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rdgrimes
06-20-2010, 02:12 PM
Recently while messing around testing some other gear, I was running some 20, 32 and 40Hz sine wave tones through my L112. Driven by Carver M-4.0t (380WPC). I kept noting an odd distortion-like sound and finally realized it was the ports. I could actually feel the wind blowing out of those ports, and putting my hand over the port stopped the sound. It would vanish at 40Hz but get progressively worse below 40. Even at fairly low amp levels it was plainly audible.

So, I've finally discovered what "port chuffing" is. Obviously it would occur pretty rarely in normal music listening, but it prompted me to wonder what might be done about it. Don't later speakers use flared ports to prevent this? Is there some port mod or replacement that might fix this?

Robh3606
06-20-2010, 02:20 PM
but it prompted me to wonder what might be done about it.

Hello rdgrimes

Use a the largest port you can to keep the velocity low and use ports that are flared on both ends. You can modify them using decent box software to get the same box tunning.

Rob:)

Loren42
06-22-2010, 05:00 AM
Well, what frequency is the port tuned to?

I think you are below the tuning of the cabinet and once you get below that frequency the excursion of the woofer gets very large and pushes high volumes of air.

The solution would be a filter on the input of the amp to cut low frequencies below the tuning of the cabinet. This will protect the woofer, too.

Flared ports help, but I bet that you are below the cabinet tuned port frequency and things get excitable very fast below that point.

rdgrimes
06-22-2010, 08:49 AM
No question that when L112 was designed, a 40Hz filter was assumed. It's pretty amazing how much air is blowing out of the ports at 20Hz, you can feel it 2' away. :eek:

Robh3606
06-22-2010, 10:34 AM
No question that when L112 was designed, a 40Hz filter was assumed.

I wouldn't make that assumption. Those are fullrange bookshelfs that were designed to handle the typical bass content available in the media at that time. 20Hz sinewaves were not on the agenda. Any bass relex box in going to have issues with a 20Hz sinewave of any real power. It's simply too low and below the box tuning.

Rob:)

rdgrimes
06-22-2010, 10:50 AM
I wouldn't make that assumption. Those are fullrange bookshelfs that were designed to handle the typical bass content available in the media at that time. 20Hz sinewaves were not on the agenda. Any bass relex box in going to have issues with a 20Hz sinewave of any real power. It's simply too low and below the box tuning.

Rob:)
The media at the time was LP, which employs a 40Hz filter more often than not. AFAIK, the RIAA EQ curve includes the filter, and most Pre-amps and receivers of the day did as well.

FWIW, my PS1400s don't have any chuffing with 20Hz sine waves. Of course they don't make much sound of any kind at 20Hz either, and just a little bit at 32.5. ;)

Robh3606
06-22-2010, 11:33 AM
The media at the time was LP, which employs a 40Hz filter more often than not. AFAIK, the RIAA EQ curve includes the filter, and most Pre-amps and receivers of the day did as well.

The only thing that was common was a subsonic filter for "rumble" below 20Hz. There is no 40hz filter in the RIAA curve. It hinges at 1K and has +20db boost at 20hz and -20db at 20Khz.



http://sound.westhost.com/project99.htm

4313B
06-22-2010, 12:08 PM
No question that when L112 was designed, a 40Hz filter was assumed. It's pretty amazing how much air is blowing out of the ports at 20Hz, you can feel it 2' away. :eek:I highly doubt any such consideration was given.

The L112 is a good example of a too high compliance woofer paired with a too small diameter port in a relatively small box tuned a bit too high. It could use a 4" to 5" diameter port tuned about 5 Hz lower. Or just put it in an L150A box with a PR300. :D

In any case, the L112 filled the need for the standard JBL 12-inch 3-way bookshelf. It could take alot more abuse than the 10-inch 3-way 4313B or L96, accuracy be damned. It was also known as the Century II.

The LE14H has roughly half the compliance that the 128H has. The LE10H has roughly one quarter of the compliance that the 128H has. Interestingly, the older 124H/2203H had an even greater compliance than the 128H but it had a 4-inch voice coil and was able to control the moving mass very well. It worked very well in that same "bookshelf" volume tuned up to around 40 Hz. It was a very dynamic performer.