2x18" IB's v's 4x15" IB's

[Wull]

Hi. Lack of confidence question.

Does anyone know if 2 x 18" IB's are equal too, or fairly close to 4 x 15" IB's output?
I'm slightly worried about bottoming out 2 x 18's, and don't really have room for 4. But could go 4 x 15" Ib's in one manifold. There is only $40. difference between the two in cost, which is a big saving if I can get away with 2 x 18.
my room is 26ft/13ft/7.5 - 2535cubic ft. Seating position 16ft from speakers

If I went with the 18's and decided at a later date to add more could I use 15" IB's?

Any advice will be truly welcome.

Thanks. Will

[chrisbee]

Hi Will

The safety of IB drivers depends on displacement. This means that you can have a large cone area which doesn't move far. Or a smaller cone area which moves a long way. Both methods can achieve the same total displacement.

IB driver Displacement = stroke x total cone area.

Run out of stroke (Xmax) on loud, low frequency peaks and your drivers might be damaged.

Add more cone area, or more Xmax, and your drivers are safe because you have increased the total displacement.

Each Fi 18" (IB318) has a displacement of over 7 litres. So two 18" = 14 litres total displacement.

A 15" has a displacement of somewhere about 3 litres. So four x 3 = around 12 litres of total driver displacement.

So the two 18" divers, with a larger total displacement, are actually safer in use than four 15" drivers.

Another advantage to using the 18" IB318s is that they have a smaller Vas than some other makes of 15" drivers. So, if it is important to you, you can use the 18" drivers in much smaller IB enclosures than four 15" drivers. Particularly since you only need two 18" rather than four 15". Because with the 18" drivers you only have to multiply Vas by 2. Instead of multiplying by 4 with the 15" drivers.

[Wull]

Cheers Chrisbee.

I was looking at making a 50cm square manifold for the 18's, which equates to 2500cubic cm's, as they have an SD 1210.

For the 15's I was looking at 57cm which equates to 3240cubic cm's.

I can go with either, this size is fine.

Why do people bother with 4 x 15's when 2 x 18's would do?

I have just had an email back from Bladeice whom I have placed an order with, they put:

'Working it out with the 10x VAS rule, yes the 18s would be closer to the mechanical limits, 4 15s or 4 18s would make better use of the airspace you have.'

So there is very little between the set ups you say, except for the £400. more to have 4 x15's

[Wull]

Can anyone explain displacement to me please.

I have been told that 4 x 15" IB's have the edge on displacement over 2 x 18" Ib's

What does this mean?

No need, I've been told.

Cheers. Will

[chrisbee]

A driver cone acts just like a piston. It can only move in and out.

The piston (cone) has an active area which can be measured.

We call the active area of the cone Sd. (measured in square cm)

We call the one way stroke Xmax.

But we must double Xmax because ir is a one way measurement.

Driver displacement Vd = 2 x Xmax x Sd.

Let's try working out an example we already know all about.

The Fi IB318 has a cone area of 1210 square cm.

It has an Xmax (one way) of 30mm.

It has a claimed displacement of just over 7 litres.

Double the 30mm Xmax and it becomes 60mm.

But we must use the same units as the area of the cone = cm.

So 60mm becomes 6cm.

1210 x 6= 7260 cu.cm.(cc)

To convert cc to litres we must divide by 1000.

7260 divided by 1000 = 7.2 litres.

[ThomasW]

Chris,

Save your energy, the OP is getting advise in the 3 duplicate threads he's started on HTS.

[Wull]

Aug 30, 2010 16:25:45 GMT -7 ThomasW said:

Chris,

Save your energy, the OP is getting advise in the 3 duplicate threads he's started on HTS.

Yes I am, the reason for this is I stupidly jumped in placed an order for 2 x 18"IB's thinking I could upgrade to 4 x 18's 'I got confused over the size of manifold I could use' Now realising that unless I make another hole in my ceiling to accommodate another manifold this is not possible, and I really don't want to do this, so I was after some quick advice as I can change my order to 4 x 15" today, but it has to be today, so I edge my bets, no harm in that is their.

All the same thanks for the advice Chrisbee and Thomas, I really do appreciate you taking time to answer, and I am very sorry if I have caused any offense.

Cheers

[Wull]

Aug 30, 2010 13:30:29 GMT -7 @chrisbee said:

A driver cone acts just like a piston. It can only move in and out.

The piston (cone) has an active area which can be measured.

We call the active area of the cone Sd. (measured in square cm)

We call the one way stroke Xmax.

But we must double Xmax because ir is a one way measurement.

Driver displacement Vd = 2 x Xmax x Sd.

Let's try working out an example we already know all about.

The Fi IB318 has a cone area of 1210 square cm.

It has an Xmax (one way) of 30mm.

It has a claimed displacement of just over 7 litres.

Double the 30mm Xmax and it becomes 60mm.

But we must use the same units as the area of the cone = cm.

So 60mm becomes 6cm.

1210 x 6= 7260 cu.cm.(cc)

To convert cc to litres we must divide by 1000.

7260 divided by 1000 = 7.2 litres.

Thanks Chrisbee, this helps a lot.

[Wull]

So using your formula:

18" Fi IB3's

1210 x 6= 7260 cu.cm.(cc)

To convert cc to litres we must divide by 1000.

7260 divided by 1000 = 7.2 litres. x2= 14.4 ltrs for 2 x 18"IB's
----------------------------------------------------------------------------------

15" FiIB3's

810 x 6= 4860 cu.cm.(cc)

To convert cc to litres we must divide by 1000.

4860 divided by 1000 = 4.86 litres. x4= 19.44 ltrs for 4 x 15"IB's

so the result is 4 x 15's have 5.o4 ltrs more displacement.

[chrisbee]

He's all over AVF too.

Never mind, information is always useful to somebody. For every question posted there are always a thousand (or more) who will never ask.

Any question which forces me to question what I only thought I already knew is valuable. The question about displacement meant I had to do some homework. To ensure I understood the concept well enough to provide an answer.

The online definitions of Xmax are actually quite vague and difficult to understand for the layman. Until I worked the example of the IB318 I would always have ended up with only half of the real displacement.

Where was I going wrong? In my head I was carrying the false knowledge that Xmax represented the full cone excursion. Just like a piston moving from BDC to TDC when calculating engine displacement.

I was carrying the "idea" that Xmax was a one way measurement had made me think that it simply meant the safe peak to peak excursion before distortion set in with a vengeance. ie. the full cone "stroke" from fully inwards to fully outwards.

My assumption about Xmax was wrong and I am quite happy to admit it. There is no loss of face in not knowing something. Only a fool fails to ask. Our greatest inventions are the result of the curious asking obvious questions. Assumptions have little value to human progress.

As long as people keep asking these "obvious" questions my assumptions can be regularly checked and corrected if necessary.

[Wull]

Aug 30, 2010 22:49:44 GMT -7 @chrisbee said:

He's all over AVF too.

Never mind, information is always useful to somebody. For every question posted there are always a thousand (or more) who will never ask.

Any question which forces me to question what I only thought I already knew is valuable. The question about displacement meant I had to do some homework. To ensure I understood the concept well enough to provide an answer.

The online definitions of Xmax are actually quite vague and difficult to understand for the layman. Until I worked the example of the IB318 I would always have ended up with only half of the real displacement.

Where was I going wrong? In my head I was carrying the false knowledge that Xmax represented the full cone excursion. Just like a piston moving from BDC to TDC when calculating engine displacement.

I was carrying the "idea" that Xmax was a one way measurement had made me think that it simply meant the safe peak to peak excursion before distortion set in with a vengeance. ie. the full cone "stroke" from fully inwards to fully outwards.

My assumption about Xmax was wrong and I am quite happy to admit it. There is no loss of face in not knowing something. Only a fool fails to ask. Our greatest inventions are the result of the curious asking obvious questions. Assumptions have little value to human progress.

As long as people keep asking these "obvious" questions my assumptions can be regularly checked and corrected if necessary.

Thanks Chrisbee. I know you have helped me on AVF as well, I have spent all weekend trying to find answers to displacement etc, it's all completely new to me.
I think I have asked questions that lots more would like to know, whom like me get lost in it all.
Again, I thank you for your time, I shall once up and running send pictures etc, so this might help the next person in my situation.

[brady]

The 15" drivers have 22mm xmax. So it's 810 x 3.5 not 6. That gets you 3.5 liters x 4 = 14 total

[ThomasW]

Aug 30, 2010 21:53:54 GMT -7 Wull said:

All the same thanks for the advice Chrisbee and Thomas, I really do appreciate you taking time to answer, and I am very sorry if I have caused any offense.

The offense is a function of HTS stealing ideas, information, etc., to create an IB forum in direct competition with the cult.

And that's why this sticky thread is posted in the 'My Project's' section of this forum
ibsubwoofers.proboards.com/index.cgi?board=projects&action=display&thread=1143

I created and have evolved the modern IB sub starting in 1999. So any information from other forums has come from the work done by the cult.

I have no issues with threads on AVF. HTS on the other hand plays bait and stitch with potential IB builders. The sticky thread where Rodney 'converts' his IB to a LLT is an example of how little knowledge those people really have.

FWIW the 'posers' on HTS have already given you questionable advise. For example, using WinISD plot to compare drivers but drawing incorrect conclusions from those plots.

But that's not my problem.

I stupidly jumped in placed an order for 2 x 18"IB's thinking I could upgrade to 4 x 18's 'I got confused over the size of manifold I could use' Now realising that unless I make another hole in my ceiling to accommodate another manifold this is not possible, and I really don't want to do this, so I was after some quick advice as I can change my order to 4 x 15" today, but it has to be today, so I edge my bets, no harm in that is their.

If properly designed it's certainly possible to add a second pair of drivers to the same manifold that already contains one pair of drivers.

[baggy]

Aug 31, 2010 6:46:23 GMT -7 brady said:

The 15" drivers have 22mm xmax. So it's 810 x 3.5 not 6. That gets you 3.5 liters x 4 = 14 total

The Fi website shows both the 15" & 18" IB3 drivers having the same 30mm xmax

[Wull]

Aug 31, 2010 6:58:51 GMT -7 ThomasW said:

Aug 30, 2010 21:53:54 GMT -7 Wull said:

All the same thanks for the advice Chrisbee and Thomas, I really do appreciate you taking time to answer, and I am very sorry if I have caused any offense.

The offense is a function of HTS stealing ideas, information, etc., to create an IB forum in direct competition with the cult.

And that's why this sticky thread is posted in the 'My Project's' section of this forum
ibsubwoofers.proboards.com/index.cgi?board=projects&action=display&thread=1143

I created and have evolved the modern IB sub starting in 1999. So any information from other forums has come from the work done by the cult.

I have no issues with threads on AVF. HTS on the other hand plays bait and stitch with potential IB builders. The sticky thread where Rodney 'converts' his IB to a LLT is an example of how little knowledge those people really have.

FWIW the 'posers' on HTS have already given you questionable advise. For example, using WinISD plot to compare drivers but drawing incorrect conclusions from those plots.

But that's not my problem.


I understand, I am sorry ThomasW..

[Wull]

Aug 31, 2010 6:46:23 GMT -7 brady said:

The 15" drivers have 22mm xmax. So it's 810 x 3.5 not 6. That gets you 3.5 liters x 4 = 14 total

I have just email Bladeice regarding the above, I asked:

Sorry to bother you once again, but someone is trying to tell me the Xmax on the IB 15's is 22mm and not 30mm as stated on your web site. Can you confirm what the correct Xmax figure is please.

And they replied:

Hi Will

I can confirm they are 30mm 1 way linear


Am Singh | Blade ICE.
www.bladeice.com
www.knukonceptz.co.uk
[phone]
[phone]

[brady]

My mistake. I was using the specs from Mike's driver comparison chart.

[baggy]

Sept 1, 2010 4:43:58 GMT -7 brady said:

My mistake. I was using the specs from Mike's driver comparison chart.

I'm glad we got that sorted. I was beginning to doubt my purchase

[moonfly]

Aug 30, 2010 22:49:44 GMT -7 @chrisbee said:

He's all over AVF too.

Never mind, information is always useful to somebody. For every question posted there are always a thousand (or more) who will never ask.

Any question which forces me to question what I only thought I already knew is valuable. The question about displacement meant I had to do some homework. To ensure I understood the concept well enough to provide an answer.

The online definitions of Xmax are actually quite vague and difficult to understand for the layman. Until I worked the example of the IB318 I would always have ended up with only half of the real displacement.

Where was I going wrong? In my head I was carrying the false knowledge that Xmax represented the full cone excursion. Just like a piston moving from BDC to TDC when calculating engine displacement.

I was carrying the "idea" that Xmax was a one way measurement had made me think that it simply meant the safe peak to peak excursion before distortion set in with a vengeance. ie. the full cone "stroke" from fully inwards to fully outwards.

My assumption about Xmax was wrong and I am quite happy to admit it. There is no loss of face in not knowing something. Only a fool fails to ask. Our greatest inventions are the result of the curious asking obvious questions. Assumptions have little value to human progress.

As long as people keep asking these "obvious" questions my assumptions can be regularly checked and corrected if necessary.

John from AE wrote an interesting piece on the subject of xmax.

The first part of this is really Xmax History 101, so feel free to skip the first 4 paragraphs if you want. Back in the mid 90's when I first got into the DIY audio thing, Xmax wasn't nearly what it is today. I was on this somewhat well known email list with something like 1800 other people called the "bass list." At that time the champion of all Xmax was the NHT1259. It was the greatest driver ever and I believe it had 11mm Xmax. Shortly after that the ACI DV12 came out with 12mm Xmax and became the new champion. Then Dan Wiggins decided to go for even more and the Shiva came out with I believe 15mm Xmax at that time. But something changed. People stopped using just the physical overhang to rate Xmax. DUMAX came out with the .707x the rest BL value to rate Xmax. This is when I started playing around with drivers as well as Nick at Lambda Acoustics.

We found that you could take a driver with the same physical overhang, say 14mm overhang, but vary the top plate thickness and get different results. For example a 10mm gap with a 38mm coil has 14mm overhang but acts totally differently from an 18mm gap plate with a 46mm coil. Both have 14mm overhang, but the BL curves are totally different and you have much more usable excursion on the driver with the thicker top plate. Likewise, taking a driver with the same 38mm coil and using the two different 18mm and 10mm mm gaps gives nearly the same results. Going by traditional standards of just coil length minus gap and divide by two, you'd think that one would have 14mm and the other 10mm, but in reality they are very similar with the edge actually going to the shorter overhang driver in terms of usable excursion. This is about the time people started throwing out the generally accepted ideas on Xmax and started investing more time and money in FEA analysis and real world testing.

The Xmax race as I remember it was on. TC sounds came out with their 3HP motor after a lot of FEA work. It had a 1.4" gap plate with a 2.3" long coil. By traditional standards the overhang was only 12mm, but by DUMAX results based on the BL curve Xmax was in the range of 23-27mm. This extremely tall gap really changed the way people looked at Xmax. Our HE15 was based on this design with 27mm. We were for a short time the Xmax champions... but it only lasted a few months.

Dan Wiggins came out with the XBL^2 motor topology that would again totally change the way people look at Xmax. There was no simple formula to give Xmax as the coil essentially traveled back and forth between multiple gaps while creating linear force on the coil over a longer range. They achieved Xmax in the 30mm+ range and no longer was the motor design the limiting factor. Now the physical clearances in the frame and suspension parts became the limits. Things were never the same again.

So that all said, what does Xmax really mean? It's a good way to tell how much range of the driver is really useful while keeping distortion inaudible. In speakers, the low end limit for being able to notice distortion is considered to be about 10%. In reality most people can't hear distortion and notice the audibility up until way over 25% even. That seems high, but while at Michigan Tech, the regional AES vice president, Tom Nousaine, came and did a discussion on distortion. Nut just subwoofer distortion, but distortion full range. He played a track of Tom's Diner by Suzanne Vega. This was a blind listening test where the track was played over and over again with progressive amounts of distortion added. The trick was to say when you could hear audible distortion, but we didn't know the amounts until later. In a room of about 20 people, all audio geek AES members, we all sat and listened and would say "ok, the 4th time i could hear it". It turned out that only a couple in the room claimed to hear distortion at the point where 10% was added. Most claimed at 25-50% distortion.

This means that if distortion isn't audible until 10% we can use the driver up until 10% distortion and it will sound clean. Therefore taking the point at which Bl drops to 70% of it's rest value is usually considered the point where the motor will cause the driver to reach 10% distortion and the reason that point is chosen for Xmax ratings.

However, the motor isn't the only thing that causes distortion. Parts physically making noise during their motion, resonances in parts, power compression, etc can all cause distortion to rise over 10% well before the motor is the limit. Klippel had all kinds of other standards to simply measure 10% distortion points. In many cases this would correlate quite well to the 70% Bl mark, but some drivers would hit 10% distortion above and below that point. IMO this is the best way to truly rate a driver by looking at all the factors that contribute to distortion and seeing which brings it to an audible point first, then rating Xmax based on that point. Not everyone has money for a Klippel though

That all said, how much Xmax is enough? It really depends on so many factors. The largest being the enclosure. You can take a driver with a given Xmax and have it reach that point with 100W or need 1000W depending on which enclosure you put it in. In a sealed enclosure, Xmax is more easy to compare. You also need to factor in the Sd of the driver though. Your maximum output is directly equal to how much air you can move before you reach 10% distortion. The more air you move, the louder you can play. How loud you play at any given point is determined by this formula:

SPL = 20 * Log (Vd * F^2) + 48.4

Vd in liters being calculated from Sd x Xmax

This doesn't tell you how much power you need to reach that level. It just tells you that if you can move the driver a certain range at 10% distortion, it will result in that SPL. The more Xmax the better, but at some point you become impractical as you need unrealistic amounts of power to reach that distance. That is when it becomes more practical to use multiple drivers, each only having to move shorter distances with less power.

In vented enclosures it's harder to compare. The driver has to move less as the port is creating the majority of output around tuning. The point then is that you need to further excite the resonance of the port, and the motor strength of the driver becomes more important at that point than the long travel. However, as you do now have other issues like group delay to contribute to "distortion" of the system, you want the driver operating in an even more linear range. You also still have the driver excursion peak above resonance to deal with, but as others said, with smaller enclosures you typically hit thermal limits before excursion limits. That is a whole different writeup to get to though....

John

Not exactly on topic, but interesting and possibly relevant all the same.

[chrisbee]

Hi, Dan! ;D

Yes, I read that, but was protected by my advancing dementia from ever having to reveal the awful truth.

Perhaps of more relevance, one must ask whether our favourite drivers are performing on a level playing field in our cosy IB world.

Do the Fi and AE drivers for IB purposes have the claimed Xmax?

The point being the relevance of enclosure size to measured Xmax...

I don't doubt the veracity of the makers.. just curious how JohnH's piece relates to our subjective hunches when making buying decisions.