# Subwoofer inverted impulse response



## vidger (Apr 12, 2008)

Hi guys,

I've been using REW for quite sometime now, and I'm currently digging a little more on the interpretation of the impulse response. Attached to this post is my subwoofer impulse response. 
As you can see the strongest peak (100%) is positive. In order to obtain that positive peak I had to invert the polarity of my sub ( jbl 2241h in a bass reflex cab). If I hadn't done the inversion, my subwoofer impulse reponse would be the same but inverted, with the strongest peak being negative.

So my question is: is this natural inverted impulse response a characteristic of a bass reflex cabinet? Should I invert the polarity of the Jbl in order to get a positive 100% peak, or should I leave it just the way it is? 
I just wanted to have your thoughts on that, because otherwise I'm really satisfy with my system bass response. 

Thanks in advance


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## brucek (Apr 11, 2006)

> Should I invert the polarity of the Jbl in order to get a positive 100% peak


You should return the polarity to its correct position. The impulse is the result at the microphones listening position. The phase it's received at in relation to the mains is dealt with after the fact (using the frequency response plot) and the subs phase control. The impulse is a raw measurement that doesn't include the soundcard or meter calibration.

brucek


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## vidger (Apr 12, 2008)

Thanks Brucek,

So, I should return the polarity to its original setting... But If I do that my impulse will be negative, which is not the case for the other channel I' ve measured... 
The impulse response attached with my previous post was capture from my sub only, and the results were the same even when I changed the mic position and put it very close to the JBL ( 1")...

All of my other speakers (jbl 2445, 2425, d123,) I've tested shows a positive impulse when the polarity is correctly set. Why is is that for a subwoofer it seems to be the other way around? 

Regards,


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## JohnM (Apr 11, 2006)

The sub amp might be inverting, or the driver connections reversed.


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## vidger (Apr 12, 2008)

OK I'll check it,

You confirm that, in theory, a normal "ported" subwoofer, should give a positive impulse response if the polarity is correctly set?

Thx


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## JohnM (Apr 11, 2006)

The response should be positive for any kind of sub. Some mic amps are also inverting, but since you indicate all the other channels have positive polarity the inversion must be in the sub path.


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## brucek (Apr 11, 2006)

> The response should be positive for any kind of sub.


Huh, interesting. 

I always figured that there were so many pieces of equipment involved in the measurement chain that could invert the signal that I had always discounted what REW showed for the impulse with respect to positive or negative.

Here's my two subs from my two different systems.

Each used the same mic, cables and mixer preamp, but the computers and AV systems are different.

First pic is my Energy EPS-150 sub showing a positive impulse and the second is my very nice Servo-15 showing a negative impulse. :huh:

To it seems insignificant, as the resultant signal when combined with the mains is cleaned up with the phase control.

















brucek


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## JohnM (Apr 11, 2006)

Perhaps I should qualify my statement by adding "when the sub's phase control is set to 0/normal". If phase is set to invert then the impulse would reflect that.


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## brucek (Apr 11, 2006)

> when the sub's phase control is set to 0/normal


And what about the receivers/processors distance time delay on the sub channel?

brucek


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## JohnM (Apr 11, 2006)

brucek said:


> And what about the receivers/processors distance time delay on the sub channel?


No effect, in the processor it really is just altering delays.


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## brucek (Apr 11, 2006)

> in the processor it really is just altering delays


But how is the phase control any different, other than it is in the analog domain?

Does the phase control (as a low Q, all pass filter) not offer linear phase response over its range and result in the same effect as the time delay?

brucek


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## vidger (Apr 12, 2008)

Hey guys, glad to see this thread encourages discussion!

I have another question, when looking at the graph posted by Brucek I noticed that the shape correspond (roughly) to my sub graph, but in the time domain, my sub peak is at + 5ms where Brucek's one is located at exactly 0ms. What hypothesis can we infer out of these observations?


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## JohnM (Apr 11, 2006)

brucek said:


> But how is the phase control any different, other than it is in the analog domain?
> 
> Does the phase control (as a low Q, all pass filter) not offer linear phase response over its range and result in the same effect as the time delay?


No. The typical normal/invert sub "phase" control is simply a switchable inversion. For variable controls it is an all pass that is reasonably linear through the middle of its operating range but not at the ends.


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## JohnM (Apr 11, 2006)

vidger said:


> I have another question, when looking at the graph posted by Brucek I noticed that the shape correspond (roughly) to my sub graph, but in the time domain, my sub peak is at + 5ms where Brucek's one is located at exactly 0ms. What hypothesis can we infer out of these observations?


That the measurements were made with 2 different versions of REW, one which used the IR peak as the time=0 reference point and the other which used the first point before the peak where the response exceeds 10% of peak level. The location of the t=0 reference doesn't make any difference to the IR or the frequency response, deciding where to place it is not important until phase or group delay are being analysed (which REW currently doesn't offer).


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## vidger (Apr 12, 2008)

*Re you working*

Thank you John,

So, if I want to align (temporally) my sub with my mains, by capturing the sub impulse and afterwards my mains impulse, I cannot use REW to draw any conclusions from potential delay between the impulse peaks of the mains and the sub?

Is it a feature you are currently working on for REW?

Regards


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## JohnM (Apr 11, 2006)

*Re: Re you working*



vidger said:


> So, if I want to align (temporally) my sub with my mains, by capturing the sub impulse and afterwards my mains impulse, I cannot use REW to draw any conclusions from potential delay between the impulse peaks of the mains and the sub?


Correct



vidger said:


> Is it a feature you are currently working on for REW?


Yes, V5 allows a left channel loopback to be used as a timing reference.


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## vidger (Apr 12, 2008)

Well... REW V5 sounds fantastic!!!

If you're looking for beta testers, you know were to find me :bigsmile:


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