Amplitude at Crossover Frequency - Home Theater Forum and Systems - HomeTheaterShack.com

Old 05-02-07, 09:36 PM Thread Starter
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Otto

Join Date: May 2006
Posts: 1,356
Amplitude at Crossover Frequency

OK, I've been looking around for info about the measured level of speakers at the crossover. I saw a graph that Wayne posted here that showed the signal level being down 3 dB at the crossover frequency. I've always thought of the crossover point as the -3 dB point as well. However, I measure my mains rolloff as 6 dB down at the crossover point. I've also read somewhere that speakers should be 6 dB down for even-order filters, and 3 dB down for odd order filters. I'm not sure if I buy that...

Anyway, discussion on the above is welcomed.

Tonight, while playing with making an LFP out of my BFD, I was playing with the rolloffs for the sub, as applied to target level. There is no house curve loaded. By playing with the crossover slope, and cutoff buttons in REW, I can get the target level to appear as either of these; 3 db down at the crossover point (40 Hz) or 6 dB down at the crossover point. The target is 75 dB, no house curve, nothing but the target as calculated by REW.

???

Graph 1: Target=75 dB, Crossover=40 Hz, Response Shown 3 dB Down at fc

Graph 2: Target=75 dB, Crossover=40 Hz, Response Shown 6 dB Down at fc

-- Otto
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Old 05-02-07, 10:05 PM
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brucek

Join Date: Apr 2006
Posts: 7,514
Re: Amplitude at Crossover Frequency

Not sure what your question is, but the REW targets are a standard 6dB down at the crossover frequency. As you know, if you combine two signals at the same level and frequency you'll get a 6dB gain as a resultant.

So in a perfect world, the red line (shown below in the pic) would be the resultant of a set of mains and sub when combined and given the standard crossovers. A perfect flat response...

brucek
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Old 05-02-07, 10:52 PM Thread Starter
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Otto

Join Date: May 2006
Posts: 1,356
Re: Amplitude at Crossover Frequency

Yeah, makes sense to have it be 6 dB down. I edited my post to include descriptions of the graphs. Note that REW has calculated two different responses for a fourth-order 40 Hz LPF.

I think it's a bug in REW. You can cause it to happen by playing with the "Cutoff" and what I'll call "slope" controls in REW. Change slope, change cutoff, change slope... Something like that. You'll see it change the x dB down point for the target.

Wonder if anyone (myself included) has taken it as the target, and later had problems integrating with mains...

-- Otto
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Old 05-03-07, 07:41 AM
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brucek

Join Date: Apr 2006
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Re: Amplitude at Crossover Frequency

Quote:
I think it's a bug in REW. You can cause it to happen by playing with the "Cutoff" and what I'll call "slope" controls in REW
That bug has already been noted before. See here.

John has fixed it and it will be repaired in the next rev....

brucek
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Old 05-03-07, 08:14 AM Thread Starter
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Otto

Join Date: May 2006
Posts: 1,356
Re: Amplitude at Crossover Frequency

Thanks/oops. Missed that, or forgot about it.

-- Otto
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