[Greg1981]

My subwoofer is located 7 feet from my seating position. Using Avia, my LFE level is properly set. I've run into problems when trying to match the sub with the crossover frequency (80hz) of the front mains. If I set my subwoofer distance within the AVR to 10 feet, I get a perfect match between high frequency, crossover, and LFE (when factoring in correction) tones with the AVIA test tones. If I set my subwoofer distance to 13 feet, I get maximum SPL at the crossover frequency, which rises ~4db (so instead of 75db, its ~79db).


I'm a little confused because I've read that the subwoofer is in phase with the mains when it produces its maximum SPL; am I doing the wrong thing setting the distance at 10 feet to keep things equalized?

 

[Wayne A. Pflughaupt]

I’d go with the setting that gets the smoothest frequency response through the crossover region (e.g. an octave above and below the crossover frequency).


Regards,

Wayne A. Pflughaupt

 

[Bigus]

You should set the distance to the actual distance of your sub and then keep adjusting level to get properly calibrated output. If you level set your sub independently and then the responses isn't flat when mains are included, go back to the sub level setting and adjust so that the total system response is flat. Your speakers need to work in harmony, not as individuals.


BTW, what kind of mains, sealed or ported, what brand and model or better yet what is the -3dB point?

 

[Greg1981]

Thanks to the both of you for your advice, greatly appreciated. Sorry for the much belated response, as I've been tweaking my audio system extensively this past week. I ended up moving the sub a foot further back (8ft, and its now slightly behind the mains), and set the sub distance to its actual distance, and there is now a better blend between them at the crossover point. I cut -2db of bass with the bass/treble control to help line it up, as I didn't want to mess with the subwoofer volume as its lined up accordingly with the help of AVIA's test tones.


In response to your questions Bigus, my mains are sony tower speakers; though rated at 45hz (-3db), I don't trust their rating and treat them instead as glorified satellite speakers, crossing them over at 100hz (as they get a bit boomy below that)

 

[brandonnash]

The distance and crossover settings are completely different thing that do completely different things to the audio signal. Moving the sub around will change the frequency response you're experiencing in room. The room itself is what changes this. Low frequencies interact with a room quite a bit more than higher frequencies.


The distance setting is essentially a delay. Using this on the subwoofer as well as correctly with the mains you should have the sound hitting at listening position at the same time.


The crossover helps with the actual frequency blend between your sub and the mains. Changing the distance setting in your receiver and leaving your sub and speakers in the same place should have no effect on how the frequencies between your sub and your mains act with each other.

 

[craig john]

The subwoofer Distance control can *definitely* have a huge impact on the response around the crossover point and the blend of the speakers with the sub. Here is a graph which shows the response with the Distance setting at 10.4 ft, and with the Distance setting at 13.8 ft.

Blue = 10.4 ft. Green = 13.8 ft.

The *only* change made between those two graphs was the Distance setting! Adding 3.4 ft to the Distance setting eliminated a 30 dB null caused by the sub and speakers being out of phase. The Distance setting corrected that out of phase condition and allowed for a much better splice.


Keep playing with your Distance setting to optimize the response. I would shoot for the smoothest response through the crossover region, as Wayne stated.


Craig

 

[craig john]



Quote:
Originally Posted by Bigus /forum/post/19516981


You should set the distance to the actual distance of your sub and then keep adjusting level to get properly calibrated output.

The subwoofer Distance should be set to the "acoustic Distance", not the physical distance. The acoustic Distance takes into account any delays caused by processing, either in the receiver or in the sub itself. If the AD/DA converters and EQ's add some latency to the signal, this needs to be accounted for in the delay, (which is what the Distance setting does.)

Quote:
If you level set your sub independently and then the responses isn't flat when mains are included, go back to the sub level setting and adjust so that the total system response is flat. Your speakers need to work in harmony, not as individuals.

This is much better done with the Distance setting than the Level setting. The Level setting affects all the frequencies the subwoofer outputs, so using the Level setting to set the levels around the crossover point can throw off the levels through the rest of the subwoofer range. The Distance setting adjusts the timing of the subwoofer's output so it arrives at the same *time* as the speaker's outputs. This only important at the frequencies the sub and speakers share... i.e., the frequencies around the crossover point.


Craig

 

[krs]

Good info and example, Craig, Thanks.


Was the out of phase condition deliberate and the cited adjustment(s) done solely to achieve balance in that condition? If so, a quick explain of the decision to use the sub out of phase?

 

[Moggie]

I have a recent experience that is in some ways similar. My problem was not the crossover response, but in the response of my subs at my problematic seating positions (near center and 3/4 of room length). Not surprisingly I had large nulls and peaks to deal with that seemed to be made worse with an accurate rear sub setup. I even tried the Audyssey EQ1 sub equalizer that is said to specialize in dual subs equalization but even it suffered from boomy bass from some of my HT seats. So I took a completely different tack and put a Berhinger DCX2496 onto my sub channel and tried the manual equalization approach. After lots of experimentation I decided to play with the delay to the rear sub relative to the front. Whilst running the RTA function of REW I was able to dial in a distance (delay) for the rear sub, that although was not technically accurate, solved 90%+ of my room mode problems at my listening positions. Then when some more traditional EQ I was able to get to a response I was happy with.


So with the caveat that I'm an audio novice who likes to experiment, it would seem to me that messing with the distance/delay of the subs relative to the mains is equally valid in solving modal resonances.

 

[craig john]



Quote:
Originally Posted by krs /forum/post/19571659


Good info and example, Craig, Thanks.


Was the out of phase condition deliberate and the cited adjustment(s) done solely to achieve balance in that condition? If so, a quick explain of the decision to use the sub out of phase?

After running Audyssey XT32 on my Integra DHC-80.2, I changed the crossovers from "Full Range" to 80 Hz. Audyssey/Integra had also set the subwoofer Distance to 10.4 ft. After I changed the crossover, I measured the response just to verify, and got the response with the 30 dB dip. I then adjusted the Distance setting until I obtained the *much* flatter response and eliminated the dip. Note that the dip is *right at the crossover point* of 80 Hz.


Craig

 

[craig john]



Quote:
Originally Posted by Moggie /forum/post/19572026


I have a recent experience that is in some ways similar. My problem was not the crossover response, but in the response of my subs at my problematic seating positions (near center and 3/4 of room length). Not surprisingly I had large nulls and peaks to deal with that seemed to be made worse with an accurate rear sub setup. I even tried the Audyssey EQ1 sub equalizer that is said to specialize in dual subs equalization but even it suffered from boomy bass from some of my HT seats. So I took a completely different tack and put a Berhinger DCX2496 onto my sub channel and tried the manual equalization approach. After lots of experimentation I decided to play with the delay to the rear sub relative to the front. Whilst running the RTA function of REW I was able to dial in a distance (delay) for the rear sub, that although was not technically accurate, solved 90%+ of my room mode problems at my listening positions. Then when some more traditional EQ I was able to get to a response I was happy with.


So with the caveat that I'm an audio novice who likes to experiment, it would seem to me that messing with the distance/delay of the subs relative to the mains is equally valid in solving modal resonances.

Actually, you were probably not solving modal response issues. When you set the timing between the 2 subs, you were probably solving reinforcement/cancellation issues between the two subs, although you may have been doing some of both at the same time. Bottom line, if you have a flatter response now, you've fixed the problem.


Craig

 

[Moggie]



Quote:
Originally Posted by craig john /forum/post/19577534


Actually, you were probably not solving modal response issues. When you set the timing between the 2 subs, you were probably solving reinforcement/cancellation issues between the two subs, although you may have been doing some of both at the same time. Bottom line, if you have a flatter response now, you've fixed the problem.


Craig

I understand the the reinforcement/cancellation effect but the null that existed with just the front sub was substantially diminished as was the subsequent length mode. Is it not possible that the timing/phase of the rear sub canceled out a portion of the otherwise reflected wave? I remember reading about an experiment in Europe where two opposing sub woofer walls were configured so that one acted as an active absorber by being exactly out of phase with the wavefront from the opposing wall...


Sorry if this is now getting off topic.

 

[craig john]



Quote:
Originally Posted by Moggie /forum/post/19579170


I understand the the reinforcement/cancellation effect but the null that existed with just the front sub was substantially diminished as was the subsequent length mode.

You changed the timing of *one* sub without the other sub playing, and it impacted a mode? That's not possible. If you change the timing of one sub without the other playing, it only impacts *when* the sub interacts with the room, not *how* it interacts with the room. You will still have the exact same modal structure; it will just be delayed slightly, (a few milli-seconds), in time.


If you changed the timing of one sub with the other sub playing, then you are impacting the interaction of the waves from the two subs, which will now be arriving at *different* times, relative to each other. This will definitely affect the frequency response, but it is due to the interaction of the waves with each other, not the inherent modal response of the room/sub placement/listening (measuring) position.

Quote:
Is it not possible that the timing/phase of the rear sub canceled out a portion of the otherwise reflected wave? I remember reading about an experiment in Europe where two opposing sub woofer walls were configured so that one acted as an active absorber by being exactly out of phase with the wavefront from the opposing wall...


Sorry if this is now getting off topic.

That experiment was totally unrelated to this discussion. It required a perfectly dimensioned, rectangular room, perfect placement of the sub drivers on the front walls relative to the boundaries, and perfect timing of the rear sub to perfectly cancel the reflected wave off the rear wall. Although some of the same principles were at work there, that is not what you are talking about here.


Craig

 

[xianthax]



Quote:
Originally Posted by krs /forum/post/19571659


Good info and example, Craig, Thanks.


Was the out of phase condition deliberate and the cited adjustment(s) done solely to achieve balance in that condition? If so, a quick explain of the decision to use the sub out of phase?

The subwoofer isn't necessarily in phase with the mains when the distance is set to the physical distance and the phase switch on the sub is zero.


In addition to the factors Craig listed, the phase response of speaker is not constant, especially near its limits of low end extension. Combine this with the phase shift induced by the cross over filter and at the cross over point the phase shift of mains is very much non-zero.


Rythmick has a pretty good set of examples and basic work flow for calculating the required phase shift needed on the sub to align it with the mains, as well as example response measurements showing what happens when the sub is out of phase:

Introduction
Measurements
Calculating needed Shift


Of course you can skip the math and measure this with a meter as well but I would think you would want to measure this near-field and adjust for room modes with EQ, maybe not. EDIT: I guess measuring nearfield would only be reasonable with the sub(s) set next to the mains which is my arrangement, in most situations i guess you would have to measure at the listening location. If there is a room mode at your listening position near your crossover point that may cloud your efforts. I guess i'd just move the sub in that case.

 

[Bigus]

Craig, yes, it should be set for apparent distance. Poor choice of words when I said actual. However for many people these days the delay is set automatically in setup using an included mic, and that accounts for any processing delays.