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Harrison Labs F-mods

 

Blair
Unregistered guest
i understand these things are used in substitution of a subsonic filter if your amp doesnt have one.

can some one explain to me
1) what they are exactly;
2) how they work
3) where can i get one
4) how much do they cost.

thanks
 

Silver Member
Username: Fishy

Tamarac Ft.Laud, FL USA

Post Number: 728
Registered: Sep-04
I'm not to familair with them but as far as I know they're simply a low level high pass filter similar to the passive high pass found on component xover networks, just smaller and with a much lower xover point. Dunno what order(slope) and type they offer, however.

-Fishy
 

Silver Member
Username: Fishy

Tamarac Ft.Laud, FL USA

Post Number: 729
Registered: Sep-04
http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=266-246

Looks like its a passive 12 dB/octave filter to me.

-Fishy
 

Silver Member
Username: Fishy

Tamarac Ft.Laud, FL USA

Post Number: 730
Registered: Sep-04
I too wonder how the things work and just how accurate this "20 Hz" figure is. Crossover point in a typical component network depends on speaker impedance. What would it depend on in a low level application? the input impedance of the amp? and doesn't this change from product to product?

hmmmmmm.......

-Fishy
 

blair
Unregistered guest
I wonder if Jonathan can help as he was the one that told me about these things...

also im trying to figure out gow low to port my subs?..Jonathan recommended 28 hz for my ID max D2's and if that were the case should i get the 20 or 30hz F mod?
 

Silver Member
Username: Fishy

Tamarac Ft.Laud, FL USA

Post Number: 734
Registered: Sep-04
Definitely the 20(the 30 would start rolling stuff above your tuning frequency) if you even need one with a box ported that low. I think the general rule of thumb is 1/3 an octave below tuning or around 22-23 Hz in your case, but there's not a lot of material out there that goes THAT low. I have a PPI frx 456 which has a subsonic filter variable from like 7-250 Hz and a pair of 10w3's tuned at 28 Hz. I can't tell any difference with my lowest playing tracks whether I have the subsonic at 7 or 25 Hz.

Subsonic filters are a lot more important for boxes with higher tuning frequencies, but I'd still like to know how these Fmod thingies work.

-Fishy
 

Silver Member
Username: Fishy

Tamarac Ft.Laud, FL USA

Post Number: 740
Registered: Sep-04
I'm curious as to what the rolloff frequency would be utilizing one of these 20 Hz Fmods off a single mono cable to a set of amplifier inputs through a y-splitter. Seems like the input impedance would be halved and if remember correctly that should halve the cutoff freqeuncy as well(10 Hz?). I know halving the impedance on a 6 dB lowpass(a coil in series) will double it and I'm pretty sure a high pass works just the opposite. Not sure about a 12 dB filter tho. It may be a bit more complicated. Darnit, I been outa school way too long.

Hmmmmm...... maybe they include TWO Fmods in that package for a reason.

-Fishy
 

Silver Member
Username: Fishy

Tamarac Ft.Laud, FL USA

Post Number: 741
Registered: Sep-04
http://www.hlabs.com/technical/crossovers/page2.html

After reading this I'm a bit wary of utilizing these things. They seem to be standardized for a 22kohm load so if thats the input impedance on your amp then you're probably fine. Otherwise I'm not sure. Frequency change/load isn't as straightforward for a 12 dB filter.

From what I gather here halving the impedance results in a 25% increase(HP) in cutoff frequency(25 Hz for the 20 Hz y-splitter scenario), but with a 12 dB crossover I'm pretty sure coils and caps are matched with impedance in mind so the slope might get a little screwy.

They claim that this kind of stuff isn't audible, but still charge $25 to research the input impedance of your amp.

Guess its really not that big of a deal with a simple subsonic filter, but I'm not sure even having one is that big of a deal either with a box tuned at 28 Hz. Just stick to music and stay way from sub-25 Hz test tones.

:-)

-Fishy

 

Blair
Unregistered guest
Thanks for the help fishy.

Tell me.. in your experience with your box, does having it tuned at 28hz cut off any typical low musical frequencies?

also can you cross check my box mesurements?

2 12' id max D2's

5 cuft box tuned to 28 hz with a 7.6in x 10 in port that is 40in long.

cheers thanks
 

Silver Member
Username: Fishy

Tamarac Ft.Laud, FL USA

Post Number: 744
Registered: Sep-04
Any ported box has a steep, "audible" rolloff below tuning frequency. That doesn't mean your sub(s) isn't still working overtime at these lower frequencies however. Its kind of hard to explain how a ported enclosure works, but if you can imagine the oscillating mass of air in the port as another "driver" with the compressing/decompressing volume of air inside the enclosure as its suspension its a little easier to comprehend.

From what I understand the driver and port operate 90 degrees out of phase at tuning with the sub cone barely moving and the port providing almost all acoustic output. As you move below tuning frequency the driver and port rapidly approach a point where they operate 180 degrees out of phase(externally) which means that when the mass of air in the port moves OUT the speaker cone moves IN and much of the acoustic output at these lower frequencies is cancelled. If you've ever played a couple of subwoofers that have been wired opposite of each other you've experienced basically this same thing.

The problem with a ported system at these lower frequencies is that while the port and driver are moving out of phase OUTSIDE the box, inside they are moving in synch and the driver is said to "unload". If you're familiar with how a sealed enclosure works you realize that the compression and decompression of the volume of air inside provides a good percentage of the suspension(spring force) for the driver. With the port and driver operating 180 degrees out of phase you lose this "support" and its very easy for the driver to reach some serious excursion which can result in physical damage to the suspension, etc.

I'm planning on upgrading my w3's to a pair of w6's and since I've almost always utilized ported enclosures was curious how they might sound sealed so I built me a sealed box(~.75 ft^3 each with polyfill) for my 10w3's which I could also use with some 10w6's(probably w/o polyfill). Altho in general I got more low bass out of my ported box I did experience some "new", very low, maybe non-musical stuff on some some tracks, especially in the quieter sections. It wasn't very loud, but it was new, and it was there.

It'll be interseting to see how the w6's sound when and if I get them, but to give a short answer to your question: Yes, with cabin gain, you may experienece some lower stuff with a sealed enclosure on a FEW tracks, but whether they're "musical" is probably a matter of opinion.

-Fishy

[note] lemme fire up WinISD and I'll get back to you on that IDmax box/port question.
 

Silver Member
Username: Fishy

Tamarac Ft.Laud, FL USA

Post Number: 745
Registered: Sep-04
Hmmm... what program/formula did you use to calculate port length?

WinISD calls for a 46 inch long port with one end flanged(44.84" with both ends flanged), but I have heard of discrepancies in port length when using WinISD. This was with the new alpha version. Using the old beta verion you don't have the flange option and I get 44.76 inches(both ends flanged I guess).

Might wanna get some input from people using other programs or formulas for a good check.

-Fishy
 

Silver Member
Username: Fishy

Tamarac Ft.Laud, FL USA

Post Number: 746
Registered: Sep-04
Here's the WinISD gain curves for a pair of 12D2v3's in my Explorer:

http://home.comcast.net/~guppyrig/wsb/media/322068/site1081.JPG

The white is your proposed allignment and the yellow is 2 drivers in ID's recommended sealed enclosure(2.6ft^3 common).

Both are modeled with a 24 dB Butterworth lowpass filter @ 80 Hz and a linkwitz transform(starting at 15 Hz) that corresponds to the aproximate cabin gain of a 96 Explorer Sport(~9ft max dimension).

The graphs are a little deceptive as far as what the actual output would be, but the shapes are probably close. Remember that you might have the gains set a bit higher with the sealed enclosure so as to get more output and better blend with your mids and highs so it would play lower relative to the ported than what these plots would suggest. The ported plot is down 3dB from max at about 22Hz whereas the sealed is down 3dB at around 16 Hz.

Regardless you can see how much faster the ported enclosure rolls off and how it would be possible to get a bit more low end extension with the sealed. Cabin gain plays a major roll. These plots look a lot more one sided(in favor of ported) w/o that transform.

Thats whats so cool about this newer alpha version. You couldn't model cabin gain or active crossovers with WinISD beta and gain plots were pretty useless in predicting output when comparing different enclosure types.

-Fishy
 

Blair
Unregistered guest
hmm.... damm, i was hoping to get a simple answer. i used the program on this site : http://www.carstereo.com/help/Articles.cfm?id=31

it seems there is a large discrepancy in port lengths ranging from 40 - 46 inch.

also because of the length of the port going to have to put it on one side of the box . will this effect the balance of the sound within my car.(sorry if thats a stuped question, i assume it wont.)
 

Silver Member
Username: Fishy

Tamarac Ft.Laud, FL USA

Post Number: 749
Registered: Sep-04
Thats the way my box is ported.
 

Silver Member
Username: Fishy

Tamarac Ft.Laud, FL USA

Post Number: 750
Registered: Sep-04
I wasn't very impressed with some of the explanations on that site but I'd go with the 41 inch port length. WinISD doesn't have allow for "slot" port calculations. You do realize that effective port mass will be different for a slot port do to OUTSIDE effects as well, i.e. whether the slot is verticle and not adjacent to an external surface or horizontal at the bottom of the enclosure and firing parallel and near the "floor".

-Fishy
 

Blair
Unregistered guest
btw... what is the difference between a slot port and a standard square port?? i was assuming they were the same thing but on that site when you click the slot port as well as the square it alters the length significantly...

also do you recommend flaring the inside end of the port ?....am assuming i just open the inside of the port with a 45deg piece of mdf?
 

Silver Member
Username: Fishy

Tamarac Ft.Laud, FL USA

Post Number: 769
Registered: Sep-04
Lol, when I originally posted I didn't realize there WAS a difference, but yes there is. I always thought the term "slot port" referred to any port that wasn't round, hehe, but apparently it refers to a port that utilizes an enclosure wall as one or more sides of a port's termination.

I've always seen it recommended to terminate the end of a port at least 1 port diameter away from an inner enclosure surface. Why? because if you run a port parallel and "flush" with a surface the effective mass of vibrating air in the port is increased. That parallel surface forces more air to be affected. Unlike a totally "free" port end the air on one or more sides has nowhere else to go and is forced to oscillate along with the rest of the column of air. Thats why ports with flanged or baffled ends produce a slightly lower tuning frequency than those of equal length with unflanged ends.

So, if your utilizing an enclosure wall for one side of a port you need less length to achieve the same tuning frequency. I'm sorry. I just automatically assumed you were going to use a plain old rectangular port.

As to your second question I would think flared port ends would always be desirable. They reduce end turbulence which could become audible as "whistling" on an external port opening. Do realize that just like the flanged and slotted ports the air mass is increased a bit which results in a slightly lower tuning frequency.

Then again this isn't an exact science. If you get it close and don't tune below the fs, or resonant frequency of the driver(free air) you should be fine. There's just so many other variables that effect acoustic output. Getting port lengths "exact" shouldn't be all that critical.

Sry bout the long winded answers. Hope they didn't do more harm than good.

:-)

-Fishy
 

Blair
Unregistered guest
Nah mate they have been great. Thanks for the help. The old man owns a trim shop and im planning on doing a custom job and i just wanted to get the box right the first time as i didnt wanna go having to change the rest of the install
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