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Discussion Starter #1
Does the use of different subs in the same system cause intermodulation distortion?

For example, if I have 4 ported TC Sounds 18's for subs and I have a ported UM18 as the bottom of a full range 3 way main speaker with no high pass filter, running them full range, does that introduce IM distortion?
 

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If the subwoofers were placed outside, and 30ft apart and/or with 30ft of delay... then probably yes.

In a room, with a 120hz LPF, and a similar tuning, with correct delay and phase, and similar amps. I wouldn't expect there to be an audible difference (beyond room modes), although you might be able to detect it with a measurement mic... if you tried hard enough.

Why, are you having problems?
 

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Does the use of different subs in the same system cause intermodulation distortion?
No, because IMD is caused by the mixing of different frequencies. Using different subs wouldn't cause that, as they're all playing the same frequencies.
 

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I do something similar with sealed UM-15's and sealed LMS-18's. I haven't encountered any problems.

That said, I run my subwoofers stereophonically, all the subs on the left-side of the room get the left channel, and the right-side the right channel.
When there is music with stereophonic bass, the subs in my mains and the dedicated sub boxes are getting nearly identical signals.
(Not a mono-summed downmix + stereo mains bass.)
Music with heavy amounts of stereophonic bass is pretty rare though, so I wouldn't expect any problems with 95% of music for mono-bass and mono-stereo-confused systems.

Technically there should be no difference between subs in your mains and dedicated sub boxes placed in the same location (so as long as it is fed stereo bass... or all 3+ are fed mono-bass.)

In movie-mode I route the LFE to the dedicated subs, and sometimes if the movie has weak mains-bass I'll route the LFE to the subs in my mains too.
Usually this isn't a problem for the subs in my center channel, since that track in most movies is usually loaded with good bass as-is.
Why there are movies with such weak mains-bass in this day and age is beyond me... is what it is though!

I'm 90% music, so I'm usually in stereo-bass mode. But when I do movies I want the LFE too, so I paid extra to have the needed cables and processing to make that happen.
99.9999% of people just run mono-bass and call it a day.
I stopped using summed-mono bass back in ~2004, and I went tri-channel bass in 2009. I will NEVER to back to mono-bass, you can't make me! (well... not unless you pay me enough to retire early! ;))
 

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Discussion Starter #5
If the subwoofers were placed outside, and 30ft apart and/or with 30ft of delay... then probably yes.

In a room, with a 120hz LPF, and a similar tuning, with correct delay and phase, and similar amps. I wouldn't expect there to be an audible difference (beyond room modes), although you might be able to detect it with a measurement mic... if you tried hard enough.

Why, are you having problems?
Thanks for all the responses.

I am building a stereo pair for a small living room system that may later use subs. The UM18 will run to 120hz as you say but the subs would not. They will cross much lower. For now the system looks to be as follows but with no subs. 100% music:

Woofer in 8 cubic feet tuned to 18hz and crossed @ 120hz: UM18
Midrange: AE TD10M running 120hz sealed to 1000hz (because I have it already)
Highs SEOS 15/BMS 4550 (BMS I have already)

The equipment in place already includes a DCX2496 with 6 channels of amplification so the system will be actively crossed.

I thought about running the SEOS 18 with the BMS because it fits the enclosure. I'm not sure if there would be much of a benefit over the SEOS 15 with the TD10M.
 
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