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This topic comprises 2 pages: 1 2
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Author
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Topic: Build a Subwoofer
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Antonio Marcheselli
Phenomenal Film Handler
Posts: 1260
From: Florence, Italy
Registered: Mar 2000
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posted 10-05-2001 02:23 PM
Hello everyone.From my theater I get one 46cm JBL speaker from our broken subwoofer. It is model 2240 (if I remember correctly) from our old 4668 (same).
I found a room where I can build my HT without problems with neighbours.  
So I would like to build my own Subwoofer using the JBL speaker.
Question: I bring the speaker in an Hi.Fi shop where they tested it and the gave me some project of speaker but the lower frequence was not enough for me (60Hz, is it a subwoofer?  )and I asked them to make others better project.
I wonder if it is more simple to copy the 4645C JBL subwoofer that has a more powerful speaker but with the same diameter.
What so you think? I saw that 4645C can handle 35Hz without EQ and 22 with EQ. So you think that I can obtain more with a bigger cabinet? Perhaps JBL optimized that subwoofer to work better with more than one cabinet...
I would like to be sure before building it! Equalization. I could buy a parametric equalizer for the subwoofer and I've read on these forum few time ago that setting parametric equalization with ears is not so difficult. Why? Last trouble: I get the theater's amplifier. It is a crown 2401. Just for fun I've tested the speaker in my bedroom to see if it works but when I turn off the amplifier I can hear a loud "TOC" and the cone goes forward in a bad way. I can't realize if the "toc" are frequencies coming from the amplifier or if is the coil that hit the bottom of the speaker; should the crown 2401 have some protect at shutdown? I didn't recall any "Toc"s when I powered my subwoofer in the cinema... Thanks for all your suggestion! Bye
Antonio
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Barry Floyd
Phenomenal Film Handler
Posts: 1079
From: Lebanon, Tennessee, USA
Registered: Mar 2000
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posted 10-05-2001 04:01 PM
When I started working as a mobile DJ back in 1984, I tried building my own subwoofer. What I found - that by the time I spent building the cabinet and buying the electronics, I could have bought one cheaper. Another downside of home-built speakers - they have NO re-sale value, no matter what they're loaded with.I've got 3 subs now (one for each of our systems). Two of them are Cerwin Vega L36P's (two reverse fold 18" drivers) - each about the size of your average washing machine. The third is an Electro-Voice subwoofer, and it is just incredible. It only has one 18" driver - but it blows the Cerwin Vega's away!! If I could afford it, I'd replace them all with EAW's. As far as your amp is concerned. An old gray-haired DJ taught me one night of the best "rule-of-thumb" I've ever learned. When powering "up" your system - the amplifier is the LAST thing to be turned on - and the FIRST thing off when powering down. Those "TOC's" can fry a voice coil in a heartbeat.
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Barry Floyd
Floyd Entertainment Group
Nashville, Tennessee
(Drive-In Theatre - Start-Up)
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Zac Manning
Film Handler
Posts: 5
From: Des Plaines, IL, USA
Registered: Oct 2001
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posted 11-04-2001 01:57 PM
My two cents on using a parametric equalizerTypically on systems that I have looked at where people used their ears to eq, they end up boosting the hell out of lows and the highs (the fletcher munson curve explains this) so that it isn't flat like you want it to be. Also, you said that with eq you were able to get it to 22hz, or 20hz. What software was used to determine this? and when it reached 22, where was it compared to the rest of the frequency spectrum. I think that is sort of fighting the natural properties of the speaker when you push a frequency out of it that doesn't want to some out, are you able to hear 20hz? If you can't why worry about it. Very few people can hear 20hz or 20khz
You might be over heating the coil by forcing the frequencies that want to roll of 10 or 15dB from maybe 30hz to 20hz.And usually it doesn't sound very good when you forse those frequencies into a speaker. And in boosting those frequencies you are in turn causing a lot of phasing issues. I would suggest trying to get it to sound the way you want by adjusting the levels on the amp first before you even touch the EQ.
Don't forget the room either, what are the dimensions of the room? You might have some standing wanves or room modes that accentuate the low end, especially if the dimensions are multiples of each other. i usually never boost with the Eq, only cut out the places where you have spikes, and a parametric eq is a great way to do that.
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Zac Manning
Film Handler
Posts: 5
From: Des Plaines, IL, USA
Registered: Oct 2001
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posted 11-04-2001 06:08 PM
I think they would sound like shit for a lot of reasons. The "Q" or directivity factor of those speakers vs speakers designed for cinema playback is going to be very different. Also, dialogue is rarely intelligible without the use of horns when you are behind screen, at least that's how I hear it. And yes, those specs are going to be a little wonky, and those amps probably don't sound that good to begin with i.e. lots of harmonic distortion, poor dampening factor etc. But you might actually be getting 1000 watts, it just matters what load they put on the amp. You can check for yourself by taking a 1000hz sine wave and sending it into the amp, on the output take a scope and look at the wave form till it clips, read the A.C. output voltage accross the output load with a VOM, calculate the R.M.S by taking that voltage and squaring it, and dividing it by the impedance. So there is not "real power" per say. If your R.M.S. value is 400, you can get as much as 400 watts if the load is one Ohm, or as little as 25 watts if the load is 16 ohms. But i still would never you anything on a car in cinema. And I personally think THX is sort of over rated.
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