The effect of a subwoofer's port spl on overerall Frequency Response.

[Exocer]

I realized something the other day about my velodyne. It is capable of playing up to 300hz at reasonable output levels. My receiver's crossover is setup to 80hz, but is is still giving a lot of output into higher frequencies. I decided to completely overhaul my set filters. I ended up switching back to a 180degree phase which killed a 10db null between 50 and 70hz. A side effect was a major dip in the 200-500hz range (sub + mains sweep)...So I disconnected my mains and tested the sub alone with REW and SPL meter in hand. The sub seems to be playing into these frequencies potentially causing cancellation. Because the sub is crossed over so much lower than this range I can only assume it is port sound. I re-created a house curve with the BFD and made sure I got nothing over 60db in the 200-300hz range from the sub. Connect my mains again and voila! that null is gone and things sound A LOT better. It seems that this may have something to do with the sub being ported as I didnt have to deal with this problem on the Rythmik sealed sub.So, does one have to keep in mind the effects of port resonances when using a BFD to EQ things? I think so. Also it is very important to run a sweep with mains after all sub EQing, you may be suprised how your sub can cause cancellation with your mains.

Moving the sub out of the corner and closer to the mains also did magic for music. IMO everyone who has this option should try it.

[Wayne A. Pflughaupt]

Interesting post, Robert. As I understand it, though, a speaker’s (or sub’s) port is tuned below the driver’s front output. As such, there’s no way what you were experiencing could be caused by the port.

If you were getting that much energy in the 200-300 Hz range, the first thing I’d suspect would be the crossover. Most Dolby Digital receivers have crossovers with 24 dB/octave slopes. This means that anything generated at 300 Hz should be nearly 48 dB below what’s generated at the 80 Hz crossover frequency (since 300 Hz is almost two octaves above 80 Hz). You should be able to verify this with your SPL meter and some sine wave tones at those two frequencies.

Assuming the crossover is functioning as it should, it can be tricky to determine what’s causing the problem. My first guess would be that a room mode is boosting output in that region and essentially over-riding the crossover. If that’s the case, the sub’s output could be added on top of what the mains are doing there. But since you say you didn’t have this problem with your other sub (assuming nothing else changed, especially the room), we have to rule that out.

Either way, you did the right thing by eliminating the excessive output with the BFD. You could have also dialed in the sub’s built-in crossover on top of the receivers to effect a functioning slope of either 36 or 48 dB/octave, depending on the sub’s crossover’s slope. This would have reduced any output at 300 Hz to 72 or 96 dB below 80 Hz.

Regards,
Wayne

[Exocer]

The sub's crossover proved pointless. All it did was create a large narrow decrease in output @ the frequency it was set to, not above or below it. I wish i saved those graphs...But thanks for letting me know I did things right I feel like this is a perfect example of the BFD being used as a crossover lol.

[Sonnie]

JohnM shows us how we can use the BFD as a crossover... you could simply adjust the filters upwards to accomodate a higher frequency cutoff. Takes up a few filters though. You might have to loop your left and right channels on the BFD and use the second channel if you end up using more than six on your sub's response.

50hzxover.jpg

50hzxover.png

aura50hzlp.req

[Exocer]

Sonnie, that is a cool looking graph