Phase Cancellation at Crossover Frequency - AVS Forum
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post #1 of 45 Old 02-10-2014, 09:55 AM - Thread Starter
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Please help.

The setup: 3.2 ht/audio system with L/C/R speakers and two sealed subwoofers (no presence or surrounds). The L/C/R speakers match each other, and the subs match each other. The auto-calibration program sets all speakers to "large" and confirmed that all speakers are in phase. I visually inspected and tested each speaker to verify their positive polarity. The mains are connected to a two-channel power amp; the center via the AVR's center channel port; and the subs via the AVR's dual mono LFE ports.

The problem: When the mains are set to "large," and the subwoofer phase in the AVR is set to normal, the SPL is great! However, when the mains are set to "small," the room loses a lot of SPL unless the subwoofer phase in the AVR is reversed – which then improves the SPL but not to the extent of the original large/normal setting. Also, at the small/reverse setting, the SPL gets worse as the crossover in the AVR is raised. At the large/normal setting, the SPL improves at the higher crossover setting.

The question: I want to set the mains to "small," as recommended on this and other forums, but the system seems to fight itself at this setting. What might be the problem? Thank you for your consideration.

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post #2 of 45 Old 02-10-2014, 11:27 AM
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Assuming you do not have a continuous phase adjust on the subs, then with the mains set to small I would reverse the sub's polarity if that is louder, then adjust the AVR's sub delay setting for the loudest sound while playing a test tone at the crossover frequency.

There are of course a myriad of other ways, most involving more equipment, but that should get you close.

It is also possible that you simply like the extra bass from you mains. There is nothing inherently wrong with setting it up that way. I am guessing you prefer the extra mid-bass boom that may provide.
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post #3 of 45 Old 02-10-2014, 11:43 AM - Thread Starter
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Thank you DonH50: Yes, the subs do have variable phase adjustment. Thank you for the recommendation. With the mains set to small, would you reverse the phase on the subwoofer(s) or on the AVR? And is the AVR's "sub delay setting" the same as the distance setting on the AVR?

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post #4 of 45 Old 02-10-2014, 05:56 PM
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1. Doesn't matter where you reverse phase. I only suggested that since you said it sounded better that way so should give you a better starting point.
2. Delay = distance, yes.

Good luck! - Don
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post #5 of 45 Old 02-11-2014, 06:03 AM
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I assume your mains are towers since they are set to large so another thing that is probably happening is since your receiver is most likely applying a 2nd order high pass to your mains and 4th order low pass to the sub, that is messing up the phase. If you have a way of crossing over the mains using a 4th order high pass they will blend in to the sub a lot better though. A couple other options are to set your crossover lower like 60hz as kind of a compromise or you could set your mains to the double bass type setting where they are full range but your sub is still on.
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post #6 of 45 Old 02-11-2014, 07:12 AM
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Originally Posted by mdaleavs View Post

Please help.

The setup: 3.2 ht/audio system with L/C/R speakers and two sealed subwoofers (no presence or surrounds). The L/C/R speakers match each other, and the subs match each other. The auto-calibration program sets all speakers to "large" and confirmed that all speakers are in phase. I visually inspected and tested each speaker to verify their positive polarity. The mains are connected to a two-channel power amp; the center via the AVR's center channel port; and the subs via the AVR's dual mono LFE ports.

The problem: When the mains are set to "large," and the subwoofer phase in the AVR is set to normal, the SPL is great! However, when the mains are set to "small," the room loses a lot of SPL unless the subwoofer phase in the AVR is reversed – which then improves the SPL but not to the extent of the original large/normal setting. Also, at the small/reverse setting, the SPL gets worse as the crossover in the AVR is raised. At the large/normal setting, the SPL improves at the higher crossover setting.

The question: I want to set the mains to "small," as recommended on this and other forums, but the system seems to fight itself at this setting. What might be the problem? Thank you for your consideration.

As I am sometime prone to do I question the question.

OK, so there is a huge amount of bass. Hifi is not about a huge amount of bass, its about a natural amount of bass. In general when you set up speakers the goal is not to maximize response, but rather to obtain smooth, flat, natural response.

There is no relevant detail in the question as presented, not one make or model. So, it looks to me like a pig in a poke. An opportunity to trip myself up by answering in the absence of sufficient evidence.

When an automated room setup program lowers the crossover frequency, it often means that the speaker location in the room is excessively maximizing the amount of bass that the L&R speakers produce.

When this happens within reason, it could be a good thing. When its so extreme that bass management is bypassed, maybe its not such a good thing.
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post #7 of 45 Old 02-11-2014, 07:51 AM - Thread Starter
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Originally Posted by aarons915 View Post

I assume your mains are towers since they are set to large so another thing that is probably happening is since your receiver is most likely applying a 2nd order high pass to your mains and 4th order low pass to the sub, that is messing up the phase. If you have a way of crossing over the mains using a 4th order high pass they will blend in to the sub a lot better though. A couple other options are to set your crossover lower like 60hz as kind of a compromise or you could set your mains to the double bass type setting where they are full range but your sub is still on.

Thank you. Right, the mains are towers. Don't know how to change the high pass filter to 4th order (how is that done? may be over my head). The crossover is usually set to 60Hz or 80Hz, as you suggest, and the double (or "extra") base setting presently sounds best (fullest). I think the system sounds great at large/normal, so perhaps I should just be satisfied, I just wanted to run the mains small to minimize the load on the main amp. Unfortunately, running the mains small also means losing some SPL. Also, running them small limits my crossover options because the SPL progressively decreases at higher crossover settings. Thanks, again.

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post #8 of 45 Old 02-11-2014, 08:20 AM - Thread Starter
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Originally Posted by arnyk View Post

As I am sometime prone to do I question the question.

OK, so there is a huge amount of bass. Hifi is not about a huge amount of bass, its about a natural amount of bass. In general when you set up speakers the goal is not to maximize response, but rather to obtain smooth, flat, natural response.

There is no relevant detail in the question as presented, not one make or model. So, it looks to me like a pig in a poke. An opportunity to trip myself up by answering in the absence of sufficient evidence.

When an automated room setup program lowers the crossover frequency, it often means that the speaker location in the room is excessively maximizing the amount of bass that the L&R speakers produce.

When this happens within reason, it could be a good thing. When its so extreme that bass management is bypassed, maybe its not such a good thing.

Thank you. It sounds like my question might have frustrated you, and for that I sincerely apologize. Obviously, I have dual subs because I want plenty of bass distrubuted evenly throughout my room - likely more than is "natural" or "flat." I certainly didn't intend to trip you or anyone up with lack of detail. I figured that sufficient information about my system can be found at the bottom of my post. I didn't elaborate on make/model information because I've read many posts (here and elsewhere) and usually the discussion focuses less on make/model and more on the principles of good bass management. In summary: I think my system is demonstrating signs of phase cancellation, and I don't know what to do about it. Any guidance or insight you're able to offer is much appreciated!

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post #9 of 45 Old 02-11-2014, 08:32 AM
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Quote:
Originally Posted by mdaleavs View Post

Thank you. Right, the mains are towers. Don't know how to change the high pass filter to 4th order (how is that done? may be over my head). The crossover is usually set to 60Hz or 80Hz, as you suggest, and the double (or "extra") base setting presently sounds best (fullest). I think the system sounds great at large/normal, so perhaps I should just be satisfied, I just wanted to run the mains small to minimize the load on the main amp. Unfortunately, running the mains small also means losing some SPL. Also, running them small limits my crossover options because the SPL progressively decreases at higher crossover settings. Thanks, again.

Most AVR don't have the option for a 4th order high pass and honestly if you like a lot of bass the 2nd order will you give you more. If you want to lighten the load on your Amp you can still use an 80hz crossover which will still be a lot of bass out of your mains, then you just adjust the gain on your subs if the bass isn't to your liking, but make sure the setting in your AVR is set to 0 or less so the signal is clean then just increase the volume in the subs to your liking. This should give you the bass you want and the mains should sound better as well since they aren't taking the full range signal.

If the problem is that you have a lot of cancellations around the room then you could look into bass traps or positioning your subs differently, experimenting with the position settings in your AVR, etc. But if you want to really tweak the system by more than just your ears you will have to get a microphone and start measuring frequency response to really see how changing things up is affecting the bass.
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post #10 of 45 Old 02-11-2014, 08:41 AM - Thread Starter
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Thank you, Aarons. I'll follow-up on your suggestions. My next venture might be REW and mini-DSP.

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post #11 of 45 Old 02-11-2014, 08:44 AM
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Or just REW. You may not need much in the way of EQ once you get the settings and positions right. If you're lucky. ;)

http://www.avsforum.com/t/1449924/simplified-rew-setup-and-use-usb-mic-hdmi-connection-including-measurement-techniques-and-how-to-interpret-graphs#post_22790110


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Did you really need to quote that entire post in your reply?
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post #12 of 45 Old 02-11-2014, 08:44 AM
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Quote:
Originally Posted by mdaleavs View Post

Quote:
Originally Posted by arnyk View Post

As I am sometime prone to do I question the question.

OK, so there is a huge amount of bass. Hifi is not about a huge amount of bass, its about a natural amount of bass. In general when you set up speakers the goal is not to maximize response, but rather to obtain smooth, flat, natural response.

There is no relevant detail in the question as presented, not one make or model. So, it looks to me like a pig in a poke. An opportunity to trip myself up by answering in the absence of sufficient evidence.

When an automated room setup program lowers the crossover frequency, it often means that the speaker location in the room is excessively maximizing the amount of bass that the L&R speakers produce.

When this happens within reason, it could be a good thing. When its so extreme that bass management is bypassed, maybe its not such a good thing.

Thank you. It sounds like my question might have frustrated you, and for that I sincerely apologize. Obviously, I have dual subs because I want plenty of bass distrubuted evenly throughout my room - likely more than is "natural" or "flat." I certainly didn't intend to trip you or anyone up with lack of detail. I figured that sufficient information about my system can be found at the bottom of my post. I didn't elaborate on make/model information because I've read many posts (here and elsewhere) and usually the discussion focuses less on make/model and more on the principles of good bass management. In summary: I think my system is demonstrating signs of phase cancellation, and I don't know what to do about it. Any guidance or insight you're able to offer is much appreciated!

Phase cancellation is usually addressed by adding absorption (bass traps for example) or moving speakers or walls around.
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post #13 of 45 Old 02-11-2014, 09:02 AM - Thread Starter
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Originally Posted by LastButNotLeast View Post

Or just REW. You may not need much in the way of EQ once you get the settings and positions right. If you're lucky. wink.gif
http://www.avsforum.com/t/1449924/simplified-rew-setup-and-use-usb-mic-hdmi-connection-including-measurement-techniques-and-how-to-interpret-graphs#post_22790110

Thanks! I'd rather avoid the filters associated with EQ. By the way, I'm by-passing the PEQ - could that be causing cancellation issues? And thanks for the link!

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Originally Posted by arnyk View Post

Phase cancellation is usually addressed by adding absorption (bass traps for example) or moving speakers or walls around.

The subwoofers are placed according to their prefered "aesthetic" location, next to the mains and equidistant to the primary listening position, but probably not acoustically optimal. I may need to tweak their placement if all else fails. Thank you for the suggestion!

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post #15 of 45 Old 02-11-2014, 09:08 AM
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Quote:
Originally Posted by mdaleavs View Post

Quote:
Originally Posted by arnyk View Post

Phase cancellation is usually addressed by adding absorption (bass traps for example) or moving speakers or walls around.

The subwoofers are placed according to their prefered "aesthetic" location, next to the mains and equidistant to the primary listening position, but probably not acoustically optimal. I may need to tweak their placement if all else fails. Thank you for the suggestion!

When it comes to speakers it seems like anything that looks good, sounds bad! ;-)

This as you may know is one of the advantages of subwoofers - they can be the most sensitive to placement, and being able to place them where they work best can really work out.
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post #16 of 45 Old 02-11-2014, 11:54 AM
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Quote:
Originally Posted by arnyk View Post

Phase cancellation is usually addressed by adding absorption (bass traps for example) or moving speakers or walls around.

Note the date, I am going to disagree (mildly) with Arny!

IME bass traps do little to control phase cancellation issues between mains and subs. They do not absorb well at LF so you typically need a lot of them, fairly thick, to do much. And, if the fundamental problem is mains and subs not in phase, I prefer to address that through either adjusting their relative phase or accomplishing the same thing by moving the speakers or subs around (ah, back in agreement, whew!)

Moving walls can be challenging, but sometimes re-arranging the room to move the listening position can be of great benefit.

I have recently decided that addressing the amount of bass desired by a given listener, i.e. reference vs. preference, is unrewarding, unfulfilling, and doomed to failure because a lot of folk want unrealistically high bass levels because they like it, and are OK with that.

Aside: My room is very heavily treated, and of course the best spots for the subs simply will not do (would block a door and really not room on the other wall, small room).
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post #17 of 45 Old 02-11-2014, 12:16 PM - Thread Starter
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Originally Posted by DonH50 View Post

I have recently decided that addressing the amount of bass desired by a given listener, i.e. reference vs. preference, is unrewarding, unfulfilling, and doomed to failure because a lot of folk want unrealistically high bass levels because they like it, and are OK with that.

Thanks. I'm presently infatuated with my subwoofers, and like a new romance, want as much of them as I can get. (My wife might get jealous if she sees this.) I'm sure my passion for excess bass will subside as "our relationship" matures. For now, I enjoy being smothered in bass - and that's why the cancellation issues are such a bother. Thank you for your understanding.
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^^^^Thanks for being such a considerate and polite poster. I'm just a bystander in this thread, but I'm thoroughly enjoying reading it. A big part of that is how thoughtfully you have posed your questions and replies to the help provided you.

A big thanks also goes to Don and Arny and Aaron, of course.
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post #19 of 45 Old 02-11-2014, 12:36 PM
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Quote:
Originally Posted by DonH50 View Post

Quote:
Originally Posted by arnyk View Post

Phase cancellation is usually addressed by adding absorption (bass traps for example) or moving speakers or walls around.

Note the date, I am going to disagree (mildly) with Arny!

IME bass traps do little to control phase cancellation issues between mains and subs. They do not absorb well at LF so you typically need a lot of them, fairly thick, to do much. And, if the fundamental problem is mains and subs not in phase, I prefer to address that through either adjusting their relative phase or accomplishing the same thing by moving the speakers or subs around (ah, back in agreement, whew!)

I'm going to take the position that since you said IME - In My Experience - you may be right, and I may be right, too. My experience is that there's a lot of questionable stuff that is called bass traps, but that is more wish fulfillment than sound absorbency..
Quote:
Moving walls can be challenging, but sometimes re-arranging the room to move the listening position can be of great benefit.

Moving the LP or the subwoofers or both may be necessary.

Quote:
I have recently decided that addressing the amount of bass desired by a given listener, i.e. reference vs. preference, is unrewarding, unfulfilling, and doomed to failure because a lot of folk want unrealistically high bass levels because they like it, and are OK with that.

I may resemble that remark! ;-)

Quote:
Aside: My room is very heavily treated, and of course the best spots for the subs simply will not do (would block a door and really not room on the other wall, small room).

Isn't there some law of physics that says that a speaker cannot be positioned so it both looks good and sounds good at the same time? ;-)
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post #20 of 45 Old 02-11-2014, 12:40 PM
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Quote:
Originally Posted by mdaleavs View Post

Please help.

The setup: 3.2 ht/audio system with L/C/R speakers and two sealed subwoofers (no presence or surrounds). The L/C/R speakers match each other, and the subs match each other. The auto-calibration program sets all speakers to "large" and confirmed that all speakers are in phase. I visually inspected and tested each speaker to verify their positive polarity. The mains are connected to a two-channel power amp; the center via the AVR's center channel port; and the subs via the AVR's dual mono LFE ports.

The problem: When the mains are set to "large," and the subwoofer phase in the AVR is set to normal, the SPL is great! However, when the mains are set to "small," the room loses a lot of SPL unless the subwoofer phase in the AVR is reversed – which then improves the SPL but not to the extent of the original large/normal setting. Also, at the small/reverse setting, the SPL gets worse as the crossover in the AVR is raised. At the large/normal setting, the SPL improves at the higher crossover setting.

The question: I want to set the mains to "small," as recommended on this and other forums, but the system seems to fight itself at this setting. What might be the problem? Thank you for your consideration.



When you change your R & L main bass management settings from large to small, yes phase gets out of sync by 180 degrees. Reverse of phase of the subwoofer is required. You may have to wire your center channel in reverse phase to get things right, but that issue has to be tested.

When you main speakers are set to large, do you have the "extra" bass turned ON or OFF?

As far as having no surround speakers is concerned, that may engage DRC in your AVR regardless of the DRC settings.
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post #21 of 45 Old 02-11-2014, 12:43 PM - Thread Starter
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Originally Posted by arnyk View Post

Isn't there some law of physics that says that a speaker cannot be positioned so it both looks good and sounds good at the same time? ;-)

"You can't have your cake and eat it too"?

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post #22 of 45 Old 02-11-2014, 12:56 PM - Thread Starter
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When you change your R & L main bass management settings from large to small, yes phase gets out of sync by 180 degrees. Reverse of phase of the subwoofer is required. You may have to wire your center channel in reverse phase to get things right, but that issue has to be tested..

Thank you. Yes, even with the center channel turned OFF, the cancellation persists.
Quote:
When you main speakers are set to large, do you have the "extra" bass turned ON or OFF?.

With the mains set to large, the extra bass is ON. The cancellation is not apparent when the extra bass is OFF.
Quote:
As far as having no surround speakers is concerned, that may engage DRC in your AVR regardless of the DRC settings.

Dynamic range control set to OFF. If DRC is still engaged despite being turned OFF, then that could be the problem? Thanks.

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post #23 of 45 Old 02-11-2014, 01:03 PM
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Quote:
Originally Posted by mdaleavs View Post

Quote:
Originally Posted by J_Palmer_Cass View Post

Quote:
When you main speakers are set to large, do you have the "extra" bass turned ON or OFF?.

With the mains set to large, the extra bass is ON. The cancellation is not apparent when the extra bass is OFF.


The large R & L setting sound good with "extra bass" turned OFF.

Leave the R & L settings on large and try reversing the subwoofer polarity, and turn the "extra bass" ON.
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post #24 of 45 Old 02-11-2014, 01:35 PM
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Isn't there some law of physics that says that a speaker cannot be positioned so it both looks good and sounds good at the same time? ;-)

"You can't have your cake and eat it too"?

Corollary: If you do find a place where it looks and sounds good, the wife/significant other will not like where it is, or where the LP is, and re-arrange so as to satisfy the initial premise (good looks, bad sound, or vice-versa).

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post #25 of 45 Old 02-11-2014, 01:37 PM - Thread Starter
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The large R & L setting sound good with "extra bass" turned OFF. Leave the R & L settings on large and try reversing the subwoofer polarity, and turn the "extra bass" ON.

If the polarity on the subwoofers is reversed, either via the subwoofer amps or the AVR, while the x-bass remains ON, then the SPL audibly drops significantly (3-4 db). This is partially recovered by setting the mains (and center channel) to small - which regains 1-2 db. So, the low-bass is most prominent (and mesmerizing, I must add) when the mains/subs remain set to large/normal. Cancellation occurs when the mains are set to small. Furthermore, when the mains/subs are set to small/reverse, a faint electronic "clicking" is audible when changing between x-over settings - that doesn't happen when the mains/subs are set to large/normal.

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post #26 of 45 Old 02-11-2014, 01:54 PM
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The large R & L setting sound good with "extra bass" turned OFF. Leave the R & L settings on large and try reversing the subwoofer polarity, and turn the "extra bass" ON.

If the polarity on the subwoofers is reversed, either via the subwoofer amps or the AVR, while the x-bass remains ON, then the SPL audibly drops significantly (3-4 db). This is partially recovered by setting the mains (and center channel) to small - which regains 1-2 db. So, the low-bass is most prominent (and mesmerizing, I must add) when the mains/subs remain set to large/normal. Cancellation occurs when the mains are set to small. Furthermore, when the mains/subs are set to small/reverse, a faint electronic "clicking" is audible when changing between x-over settings - that doesn't happen when the mains/subs are set to large/normal.



I leave my R & L mains set to large, and have the other speakers are set to small with various crossover frequencies (80 Hz center and 50 Hz surround).

In my system if I set the R & L mains to small, I have to reverse subwoofer phase and raise the subwoofer volume setting about 5 dB to get equal SPL out of the system. However, the all small settings do not sound the same as the large R & L settings.

You should learn how to do in room audio measurements with your PC/laptop. That way you can see what is going on.

I am sure that you know this, but you should note that the "extra bass" does not work with the main speakers set to small.

Don't forget about the surround DRC issue. If you have an old pair of speakers lying around you could connect them to the surround outputs and see if DRC is deactivated. DRC only works on DD content. DTS content does not use DRC.
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^^^^Thanks for being such a considerate and polite poster. I'm just a bystander in this thread, but I'm thoroughly enjoying reading it. A big part of that is how thoughtfully you have posed your questions and replies to the help provided you.

A big thanks also goes to Don and Arny and Aaron, of course.

Yes, thank you to everyone who is helping me. Very helpful. At this point, as suggested, I will reverse the phase on the subwoofer amps, then run the auto-calibration, and see what happens. My second option is to reverse the subwoofer phase in the AVR, and then adjust the subwoofer distance to maximize the SPL. I suppose a third option is to set the mains to small and slowly adjust the subwoofer phase to maximize the SPL. I might be chasing my tail...

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post #28 of 45 Old 02-11-2014, 02:08 PM - Thread Starter
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You should learn how to do in room audio measurements with your PC/laptop. That way you can see what is going on..

Yes, that is probably what I need to do. Thanks. What is the basic difference between REW and a RTA (real-time analyzer). Can they serve the same purpose? I already have a SPL meter.
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I am sure that you know this, but you should note that the "extra bass" does not work with the main speakers set to small..

Yes, thank you, I discovered this. smile.gif
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Don't forget about the surround DRC issue. If you have an old pair of speakers lying around you could connect them to the surround outputs and see if DRC is deactivated. DRC only works on DD content. DTS content does not use DRC.

Interesting idea. Thank you.

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REW generates a test signal, captures the sound using a mic (prefereably calibrated), then processes and displays the results on your PC. You can do a lot with the data to present different things like frequency and time domain response, reverb decay/waterfall plots, etc.

RTA = real time analysis and usually just shows the magnitude of the frequency response but in real time. You can instantly see changes in FR, convenient if you are moving things around and watching the results on the fly. Some RTAs also have an oscilloscope mode that allows you to watch the time varying waveforms. Like watching a fire, can be mesmerizing.

For our purposes either would do, but I would suggest REW for its low price (free SW, $100 mic) and flexibility. Only takes a few seconds to capture and display the data, then you can move things around and try again.

An SPL meter also works in real time, but is a broad-band device so only gives the average level for everything that comes into it. To do frequency response graphs you need to provide test tones into your system and manually plot the data (unless you have a logging meter, a more costly device that most have).

"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
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post #30 of 45 Old 02-11-2014, 07:03 PM
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You should learn how to do in room audio measurements with your PC/laptop. That way you can see what is going on..

Yes, that is probably what I need to do. Thanks. What is the basic difference between REW and a RTA (real-time analyzer). Can they serve the same purpose? I already have a SPL meter.



True RTA is an RTA program. Includes an oscilloscope function. Download it and play with it. You can use your SPL meter for the microphone to start. Plug the SPL meter into the line in on your PC / laptop.

The free version of the program has limited functionality, but you can play with it to get a basic feel of the program.

I purchased the 1/24 octave version. Does not take much time to master.

http://www.trueaudio.com/rta_abt1.htm

http://www.trueaudio.com/rta_down.htm



Don mentioned REW. I also have that program on my system.



A third program that I use is SpectrumLab. You can check that one out in the subwoofer thread. Takes a lot of time to master, but is a fun program.

http://www.avsforum.com/t/1333462/the-new-master-list-of-bass-in-movies-with-frequency-charts





I forgot to mention the the subwoofer distance setting works together with the subwoofer polarity setting. Check out the subwoofer distance as compared with the adjacent main speaker distance. In my system, the front speakers and the subwoofer are all located on the front wall and have the same distance setting.
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