Impedance of woofer over the xmax - diyAudio
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Old 3rd December 2015, 11:02 AM   #1
zedda is offline zedda  Italy
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Default Impedance of woofer over the xmax

Hello everyone

I have a little professional system with 2 TL sub 15"
The tuning frequency is 37hz
The low pass is set to 80hz

I want to increase the output of the low frequency.

The woofer haven't a very long excursion
(7mm mathematical xmax)
But they are very sensible (99db)

I think when the excursion become to big, the amplifier (1000 watt 4 ohm load each channel) start to clip because the woofer coil are out from the magnet and the amplifier "view" to low impedance...

In open space, for a little pa system, isn't very important to have a very low frequency sub...

Very low frequency woofer means high mms, and low sensitive woofer... And I haven't more power for pilotate a low sensitive woofer...

In yours opinion:

I need a 15" woofer that matches my needs? (More spl and more xmax)

Or I need a lowcut for increase the output power?

My old active crossover have only a 40hz lowcut and I never activated this... (Dbx234xs)

Thanks a lot
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Old 3rd December 2015, 12:36 PM   #2
JMFahey is offline JMFahey  Argentina
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Quote:
I think when the excursion become to big, the amplifier (1000 watt 4 ohm load each channel) start to clip because the woofer coil are out from the magnet and the amplifier "view" to low impedance...
Well, it does not work this way, the cone (and coil) move back/forth all the time, but it's never ever fully outside the magnetic gap.

And at resonance, impedance is *maximum* (think 10 to 30 ohms) , even if damped by cabinet resonance it's still higher than "nominal" impedance.

I'd use the system as-is , a high pass filter may help to avoid crushing it, and if you need more, get more cabinets and power amps.

Trying to tweak your system to get , say, an extra dB or less, won't compensate the time and $$$ invested in it.
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Old 3rd December 2015, 01:10 PM   #3
zedda is offline zedda  Italy
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Thanks for the reply JMF
but far from the resonance, if I decide measure the impedance near the xmax?

If there is less magnetic flux the impedence don't change?

Electrical and mechanical limits

And

ABSTRACT: Recent work by Klippel [1] and Voishvillo [2] has shown the significance of voice coil inductance in respect to the non-linear behaviour of loudspeakers. In such work the methods used to derive distortion require the inductance to be represented by an equivalent circuit rather than the frequency domain models of Wright and Leach. A new technique for measurement of displacement and frequency dependant impedance has been introduced. The complex relationship between coil impedance, frequency and displacement has been both measured and modelled, using FE, with exceptional agreement. Results show that the impedance model requires that its parameters vary independently with x to satisfactorily describe all cases. Distortion induced by the variation of impedance with coil displacement is predicted using both parameterised and FE methods, these predictions are compared to measurements of the actual distortion. The nature of this distortion is discussed.

Assessment of Voice Coil Peak Displacement Xmax. Available from: Assessment of Voice Coil Peak Displacement Xmax [accessed Dec 3, 2015].

Fourth page of the pdf in this link

Last edited by zedda; 3rd December 2015 at 01:15 PM.
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Old 5th December 2015, 04:57 PM   #4
sine143 is offline sine143  United States
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zedda comparing graphs of excursion vs frequency and impedance vs frequency will help shed some light on this situation
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Old 5th December 2015, 06:41 PM   #5
TBTL is offline TBTL  Germany
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Quote:
Originally Posted by zedda View Post
I have a little professional system with 2 TL sub 15"
The tuning frequency is 37hz
I interpret 'TL' as transmission line. If you can handle larger cabinets, you could make a tapped horn which is more efficient. Think twice the size, +3dB output. Tuning to a higher frequency also increases efficiency. Increasing to 50Hz would gain you 4 dB for equal cabinet size.

Quote:
Originally Posted by zedda View Post
I want to increase the output of the low frequency.
[...]
In open space, for a little pa system, isn't very important to have a very low frequency sub...
Could you be more specific? What frequency range must be louder?

Quote:
Originally Posted by zedda View Post
Or I need a lowcut for increase the output power?

My old active crossover have only a 40hz lowcut and I never activated this... (Dbx234xs)
Yes, low bass only wastes power in small PA applications. 40Hz is OK but slightly lower would be better with your cabinet tuning of 37Hz.

Last edited by TBTL; 5th December 2015 at 06:46 PM.
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Old 5th December 2015, 06:44 PM   #6
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Quote:
Originally Posted by zedda View Post
Or I need a lowcut for increase the output power?
This. Does your amp have a lowcut? If not you need a better crossover.
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Old 5th December 2015, 08:12 PM   #7
zedda is offline zedda  Italy
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Wow how many answers
Sine143 in this photo you mean?
Click the image to open in full size.

Thanks TBTL
really a tapped horn is more efficient?
Years ago I wanted to try the TH but nobody help my to do a simulation (with hornresp...)

The TL simulator gave me a response very similar to TH graphics response on the web...
I know that the smaller duct of the TH tune the box low for equal cabinet size... Isn't true?
can you simulate a 37hz tapped horn for extrapolate the cabinet dimension?
But now it's useless create a new cabinet for this woofer...

In your opinion I need to buy a specific woofer? (Now i have a pair of 15ndl76 4ohm)

I want a 40/45hz louder without lose cone dispacement control (displacement that now with a new digital crossover it's possible to govern)

Yes 40 lowcut is to high for 37hz TL...

Thanks conanski
Yes I need to buy a new crossover, a driverack pa+ or a dcx2496
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Old 5th December 2015, 09:09 PM   #8
zedda is offline zedda  Italy
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and if I use a single 18 woofer in a tapped horn?
in italy the faital 18hp1010 is in sale at 130 euro
or 18nlw9400 at 200 euro used in good condition
or rcf lf18n451 at 250 euro used
or ciare fxh 18

I have only a 2000w amplifier (2x1000 4ohm or 1x2000 8ohm)...
and non more space in my car...
the faital it's possible to use in tapped horn?

Last edited by zedda; 5th December 2015 at 09:37 PM.
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Old 5th December 2015, 09:57 PM   #9
zedda is offline zedda  Italy
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uff... my TL are too beautiful... I don't want to put away them...
Click the image to open in full size.
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