How Is Bass Divided Among Speakers If No Sub?

That headroom mentioned by Drew_eckhardt helped out finding more with Google on my end. Some THX processor has some features that others don't. No wonder this can make someone frustrated. This link has a little more. [http://www.hometheaterhifi.com/technical-articles/163-the-misunderstood-01-lfe-channel-in-51-digital-surround-sound.html]

It depends.

Dolby suggests discarding the LFE channel when there is no sub-woofer for headroom reasons.

(The LFE channel has 10dB more headroom than the screen channels, so you're looking for reference level SPLs of up to 115dB SPL at the seats which becomes 120dB at the speakers accounting for propagation loss. With a 90dB efficient speaker that'll take a 1000W amplifier and more displacement than most speakers are capable of).

Mitch, I would check what your processor does. THX may even change whether,or how much bass goes to your surrounds. My processor says full bass to the rears, but without some Thx setting, the way I read it. Maybe that's part of the reason for different info being given when full size is selected.

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My room is big enough to have large full range speakers as a center and surround channels without crowding the seating area. Power would not be an issue to the surround speakers..they will have plenty of power.
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Obviously, only the channel(s) getting the bass will produce it if all speakers are set to large. Although bass is generally non-directional, so regardless of which speaker it comes from it will likely sound as if it's coming from everywhere.

The downside to setting all speakers to large is that bass frequencies really suck a lot of amp power, so typically the mid-band will suffer. That's why most people use a sub, it lessens the load on the amp. I expect also that most low bass sound effects are mixed to the main speakers anyway, so running centre and rears on large doesn't make a lot of sense. Very few centre/rear speakers can handle deep bass.

If a 5.1 system has identical speakers all around with 15 inch woofers and all of the speakers are set to "large" on the processor, does the serious bass get evenly distributed to all speakers? Say, like an explosion or a dinosaur stomping?

My internet is giving problems, so if I don't post, that's the reason. With Dolby Digital, I would think that if you have the processor set to large for all five, only that one channel should get that bass, if they mixed it this way. More channels will, if they mixed it to do that. IOW, if an explosion goes off in the left rear, it should give all the bass if, it was mixed/made to do so, is my understanding. The channels are supposed to be totally isolated from each other. With the sub on, it will probably get bass from that one channel explosion, if they intended it to do so, during the making/mixing of the movie. Dolby Pro-Logic could not do a lot of this, like true separate channels, and it had limited frequency response to the mono rears.

Yes, your processor controls bass distribution. If you have all your speakers set to large, all get full range sound. If you have any of the centre or rears set to small, their low frequencies are sent to the mains. If you do not have bass processing on your amp/receiver, then all speakers work full range.

As mentioned it will depend on the processor you have. But when you set your processor up and select large fronts and small rears I would say most of it goes to the fronts.

Another Dolby link.[http://www.dolby.com/uploadedFiles/Assets/US/Doc/Professional/42_DDFAQ.pdf]

I've heard that it can go to all five, and sometimes it is directed to the fronts. Size settings may cause this variation from Dolby, I'm guessing. IF you read different info on the net, you can see that Dolby varies on it. It may depend of your processor. The processor manual may give the best info. [http://www.dolby.com/uploadedFiles/zz-_Shared_Assets/English_PDFs/Professional/L.mn.0002.5.1guide.pdf] [http://www.dolby.com/uploadedFiles/zz-_Shared_Assets/English_PDFs/Professional/38_LFE.pdf]