New build for shop system

Mohawk

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Your thoughts on measuring AC current to find the Box resonant fq ?

I just put a DMM set for AC current in series with the driver and ran fq (tone)
at reasonable volume every 10 hz via REQ software .....
Started at 10 hz and went up to 100 hz...
Highest current was recorded @ 27 hz.

JBL 2235 driver in a 178L cabinet with 6"dia port around 11 1/4" long

Sound right to you guys ?
 

AngrySailor

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Procedure sounds right, the resistor method is said to be more accurate. How much I have no idea and it’s probably not enough to matter. Ball park is fine unless you’re going for competition SPL with music based on your tuning frequency, a few Hz isn’t a big deal.
 

Billboard

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Thanks, always wanted to build some speakers, don’t have the need or the room.

Do you design a box to match the resonant frequency of the driver? target freq, perhaps the box designed at the resonant freq will be the most efficient?

Any recommended reading?

I have a lovely house now, but one day I may have a barn that will need to be filled with music.

Bill
 

roccus

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You use any insulation or stiffing?? Bill build them then worry about where to put them :)
 

AngrySailor

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Billboard

You don’t necessarily build to the resonant frequency of the driver. That specification is the resonant frequency of the cone in free air... it’s a function of the moving mass of the speaker, mass of the air it’s displacing and the stiffness of the speakers suspension. This along with other specifications determine how the speaker will act when placed in a box.

To use a box calculator you need the following driver data:
Vas
Qts
Fs

Plugging these is will give you an “optimum” box volume which in my case was far to physically large to actually build. (They’re effin big as is). The Box volume (Vb) will give you an idea what the driver wants by its specs, you can alter this within reason, then you move on to port calculations which are based on the chosen Vb. Within reason you can tune different Vb’s to the same frequency although the response graph may be different. An online box calculator/simulator will show you the effects of these changes.

Here’s a good site for some reading. This fella gave me some excellent advice by email too.
(Thanks)

http://audiojudgement.com/

Roccus, no, I didn’t put any damping material in my boxes. That is used in ported boxes to dampen peaks in response caused by standing waves inside the cabinet. This is why you never build a speaker box as a cube, always three different dimensions. There are “golden ratios” which can help eliminate standing waves but it’s not really a concern on a sub only running up to a couple hundred Hz. (Still don’t build a cube!) There’s not much opportunity for octaves and harmonics. This is more of a problem on oddly shaped speaker boxes and drivers which run a wider range. Think tower speakers (tall and skinny) with a two way set up where the woofer runs up to say, 5kHz.

In a sealed box, box volume sets the resonant frequency, filling the box with stuffing lowers this frequency. Ported boxes are lined only, not stuffed and is only beneficial if needed to eliminate standing wave peaks.

What are you going to use for a crossover on your subs? An active filter or a passive low pass?
 

roccus

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I am using a DBX 234 active xover, I made some frames to hold up my Altec mid horns, I am just going to enclose them with 3/4 mfd for my subs
r57.JPG
r67.JPG
 

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AngrySailor

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That’ll work. You might want to put a few stringers to stiffen the sides. It’s amazing how much mdf will move over large unsupported panels.
 

AngrySailor

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That’ll work. You might want to put a few stringers to stiffen the sides. It’s amazing how much mdf will move over large unsupported panels.
 

roccus

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Called local hardware store today to see about some MDF, they said they don't stock it so it would need to be special ordered and they can't order under $1500 worth, but they may be able to get 3/4 countertop particle board. What did you tune your subs to??
 

AngrySailor

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Particle board will be tough to work with... any of the box stores should stock 3/4” MDF...

I believe I settled on 33hz tuning. If you tune too low, unless you’re playing music that reaches down there, the cabinet will never get into resonance and act like a large sealed box. When the cabinet approaches resonance is where you gain the +3dB as the port begins to resonate.
 

roccus

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Looks like I will have to make the trip to walmart.... in communist face mask country :( . You went to 33 I was thinking 31.5 or 32 for these and will go with a 6 port...
 

AngrySailor

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Wal-Mart for mdf? I meant Home Depot or Rona or whatever. Doubt Wal-Mart will carry construction stuff like mdf?

I don’t think you’ll tell the difference between 30,33 or even 36hz without measuring equipment.
 

roccus

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Wal-Mart for mdf? I meant Home Depot or Rona or whatever. Doubt Wal-Mart will carry construction stuff like mdf?

I don’t think you’ll tell the difference between 30,33 or even 36hz without measuring equipment.
Yes meant home depot.... Got my truck ready to register it's all insured and ready to pass insection but because of china flu I can only go to town hall with an appointment been calling for 3 days to get one but they never answer phone they say leave message and they will call back... still no call back. I need the truck to carry the MFD sheets... almost tempted to say screw it just go but the nearest home depot is in Bangor and the cops there are very nasty not to mention the gauntlet of staties I will have to go through on RT95 to get there
 

J!m

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Even during the end of days?

If you have a license and insurance card for your truck, take the bus to HD and rent their truck for $20 for 45 minutes.

I’ve rented it to get scaffolding from another company- they don’t care. Cheapest truck option.
 
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