Subwoofer Low Pass Cutoff Question - General Theory

[RoboKlipsch]

To better understand much of what was written above, let me ask a basic question about sound mixing in a movie studio.

 

The sound engineer has the ability, I would assume, to send the full frequency range to any of the channels being used in the recording.  Chris you refer to this as being the way movies were recorded back before the implementation of the subwoofer concept in a commercial theater.  So back then, were they only recording 2 channels for movies, or 3?

 

I don't know the history of theater sound development, but am guessing that at some point, it was discovered how immersive it could be to have a channel to the sides of or behind the listener for fuller envelopment.   Since a listener is almost for sure NOT sitting in the center of the theater, a center channel "anchors" the sound to the center of the screen, even if sitting to the side.   For both the theaters and home theaters, the concept then of trying to match the bass to each channel (i.e. have full range speakers) became to costly and/or size prohibitive, and so subwoofers were implemented as a cost/size solution for theaters and the home.

 

Because there is no "universal" crossover point for lower frequencies, I guess a sound engineer has no choice but to send the full signal to each channel and then let bass management handle it being crossed over.  

 

If some/most of what I posted is on track, then why is there an LFE channel?  Why bother having that extra channel when, for example, a desire to have a low tone come from the "center" simply would be a tone that is equal across all channels, and handled by bass management?  

 

If I can understand these concepts,then my question turns to the 1/4 wavelength, or approx. 3.5 feet.  3.5 feet from the mains....or more than likely, from anywhere within the "circle" of the surround setup?  

 

As always, please forgive a lack of proper terminology or comprehension.  Correct away as my goal is to learn.

[RoboKlipsch]

I found that article by Dr. Earl Geddes. It's worth a read:

 

Setting Up Multiple Subs

 

and another of his articles:

 

Why Multiple Subs?

It always makes me a little uncomfortable when one of the authorities on a subject says something seemingly controversial, such as that correcting the frequency response of the fronts is a bad idea!  I'm not saying he is wrong, as I have no authority to say that, but it goes against much of what I hear elsewhere.  

[RoboKlipsch]

For me, as  someone who has been interested in these topics seriously for less than 1 year, I have found this study to be the most enlightening for me:

https://www.harman.com/sites/default/files/white-paper/12/11/2015%20-%2006%3A12/files/multsubs.pdf

[Chris A]

The sound engineer has the ability, I would assume, to send the full frequency range to any of the channels being used in the recording. Chris you refer to this as being the way movies were recorded back before the implementation of the subwoofer concept in a commercial theater. So back then, were they only recording 2 channels for movies, or 3?

 

Actually the answer is "all over the map".  There were many mono recordings that persisted even into the 1980s, while there were multichannel recordings as early as 1935 (Fantasound Mk. 1, etc.), there also were 7-track (1952) and 4-track (1953, CinemaScope), and 6-track (1950s, Todd AO) soundtracks.  These persisted until the THX standards came into acceptance in 1983.  Much more information on this subject is easily available on Wikipedia.

 

 

...I am guessing that at some point, it was discovered how immersive it could be to have a channel to the sides of or behind the listener for fuller envelopment. Since a listener is almost for sure NOT sitting in the center of the theater, a center channel "anchors" the sound to the center of the screen, even if sitting to the side. For both the theaters and home theaters, the concept then of trying to match the bass to each channel (i.e. have full range speakers) became to costly and/or size prohibitive, and so subwoofers were implemented as a cost/size solution for theaters and the home.

 

THX and later additions to the standard addressed the basic psychophysics of multichannel immersive sound, and, most importantly, provided a standard by which all studios and movie theaters (and later, home theaters) could single-up on a preferred implementation.

 

 

If some/most of what I posted is on track, then why is there an LFE channel? Why bother having that extra channel when, for example, a desire to have a low tone come from the "center" simply would be a tone that is equal across all channels, and handled by bass management?

 

LFE was an "applique" channel that I believe originated with the Earthquake film, and continued due to the film industry's reliance on optical sound tracks.  It is a legacy system that today is really OBE in my considered opinion.

 

Chris

Edited June 15, 2016 by Chris A

[Chris A]

If I can understand these concepts,then my question turns to the 1/4 wavelength, or approx. 3.5 feet. 3.5 feet from the mains....or more than likely, from anywhere within the "circle" of the surround setup?

 

I think that my point is to minimize the lateral distance between your subwoofer and L/C/R channel loudspeakers if you are using an 80 Hz crossover point or higher.  If you're using something lower, like 40 Hz, you can get away with having the sub split out further away from the L/C/R channels.

 

Some soundtracks are crossing to a single mono bass channel at 80 Hz (sort of like mono bass found on phonograph records), so crossing to your mono sub at a lower frequency gains you little but spreading out the mono bass channel load to multiple loudspeakers--like the excellent Harman Kardon white paper on using multiple subwoofers that you referenced.

 

The key here is to pay attention to the crossover frequencies inherent in the soundtracks, and use as much multichannel/stereo information as possible, thus delaying conversion down to mono bass as far as you can in frequency.  Your AVR/AVP basically does this for you if you select a low crossover frequency to your subs.  Your choice of selection of "small" (i.e., hard crossing to the sub channel) or "large" (parallel the bass information to the surround channels and to the sub at the same time) is basically a room acoustics and subwoofer capability issue, and also where you set your subwoofer-surround channel crossover point(s).

 

Discussion on how this all affects the surround (i.e., "surround front" and "surround back") and height channels is probably beyond the scope of this thread, but note that most surround sound multichannel recordings use the surrounds as "echo channels" and not as full-fledged music channels, equal in content to the L/C/R channels. There are a few recordings using this approach--including "Gordon Goodwin's Big Phat Band" DVD-As, etc., which use each channel separately to form true "surround sound" images.

 

Chris

Edited June 15, 2016 by Chris A

[RoboKlipsch]

Thank you Chris and the others in the dicussion for sharing.  Please feel free to continue on discussing as there's a lot here.

 

I had originally thought to post that maybe, the engineers build a soundtrack with a crossover in mind, but then decided not to suggest it because it seemed to me to presumptive.  Even though I considered that, I had not considered what you said, which is if you cross lower, you are in essence using all the speakers then to create those frequencies between the soundtrack's crossover to mono and the sub's actual crossover.  Brillant stuff if you ask me.

 

My mains and back surounds are spec'd down to 30s/40s, but my center and surrounds are in the 60s so 80 has seemed to be best theoretically.

I do have the ability to cross them separately but don't have enough to knowledge to know if it would be better than a global crossover.

[mustang guy]

 

I found that article by Dr. Earl Geddes. It's worth a read:

 

Setting Up Multiple Subs

 

and another of his articles:

 

Why Multiple Subs?

It always makes me a little uncomfortable when one of the authorities on a subject says something seemingly controversial, such as that correcting the frequency response of the fronts is a bad idea!  I'm not saying he is wrong, as I have no authority to say that, but it goes against much of what I hear elsewhere.  

 

I get what you mean. I am dying for somebody here to actually try this. I would, but I don't have a place to.

 

I went onto DIY and have read a bunch of Earl's posts. He's a smart guy, but he has difficulty with people who don't roll over and believe him. I get the feeling PWK might have a thing or two to say to him if he were still alive. 

 

An interesting point of note is that in their current website, they don't show either of these documents. Perhaps there is a story to that... Still, it would be a cool experiment if you have a pair of subs in an HT room. 

[derrickdj1]

I get what you mean. I am dying for somebody here to actually try this. I would, but I don't have a place to.

 

Pioneer's MCACC has a setting for not correcting the Mains,  It also will adjust all channels and symmetry as it's other two settings done in the calibration process.  Not much difference between the 3 settings in playing with these setting over 5 years.

 

Rob, over the past 10-15 years, home speakers have become smaller.  This trend started after the adoption of subwoofers for the home.  One commonly misunderstood notion is the .1 or LFE channel is the subwoofer channel.  The subwoofer is a playback device, plain and simple.  What does it play, bass and LFE.  The LFE is a production channel for sound engineers and is 120 Hz and down.  THX requires 105 db from the speaker and 115 db from the subwoofer due to the LFE.  Add redirected bass and now the sub may be called upon to reproduce up to 121 db.

 

Most systems can't do 121 db cleanly and most movies don't tax a system that much.  I can't think of one heavy hitter movie that has peaked to 121 db.  Has you are starting to see, the subwoofer is a very important piece in the HT and for 2 ch. systems.  Today, for most people it is much easier and more acceptable to use smaller speakers and sub than big huge speakers in the home or apt.  Now, when it comes to this forum, we like the best of both worlds, Big Speakers and Big subs, lol.

Edited June 16, 2016 by derrickdj1