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Author Topic: JBL SRX718 - Basic Distortion Measurements  (Read 38868 times)

Art Welter

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Re: JBL SRX718 - Basic Distortion Measurements
« Reply #20 on: April 27, 2010, 03:25:44 pm »

The 2268 used in the SRX 728 seems to get rid of heat quite well, but from the sine wave testing Phil did at high levels it is obviously not immune from power compression. It is only rated for 800 watts continuous, so it would be unwise to test it for much time with continous levels above that.

Dynamic music would probably benefit from 1600 watt peak potential, as long as that power was at or above Fb (the box tuning, 40-41 Hz for the SRX 728).
That said, Phil’s tests show the speaker is producing about 18% distortion at 50 Hz with 900 watts, so more power may result in more distortion and little increase in level, depending on the frequency of the peak.

The JBL distortion chart shows it hits 10% distortion (and probably Xmax) at 33 Hz with only about 100 watts,

40 Hz is the low E on a bass guitar, the low B is 31 Hz.

Hit the box with 1600 watt peaks much below Fb  and you will get to hear the buzz saw tones you don’t like. You might also tear the suspension up.

If you look at the charts I posted, you will notice that excursion is at minimum at Fb, below Fb output drops rapidly, and excursion and distortion also  increase rapidly.
Excursion also increases above Fb to a maximum about 1/2 octave above Fb, then reduces at higher frequencies.

Both the cabinets I tested and the SRX 728 are too small to have flat response down to Fb. Larger boxes would allow the speaker to put out more bottom end at lower frequencies, but above Fb the speaker would hit excursion limits before it ran out of power.

Most box designers for the general public design boxes a bit small so people burn, rather than tear the speakers up.

“Advanced Speaker Systems” by Ray Alden (available at Radio Shack) is a good book explaining ported and sealed speaker cabinet designs.

Web research into Thiele and Small and D.B. Keele Jr. will get you most of the pertinent information regarding ported cabinet design.

Art Welter
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Jeff Babcock

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Re: JBL SRX718 - Basic Distortion Measurements
« Reply #21 on: April 27, 2010, 05:17:29 pm »

Great posts Phil and Art.  These have great educational potential for many of the users here in the lounge, of whom I would estimate many have very little understanding of the significance distortion has.

This reminds me of many many years ago in my newbie days, doing a show with some truly terrible subs.  The rig was really boomy and the subs sounded awful, and I could not figure out why I was getting so much sound out of the subs above the crossover point, and why I couldn't really fix it by adjusting the crossover or messing with EQ.  I was so frustrated that I just unplugged the subs and ran without them.  At first I thought maybe the crossover was broken.  But it wasn't.... my world of understanding was about to be opened.

In my ignorance at the time I had no understanding of the contribution distortion has, so I incorrectly ASSUMED the crossover would keep ALL frequencies from the sub contained within the specified HPF and LPF ranges.  That gig was a wake up call to the vast world that I was ignorant of.

Art Welter

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Re: JBL SRX718 - Basic Distortion Measurements
« Reply #22 on: April 30, 2010, 12:58:59 pm »

Jeff Babcock wrote on Tue, 27 April 2010 15:17

Great posts Phil and Art.  These have great educational potential for many of the users here in the lounge, of whom I would estimate many have very little understanding of the significance distortion has.

This reminds me of many many years ago in my newbie days, doing a show with some truly terrible subs.  The rig was really boomy and the subs sounded awful, and I could not figure out why I was getting so much sound out of the subs above the crossover point, and why I couldn't really fix it by adjusting the crossover or messing with EQ.  I was so frustrated that I just unplugged the subs and ran without them.  At first I thought maybe the crossover was broken.  But it wasn't.... my world of understanding was about to be opened.

In my ignorance at the time I had no understanding of the contribution distortion has, so I incorrectly ASSUMED the crossover would keep ALL frequencies from the sub contained within the specified HPF and LPF ranges.  That gig was a wake up call to the vast world that I was ignorant of.

The interesting thing is  many well respected subs still have over 100% distortion if full power is applied down low, anything below the -3 dB point.

Many manufacture’s HP recommendations are far too low to avoid gross distortion when music with lots of low frequency content is played at rated power levels.

As you found out, there is no way to equalize the distortion away once it has been generated.

Art Welter
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Phillip_Graham

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Re: JBL SRX718 - Basic Distortion Measurements
« Reply #23 on: April 30, 2010, 04:13:52 pm »

Phil,

Very interesting little experiment.  Doing it at 10m was a good decision, as the box itself will play less of a role, and provide a more accurate measurement.  Doing it as close as Art was forced to will inflate the SPLs

The even order components are lower for the JBL because of the differential motor design.

Very few drivers could sustain 100V sine wave tones like the JBL did without breaking and/or burning.  JBL has every right to be proud of this driver.

Let Art's graphs showing lots of power compression be a lesson to everyone at how difficult pure sine tones are for real loudspeakers.

The other takeaway component, one that I have been stating here for years, is that it is fully possible to have a vented box reach its excursion maxima well above Fb, as result of tuning too low in the name of extension.  Just because you can download and play with WinISD doesn't mean you know what you're doing...
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Art Welter

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Re: JBL SRX718 - Basic Distortion Measurements
« Reply #24 on: May 01, 2010, 11:10:40 am »

Phillip Graham wrote on Fri, 30 April 2010 14:13

Phil,

Very interesting little experiment.  Doing it at 10m was a good decision, as the box itself will play less of a role, and provide a more accurate measurement.  Doing it as close as Art was forced to will inflate the SPLs

The even order components are lower for the JBL because of the differential motor design.

Very few drivers could sustain 100V sine wave tones like the JBL did without breaking and/or burning.  JBL has every right to be proud of this driver.

Let Art's graphs showing lots of power compression be a lesson to everyone at how difficult pure sine tones are for real loudspeakers.

The other takeaway component, one that I have been stating here for years, is that it is fully possible to have a vented box reach its excursion maxima well above Fb, as result of tuning too low in the name of extension.  Just because you can download and play with WinISD doesn't mean you know what you're doing...


The 4015LF and Lab 12 showed various amounts of power compression between half power to full power, from none (the level rose 3 dB) to 1.5 dB.

Without a half power test, hard to say what the JBL’s power compression is, though the one high power test at 1250 watts yielded the same output level as 900 watts.

Looking at the SRX 728 excursion, do you have any opinion of what Xmax actually is for the 2268?
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Phil Lewandowski

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Re: JBL SRX718 - Distortion Measurements - Cont. At and Below Fb
« Reply #25 on: May 25, 2010, 05:22:00 pm »

Hey All,

So I was able to get out for a couple hours to finish off the testing of the SRX718 at and below Fb.  All test procedures where the same, and I just did 40, 35, and 30hz.  

With 40hz I ended up just using 85V, and the 35 and 30hz I pushed the level until I got the distortion around 20dB below the fundamental.


*Note: I did want to make a note that since the SPL meter I am using only has dBC, by the frequencies I was testing, I am sure some of the roll-off started to take place.  So I wouldn't say the SPL measurements are super accurate.*





40hz:



Here is distortion of 40hz at 85V.  The SPL at 10M was 100dBC. Excursion was about 11mm P-P.

http://i717.photobucket.com/albums/ww173/beatmasterphil/SRX718-40hz-85V.jpg






35hz:


Here is distortion of 35hz.  It took 67V to get to this distortion level.  The SPL at 10M was 90dBC.  The excursion was about 13mm P-P.

http://i717.photobucket.com/albums/ww173/beatmasterphil/SRX718-35hz-67V.jpg








30hz:



Here is distortion of 30hz.  It took 37V to get to this distortion level.  The SPL at 10M was 81dBC.  The excursion was about 12mm P-P.


http://i717.photobucket.com/albums/ww173/beatmasterphil/SRX718-30hz-37V.jpg






Next up is the 2010 Growler.  I am planning to do distortion measurements on it on Thursday.  

I also plan to do the comparison of SPL fall-off over distance between the SRX718 and Growler on Thursday.  I did some preliminary tests of this but plan to do more in-depth testing then!


Take Care,
Phil
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Bill Burford

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Re: JBL SRX718 - Distortion Measurements - Cont. At and Below Fb
« Reply #26 on: May 26, 2010, 03:00:03 am »

It looks like harmonic multiples of the frequency for the peak distortions at least...

i.e.  40, 80, 120, 140... etc..
or 30, 60, 90 120
or 35, 70, 105.. ...


280 seems to spike a bit for both 35Hz and 40Hz.. but not so much for 30Hz

is it even possible to have clean bass??? with the density of air and gravity and everything?
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Art Welter

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Re: JBL SRX718 - Distortion Measurements - Cont. At and Below Fb
« Reply #27 on: May 26, 2010, 10:55:30 am »

Bill Burford wrote on Wed, 26 May 2010 01:00

It looks like harmonic multiples of the frequency for the peak distortions at least...

i.e.  40, 80, 120, 140... etc..
or 30, 60, 90 120
or 35, 70, 105.. ...


280 seems to spike a bit for both 35Hz and 40Hz.. but not so much for 30Hz

is it even possible to have clean bass??? with the density of air and gravity and everything?


The even order harmonics don’t sound bad, odd orders do.

The 280 Hz “spike” is around 40 dB down, 1% distortion.

Low distortion in woofers is possible, the Lab 12 has only about 5% distortion in the 30-45 Hz range.

Most bass guitar amp/speakers have over 100% distortion down low.
The 10% distortion the JBL has is not bad.
However, the output below 40 Hz (Fb) drops off dramatically while the distortion rises, at 30 Hz the speaker can only handle 171 watts before it hits 10% distortion.

At the same distortion level, the SRX 718 can put out about 34 dB more SPL at 60 Hz than 30 Hz.
At 35 Hz, output is down by about 21 dB from 60 Hz.

As long as the SRX 718 is operated above Fb, output is decent and distortion is not excessive.
Just don’t expect much clean output below 40 Hz.

It will be interesting to see the Growler results.
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Bill Burford

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Re: JBL SRX718 - Distortion Measurements - Cont. At and Below Fb
« Reply #28 on: May 26, 2010, 01:55:56 pm »

I have heard so many great things about the Lab amps and drivers.
But I have never seen or heard one in person.
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Brandon G Romanowski

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Re: JBL SRX718 - Basic Distortion Measurements
« Reply #29 on: May 26, 2010, 02:28:11 pm »

How did you measure excursion ?
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Brandon G. Romanowski
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