Dance Club Sub Box Help!

[SMKSoundPro]

Dear Forum Members,

I am experiencing quite a few failures of McCauley 6256 18" drivers and changed the drivers to my personal favorites - 2240H. The DJ drives it hard, and I've got him squeezed down a bit with a DBX 2215 with peak stop limiting. The cabinets are homebrew boxes I built 15 years ago, modeled after a box we pulled out of a nightclub retrofit. The chief engineer I worked for at the time said these are good enough and to build three more of them. The JBL 2240's are not lasting any longer.

I just bought Harris BassBox Pro from Parts Express. The box is (outside dimensons) 32" high X 23.5" wide X 20" deep. It has a rectangular vent with the port opening of 5.125" X 15.875" on the lower half of the baffle board, with a total duct length of 13.875".

It seems as if the drivers are over-excurting, and burning up voice coils. There are three of these sub cabinets in a small 25' X 25' dance floor. They are powered by a Mackie 2600. The room is biamped with a homebrew Cerwin-vega box with a 12 and Peavey HT-94 tweeter using a Dayton 3.5K box-ready crossover from PartsExxpress. They work fine as a high-mid, powered by Mackie 1400i amps. Everything is a simple 8 ohm load.

I am running out of ideas and out of money for recones.

Has anybody ever had this problem before?

I read all of your posts, and live for and love my own JBL collection of pieces and parts!!!

Questions:
1. Do I rebuild the baffle board with a different size vents?
if so... what size should it be?

2. Do I look for 4520 scoops w/ 2225s?
3. Do I change to Crown Macro 2400 amps for subs?
4. Do I fire the DJ? ; or teach him how it works?

Please help as my wife, although an incredible understanding strong person, is kind of tired of listening to me talk about Q, free air resonance, box loading and tuning, torn surrounds....

Thank You.
Scott.

[duaneage]

The first thing I would do is run simulations on what the 2240H or even the McCauley drivers should be mounted in and tuned to. Then add about 20% box volume becuase as the drivers heat up the box requirements go up as well (Qts rises with hot voice coils).

Tune the box according to the simulation. Your going to need a signal generator and a voltmeter with frequency display to do this. Run the boxes hard with about 40 watts of power for about an hour or so and then retest quickly to see if the tuning frequency changed. Adjust the vent if needed.
Personally I would build a proper box and set the vent tuning a little higher than recommended. This will increase the loading on the drivers and maybe even give the speaker a bit more bump. If the DJ hears more thump he won't reach for the bass control as much,

Vents should be big. For 2 18s I would go with 2 x 6 inch tubes or my favorite : a shelf port of biblical proportions. It has been written that ultra high output systems need a port area almost equal to the driver area for maximum output with minimal power compression, At high power the port cannot move enough air and becomes restricted This means you have more of a sealed box than a vented box, and the drivers tend to move too much.

Watch a driver at resonant frequency in a tuned box. The cone hardly moves but the vent is really moving air. That is the vent damping the driver and limiting cone excursion.

In summary, design a proper box and tune it properly.

[SMKSoundPro]

Thanks for the advice.

I have put the box dimensions into bassboxpro and if I'm reading it right, the port opening is too large in area.

Maybe a new baffle board is in order. A single 2240 with the right port and duct length, I think, is ard to beat.

Using the external box dimensions of 32" x 23.5" x 20", with 3/4" x 3/4" internal bracing on all inside walls, and 1x3 side stiffeners, what results do you get for size of tuning ports?

Yes, I do have a older tube HP audio freq generator, and fluke 8060a bench meter. Please show me how to sweep for the impedance curves and tuning. I've never been confident about my hookups.

(I would never admit to anyone here at the club, what a newbie I am about some things)

Thank you all for your help!
Scotty.

[Zilch]

In each of your simulations, click down the plots at the left to check the acoustic power and cone excursion graphs, as well.

Get a high pass filter working on there to stop infrasonic frequencies the system can't handle. Set the frequency according to what the BB6P plots show is safe for your final alignment.

Consider 2241 and 2242 as potential upgrades. Run their sims to see how they'll play. It's not overexcursion that's smoking voice coils.

Definitely teach your DJ what's up and how it does it. He buys all recones after that.

[moldyoldy]

Nothing personal, Scott, but I can't help but feel your question seems to be an unusual one to come from the representative of a company with "SoundPro" in its' name.:dont-know

[duaneage]

Thanks for the advice.

I have put the box dimensions into bassboxpro and if I'm reading it right, the port opening is too large in area.

Maybe a new baffle board is in order. A single 2240 with the right port and duct length, I think, is ard to beat.

Using the external box dimensions of 32" x 23.5" x 20", with 3/4" x 3/4" internal bracing on all inside walls, and 1x3 side stiffeners, what results do you get for size of tuning ports?

Yes, I do have a older tube HP audio freq generator, and fluke 8060a bench meter. Please show me how to sweep for the impedance curves and tuning. I've never been confident about my hookups.

(I would never admit to anyone here at the club, what a newbie I am about some things)

Thank you all for your help!
Scotty.

Based on your box dimensions I get 7.18 cu ft. If your running two drivers in this box that is too small, not enough volume to properly load the drivers. Tuning a small volume like that is impossible. A single 2240H works into about 8 cu ft tuned between 30 and 40 Hz, there is a lot of leeway there. When you factor in the bracing, your looking at less than 7 cu ft. Too small a box can cause bad transient response as well. It is hard to get proper low frequency response with a small volume, the port is usually lengthened to compensate and that just makes it worse.

Stuff fiberglass in the box. Put 6 inches of fiberglass along the back wall, one side wall and one more side wall. You should have three of the six "insulated". This will add about 15-20% of volume to the box by increasing the damping of the air.

Next set up the oscillator and run a sweep from 20 Hz to 100 hz. Don't use and amplifier, just connect it to the speaker directly and give it about 2 Volts of output. Observe where the speaker actually moves the LEAST and the sound from the vent is the loudest. This is the Fvb or vent tuning frequency. If it is above 40 hertz, measure the length of the tube and add 20 percent ( you can also sometime slip another tube inside for testing to make it longer) If the tuning is below 30 or close to it cut 10 % off and retest. I would shoot for 35-40 Hz.

Measuring the impedance with the meter is trickier but more accurate. Get a small 10 ohm resistor and ACCURATELY measure it. I have the same meter you have so first hit the rel button when measuring resistance and touch the leads together. Hit the rel again and this zero's the meter. measure to the tenth of an ohm what the 10 ohm resistor is.

Connect the resistor across the leads of the oscillator ( remove it from the speaker) and connect the 8060a across the resistor as well.

Set the 8060a to volts around 2 volts scale.

Turn on the oscillator and increase the output until you read 1 volt on the meter.

Now, if the resistor measured 9.9 ohms adjust the oscillator until you have .99 volts. If it measured 10.5 ohms adjust until you have 1.05.

Remove the resistor and do a sweep of the speaker. You connect the leads to the speaker just like the resistor. Now you have a precision impedance meter that you can use to measure the driver impedance and see where the speaker resonates. we here on the forum can then offer more advice and ask what you see when you test.

PM me if you want to take this offline or have other questions. If you were not half wy around the world I would come over and help you with it.

[duaneage]

Nothing personal, Scott, but I can't help but feel your question seems to be an unusual one to come from the representative of a company with "SoundPro" in its' name.:dont-know

nothing personal, but I can't help but feel your post contributes nothing to solving Scotts problem or educating him about something he admits to not understanding.

[louped garouv]

this forum is awesome....

between Scott and Duanage, you'll have the means to solve that woofer blowing issue....

[mikebake]

nothing personal, but I can't help but feel your post contributes nothing to solving Scotts problem or educating him about something he admits to not understanding.

It's still a good point.

[Zilch]

I'm seeing a decidedly professional approach here, a desire to figure this out and develop a considered solution to the problem based upon knowledge and experience through consultation with others.

It's all good....

[SMKSoundPro]

Thank you Duaneage and Scott for the helpful replies.

These are single 18 cabs. One 2240h per box in three corners of the room, the fourth corner being the dj booth.

I have four en8 cabs that I rebuilt with a 2240, e110 and 2404 cheek tweeter that I use for mobile dj work. I drive them with Crown macro 2400 and 1200, crossed at 250hz, and 3.5k. My rig just sounds and looks great. It has funded my lifestyle quite well, and I would never consider replacing this "tool" as it really stands tall.

That being said... I agree with you regarding the sometimes less than punchy sound of a single vented 18, but in this small dance floor, this is all I have room for. I have quite a few projects on my plate and was looking for some simple advice and help with this speaker problem. I did not mean to imply that I am incapable of hooking up my own test bench. I was asking for a simple refresher course.

I keep looking at the plans I have for JBL 4719s. I'd like a pair of those in our larger ballroom at the club. Dual 18s in those cabs are big and appropriate for the bigger room and in a few weeks, I am hoping that our loud dj will go into that room with the bigger rig. Its all a case of politics right now, and firing the resident dj in that big room.

There are a pair of 4520 scoops next door at the music store for sale. I don't think they are a real jbl cab, just a knock off. In my dealings with my next door neighbor music store, it seems as if the employees are just looking for their next hit on the proverbial bong of life, rather than having a serious discussion about pro sound. The pity is, they are the only game in town. I just can't see how I would build scoops into this room because of the drink rail at 39" off of the floor that rings the room that I built in there 8 years ago. In reading on this forum about 4520 single scoops, I recall someone posting that in the horzontal position this particular writer did not like them or their properties.

Please accept my apologies if I have offended any of you that feel my questions were too elementary for someone with soundpro in his name. It started out as sound productions, and got shortened along the way.

When I worked at Alaska Sound Labs for 17 years, I learned ALOT! The owner/eng was my mentor, sometimes father figure, and then my aa sponsor. When I took the fulltime maintenance man, builder, resident sound guy job at the newly reopened largescale gay bar/ nightclub, he thought I had sold my soul to the devil and we parted as business partners. We don't talk and I am learning on my own what he knew, one little piece at a time.

Way more than any of you care about, I'm sure.

Thank You!
Scotty.

[SMKSoundPro]

Oh and by the way,

I am using a Mackie 2600 to drive these three 8 ohm subs, and am using the 35hz low pass filter on the back of the amp.

A friend has two racks of older crown macros available. 6 -1200s, and 2 - 2400s, but money is tight. I am trying to work a deal with him, and change both dance floors to these crowns.

smk.

[johnaec]

These are single 18 cabs. One 2240h per box in three corners of the room, the fourth corner being the dj booth.

I have four en8 cabs that I rebuilt with a 2240, e110 and 2404 cheek tweeter...

OK - 'first question - are the E110's in their own "doghouse", (inside cabinet)? What is the volume, (or at least the measurements), of this doghouse cabinet, (which must be subtracted from the total inside volume to get the true net volume the 2240 sees)? Four 2240's should be plenty for a 25x25 area! I'm thinking your port volume is way off, but we need the *net* inside volume before drawing any real conclusions.

John

[Zilch]

I am using a Mackie 2600 to drive these three 8 ohm subs, and am using the 35hz low pass filter on the back of the amp.

You mean high pass, I presume. Low pass would be set consiberably higher, at the crossover frequency.

The subs are separate from the EN8s, right? I think there may be some confusion here, now....

[SMKSoundPro]

Dear Johnaec,

The single 18 cabs in question are:

(external Measurements)

32" tall x 23.5 wide x 20" deep

A single, simple sub cab with a 2240h.

A rectangular vent of (interior) 5.125h x 15.875w and 13.875 duct length.

.75 AB fir plywood. 1x3 braces on opposite sides. .75square bracing at all intersections.

filled with 1.5' fiberglass stapled all around interior sides and back. none on the baffle board.

braced baffle to back with a 1x2 attached also to the duct.

duct made from .75" plywood

2240 is face mounted to the baffle with t-nuts and allen head screws, not inset or rear mounted.

If I am using this new bandboxpro software correctly, it seems that the tuning is way off.

If I cut/bust out the plywood duct, and install round port tubes in a plywood cover plate to cover the rectangular vent cut-out...seems like a good fix.

What to you get for the recommended tuning length of 2 or 3 - 4" ports

by the way,
I saw Scott Fitlin's 2x18's on this forum for his bumper car ride and they look outstanding!

I would love the plans to build those for our larger ballroom/showroom/dancefloor.

Thank You.
Scotty.