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FTW 21 ported design questions

post #1 of 18
Thread Starter 
I'm looking for advice on a ported design for this driver. I've searched and other than mention that it likes 25 to 30 cf I have not found anything.

Room: Small poured concrete bunker (aka apartment). main room, 20x20x8

Design considerations:
- flat down to 12 - 15 Hz at 115db
- No highpass filter required

Design style: sonotube (3rd story of a 3 floor walk-up, no 400 lb refrigerators allowed smile.gif)

I've been playing with WINISD and have looked at 3 sizes tuned to 12 or 15 Hz: 20cf 22cf and 25cf. Larger than that will not work.

Given the stupendous room gain I get in my 'bunker' flat to 12 Hz is probably a non issue even at 20cf and 15 Hz tune.

Port velocity and the first resonance is a concern.

First question. For a 10" port (lets assume flaired) what is the max velocity before turbulence occurs? I'm guessing max velocity will go up as port size increases. I've done some searching, but there does not seem to be any definitive answer on port size and velocity.

At 25cf and 12 Hz tuning a 10" port would be 28 meters/sec at max power, 20 m/sec at 600w. I'm guessing I'll never get near max power, but don't know. With a flared port, is 28 m/sec acceptable?

Anything smaller than 25cf it looks like I'm forced to go to a 15 Hz tuning to avoid port resonance and velocity issues.

Tuning:
Can I get away without a highpass filter with a 15Hz tune? That would make port length and the first resonance more manageable (between 170 and 220 Hz for the three volumes) What is the lowest acceptable 1st port resonance assuming an 80Hz crossover?

Driver and port clearance:
I understand I will need a minimum of 10" clearance for the port, what about the driver? With an 8' ceiling, I was guessing a max tube length of 6.5' to give proper port and driver clearance.

Anybody know the volume for this driver?
post #2 of 18
Just out of curiosity, with amp power cheap these days and the stated massive room gain you have - why not go with a sealed box? It would be much smaller, easier to build, and (to some) sound better.

Jim
post #3 of 18
Thread Starter 
a 10 cf sealed sub would be down 18 db at 12 Hz. I think that would be a little much for my room to overcome. With 1,200 w of power handling, I don't know how much room I have to apply EQ.

My current sub has an f3 of 27 Hz and should be down 12 db at 22 Hz. In fact it plays flat to about 21 Hz.

The idea of a sealed sub is appealing, though it would be fun to watch my landlords eyes bug out when I hauled 6.5' of 28" sonotube into my apartment biggrin.gif
post #4 of 18
I don't think you have as much LFE lift as you think. The FTW-21 in a 10 cu.ft. box should have a gentle slope, which combined with room gain should be impressive. My CHT 18.2 subs are 3db down in the high 30's and get me flat to around 10hz with a little eq. The FTW-21 should be able to handle much more than 1,200 watts on a short term basis (in a sealed box).

Jim
post #5 of 18
Thread Starter 
Quote:
Originally Posted by zora View Post

I don't think you have as much LFE lift as you think.
Why do you say that?

I don't understand driver power handling, but I thought that the number the manufacturers gave, in this case 1,200w, was the max safe power you could apply before coils started to burn.
post #6 of 18
There's thermal and mechanical limits. Reach either of those and your speaker will not be happy and might even die. Now, thermal limits are a little enigmas. For example, and I am just making up numbers, but it might be something where the driver can handle 1.2kW for 10 seconds at 20hz, but maybe only 700W for 10 seconds at 300Hz due to a decrease in air moving over the coil from less excursion. That might also mean it can handle 3kW for 1 second at 50Hz or maybe 8kW for 50ms? It's all about keeping the voice coil under a certain temperature and it can only wick away heat so fast, but hard transients may mean sending 2-3kW peaks for a very short duration, around 40-50ms with maybe 10-20x of minimal power where that heat can be wicked away.

A voice coil's temp acts similar to a rectangle function (input signal) convolved with a decaying exponential (heat sinking ability). This results in the voice coil temp being the blue line.

Edited by Looneybomber - 4/24/13 at 12:28am
post #7 of 18
Thread Starter 
Thanks for the reply Bomber. If that peak is above what the driver can handle, bad things may happen, correct? That is what I thought the manufacturers power handling number was an indicator of.
post #8 of 18
Thread Starter 
I thought I would post some graphs. Here is a 25 cf enclosure, 1,200w applied. You can see that with a 10" port, the first port resonance is awfully low.



Here's the port velocity:



Doing the image capture, I noticed that at 12Hz the port velocity is not that bad. How often do you suppose there is going to be 10Hz content? Is a peak velocity of 26 m/s going to cause audible chuffing with a 10" flared port?

25 cf is rather large... eek.gif
post #9 of 18
Thread Starter 
Here is 10 cd sealed.



Just how much extra power is this sub going to need in a sealed configuration to play flat to 12-15 Hz?
post #10 of 18
Quote:
Originally Posted by fredfish View Post

Thanks for the reply Bomber. If that peak is above what the driver can handle, bad things may happen, correct? That is what I thought the manufacturers power handling number was an indicator of.
Well, yes and no. Yes it's an indicator of how much power it can handle, as in a ball-park idea. If one woofer was rated for 100w and another 1000w, which one do you think will likely be able to handle more power? Ok, I realize that's a stupid question, but what I am getting at is a speaker can easily handle more than it's RMS or peak value, but the length of time it can handle that amount of power becomes very short, maybe tens of milliseconds! But, a hard kick drum beat is centered between 60-65Hz. A single cycle (think delta function at that frequency), is only 1/60Hz or 17ms. So it's possible for a speaker rated at 1000w peak to be able to handle a 2-3kW transient at 60Hz provided it stays within its mechanical limits. If a manufacture tested a speaker to 2-3kW in 10mS pulses and said it was rated at a peak of 2.5kW and a person played dub step at that level, it'd cook the speaker and there would be a warranty claim. If a manufacture says a speaker is then rated at 700W rms 1000w peak, but it can handle 3-4kW transients, then that company won't have very many cooked speakers for warranty.
post #11 of 18
Quote:
Originally Posted by fredfish View Post

I thought I would post some graphs. Here is a 25 cf enclosure, 1,200w applied. You can see that with a 10" port, the first port resonance is awfully low.



Here's the port velocity:



Doing the image capture, I noticed that at 12Hz the port velocity is not that bad. How often do you suppose there is going to be 10Hz content? Is a peak velocity of 26 m/s going to cause audible chuffing with a 10" flared port?

25 cf is rather large... eek.gif
That box is either too small, or tuned too low. That transfer function does not look that great. Remember folks, an 18" in an LLT is going to need 16-25ft^3, a 21 is going to need 20-30ft of space, more if you want to tune below the teens! Don't be scared of a big box, that's why it's called LARGE low tuned.biggrin.gif

*edit*
Ok, I had to model this driver. 35ft tuned to 13Hz looks pretty good. Feed it 2kW and you will not run out of mechanical excursion. >120db from 13Hz on.
Edited by Looneybomber - 4/26/13 at 9:01pm
post #12 of 18
I run 4K through each of my FTW 21's for a year or 2. No problems smile.gif
post #13 of 18
Quote:
Originally Posted by Looneybomber View Post

There's thermal and mechanical limits. Reach either of those and your speaker will not be happy and might even die. Now, thermal limits are a little enigmas. For example, and I am just making up numbers, but it might be something where the driver can handle 1.2kW for 10 seconds at 20hz, but maybe only 700W for 10 seconds at 300Hz due to a decrease in air moving over the coil from less excursion. That might also mean it can handle 3kW for 1 second at 50Hz or maybe 8kW for 50ms? It's all about keeping the voice coil under a certain temperature and it can only wick away heat so fast, but hard transients may mean sending 2-3kW peaks for a very short duration, around 40-50ms with maybe 10-20x of minimal power where that heat can be wicked away.

A voice coil's temp acts similar to a rectangle function (input signal) convolved with a decaying exponential (heat sinking ability). This results in the voice coil temp being the blue line.

Nice visual...
post #14 of 18
Thread Starter 
Well, I've booked the driver before I talk myself out of it again. I can't see a better opportunity than this coming along for a guy north of the 49th.
Quote:
Originally Posted by Looneybomber View Post

That box is either too small, or tuned too low. That transfer function does not look that great. Remember folks, an 18" in an LLT is going to need 16-25ft^3, a 21 is going to need 20-30ft of space, more if you want to tune below the teens! Don't be scared of a big box, that's why it's called LARGE low tuned.biggrin.gif

*edit*
Ok, I had to model this driver. 35ft tuned to 13Hz looks pretty good. Feed it 2kW and you will not run out of mechanical excursion. >120db from 13Hz on.

Its not the not the box that scares me per se. It the three floors up in a narrow stairwell that scares me. smile.gif I would have to haul sonotube up over the balcony

I could get up close to 30cf with 30" diameter sonotube but that's starting to occupy a lot of space. Hmm... you know, with a few tapcon screws I could just strap that sucker to the ceiling. biggrin.gif

OTOH, I'm probably moving in a year, so a sealed sub would be a little more sensible.

It sounds like power handling is not going to be an issue.

On the ported option, I still have a number of questions:

1. How low a 1st port resonance is acceptable?
2. For a 10" port is a port velocity approaching 35 m/s acceptable? What is my practical limit here?
post #15 of 18
Quote:
Originally Posted by fredfish View Post

Well, I've booked the driver before I talk myself out of it again. I can't see a better opportunity than this coming along for a guy north of the 49th.
Its not the not the box that scares me per se. It the three floors up in a narrow stairwell that scares me. smile.gif I would have to haul sonotube up over the balcony
Ooohhhh. Well, if mobility is at all a priority, you can build a sealed enclosure with sonotube to save weight.
Quote:
Originally Posted by fredfish View Post

On the ported option, I still have a number of questions:

1. How low a 1st port resonance is acceptable?
2. For a 10" port is a port velocity approaching 35 m/s acceptable? What is my practical limit here?
1) Normally at least 1 octave above your tuning point, but I like to play it safe and say about 1.5 octaves. (Close to 3x the XO frequency)
2) I don't know. I know as the port gets bigger, the acceptable port velocity also increases. You could try it and see?
post #16 of 18
Quote:
Originally Posted by Looneybomber View Post

2) I don't know. I know as the port gets bigger, the acceptable port velocity also increases. You could try it and see?
I didn't know that! (but I am noob tongue.gif)

I thought one should aim for <25 m/s, generally speaking?
post #17 of 18
I'll have to find the website again, but a guy tested multiple port sizes and flares at different frequencies. It was found that flares help, but only so much - they couldn't make up for a well undersized port. It was also found that the largest port - 6" I think - was able to have a good bit higher port velocity before audibility compared to much smaller ports.
post #18 of 18
Thread Starter 
Bomber. Is this the site?

He only went up to 6" ports so I don't know how that translates to larger ports.

I'm still pondering the advantages/disadvantages of a sealed build.
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