# Null vs Dip



## counsil (Mar 1, 2009)

What is the difference between a null and a dip?

Here is the frequency response of my dual SVS PB13 Ultras. I know I need to get the subs out of the front corners, but I can't do that right now because my entertainment system takes up 3/4 of my front wall. I know I have a lot of room gain because of it.

How does the graph below look? Do I have a null between 35Hz and 70Hz or is a more of dip because it is less than 5dB under 75dB? What about between 80Hz and 120Hz? That region is a bit narrower.










Just in case you are curious, here is my current frequency response. Also, I don't have any room treatments. I have a buddy that wants to help me make some "free" room treatments out of fiberglass insulation boards. He is an electrician for a company that makes fiberglass insulation, so he can get me all the scraps I want. I have taken advantage of this many times!


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## tonyvdb (Sep 5, 2007)

If you play with the phase on either of the subs and redo the test with REW if the dip moves you have a Null if it stays the same it is just a dip if I remember correctly.:hide:


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## counsil (Mar 1, 2009)

Here is my frequency response as REW and my digital RS SPL meter shows it...










Maybe this tells a much different story because there is no smoothing going on. However, the SPL meter is not very accurate.


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## counsil (Mar 1, 2009)

tonyvdb said:


> If you play with the phase on either of the subs and redo the test with REW if the dip moves you have a Null if it stays the same it is just a dip if I remember correctly.:hide:


Interesting. I will try that.


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## Sonnie (Apr 11, 2006)

If you are using our correction calibration file for your RS SPL meter from the Downloads Page, then it should be fairly accurate from 15-200Hz. What is not accurate is the "After" you are seeing with the SVS graph... that is only what that system is telling you it should be, not what it actually is. You need REW to tell you what the before and after is to be more accurate.


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## counsil (Mar 1, 2009)

Sonnie said:


> If you are using our correction calibration file for your RS SPL meter from the Downloads Page, then it should be fairly accurate from 15-200Hz. What is not accurate is the "After" you are seeing with the SVS graph... that is only what that system is telling you it should be, not what it actually is. You need REW to tell you what the before and after is to be more accurate.


Sonnie,

Here is my after REW graph...










Here is my before (same as above) and after combined...










I guess I kinda got lazy when I was uploading the graphs. I was trying to focus more on the dips/nulls. Can you tell whether or not I have dips versus nulls from the REW graphs?


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## Sonnie (Apr 11, 2006)

It does not appear to me that you have any serious nulls, no. The response looks pretty good. 

I do think you are a bit aggressive on that filter around 22Hz... it appears to be too narrow and/or too much cut. If raising it increases the areas around it, then try increasing the bandwidth.

I am going to move your thread over to the REW Forum for more professional help... :bigsmile:


EDIT: Okay... maybe the EQ1 is a bit aggressive... I quickly forgot you were not using the BFD. :duh: The professional help for the SVS Sub-EQ is in the SVSound forum. :whistling: I will move it back!


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