Info on crossover slope?

[Dr. Bob]

I just got a new subwoofer, and am trying to set it up with my Marantz SR5008 AVR. I initially set the crossover to 80 Hz, then played a series of bass test tones to check for room modes. To my surprise, at 60 Hz there was still a lot of sound coming from the main speakers. To check what was going on, I turned off the sub and began raising the crossover. Even up at 200 Hz I was still getting significant sound from the mains.

I was expecting (not sure why) that the crossover slope would be 24 dB/octave. From 200 Hz, 60 Hz should be almost 2 octaves down, so I shouldn't be hearing anything at all from the mains. My Marantz manual doesn't say one word about slope, so I started looking around online. It seems that there isn't really a standard: slope can be 24, 12, or even 6 dB/octave. It can even be different for the low-pass filter that sends bass to the sub vs. the high-pass that sends sound to the mains. (One thread suggested 24/12 for these was the usual, but someone else contradicted that.)

Is there any way to get information about the slope? I don't suppose there's any way to change the slope?

 

[herbu]

there was still a lot of sound coming from the main speakers.

1. Are you able to measure the freq of the "sound"? There is, indeed, a lot of bass "sound" above 80Hz.
2. Are you sure your mains are set to "Small"?

 

[slipperybidness]

Unlikely that there is a way to change the slope, but it depends on the sub. I'm pretty sure my Rythmik has some options to alter the slope, but I haven't fiddled with it in a long time.

You could get a SPL meter, take measurements, plot the results, and determine the slope from real world data using you plot.

Note that the slope is only constant from the -3dB point on down. Often, you could do a Bode Plot to simplify the plotting a little.

 

[j_garcia]

On the speaker side it is usually 6dB/octave while on the sub side it would be 12dB/octave. AFAIK, this is done intentionally to allow the mains to blend smoothly as it more gradually rolls off, while the sub gets filtered faster to minimize localization. I can't recall a unit that allowed this to be changed and some may use steeper slopes.