I always sit. Remember that it is NOT all about a flat frequency response. The decay time, among other issues,  also becomes very important especially in the lower octaves where it becomes increasingly difficult to control decay through absorption alone.

Preferred Sub Locations

There are preferred locations for all quantities of subs, be it one, two or more. Ideally you should use an even number and they should all be the same make and model – as you will read in the attached PDF, mixing subs can turn into a nightmare. There are several issues that need to be resolved if the subs are not only to be optimally matched to each other and the room, but they also have to be matched to all the satellite speakers over the crossover region.

In small rooms trying to control bass through absorption alone becomes extremely difficult due to the available space. The addition of multiple subs together with Sound Field Mangement (SFM) is often a more effcetive way of achieving the desired LF response.   Placement of the sub(s) and the methods and techniques used to equalize them can overcome room absorption problems.  There are several articles and good books on these topics by Dr. Floyd E. Toole, Vice President Acoustical Engineering, Harman International Industries, Inc. – Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms and Harman International.

The optimal locations for one, two or four subs are as follows:

• One sub in any corner. The room layout may provide better performance for one particular corner.
• Two subs may be placed, in order of performance:
  • In the middle of the two side walls.
  • In the middle of the front and rear walls.
  • One third of the rooms width in along the rear or front walls.
• Four subs may be placed, in order of performance:
  • In the middle of each wall.
  • One third of the rooms width in from the side walls along the front and rear walls
  • In each corner.

The acoustical reasons for these placements are to do with cancelling out, energizing or not, room modes (standing waves). For those that want additional information see this document from Harman.

With a single sub it is possible to optimize its location relative to the MLP by using the crawl method. This is where you place the sub at the MLP, play some bass heavy material, and crawl around the room until you find a location where the bass is the most even. You then place your single sub at that location. Unfortunately this location is not always the most convenient and our ‘other half’ will often let us know so.

Acoustic Measurements

All of what follows was measured using Room Acoustic Wizard (REW), a calibrated Dayton EMM-6 microphone from Cross Spectrum Labs , a dual channel USB mic amp from ART Pro Audio and a MacPro Laptop. There are other room equalization packages (hardware and software) besides Audyssey, that according to their users do as good as, if not a far better job, of equalizing speakers. One such popular system is made by miniDSP, who also support and can provide the DIRAC room equalization software. See the AVS Forum here for further details on this hardware and software.  Also Anthem ARC (Anthem Room Correction) is another well established room equalization system.

As Audyssey did not seem to handle my dissimilar sub configuration I ultimately returned to using the Velodyne SMS-1 for pre-sub equalization as it was supported by, and used the data provide by REW.  Even though this equalizer is now discontinued there are several others that are supported by REW and that can be interfaced to, in order to download the room equalization data directly into the hardware.

Technical & Acoustical Issues that need to be Resolved

Without getting too technical there are several issues that need to be examined when aligning more than one sub:

1. Each sub must have the same acoustical distance to the MLP. This is achieved by physical placement or by adding electronic delays to the closer subs. I used a Rane AD22d for this purpose.
2. Ideally all subs should be capable of creating similar SPL’s for the same input and/or handle the same power.
3. Ideally all subs should have the SAME bass extension. One MUST NOT roll off before the other. (Or Audyssey will use the lesser of the two subs)
4. Ideally all subs should have the same frequency response at least an octave or two above the selected crossover.
5. Ideally all subs should have the same or very similar group delay characteristics.
6. Ideally all satellite speakers should have similar group delay characteristics over the crossover region.

Just to recap. What is Group Delay I hear you say? Its the time taken for an electrical signal feeding a speaker  to be converted to sound, and it changes at different frequencies. In other words it takes longer for a low frequency sound wave to leave the enclosure than it does for a higher frequency one. This is due to the speakers design and associated built in electronic or passive speaker equalization and/or crossover and filtering , see the following graph.

Group Delay curve for the combined response of my Four Subs

The above graph clearly shows that the time taken for the electrical energy to be converted to acoustical energy begins to increase below about 40Hz. Remember that group delays that are less than 1.5 times the period of the frequency do not generally impact what is heard, and are not generally detectable by the ear. e.g. The delay should not exceed 75mS at a frequency of 20Hz.  Delays less than 1x the period of the frequency can never be heard. The above graph shows a 75mS delay at 20Hz, just on the limit. Overall the above group delay rarely exceeds 1.5x the period of any sub frequency below 40Hz.

The room and AV receivers equalization and crossovers can also add significantly to this effect and all need to be accounted for when the speaker system is correctly timed and equalized to the room.

Mixing subs with different group delays, together with satellite pairs having different group delays, will result in a combined group delay response that is often problematic to equalize. You now have sounds arriving at the MLP from different directions and distances based upon the speakers group delays which change at different frequencies! Theoretically, as all these changes are ‘fixed’, the resulting SPL at the MLP should be able to be effectively equalized. Not so. Fixing one issue with EQ often ends up affecting another. Incorrectly applied EQ, even it it creates a flat frequency response, can mess up the time/impulse response, thereby having an adverse affect in the rooms decay at lower frequencies. Remember that filters have delay properties too!

What the above tells us is that all subs should be the same make and model, mixing them will ALWAYS result in difficulties in obtaining the optimum frequency response over the subs response AND over the crossover region to the satellite speakers. Inorrectly equalizing this problem also impacts the decay time for the room, as I well found out.

Unfortunately I did not stick to this dogma, even though I was clearly told by Ed Mullen, the CTO of SVS , that “mixing different models of subs would cause me grief’……and it did, so much grief! Ed was very understanding of my resulting problems and even gave me an additional month to keep the subs, in order to get things right, which obviously, I eventually did.

My room could not support two more subs at the rear that were the same size and model as those at the front. So why did I want more you ask? I like bass, lots of it, and a really deep clean TIGHT LF extension, so I wanted to improve what I had and have MORE! Also I hoped that the addition of another pair of subs would ultimately improve the bass decays as the addition of any more absorption was just not possible, That meant, because of my room design, adding two more physically smaller subs in the rooms rear corners. The pair I selected would provide another octave response below my front subs and handled the same power. However, they where sealed models, my front two being ported; and so started my journey into sub equalization hell!

My Equalization Journey Down a Very Dark Path

The two pairs of subs that were equalized are both made by SVS – the PB12-NSD and SB2000

SVS PB12-NSD Sub                                                            SVS SB2000 Sub

Below, in pdf format, is my article about the trials and tribulations of aligning dissimilar types of subs and aligning those subs with the satellite speakers over the crossover region. The document is a little technical and not for the ‘faint of heart’ or inexperienced. I am happy to answer any questions that you may have after reading it.

This was a labor of love! Get a cup of tea or coffee and settle yourself down to a bumpy journey.

Click on the following link to download the pdf: Sub Sonics V2P

This pdf, Sub-Sonics V2P, is based on my work in my room. If you wish to use any part of it I would appreciate a link back to this page and an acknowledgment.