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18TBX100 8 ohm and 4 ohm differences

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Foca_Dacian View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Foca_Dacian Quote  Post ReplyReply Direct Link To This Post Topic: 18TBX100 8 ohm and 4 ohm differences
    Posted: 23 May 2018 at 8:31pm
I have purchased 2 x 18TBX100-4 ohm version a few weeks ago, and today I was looking at the T/S.

The 8 ohm version has 97dB @1W/1m and a no of 2,2%
The 4 ohm version has 94 dB @1W/1m and a no of 1.96%
Voltage used for measurements was 2,83 V for the 8 ohm and 2 V for the 4 ohm.

Can some one explain ?
Simulations confirm that the 4 ohm is 3 dB less efficient than the 8 ohm version.

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FrederikMA View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote FrederikMA Quote  Post ReplyReply Direct Link To This Post Posted: 23 May 2018 at 9:27pm
in simple terms, sensitivity is a function of moving mass (mms) and motorforce, (BL)(Re). So resistance is directly related to sensitivity. Motorforce and resistance are also both related to the max spl however. A high BL driver tends to enter xmax at lower input levels in my experience. Sound quality favors the 8ohm for the high BL figure, often reffered to as "high damping". Resistance obviously affects amplifier voltage swing too, so one has to look at the entire system.


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Fred
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Post Options Post Options   Thanks (1) Thanks(1)   Quote mobiele eenheid Quote  Post ReplyReply Direct Link To This Post Posted: 24 May 2018 at 6:22am
@Frederik MA:

Quote sensitivity is a function of moving mass (mms) and motorforce, (BL)(Re)
To my knowledge, sensitivity is calculated directly from n0, which can be calculated from Fs, Qes and Vas. Now if I try to arrive at Fs, Qes and Vas, based on Mms, BL and Re. I come Cms and Sd short (I had to look that up too).

Quote So resistance is directly related to sensitivity
As only Sd is similar for these drivers, how do you get to that point?

Now back to that n0, from which you can calculate sensitivity. (10*(log (n0/100)) + 112 (half space). A doubling of n0, means 3 dB higher sensitivity. If one is 1.96% and the other is 2.2%, there is no way that both the quoted sensitivity figures are correct (based on the formula), unless the variation from the actual measured T/S-parameters is quite large.

@Foca: What are you using them in and how are you simulating them?

Johan


Edited by mobiele eenheid - 24 May 2018 at 6:23am
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FrederikMA View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote FrederikMA Quote  Post ReplyReply Direct Link To This Post Posted: 24 May 2018 at 7:03am
Fair point, I was being over-simplistic I guess.

I see nO as the "real" efficiency for the stated passband and sensitivity as a value the manufacturer may extract from any frequency in (or sometimes even outside) the drivers passband.
This correlates well with your calculations, revealing a mismatch in nO and sensitivity.

And yes, the variation between actual measured TS-parameters of the two drivers are quit large, for the reason (resistance) stated above. They are in a TS-sense two entirely different drivers.

My intention was not to investigate efficiency and sensitivity in general, but rather the parameters affected when changing the resistance and their relationsship to the sensitivity/efficiency.

Regarding the resistance/sensitivity relationship, i should have added "all other things being equal" (to the extend a 4ohm and a 8ohm driver can be equal), but i assumed the productname being the same would suffice.

Best regards,
Fred
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mobiele eenheid View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote mobiele eenheid Quote  Post ReplyReply Direct Link To This Post Posted: 24 May 2018 at 10:03am
If you look at this formula:

Qes = 2Pi x Fs x Mms x Re / (BL)^2

If you halve Re, you halve Qes, therefore doubling n0. If everything (including Qes) stays the same while Re changes, sensitivity changes. But not everything stays the same when you switch from the 8 to the 4 Ohm version. A common way is to reduce BL (by a factor 1.4). So if you lower BL, you can keep Qes and Fs the same, therefore keeping efficiency the same, therefore keeping calculated sensitivity the same.

In this particular case B&C has also changed Qes and Vas but have made sure that the product of those, for both the 4 - 8 Ohm is in the same league. The combination of the Fs, Qes and Vas could perhaps be considered as the prime T/S-parameters for determining how a driver will respond in a certain enclosure. As you can see EBP (Fs/Qes) is also very much the same.

So you could say, these drivers will respond the same because they are different.

Quote And yes, the variation between actual measured TS-parameters of the two drivers are quit large, for the reason (resistance) stated above
I was referring to the difference in T/S-parameters, between two of the same drivers of the same 'manufacturer - batch'. And more specifically to the difference between the 4 - 8 Ohm version if these drivers would be picked from opposite variations in their respected batches.

Anyway, I feel we are drifting off here ;) and for my particular purpose the 4 Ohm driver would be preferred.

So what enclosure and cabinet volume are we talking about here?




Edited by mobiele eenheid - 24 May 2018 at 10:06am
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FrederikMA View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote FrederikMA Quote  Post ReplyReply Direct Link To This Post Posted: 24 May 2018 at 1:12pm
Interesting, I have not considered this design approach on the manufactures part. I think my explanation was insufficient, but in line with yours still. Thank you for elaborating.

Apart of the technical explanation, I think the main focus should be on the system integration with regards to amps and max spl this being a PA minded forum.

Best regards,
Fred
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Hemisphere Quote  Post ReplyReply Direct Link To This Post Posted: 24 May 2018 at 1:25pm
Does this mean that mixing and matching 4ohm and 8ohm drivers with the same model number is a bad idea? (I mean, assuming you correct for impedance at the amplifier stage, but deliver the same amount of Watts to each driver).

I must admit I don't often sim 4ohm and 8ohm variants of the same driver and compare plots, but I'm fairly sure they usually sim about, if not exactly the same. 

The last time I saw them differ greatly was with the Seas L26ROY and the L26RO4Y, which are wildly different drivers despite looking identical (not just in sensitivity but in appropriate enclosure size for a given response), but since the '4' is incorporated directly into the model number rather than as a suffix, my understanding was that they're intended to perform differently.


Edited by Hemisphere - 24 May 2018 at 1:31pm
Phase 1: Post on Speakerplans
Phase 2: ?????
Phase 3: Profit!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Foca_Dacian Quote  Post ReplyReply Direct Link To This Post Posted: 24 May 2018 at 7:32pm
I have simmed the drivers in BB6Pro, using the same enclosure of 175 liters tuned to 34 Hz,the response trance is roughly the same, only efficency differs.

If i understood correctly the explanations above , then the two drivers should be almost equally efficient.

I'm in a bit of a dillema regarding the enclosure.

It will be either the B&C sub 18 plan(if the two drivers are the same regarding efficiency)or the MTL46 from Marc O, I have spoken with him , he simmed the driver in the MTL46 and said it will perform well.
With the MTL I will lose some LF extension but it would be more efficient.

The main use for these subs will be indoor live sound with cover bands.(bass guitar,drums,keyboard,trumpet ,trombone,and vocals,with various genres played)

Maybe you guys can tell me what to chose from the two types of subs and why.

It will be four subs in total, accompanied by two double 12 inch+1.4 inch tops, and single 12 inch+1.4 inch as side fills when needed for crowds up to 3-400 people.




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FrederikMA View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote FrederikMA Quote  Post ReplyReply Direct Link To This Post Posted: 24 May 2018 at 7:41pm
Good question, and not one I can answer with absolut certainty.

Based on the formula and claims posted by Mobiele eenheid, the answer would be NO, you can not "average" the performance of two different drivers. If it was only the resistance which affected the other TS values then you probably could, but if manufacturers re-optimize the driver's TS parameters around Re, then they would never perform the same regardless of them averaging in impedance (which is also affected by the re-optimized parameters).


As mentioned, simulations reveal differences in performance between a 4 and 8 ohm driver. I think your observation of similar performance/simulated behaviour is in line with Mobiele eenheid's claims of manufacturers re-optimizing the driver to remain driver characteristic between the two.

Best regards,
Fred
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DMorison Quote  Post ReplyReply Direct Link To This Post Posted: 24 May 2018 at 9:03pm
Originally posted by Hemisphere Hemisphere wrote:

The last time I saw them differ greatly was with the Seas L26ROY and the L26RO4Y

Seas are a bit different - a 4 in the model name like that does not simply denote a 4Ω version of a driver - note that both of the models you suggested are 4Ω.

That 4 refers to having a 4 layer voicecoil, so typically higher BL with the attendant changes in response/box size & tuning requirements.


Edited by DMorison - 24 May 2018 at 9:04pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Foca_Dacian Quote  Post ReplyReply Direct Link To This Post Posted: 25 May 2018 at 9:19pm
I have mailed B&C about the specs and they said that the sensitivity from the data sheet is a voltage sensitivity, and it depends on the speaker impedance.

The 4 ohm's version minimum impedance is 4,4 ohms, and the 8 ohm's version is 6.2 ohm.
4 ohm version driver with 2 Vrms recieves 0.9W on minimum impedance and the 8 ohm with 2,83 Vrms receives 1,23 W.
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