# Dual Sub Calibration Q's



## gary thomas (Dec 6, 2007)

Hi. I have two subs: and DIY SS RL-P15 & a SVS Ultra. Both are tuned to 17hz. I've been moving the subs to different locations to get the best response, but have a few questions as to the best way to take the measurements. I'm using a Radio Shack meter w/ correction values. 

1. Should I measure the response of both subs with or without the other speakers?
2. Should I measure the subs individually or together?
3. I took measurements from 3 listening spots...should I calibrate based on the average of the three or from 1 prime listening spot?


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## brucek (Apr 11, 2006)

> have a few questions as to the best way to take the measurements


I would certainly forget about the 'other' speakers until I had found the best spot for the two subs. 

If you have them in different positions, presumably this is to produce an overall response at the listening area that neither single position provided. 

Alternatively, if you've found a single position in the room that provides a fairly smooth response, then I would co-locate the two subs.

Once you've found the best position(s) for the two, then it's really a bit of trial and error when equalizing. In the end, the two subs are acting as a single unit, but you may have to measure each at the same time and offer each a bit of equalization to end up with the two creating an overall satisfying result.

You can certainly measure and equalize for a single listening position, but if you have more than one person listening, it's best to spread out the measures to a bit of a wider area and average the equalization.

brucek


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## gary thomas (Dec 6, 2007)

Thanks Bruce. The reason I asked about the other speakers is that I'm getting an upward trend on SPL between 80 & 100...I'm thinking that it may be caused when my mains (crossover @ 80) and center/surrounds (crossover at 100) kick in.

I initially had both subs stacked in the corner when I added the DIY sub & output become a non-issue. The levels were great in the center room listening area & at my main listening area, but the couch seats along the wall were blown away. Separating the subs gave me a tighter response when averaging my three main listening areas. (I also tried 1 of the subs along a side wall, but things got worse)

I had a BFD at one time, but sold it. My goal is to get a good average response in all three listening positions by deteriming ideal position. 

The picture shows the stacked position...I now have 1 located behind & outside each main speaker. I plan to experiment by placing them behind & inside the mains...or 1 inside & 1 outside the mains. If possible, I'd like to keep them on the stage.

After finding this great forum...and reading about subwoofer calibration, I'm wondering if using the Rat Shack w/ correction values is even worth it. I wanted to make sure I'm taking the proper measurements. I'm considering jumping in w/ REW...mostly because I get bored without a home theater project!


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## brucek (Apr 11, 2006)

> I'm getting an upward trend on SPL between 80 & 100...I'm thinking that it may be caused when my mains (crossover @ 80) and center/surrounds (crossover at 100) kick in.


Absolutely. This where REW can help show the best sub phase and distance setting to get that rise down.



> I had a BFD at one time, but sold it.


Not many people can get by without it..



> I'm wondering if using the Rat Shack w/ correction values is even worth it. I wanted to make sure I'm taking the proper measurements. I'm considering jumping in w/ REW


The Radio Shack meter combined with REW and our downloadable correction factors is perfectly suitable for subwoofer measurements. REW is light years ahead of simple tone measuring......

Better start reading 

brucek


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## DrWho (Sep 27, 2006)

brucek said:


> Absolutely. This where REW can help show the best sub *phase and distance setting to get that rise down.*


I'm not sure that is the approach I'd pursue...

Generally speaking, you're trying to optimize acoustic summation through the crossover frequency so that you can also maintain a flat power response. I would be willing to wager that the rise might be the result of processing inside the receiver. For example, the difference between "Direct" and "Stereo" on my Denon AVR is a 3dB rise at the crossover frequency. I've seen this show up on other systems too.

Nevertheless, if the rise is in fact the result of acoustic summation, then the best approach would be to implement a cascaded filter on the subwoofer, which would increase the steepness of the crossover, thus reducing the polar lobing that occurs from the subs' interaction with the mains. In other words, this should improve the power response of the system.

In order to know what kinds of interactions are going on, you'll need to measure each speaker separately, then do a final combined system response. If your phase and time-alignment is correct, you should see a 6dB rise at the crossover intersection, which indicates perfect summation.

Rane has an interesting article that addresses some of the concerns for crossovers:
http://www.rane.com/note160.html
Of interest here is the notion that mis-aligning the phase (like what occurs with a butterworth filter) *results in the main lobe being redirected - not attenuated*. This means that somewhere else in the room you have more bass, which is rarely ever going to be conducive when trying to optimize the sound over various listening positions.

Likewise, you should think of the subs' positions in the room as a line-array, where the physical offset between drivers changes the shape of the lobe (note that it is also frequency dependent). Here's a picture that describes the change in shape of the polar lobes, which is shown as a ratio of wavelength so that you can calculate the different lobe at each frequency for a fixed distance between sources:


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