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Author Topic: RCF 21" Sub Build Pics and Impressions  (Read 22846 times)

Art Welter

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Re: RCF 21--Designer comments
« Reply #30 on: November 04, 2010, 12:19:18 pm »

Phillip Graham wrote on Thu, 04 November 2010 09:47


Cabinet impedance testing at different values of large signal V_in is the most straightforward way, in my mind, to see the port compression effects, and well as shifts in port tuning frequency due to air behavior in the port with increasing Reynolds number.



Phillip,

Could you clarify a few points:

How do you define "port compression effects" ?

My understanding is the port tunes the enclosure fB (box frequency), when you say "shifts in port tuning frequency" are you referring to a fB shift or something else?

Does that frequency shift go up, or down with increased turbulence (higher Reynolds numbers) ?

Art Welter
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Phillip_Graham

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Re: RCF 21--Designer comments
« Reply #31 on: November 04, 2010, 01:07:08 pm »

Art Welter wrote on Thu, 04 November 2010 12:19

Phillip Graham wrote on Thu, 04 November 2010 09:47


Cabinet impedance testing at different values of large signal V_in is the most straightforward way, in my mind, to see the port compression effects, and well as shifts in port tuning frequency due to air behavior in the port with increasing Reynolds number.



Phillip,

Could you clarify a few points:


Sure

Quote:


How do you define "port compression effects" ?


It is a collection of effects.  Let me see if can mention a few:

First effect:
Lets call the acoustic impedance of the port is R + j(w)M where R is acoustic resistance, (w) is angular frequency, and M is mass.  

Mass of air in the port resonates in concert with the compliance (1/stiffeness) of the air in the box as a mass/spring system.  The mass of air in the port is the port's reactive component, and R is the lossy component.

As the Reynolds number increases above about 2000, there is a transition range to fully turbulent flow and eventually at very large Reynolds numbers (200,000+) to a turbulent boundary layer, too.  R increases as the Reynolds number (Re) increases.

Second Effect:
The dramatic increase in R in effect one reduces the suppression of the driver excursion.  Ideally the port air mass resonates out of phase with the driver at Fb, suppressing its movement.  If R is high, the effect is damped and the driver moves more.  This leads to the driver itself producing more harmonic and IMD components.

Third Effect:
Flow out of the end of a open tube does not behave the same way as flow into the end of a open tube, so if you feed, say, a sine wave to a driver, the airflow "waveshape" from the port can be distorted asymmetrically on each half of the waveform.

Fourth Effect:
Vortex formation at the edges of the port to cabinet transition, while technically a subset of the first effect, are significant enough to mention separately.  These contribute to increasing the R in effect one.

Fifth Effect:
The port itself makes extraneous noises (e.g. "chuffing" and "whistling") that contaminate the original acoustic output.

Quote:


My understanding is the port tunes the enclosure fB (box frequency), when you say "shifts in port tuning frequency" are you referring to a fB shift or something else?


Yes, the reactive component of R + j(w)M above can change with different Reynolds numbers, and can change the apparent box Fb slightly at different output levels.

Quote:


Does that frequency shift go up, or down with increased turbulence (higher Reynolds numbers) ?



While technically you might be able to make boxes that exhibit either a raising or lowering effect, generally the behavior of highly undersized ports is to have M decrease as a function of Re, which has the effect of raising the box Fb.
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Art Welter

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Re: RCF 21--Designer comments
« Reply #32 on: November 04, 2010, 02:50:25 pm »

Phil,
Thanks for the detailed explanation.
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Phil Lewandowski

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Re: RCF 21--Designer comments - Quick side question-2268H
« Reply #33 on: November 04, 2010, 04:18:21 pm »

Phillip Graham wrote on Wed, 03 November 2010 10:43



I found the JBL 2268H on the Altec Heritage website:
     http://www.audioheritage.org/vbulletin/showthread.php?11346- 2268H





Hey Phil,


Quick question on the 2268H driver.  On that spec sheet, I see that with a 8 cubic ft. sealed box in 1/2 space and 2.83v it averages around 95dB under 100hz.

Now when you take the SRX718 outside in 1/2 space and measure it with 2.83v you get closer to around 100dB.


Would this raise in efficiency is attributed to the porting?  Because the SRX718 is still about 8 cubic feet.  I don't remember porting having an effect like that, but I could have missed it.



Just curious because the SRX718 spec sheet shows similar to the 2268H sensitivity, but when I have measured the SRX718 both at 28.3v at 10M and 2.83v at 1M, it averages about 99-101dB under 100hz.  (SRX728 between 102-104dB with 2v)



Thanks,
Phil
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Phillip_Graham

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Re: RCF 21--Designer comments - Quick side question-2268H
« Reply #34 on: November 04, 2010, 04:52:41 pm »

Phil Lewandowski wrote on Thu, 04 November 2010 16:18

Hey Phil,

Quick question on the 2268H driver.  On that spec sheet, I see that with a 8 cubic ft. sealed box in 1/2 space and 2.83v it averages around 95dB under 100hz.

Now when you take the SRX718 outside in 1/2 space and measure it with 2.83v you get closer to around 100dB.

Would this raise in efficiency is attributed to the porting?  Because the SRX718 is still about 8 cubic feet.  I don't remember porting having an effect like that, but I could have missed it.

Just curious because the SRX718 spec sheet shows similar to the 2268H sensitivity, but when I have measured the SRX718 both at 28.3v at 10M and 2.83v at 1M, it averages about 99-101dB under 100hz.  (SRX728 between 102-104dB with 2v)



I am inclined to think that your microphone is improperly calibrated, or your voltmeter is showing an incorrect RMS voltage out of your testing amplifier, or that your measured spl are peak values.  Your numbers seem a little too high.

I would check your mic against someone else's mic calibrator, or make sure the calibrator adapter for your mic diameter is inserted properly into your calibrator.  Similar advice for the voltmeter.

The JBL measured datasheet reflects what the T/S models show as the driver efficiency.  If those are your half space numbers, then they appear too high.

The only other alternative is JBL using a different driver wiring on the datasheet than in the released product.

The T/S model predicts the JBL driver to have an no of 2.7% and a 2.83V/1m sensitivity of 98.2dB in half space.

The RCF 21 comes out at no 3.2% and 2.38V/1m of 98.8dB (also in half space).

Also remember that 2.83V sensitivity is a game, especially with the stout modern amplifiers.  Ultimately a few core parameters determine the output of vented boxes.  Output limits are essentially determined by box volume, tuning frequency, driver Sd, driver motor strength, and driver linear excursion-assuming the port behaves linearly enough to let a stronger motored driver use it.

The RCF 21" has 1.9x the Vd of a single 2268H, and a stronger motor (Bl^2/Re of 212 for RCF vs. 87 for JBL).  Similar Vd means similar output, and the stronger motor of the 21 gives the extra output near the low corner frequency for a given box volume.


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Steve Hurt

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Re: RCF 21--Designer comments - Quick side question-2268H
« Reply #35 on: November 04, 2010, 04:55:06 pm »

I thought JBL specs were full space.
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Phil LaDue

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Re: RCF 21" Sub Build Pics and Impressions
« Reply #36 on: November 04, 2010, 04:57:33 pm »

Ned Ward wrote on Mon, 01 November 2010 13:48

Bennett Prescott wrote on Mon, 01 November 2010 10:16

Loren,

P.S. Beautiful carpentry.

+1 on that - love all the pocket screws. Norm Abrams from the New Yankee Workshop would be proud.

Kreg makes some very useful tools.
http://www.kregtool.com/default.html

Phillip_Graham

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Re: RCF 21--Designer comments - Quick side question-2268H
« Reply #37 on: November 04, 2010, 05:03:59 pm »

Steve Hurt wrote on Thu, 04 November 2010 16:55

I thought JBL specs were full space.


That would make little sense for LF drivers, as it would be unnecessarily difficult to achieve in the test environment.

I have good collection of AES papers written by JBL engineers, and the pictures of every test structure in them for LF devices shows half-space loading.

They may measure their full range products in full space?
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Steve Hurt

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Re: RCF 21--Designer comments - Quick side question-2268H
« Reply #38 on: November 04, 2010, 05:08:49 pm »

It would make sense, but things don't always make sense!

I'm pretty sure the 718 and 728 are full space numbers.
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Phillip_Graham

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Re: RCF 21--Designer comments - Quick side question-2268H
« Reply #39 on: November 04, 2010, 05:13:49 pm »

Steve Hurt wrote on Thu, 04 November 2010 17:08

It would make sense, but things don't always make sense!

I'm pretty sure the 718 and 728 are full space numbers.



It is highly, highly unlikely.

Consider that all other manufacturers measure the LF enclosures in half space, so JBL would put themselves at a 6dB efficiency penalty on their spec sheets relative to the competition.  This is highly improbable.

Regardless, I am curious to find out, ideally with a well calibrated microphone.
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