For a 3-way designed for sub crossing at 80Hz - what are the implications of multiple woofers? - AVS Forum
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post #1 of 37 Old 04-17-2013, 12:51 PM - Thread Starter
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Hi - I'm trying to spec out my dream next 5.0 upgrade; although i won't be the one building it, I figured the DIY crowd would be the best to help me understand some of the issues and trade-offs.

I am currently thinking of a 3-way using a ScanSpeak Revelator 7" for the woofer. They have a 4ohm version with 90db sensitivity which is what I was thinking in a single woofer T-M-W.

So for a 3-way bookshelf/mini-tower designed to be crossed over to a sub at 80Hz...

...what are the pluses and minuses for using two woofers (T-M-W-W vs T-M-W)?

...if you were to use two woofers, what are the pluses and minuses for using 4ohm or 8ohm speakers?

Thanks for the education redface.gif

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post #2 of 37 Old 04-17-2013, 01:49 PM
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Do you mean a 4-way with two woofers or a 3-way with two woofers?

With one good sub, the difference is probably marginal at best; and only noticeable in music-mode with mains only activated.

Given the number of subwoofers most people in DIY have, I don't think it would make a whole lot of difference one way or the other.
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post #3 of 37 Old 04-17-2013, 02:16 PM - Thread Starter
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Quote:
Originally Posted by BassThatHz View Post

Do you mean a 4-way with two woofers or a 3-way with two woofers?

Since the one or two woofers would be playing exactly the same frequency range I consider a 3-way whether it has 3 or 4 drivers.

So the questions have to do with a design where the woofer is responsible for 80 - 650Hz and the drawbacks/advantages to using one or two drivers for this frequency range.

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post #4 of 37 Old 04-17-2013, 02:28 PM
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Advantage for a single woofer driver:
Cheaper

Advantage for two or more woofers:
Less distortion at equal outputs.

There are more, but then it becomes application dependent.

YID DIY
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post #5 of 37 Old 04-17-2013, 02:31 PM - Thread Starter
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Originally Posted by Looneybomber View Post

There are more, but then it becomes application dependent.

The application is for front L/R in a 5.2 system for both HT and multi-channel music.

The centre will be a W-T/M-W, so dual woofer mains would exactly match the centre.

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post #6 of 37 Old 04-17-2013, 02:34 PM
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Originally Posted by rick240 View Post


...what are the pluses and minuses for using two woofers (T-M-W-W vs T-M-W)?

The 80 to 650hz (as you pointed out) region is the most demanding bands you'll be asking the speaker to reproduce. Sub 80hz might be more demanding, but you're using subs. So doubling up the woofers halves the demand on the woofer, therefore increasing fidelity. Also increases sensitivity. This is also the baffle step region (guessing you're wanting a narrow speaker) so that added woofer will help overcome baffle step losses.

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Originally Posted by rick240 View Post


...if you were to use two woofers, what are the pluses and minuses for using 4ohm or 8ohm speakers?

As long as the amp can handle it, 4 or 8 ohm doesn't really matter. If it's 8ohm, the amp can just run along like it's not big deal. The 4ohm load will draw more current. With two woofers and a good amp, I'd maybe go with 2 x 8ohm woofers in parallel.



As an aside, if crossing to a subwoofer I wouldn't waste the performance of the scan speak on this speaker. The scan speak excels at producing bass while sacrificing effeciency. Consider something better designed for 80 to 650hz if that's the intended pass band. Scan speaks are really nice, just not for this application, imo. Although they do have a few models that fit this application well I suppose.
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post #7 of 37 Old 04-17-2013, 02:35 PM
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Quote:
Originally Posted by rick240 View Post

The application is for front L/R in a 5.2 system for both HT and multi-channel music.

The centre will be a W-T/M-W, so dual woofer mains would exactly match the centre.

Posting while you posted.

If this is the case, I'd spring for the duel woofer setup. That way everything stays the same.
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post #8 of 37 Old 04-17-2013, 02:37 PM
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Quote:
Originally Posted by rick240 View Post


...what are the pluses and minuses for using two woofers (T-M-W-W vs T-M-W)?
Two woofers will have higher sensitivity and output capability.
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...if you were to use two woofers, what are the pluses and minuses for using 4ohm or 8ohm speakers?
You don't want to build a speaker that you amp can't comfortably drive, so parallel wired 4 ohm drivers are probably not a good idea.

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post #9 of 37 Old 04-17-2013, 02:45 PM
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is you r going to use revelator in bookshelf crossover at 80hz, i would just go 2 way. i am using a set of zaph zrt and they are phenomenal and can go quite loud with a good sub.
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post #10 of 37 Old 04-17-2013, 02:49 PM - Thread Starter
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Quote:
Originally Posted by Bill Fitzmaurice View Post

You don't want to build a speaker that you amp can't comfortably drive, so parallel wired 4 ohm drivers are probably not a good idea.

The specific driver I'm thinking of is the 7" Revelator - the 4ohm is 90db sensitive and the 8ohm is 87db.

So with two woofers you'd use the 8ohm drivers and in the one woofer the 4 ohm; do I have the general idea?

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Who is designing your crossover? As good as revelator woofers are, they will still sound bad with a poorly designed crossover. Also, while I love the revelator woofers, they would not be my choice for a three way. The benefit of the revelators (other than the low distortion motor) is the very smooth response which means you can cross over higher or use shallower slopes in a two way. If I were designing a three way with the woofer only running up to 400hz or so, there are other woofers that will cost less and give as good or better performance.
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post #12 of 37 Old 04-17-2013, 03:06 PM - Thread Starter
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Originally Posted by djarchow View Post

Who is designing your crossover?

If things go as per my last project it'll be a Jim Salk/Dennis Murphy collaboration.

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As good as revelator woofers are, they will still sound bad with a poorly designed crossover.

Assuming it's Dennis, i don't think there's a risk of a poor design.

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Also, while I love the revelator woofers, they would not be my choice for a three way. The benefit of the revelators (other than the low distortion motor) is the very smooth response which means you can cross over higher or use shallower slopes in a two way. If I were designing a three way with the woofer only running up to 400hz or so, there are other woofers that will cost less and give as good or better performance.

I picture the speaker will cross to a sub at 80Hz and the woofer would cross to the mid somewhere in the 600 - 700 range. What other drivers would you consider?

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post #13 of 37 Old 04-17-2013, 03:29 PM
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http://www.parts-express.com/pedocs/specs/294-696-bc-speakers-8bg51-specifications.pdf

or

http://www.parts-express.com/pdf/294-649s.pdf

Both can be bought in Canada from loudspeakers.ca

I'd also look closely at the SB Acoustics stuff. They tend to trade off bass for effeciency, but probably only a couple db over the scan. There's actually a dissapointing number of options for the application you're describing. Especially in your price range. And I'm betting looks matter, so ugly pro woofers are out. How about a 10"? If you could cover 80 to 650hz with a pair of 10"ers you'd have quite a few options. That might be to wide for you though.
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post #14 of 37 Old 04-17-2013, 03:45 PM - Thread Starter
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Originally Posted by tuxedocivic View Post

http://www.parts-express.com/pedocs/specs/294-696-bc-speakers-8bg51-specifications.pdf

or

http://www.parts-express.com/pdf/294-649s.pdf

Both can be bought in Canada from loudspeakers.ca

I'd also look closely at the SB Acoustics stuff. They tend to trade off bass for effeciency, but probably only a couple db over the scan. There's actually a dissapointing number of options for the application you're describing. Especially in your price range. And I'm betting looks matter, so ugly pro woofers are out. How about a 10"? If you could cover 80 to 650hz with a pair of 10"ers you'd have quite a few options. That might be to wide for you though.

Thanks for the suggestions, although size wise I am hoping for a 6" - 7". Might go to 8" if needed, but I suspect there are several good 6 1/2" or 7" options.

Available in Canada doesn't matter as i am not DIY-ing and both Jim and Dennis are US based (likely a purchase through Madisound).

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post #15 of 37 Old 04-17-2013, 03:51 PM
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Quote:
Originally Posted by rick240 View Post

If things go as per my last project it'll be a Jim Salk/Dennis Murphy collaboration.
Assuming it's Dennis, i don't think there's a risk of a poor design.
I picture the speaker will cross to a sub at 80Hz and the woofer would cross to the mid somewhere in the 600 - 700 range. What other drivers would you consider?

You are good then. Dennis does fantastic crossover work, and Jim's cabinets are works of art.
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post #16 of 37 Old 04-17-2013, 03:51 PM
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Dang, the size is really gonna limit you.
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post #17 of 37 Old 04-17-2013, 04:14 PM - Thread Starter
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Quote:
Originally Posted by tuxedocivic View Post

Dang, the size is really gonna limit you.

The Revelator 18W/4531G (single driver 4 ohm, or 18W/4531G-00 *ohm for dual driver) should both do the trick and sound incredible.

So along with the dual vs single question that this thread was originally about, it's sort of morphed to include what alternatives to the Revelator might would contribute to a great sounding 90db-ish speaker in the 80 - 640Hz range.

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post #18 of 37 Old 04-17-2013, 04:22 PM
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In the 80 to 650hz range, I look for sensitivity. Especially with a 3 way where you can get fairly sensitivity out of it. Even using a dome mid and tweeter. I just did one (with an 8" woofer) and I got about 88 or 89db at 8 ohms and ample baffle step compensation. If the woofer isn't sensitive, you waste a lot of power padding down the mid and tweet. And without needing the sub-80hz content, why not take advantage of the sensitivity.

The one I just did for a local guy was pretty small even with an 8". Didn't look out of place sitting on a stand at all. And if you're debating 2 x 7" woofers, I'd think 2x 8" woofers would look asthetically acceptable.
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post #19 of 37 Old 04-17-2013, 04:51 PM - Thread Starter
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Quote:
Originally Posted by tuxedocivic View Post

In the 80 to 650hz range, I look for sensitivity. Especially with a 3 way where you can get fairly sensitivity out of it. Even using a dome mid and tweeter. I just did one (with an 8" woofer) and I got about 88 or 89db at 8 ohms and ample baffle step compensation. If the woofer isn't sensitive, you waste a lot of power padding down the mid and tweet. And without needing the sub-80hz content, why not take advantage of the sensitivity.

The 7" Revelator 4ohm is 90db sensitive and the 8ohm is 87 (so a pair should get up around 90, right?). Are many other options much higher?

As an aside, how much of a drop is speaker sensitivity relative to driver sensitivity due to BSC?

Quote:
The one I just did for a local guy was pretty small even with an 8". Didn't look out of place sitting on a stand at all. And if you're debating 2 x 7" woofers, I'd think 2x 8" woofers would look asthetically acceptable.

I'm preferring the look of single woofer in the mains and dual in the centre - but am querying here to determine if it makes sense to push the WAF and do duals (turning them from large bookshelves to mini-towers).

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post #20 of 37 Old 04-17-2013, 05:08 PM
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With 2 oh the 8ohms, you'll actually be at 93db/m/2.83V, but at 4ohms. What you're not considering is baffle step losses will take down that single 4ohm woofer to 84db. And it'll be sucking up twice as much power to make 84db as an 8ohm version. Think of it like this and compare it to my single 8" woofer 3 way that does 88db at 8ohms. If I am listening at the listening position 4m away at an average of 75db, that means that speaker is making about 87db average (6db loss per doubling distance). Ok, so on average it's using 1 watt. Suppose There's a 15db above average peak, that's 90db at the LP, or 102db at the speaker. That's 14db above the speakers sensitivity and my amp will have to make less than 32 watts to reproduce that peak.

Ok now lets look at the 4ohm load with 85db sensitivity (assume not full baffle step compensation). Again, at the LP you're listening at 75db average with a 15db peak. That's 90db at the LP and 102db at the speaker. Nothing has changed except that it's 17db above your speakers sensitivity and your speaker is 4ohms, therefore your amp has to produce just under 128 watts to reproduce that peak. Add 3db more dynamic range and your amp needs 250 watts! The poor little scan speak is gonna struggle with that. That's what an extra 2" of cone diamter can do for you. Or an extra woofer, your choice. And most guys around here don't consider 75db average with 90db peaks at the LP at all good enough. I probably would be satisfied with that, but if you like action movies and to have a good time with your buddies over watching things blow up, a single scan speak 18w will not have fun doing what you ask it to. If you were spending a couple hundred on a pair of bookshelves for you livingroom, I'd say don't sweat it, but it sounds like you want quality.
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post #21 of 37 Old 04-17-2013, 06:40 PM
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Quote:
Originally Posted by rick240 View Post

The specific driver I'm thinking of is the 7" Revelator - the 4ohm is 90db sensitive and the 8ohm is 87db.
I believe that's at 2.83v, not 1 watt. 2.83v into 4 ohms is 2 watts, into 8 ohm it's 1 watt. The sensitivity per watt should be the same.

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post #22 of 37 Old 04-17-2013, 07:42 PM
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Why stop at two woofers when you can have four!?

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post #24 of 37 Old 04-17-2013, 09:44 PM - Thread Starter
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Quote:
Originally Posted by Bill Fitzmaurice View Post

I believe that's at 2.83v, not 1 watt. 2.83v into 4 ohms is 2 watts, into 8 ohm it's 1 watt. The sensitivity per watt should be the same.

This is another one of the things that confuses me at times - multiple sensitivity scales confused.gif

And then complicated again because most amps can deliver double the power into lower impedance, sigh.

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post #25 of 37 Old 04-17-2013, 09:52 PM - Thread Starter
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Quote:
Originally Posted by tuxedocivic View Post

With 2 oh the 8ohms, you'll actually be at 93db/m/2.83V, but at 4ohms. What you're not considering is baffle step losses will take down that single 4ohm woofer to 84db.

So is it fair to say that BSC will bring sensitivity down by 4 - 6 db then?

Quote:
And it'll be sucking up twice as much power to make 84db as an 8ohm version. Think of it like this and compare it to my single 8" woofer 3 way that does 88db at 8ohms. If I am listening at the listening position 4m away at an average of 75db, that means that speaker is making about 87db average (6db loss per doubling distance). Ok, so on average it's using 1 watt. Suppose There's a 15db above average peak, that's 90db at the LP, or 102db at the speaker. That's 14db above the speakers sensitivity and my amp will have to make less than 32 watts to reproduce that peak.

Ok now lets look at the 4ohm load with 85db sensitivity (assume not full baffle step compensation). Again, at the LP you're listening at 75db average with a 15db peak. That's 90db at the LP and 102db at the speaker. Nothing has changed except that it's 17db above your speakers sensitivity and your speaker is 4ohms, therefore your amp has to produce just under 128 watts to reproduce that peak. Add 3db more dynamic range and your amp needs 250 watts! The poor little scan speak is gonna struggle with that. That's what an extra 2" of cone diamter can do for you. Or an extra woofer, your choice.

And sometimes getting good specs for power handling can be hard.

Quote:
And most guys around here don't consider 75db average with 90db peaks at the LP at all good enough. I probably would be satisfied with that, but if you like action movies and to have a good time with your buddies over watching things blow up, a single scan speak 18w will not have fun doing what you ask it to.

Being in my fifties and having been too close to the front for many a concert I suspect that the 75/90 is probably typical, but crank it to 80/95 or 85/100 for the odd movie or music disc (when the wife's not around wink.gif).

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If you were spending a couple hundred on a pair of bookshelves for you livingroom, I'd say don't sweat it, but it sounds like you want quality.

I definitely want quality - but am always open to finding great value.

Current 5.0 is three KEF iQ3 across the front and a pair of iQ1 for the rear - absolutely incredible value. Paired with a Rythmik F12, when I play Dark Side of the Moon m-ch SACD it sounds great.

I just want to make sure that whatever is my upgrade, is really and truly an upgrade all around.

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post #26 of 37 Old 04-17-2013, 10:29 PM
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How do you find the volume of the kefs? Are they clean at high output?

Yes, BSC will knock 4db off with a narrow cab.
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post #27 of 37 Old 04-18-2013, 06:14 AM
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Originally Posted by rick240 View Post

And then complicated again because most amps can deliver double the power into lower impedance, sigh.
They don't. At full power an amp can deliver about 1.7 times the power into a halved impedance load. More important, it doesn't matter. What limits the output of a woofer is usually the cone displacement (Vd), not power. A four and eight ohm driver with the same Vd will have the same output capability.
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Yes, BSC will knock 4db off with a narrow cab
Not necessarily. Where the baffle is less than 1/4 wavelength from the rear wall all of the energy that wraps around the cab below the baffle step frequency reflects off the rear wall to fully recombine with the direct wave. Even where the distance is greater than 1/4 wavelength the reflected wave will recombine, mostly constructively. The only problem frequency is at 1/4 wavelength, where the direct and reflected waves will meet 180 degrees out of phase, resulting in an Allison Effect notch. The full effect of the baffle step is only seen where there is no reflection off the rear wall, ie., outdoors. Indoors that energy is not lost, it's just heard as reflections rather than directly.

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post #28 of 37 Old 04-18-2013, 06:14 AM - Thread Starter
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Quote:
Originally Posted by tuxedocivic View Post

How do you find the volume of the kefs? Are they clean at high output?

They get loud enough for me; as i mentioned, I probably play m-ch music the loudest. They seem clean to my untrained ears.

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Yes, BSC will knock 4db off with a narrow cab.

OK, thanks for the education on this.


btw. I have some good friends living in Ladysmith, small world. If you're into pottery you should visit their studio, http://jovicpottery.com/

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post #29 of 37 Old 04-20-2013, 12:28 PM - Thread Starter
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Quote:
Originally Posted by Bill Fitzmaurice View Post

Two woofers will have higher sensitivity and output capability.

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Originally Posted by Bill Fitzmaurice View Post

They don't. At full power an amp can deliver about 1.7 times the power into a halved impedance load. More important, it doesn't matter. What limits the output of a woofer is usually the cone displacement (Vd), not power. A four and eight ohm driver with the same Vd will have the same output capability.

I think I get it (sometimes I'm a bit slow redface.gif).

So, if you have 2 woofers in your design, and your calculation for dynamic peak handling says that there would be an instantaneous power need of 300 watts, would each driver see 150watts or 300?

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post #30 of 37 Old 04-20-2013, 04:37 PM
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Originally Posted by rick240 View Post


I think I get it (sometimes I'm a bit slow redface.gif).

So, if you have 2 woofers in your design, and your calculation for dynamic peak handling says that there would be an instantaneous power need of 300 watts, would each driver see 150watts or 300?
I don't know what you mean by instantaneous power need or dynamic peak handling. Those are mainly marketing terms, not engineering terms. Speaker output is measured in decibels, not watts. The decibel output limit for a woofer is determined by the maximum cone displacement. If one driver lacks adequate displacement adding a second driver doubles the displacement, allowing for 6dB additional output.

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