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Old 23rd November 2008, 17:41   #1
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Question What is resonance (tuned) frequency?

Hi all,

I just saw the manual of JBL GTO3501 mono amp. Here it mentions to set the HP filter frequency to a value 10 Hz below the enclosure's (i.e. the sub-woofer with enclosure) resonance (tuned) frequency.

So what exactly does this resonance (tuned) frequency mean? I searched on TBHP but couldn't find anything. I have a JBL GT5-1204BR sub driven by this amp. So how exactly should I set the HP filter frequency of this amp?

Could the guru's please shed some light on this?

Thank you.
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Old 24th November 2008, 11:16   #2
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Originally Posted by m2avg View Post
Hi all,

I just saw the manual of JBL GTO3501 mono amp. Here it mentions to set the HP filter frequency to a value 10 Hz below the enclosure's (i.e. the sub-woofer with enclosure) resonance (tuned) frequency.

So what exactly does this resonance (tuned) frequency mean? I searched on TBHP but couldn't find anything. I have a JBL GT5-1204BR sub driven by this amp. So how exactly should I set the HP filter frequency of this amp?
Does your amp have a HP filter and LP filter that can be set for each channel or each bridged channel?

Say you set the LP filter at say 80hz can you also set a HP filter at say 20Hz?

I did not see Fb (box resonance freq) mentioned anywhere in the User manual of the JBL sub. If it were you could HP at say 10Hz below this frequency it would prvent your sub from being pushed hard by too much low frequency information. For now leave it alone. If you find you sub loosing control then HP at 25Hz and listen again.
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Old 24th November 2008, 19:53   #3
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Yes this amp has individual LP and HP. As you said, I can set each at two different frequencies. And this is what the manual says exactly

"For woofers in tuned (vented) enclosures, set the Frequency control to a value 10Hz below the enclosure's resonance (tuned) frequency."
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Old 25th November 2008, 09:38   #4
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GURUS and m2avg correct me if i'm wrong i believe that the amp has a 12 db/oct filter. so if the sub's box has a tuned frequency of say 40hz then setting the hp on the amp to lets say 28 or 30 hz will make the amp push the sub from that frequency upto the cutoff of the lp filter. So effectively this is channeling the amps power only to where it is needed and not across the range as you would do for say a free air or sealed enclosure unit. Thats the explanation on the amp side. i think here they expect that when you buy such an amp and use a vented/ported you have got the box and vent/port tuned to the point or frequency where you want most output or punch. i haven't seen many people actually using most of these theories in india but i can't say much about my travelling to make such comments. internationally box tuning is a lucrative business i dunno how it is here. most installers I've met (not too many) don't seem to know much about the thiele small parameters which indicate to you the volume and kind of box necessary for a sub. but correct me if i'm wrong.
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Old 25th November 2008, 12:29   #5
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Originally Posted by valhallen.282 View Post
GURUS and m2avg correct me if i'm wrong i believe that the amp has a 12 db/oct filter. so if the sub's box has a tuned frequency of say 40hz then setting the hp on the amp to lets say 28 or 30 hz

i haven't seen many people actually using most of these theories in india but i can't say much about my travelling to make such comments.
Actually the real reasoning for this is as follows.

For ported/bass reflex subs when a sub is pushed below it's Fb the sub decouples from the box (acoustically). This means that the sub is not getting any acoustic support from the box and it's suspension is the only thing keeping it from falling apart.

By filtering out information below Fb one is protecting the sub's suspension.

I hope this helps. It is as simple as I can make it. Sam where are you?
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Old 25th November 2008, 13:55   #6
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Originally Posted by valhallen.282 View Post
So effectively this is channeling the amps power only to where it is needed and not across the range as you would do for say a free air or sealed enclosure unit.
i haven't seen many people actually using most of these theories in india but i can't say much about my travelling to make such comments. internationally box tuning is a lucrative business i dunno how it is here.
You are right in a way. If you dont care about the lowest octaves, "bandpassing" a sub this way will not unnecessarily use up amplifier power to drive under the HPFed frequency. This is often also done in professional audio installs like discos etc where frequencies under 35-40Hz are not desired, and the drivers themselves are not designed to play them.

However in this case, as Navin mentions, the idea is to have a subsonic filter (infrasonic actually) so that the subwoofer in vented enclosure is prevented from going haywire below the tuning frequency.

And yes, even in India, good installers use all these theories in their practice.
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Old 25th November 2008, 14:41   #7
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i believe that the amp has a 12 db/oct filter.

I thought Amps also came with a 8db/oct filter and could be preset.
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Old 25th November 2008, 15:51   #8
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I thought Amps also came with a 8db/oct filter and could be preset.
12, 18 and 24db are what is most common.
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Old 26th November 2008, 20:09   #9
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That was certainly some high-tech information!!!

Quote:
Originally Posted by valhallen.282 View Post
GURUS and m2avg correct me if i'm wrong i believe that the amp has a 12 db/oct filter.
Can't really see the db/oct filter anywhere. Maybe it's there but I don't know where to look or it has some other terminology.



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Originally Posted by valhallen.282 View Post
i think here they expect that when you buy such an amp and use a vented/ported you have got the box and vent/port tuned to the point or frequency where you want most output or punch.
Never knew that a box is also TUNED. Couldn't find anywhere on this forum either. How exactly is it done?


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I did not see Fb (box resonance freq) mentioned anywhere in the User manual of the JBL sub.
Navin, so is there no way to find out the box resonance frequency? Sorry for my ignorance, but what does Fb stand for?


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Sam where are you?
Yes the JBL man can certainly shed some more light on this. Do I need to PM him?


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Old 27th November 2008, 08:59   #10
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@Navin sir thanks that was a clear enough explanation and simplistic by my standards too i looked into it and got more info from some books and online.
@B&T sir i wasn't actually talking about bandpassing a sub but merely giving an example. there is a program infact that works with winamp called the jammix enhancer that gives fair measure of control on audio. i got the idea from there. i was using it to run a similar bandpass for boosting specific frequencies for my in-ear headphones. they can't handle below 28hz and the best they take are at about 32hz.
@M2AVG no there is no other terminology. the db/oct filter where db-decibel and oct-octave or frequency in layman's terms. it is basically a slope drawn on a frequency graph to show you how much volume is reduced as the frequency increases. thats again in layman's terms. a shallow slope is 8. regular slope is 12. 18 is a steep slope and 24 is a very steep slope. no your amp cannot be adjusted for these settings it is inbuilt. you get these settings however on certain high end amps and some of the more expensive headunits. IMO if you don't want your budget shot i suggest you stick with only plugging in the amp and sub. but a headunit with 3preouts is an excellent idea. or change the front speakers to components.
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Old 27th November 2008, 10:29   #11
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Guys sorry about the last part of this post i think this and another post of mine have got clubbed together. please ignore the part about changing speakers and adding amps etc it was for another thread. seems to have got pasted here by my cell. truly sorry.
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Old 27th November 2008, 11:24   #12
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Never knew that a box is also TUNED. How exactly is it done?

Navin, so is there no way to find out the box resonance frequency? Sorry for my ignorance, but what does Fb stand for?

Yes the JBL man
1. A box is tuned by being properly designed (damping, volume, dimensions, port dimensions/ PR loading, etc..) and then calibrated using software like MLSSA, LEAP/LMS, CalSod, Clio, etc. The equations are all well known and one can go this manually but a computer is so much faster.

2. Yes there is. If you have T/S specs (esp Vas, Fs, Qts) and the Box volume (and port dimensions if any) you can find programs (Box Plot, Box Design, etc..) that will give you a computation of the same.

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Originally Posted by valhallen.282 View Post
a shallow slope is 8. regular slope is 12. 18 is a steep slope and 24 is a very steep slope.
I assume you meant "a shallow slope is 6db/oct".

One can design a XO with a slope of 8db / octave but is wont be a "regular XO". In fact most well designed XOs are not "regular XOs". These XOs take in to account for the impedance curves, resonance peaks, as well as the driver's natual roll off as well and peaks that might occur from the driver's natural response (break up nodes, etc..) and difraction caused by the box's baffle.
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Old 27th November 2008, 13:19   #13
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Well navin sir you read my mind in both cases. yes i was talking about a 6db major typo's happening with me today and i guess the 8db xo's are very specific a little too much to discuss here. BUT someone should dedicate a thread to educate people clearly about the finer nuances of audio, tuning, design etc and help in directing those who want to move further in the field.
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Old 27th November 2008, 19:36   #14
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Originally Posted by valhallen.282 View Post
someone should dedicate a thread to educate people clearly about the finer nuances of audio, tuning, design etc and help in directing those who want to move further in the field.
Guess thats why this section is there on TBHP.
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Old 28th November 2008, 12:36   #15
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BUT someone should dedicate a thread to educate people clearly about the finer nuances of audio, tuning, design etc and help in directing those who want to move further in the field.
1. I am not equipped to do this; the Pros (Sam, Gunbir, LBM, B&T, etc..) might be more able.

2. There are many forums from diyaudio, diymobileaudio, etc... that cover audio and ICE in greater detail.
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