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Discussion Starter · #1 ·
I used Win ISD beta from LinearTeam to design and build a box. The Vas for the driver was 3.4 cubic ft and Fs is 35 Hz. My box is a ported 7 cubic ft and tuned to 30Hz. Will that be bad for the speaker? I have noticed that at some higher volumes, the speaker sounds like something is rubbing it at low frequencies. Like a plastic sheet almost or as if the speaker had no box. But it's only at low frequencies. Is it because of the large box and box being ported?


BTW, the sub crossover is at 80Hz and the vent mach is calculated as .03. Other than that one probelm, the sub sounds and feels awsome.
 

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Discussion Starter · #2 ·
I checked it last night and it only happens at the lowest frequencies. When the sound is almost inaudible, I would hear the cone moving more than the sound coming out of it. I'll pull the speaker out to see if maybe the wire or polyfill is touching the speaker.
 

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That sounds like the 60-80Hz region, an octave or so above where it was tuned and where the crossover is. It could be the large box and box being ported.


I'm going to try a sealed sub design Q.707, and the 18" Maelstrom driver puts out 20Hz I think in a 22" box.
 

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Discussion Starter · #4 ·
The 60-80Hz region is fine. It's down near the 30Hz area that the noise appears. It could be the large box, but there are so many that use subs like SVS and infinite baffles that are bigger than mine. The ports are a possibility. There are 4 of them on 4 sides of the speaker.


I pulled the speaker out so I could paint the box. Nothing touching the speaker inside, but I did notice that the rubber ring around the speaker is loose in one spot. The rubber ring is there if you want to put a grill cover over the speaker. I may try to reattach the rubber or just remove it all. I think that might have been the culprit.


I didn't go with a sealed design because the lower frequencies drop off too much. I'm using my old car audio competition speakers. They are 12" Blaupunkt Velocity Pro Component dual voice coils. I thought about building a sonotube design, but figured it wouldn't matter because it's going to be hidden anyway, so a box design would be fine. Plus an 18" sonotube only comes in 12' length and costs $60. I also thought about using 2 subs mounted on adjoining side of one box. Then wire them up like an iso-baric push-pull design. But that may come later.
 

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Looks like the rubber ring logically might be the culprit - the 30Hz is the Fs resonance driver freq (tuned) right?


With an 18" sealed design I want to cross my sub @40-50Hz with the mains (20-25Hz +/-.5 to 3dB). With an overlapping octave the sub should be "clean" to double the crossover point - or 80-100Hz. If I use an Fs of 32Hz, sub is down 3dB at 80.8Hz and 16Hz... Crossovers are 6dB passive caps and inductors in addition to sealed 12dB/octave design.


Right now I have a Tempest 15" sealed active sub w/250 watt plate amp crossed at 70Hz with 35Hz main JBL S312's. The sub amp is run from LFE outs on Class AB NADC370 int amp. I'm going to bypass the NAD C370 Class A active preamp section with 2 channel pairs from an "audiophile" sound card – the RME HDSP9632 + expansion board 1st.


My current system and room setup do very good already for a 18-20Hz-20KHz “flat†response (+/-3dB), but there isn’t much I can do about getting that 15-16 Hz at +/- 1dB – I’d have to tune it for Fs 20Hz and it would drop off 3dB at 50.5Hz (on the other end)…


But I think it is adequate for getting WAY down as I want... ;)
 

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you might try WinISD Pro Alpha and look at the cone excursion numbers. Given your port tuning it seems likely to me the frequencies at which you're hearing the noise at just about where max movement will occur (well excepting well below the port tuning of course).
 
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