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Discussion Starter · #1 ·
Well, I just spent the evening going through my components connecting and disconnecting wires in an effort to find a very nasty humming noise going through my left front channel speaker and subwoofer. I finally found the culprit: my Audio Control Phase Coupled Activator. It's a stand alone component which houses a crossover circuit with a base enhancing circuit. I don't know what's wrong with it, but when I remove it from my system, everything's fine. This dismays me because I've had if for years and have always been quite happy with it.


After I took it out of my system and began to recalibrate, I came upon the real reason for why I'm now writing this thread. I set up my subwoofer using a Paradigm X-30 crossover unit that came with my Servo-15 a long time ago. Calibration was done with the Avia test disc using all the test tones. I just got this disc recently, so have never really completed the audio test tones with my setup. I got to the "Low Frequency Sweep" which outputs a test tone starting at something like 200Hz and works its way down progressively to 20Hz. The idea is to have a smooth transition between your satellites and subwoofer in the crossover range, with no significant dropouts, as well as testing the ability of the subwoofer to reach the lower range. My crossover is set at for all channels. To my astonishement, as the sweep neared approximately 105Hz, the sound level dropped significantly, and didn't rise again until about 70Hz. I refused to believe that this could be due to my satellites (Paradigm LCR-450 for all channels) not reaching down that low since the spec on them says +/- 2dB from 70Hz-22kHz with a low freq extension of 45Hz (DIN). From that I would gather the response of the speaker would be rock solid down to at the very least, 70Hz. To get to the point, I went against my better judgement and moved the crossover up to 120Hz. The frequency dip disappeared and the transition for the test tone was smooth.


Normally I wouldn't care about this, but I have to wonder why the speakers don't reach down to even near where the spec claims. I also would prefer to have a crossover near 80Hz or lower, as per the THX requirements. I would just feel better about my inablility to locate the base source with the crossover being lower. I'm worried that if I buy a new receiver that it might not give me the ability to crossover at 120Hz (like my friends Yamaha which crosses everything over at 90Hz.) I was interested in the Pioneer Elite 45TX, or the Sony Da4ES.
 

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What you have discovered is probably a room "mode," also known as a "resonance." Your speakers interact with your room to create an acoustic field, and at some frequencies, the room will tend to increase output, while at others, it will cancel. A resonance is a frequency where the room interactions dramatically cancel the sound waves produced by the speaker. It looks like your bass resonance is about 85-90 Hz. If you want to prove to yourself that the culprit is the room, then just rearrange the speakers, and perform the sweep again. Or, move something else, like the couch or the TV.


The fact that you raised the crossover and removed the dip is consistent with this hypothesis. This is because when you have all of the sound below 120 Hz going through the subwoofer, the room interactions are different from when all speakers are handling these frequencies.


Tim
 

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Discussion Starter · #3 ·
Thanks for the input Tim. I'm gonna go experiment with your suggestions when I get a chance and see if it brings my speakers back to life.
 

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The 4ES will let you crossover at a variety of frequencies, including the 120 Hz mark. Not that its a good solution, as it would be better to be able to tune them down to the good 80Hz mark :)


I am also interested to hear how the results are with the speakers set up differently.
 

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I agree with Tim about the cause.


It's probably related to bass cancellation at the listening position.


To test this hypothesis, it's probably easier to move the listening position than the speakers. But ultimately some room treatments (ie-bass traps) and manipulation of your setup will probably fix it.


For $50 you cay buy the CARA room acoustics software. Search on yahoo and read about it at the website. Very detailed. So much so it can take your computer a couple of months of full time calculations to come up with the ultimate setup. You can also get a little less sophicated and get it done more quickly.
 

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If I may suggest another possibility: maybe it's a phase issue. If the sub has a crossove switch or knob, play with it. If not, try switching polarity on the other speakers. Relative polarity is more important than absolute polarity here.
 

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Discussion Starter · #7 ·
Well... I did some experimenting. I fear I'm sliding down a slippery slope. I experimented with the following:
  • moving the coffee table out of the room
  • moving my satellite speakers
  • adjusting the phase of the sub (continuously from 0 to 180)


The endeavor that made the most difference was moving the coffee table. The other two experiments were inconclusive as far as I was concerned. They did seem to make some difference, but often the result seemed to be a shift of the dropout from one spot in the freq band to another. Often whatever I did would create an increase, or resonance, in another freq band regardless of whether the original dropout was mitigated or not. Maybe I should try this experiment with the sub turned off.


Oddly enough, the thing that still seemed to produce the best results was to crossover the sub at 120Hz. It's funny, but if I had never done this Avia freq sweep, I would have been blissfully ignorant of this.
 

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Let me ask you this:


Does it sound better with the x-over set high? My guess is "not really". I'm going to be that it's a room-mode issue. Here's what will sound like a nutso idea, but try it:


Place your sub, with the x-over set back low, where you sit, and play a tone in the middle of your 'hole' (85-90 Hz). Crawl around the room, with your head at the height above the floor that the sub sits.


Where the bass sounds best, place the sub. Now, when you sit in your usual seating position, the bass should also sound best. Tweak the phase again. Sounds crazy, but works.
 

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Discussion Starter · #9 ·
I indeed might try this method. I have heard of it before. How will I play a tone right in the frequency hole? I don't have a "tone generator" that I know of.


I'm also worried that after I do all this moving around, I might perform a sweep and find out I have a hole at another frequency.
 

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Discussion Starter · #10 ·
Quote:
Originally posted by Larry Fine
Let me ask you this:


Does it sound better with the x-over set high? My guess is "not really".


Well... it doesn't have a hole in it any more through the entire sweep, and it sounds more punchy. More midbass?
 

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If you like it, don't worry about it. But you might like it better if you experimented with placement and got some room treatments/bass traps.
 

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Discussion Starter · #12 ·
I feel uncomfortable about the crossover being so high. I will try to experiment, but I have almost no latitude in where I put the speakers or the sub.
 

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Discussion Starter · #14 ·
What is the best way to go about figuring out what I need, the CARA software?


Where is the best place to purchase something like that?
 

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CARA does all kind of cool stuff. You could model your room and speakers and then run the simulation with and without various treatments and assess the effect of treatments. The website has all the details. I think it costs about $50 and can be ordered online. Do a search on CARA acoustics and you can find it.
 

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Oh- another possibility is www.etfacoustic.com I don't own it, but it is a bit different from CARA. ETF connects into your computer/laptop and MEASURES your room and can help you calculate reflection points and room modes and offer up help for improvments. It is probably better for existing rooms with existing problems. CARA is better for assisting with the best layout. Compared to the price of good HT gear, both are minimally expensive and a well-designed room with average equipment will sound better than an average room with good equipment.
 

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Discussion Starter · #17 ·
The eftacoustic software looks interesting, but can I use that as a simulator on my desktop pc (which isn't near my tv) or is it only an analyzer that is intended for laptop use? I visited the site, but it is not clear to me.
 

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I can comment on use of the ETF software, but it's best to have a PC soundcard that supports full-duplex operation (play and record at the same time);


1) For a data collection microphone/preamp with the ETF software, you can use the RCA output plug (MONO) on the Radio Shack SPL meter into one of the RCA stereo inputs (say the LEFT LINE-IN) on your Desktop PC's soundcard.


2) You then connect the same (LEFT LINE-OUT) on your Desktop PC's soundcard to the amplifier channel input driving the speaker under test.


3) To complete the connections, connect the PC soundcard's RIGHT LINE-IN to the PC soundcard's RIGHT LINE-OUT.



If you want to test two speakers running together (like left and right mains, or left main and sub, or right main and sub), simply put a Y-connector on the LINE-OUT from the PC soundcard to the amp inputs.


By the way, instead of connecting the PC soundcard LEFT LINE-OUT directly to a speaker's amp, you could connect it to a spare analog preamp input (Left, Right, or both with a Y-connector) instead. Then you simply use the preamp's analog output when testing and disconnect those speakers you don't want to test.


I've heard of people using 50 foot cables for these connections. Plus if you put the SPL meter on a tripod at your listening position you don't need to be in the listening room while you do the testing.
 

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Discussion Starter · #20 ·
My pc is in an upstairs room, while my HT is downstairs. Are you saying that to use this tool I would have to run a cable all the way upstairs? I guess I could do it, but it would be mighty long.
 
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