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in / out of phase (1 Viewer)

Ben Yang

Grip
Joined
Jan 21, 2001
Messages
22
After three months of loving a DIY home theatre, I have started to do some critical listening now that my speakers are well broken in. I noticed a dip in SPL from the Stereophile test CD from ~80hz to 40hz. My shiva sub is down ~3db at 20hz. It's tuned that way to account for inroom bass reinforcement, according to Dan's recommendations. I'm running my shiva sonotube sub using an NAD 7175 receiver with the preouts pulled and my HK receiver sending the l+r / lfe signal to the receiver in stereo mode using a signal Y splitter. The receiver is only 75wpc but is summed at the shiva using both voice coils independently. I experimented with the setup today and tried a phase inversion test. I reversed both voice coils inputs on the sub and noticed a huge improvement in bass output. In retrospect, I was cancelling out the bass from my main speakers and my sonotube. Once my mains rolled off, there wasn't any sound to cancel and my sub would be loud again. I assumed it was inroom peaks and valleys, but am unclear as why it would be out of phase.
Question: What causes in/out phase discrepancies when all circuitry is correctly wired from input to output?
I was very impressed with my sub before. Now, it's downright scary. Beware of painting / picture frames in the same room. Almost shook one off the wall today with this test at -20db. Furthermore, noticed high pitched "breathing" sound from the port. A grill I placed over it is causing some "whistling" at the ports tuned frequency. I will need to fix that, but can't go without some sort of grill risking having my 1.5 year old son stuff various objects down the port! I used a staight piece of 4" PVC. Now I wonder if I had used a flared port would this be an issue? Without the grill, it doesn't breathe. In addition, next sonotube will be weighed down, sand, lead shot or something. At high output, it wants to vibrate and "move" around the floor a bit. Sorta like those old football games with the plastic pieces that vibrated across the playing field. Anyone here old enough to remember those games?
Ben.
http://members.bellatlantic.net/~wpsg/
 

Julian Data

Second Unit
Joined
Oct 5, 1999
Messages
408
Congrats on the "tweaking nirvana"! :)
On your question, do you just mean via electrical or by the room?
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Shade Watson

Stunt Coordinator
Joined
Jan 24, 2001
Messages
135
ELECTRIC FOOTBALL!
My brother and I loved that game when we were kids. We spent at least half the time trying to adjust their stands so that they would go straight.
We also played our fair share of Rock'em Sock'em Robots.
 

Ben Yang

Grip
Joined
Jan 21, 2001
Messages
22
Julian,
I am interested in the explanation for both. Acoustically, I assume the + and - portions of the sound wave have an impact on where in the room you feel strong bass and where it is weak. It's pretty easy to tell, by walking around my listening area where the areas are. Fortunately, the couch is a peak area. Again, it's something to do with the wavelength and it's relationship to the size of the room, correct?
Is my electronic "fix" really just accounting for inroom standing wave/cancellation? I don't think that's the reason, because it is clearly in the mid/lower bass that the cancellation was most significant. With very deep bass, where my mains don't reach, there wasn't any wave to cancel out the sub, hence it was "unopposed".
 

Ben Yang

Grip
Joined
Jan 21, 2001
Messages
22
Shade,
Where have all the GOOD games gone?
Do they even make rockem sockem robots anymore? I guess it's all been replaced with computer versions of the same.
 
Joined
May 22, 2001
Messages
19
My son loves Rock em Sock em robots. He has the Remote controlled kind. They are pretty cool. HE has several sets but they only operate on 2 frequencies so he lines them up and charges them at each other then about 75$ of them fly up in the air.
Not quite like the ones I had when I was a kid. These break often and are $20 for one set.
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Julian Data

Second Unit
Joined
Oct 5, 1999
Messages
408
Ben,
Your fix could be alleviating the cancellation interaction with your mains, or room interactions at that particular spot in the room.
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