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Old 25th March 2010, 09:50 PM   #1
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Default How does this work...?

Ok, so some background to begin.
A few years ago, Dad bought a Sony HTIB system. The satellites were Ok, but the sub is amazing for what it is. It uses a 6.5" driver is a ported box (tuned approx 35Hz, calculated by winISD)

Connected to the DVD player/amp all in one thingy, it will vibrate the sofa. I can feel it in passages such as those in the Dark Knight. My sub (pair of 8"s, sealed, LT to 28Hz) won't do that.

However, connected to a straight amplifier, it sounds like it bottoms out at up to listening levels. A 6.5" driver bottoming out should have reasonable output, but it just didn't.

So, what do we think's going on?

Thanks for any ideas
Chris
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Old 25th March 2010, 09:56 PM   #2
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When connected to the dedicated amp system, EQ and filtering is applied, meaning it can produce high SPL but only over a narrow frequency band. Also, sofa-vibrating bass isn't low bass but more in the 50-80Hz range.
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Old 25th March 2010, 10:01 PM   #3
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It all falls down when it bottoms out playing very quietly, on a straight amp. Any 6.5" driver would have some output when pushed that far, but, as I say, it had minimal output, with minimal power input. This shows it's no longer a straight amp, or even a band-restricted amp. It bottomed out at 60Hz, with the volume at 8 oclock.
I'm not sure on your last comment. I know bass that hits you in the chest is 50-100Hz, but sofa vibrating - I expect it's lower. After all, it has much more compliant suspension than a ribcage, thus lowering it's resonance considerably.

Chris
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Old 26th March 2010, 11:53 AM   #4
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Hi Chris I have just such a system lying around, i dont use it much but i do use the sub and the amp until i build a better one.

I have a feeling that the sofa shaking your getting is purely down to playing certain high volume notes at the resonance of your sofa.

I get something similar with my workbench which will vibrate but its with relatively high frequency bass notes.

I you run a low frequencyslow sine sweep through the amp say 30hz to 80hz via your computer of whatever you will find the resonance.

It does appear as if the little sub is kicking out some serious power but it's an illusion.

I'm guessing the sub is approx 2' X 2' by 8 " external with a 3" ish port?
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Old 26th March 2010, 04:16 PM   #5
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It's smaller than that.
My point is that the amp is doing something very odd - with a normal amp, you get almost no output, it just distorts. I have tweeters that do better on a straight amp.
I've done sweeps and test tones on it before. On it's amp, it goes to 30Hz and keeps going, even when going well above listening levels. On a straight amp, there's no reprive from the frequency independant bottoming out.

Here's what I've deduced so far about the amp it uses.
It compresses the bass at high volumes. You still get thuds, but the kick disappears. At low volume, it plays full dynamic range.
It does something very clever to turn a driver that's useless on a straight amp into something amazing (for it's size, cost etc). I'm thinking AceBass has been applied, but I could be wrong.

The problem is that, no matter how you look at it, on a straight amp it bottoms out when it really shouldn't, but gives almost no output when it does run out of excursion. I could feel the box vibrating, so it was obviously moving a fair amount, but still no output........

That's what's confusing me
Chris
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Old 26th March 2010, 07:32 PM   #6
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Take it apart is the only way It's possibly a dual voice coil and one coil is used for feedback, compressing the signal when the limit is near. There are some Wharfedale PA speakers that do this, I know of them as I worked at the company that partnered with them for the design.
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Old 27th March 2010, 05:09 PM   #7
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It's Dad's sub from the HT system in the lounge, so I can't take it apart until he gets a new one.
I'm not sure if it would do the feedback idea - there's only a two core cable running to it...
I look forward to dismantling the subwoofer, when Blu-Ray becomes prominent over the DVD-only system he has.
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Old 27th March 2010, 05:29 PM   #8
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It's possible the amplifier has a negative output impedance, thereby giving extreme damping to the woofer cone. On a normal amplifier, the compliance & resonance will allow the woofer cone to achieve excessive movement, & 'bottom out'.
The SPL of a speaker is determined by its cone velocity, so by having a very highly damped cone, you can still achieve high SPL without excessive cone movement.
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Old 27th March 2010, 08:18 PM   #9
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Acceleration, not velocity.
And what you are saying would mean higher frequency. (or added harmonic content if you wish to stay with the same frequency)
No way around the relation between SPL, excursion and frequency.

It is possible that the amp it adding Q (making it more resonant) at tuning frequency to give it more output and allowing port resonance to build up, adding more time so to speak.
It would also excite room modes more as you have longer time to build up their resonance. And also make other things shake more easily.
Impressive at first but horrible in the long run as it does it every time, even when it is not suppose to.


btw Dark knight is centered at around 35Hz and looks like a tent on both sides from there, pulsating.
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