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Multi-Way Conventional loudspeakers with crossovers

4ohm vs 8ohm midwoofer ...?
4ohm vs 8ohm midwoofer ...?
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Old 8th July 2019, 06:13 PM   #1
animal farm is offline animal farm
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Default 4ohm vs 8ohm midwoofer ...?

For the fun of it, I compared 13 different 5"-6" midwoofers from Wavecor, SB Satori and Scan-Speak where the same model has both a 4ohm version and an 8ohm version; MW13 4/8ohm, 15WU 4/8ohm, WF152 4/8, ... etc.

Based on spec t/s-parameters, I then calculated the sensitivity (with fixed impedance), the force/watt, the accelleration-factor ++, and ended up with the following typical/average difference between 4ohm and 8ohm:

Average for the 13 midwoofers:
• 4ohm has sligtly higher sensitivity: average 82,54dB for 4ohm and 82,25dB for 8ohm (with fixed, highish impedance). High sensitivity is always a good thing - all else equal.

• 4ohm has higher mms: average 8,04cm2/gram for 4ohm and 8,71cm2/g for 8ohm. Mass is good - all else equal, mass helps a midwoofer produce deep, powerful bass. And in this case, higher mass may indicate more copper (less resistance) in the voicecoil, which is always a good thing.

• 4ohm has lower voicecoil inductanse: average 0,16mH for 4ohm and 0,21mH for 8ohm. Low mH is always a good thing when a midwoofer is producing bass and midrange at the same time. Low mH = low distortion / lots of copper in the magnet field.

• 4ohm has around 10% higher force/watt and around 4% higher accelleration-factor than 8ohm. Efficiency is always a good thing, and speed is always a good thing when a midwoofer is producing midrange frequencies.

I may be way off, I'm very far from beeing an expert, but these numbers tells me that - as long as the amplifier is happy-happy with 4ohm speakers - the 4ohm version will (always) be the better choise. You get more of just everything.

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Old 8th July 2019, 06:54 PM   #2
adason is offline adason  United States
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Even if the amplifier is capable of driving 4 ohms, it will always have higher distortion as at 8 ohms. I have made MTM recently, high efficiency ~98 dB/w/m, and purposely wired it
at either 4 ohm, or 16 ohm. Offcourse I preferred 16 ohm, as all my ClassA amplifiers prefer higher impedance loads. Just my experience.
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Old 8th July 2019, 11:34 PM   #3
PeteMcK is offline PeteMcK
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Modelling 4 ohm vs 8 ohm versions of the same driver in Unibox, you'll find that subtle differences in Qts will make one or the other come out with a flatter LF response, it isn't consistent. There isn't usually much in it, & tweaking the tuning of one or the other you can often get them to match.
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Old 9th July 2019, 12:28 AM   #4
Cal Weldon is offline Cal Weldon  Canada
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4ohm vs 8ohm midwoofer ...?
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Originally Posted by animal farm View Post
Opinions, please.
My thought is to thank you for doing this, whether I get to use the info directly or not.
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Old 9th July 2019, 02:23 PM   #5
ICG is offline ICG  Germany
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Quote:
Originally Posted by adason View Post
Even if the amplifier is capable of driving 4 ohms, it will always have higher distortion as at 8 ohms. I have made MTM recently, high efficiency ~98 dB/w/m, and purposely wired it
at either 4 ohm, or 16 ohm. Offcourse I preferred 16 ohm, as all my ClassA amplifiers prefer higher impedance loads. Just my experience.
The distortion of the amp is laughably low compared to the distortion of the speakers. On class A the impedance doesn't matter as long as it can deliver the voltage and/or the current. The impedance of the speakers on class D is a completely different matter. Most of these amps are optimized for either 4 or 8 Ohm and get a bump or dip on high frequencies. Since practically none are built for 16 Ohm I'd avoid high impedances for tweeters and also for the midrange if they are crossed over high or they have a higher inductance which increases the impedance at their upper end often a lot.
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Old 10th July 2019, 08:12 PM   #6
Dave Bullet is offline Dave Bullet  New Zealand
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What frequency did you choose to assess sensitivity and what data did you use? Manufacturer graphs? were they all on the same IEC type baffle conditions and did you choose 1KHz or something above the bafflestep allowing for possible ripple?

I ask since I am surprised at 4 ohm drivers only being 0.29dB more sensitive (negligible). I was expecting ~3dB gain all other things being equal 4 and 8 ohm versions.
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Old 10th July 2019, 09:28 PM   #7
ICG is offline ICG  Germany
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Quote:
Originally Posted by Dave Bullet View Post
I ask since I am surprised at 4 ohm drivers only being 0.29dB more sensitive (negligible). I was expecting ~3dB gain all other things being equal 4 and 8 ohm versions.
Uhm, that's not correct. You only gain 3dB if you measure it at 2,83V, which translates to 2W on 4 Ohm (instead of 1W@8 Ohm). If there's only 0,29dB difference, both were measured at 1W and the tad more spl comes from i.e. a bit less dampend resonance or lower Mms.
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Old 11th July 2019, 09:16 AM   #8
animal farm is offline animal farm
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As I wrote, I used the manufactorers t/s parameters and known formulars. I used Fs, Vas and Qes to calculate the dB, and I used Rdc, Bl and Mms to calculate force/watt and akselleration.

I think the 4ohm models came out best (in this theoretical comparison) because of the added copper in the voicecoil. More copper gives more mms, lower mH, higher dB, higher force/watt and faster akselleration. Its the amount of copper that counts, not how its wound.
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Old 11th July 2019, 01:08 PM   #9
ICG is offline ICG  Germany
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Quote:
Originally Posted by animal farm View Post
I think the 4ohm models came out best (in this theoretical comparison) because of the added copper in the voicecoil. More copper gives more mms, lower mH, higher dB, higher force/watt and faster akselleration. Its the amount of copper that counts, not how its wound.
No, that's wrong. Firstly, it's exactly the way it's wound what determines how high the inductance is. That doesn't matter though because the MW13 got excellent impedance control and the rise in impedance is next to nothing within the usable range. Despite the 8 Ohm version having a higher inductance, both are extremely low. Secondly, more mass means it takes more power to force it to the same movement, lower mass often (not always) means it can follow subtle details more easily and the BL is higher on the 8 Ohm version. Thirdly, the control of the amp over the membrane is determined by the damping factor and on 4 Ohm it's always the half of 8 Ohm. Plus, on lower impedances the serial resistances 'weight' much more, the same serial resistance of the crossover causes much more impact on the sound. With 4 Ohm it's still not critical for most amps except tubes but if that's a factor to decide which version you want to buy, it actually is a determing spec which outranks others.

You've only taken the parameters to compare the drivers and completely ignored other very important things that impact the sound much more. The parameters of the 4 Ohm version of most drivers result in a lower Qts. That can be an advantage, but can also pose a disadvantage, i.e. because it will not go as deep than the 8 Ohm driver. You have to look at the response and the distortion too, that also often changes with the voice coil. If you look at the SATORI MW13P-4, you'll notice it got a peak at ~310Hz, which the -8 does not have. Since it's ~2,5dB high, I'd strongly prefer the non-resonant-peak-version, meaning the -8 got a very good advantage there.

You've also ignored the use of the drivers. For a D'Appolito setup two 4 Ohm drivers in parallel is usually not acceptable, the 2 Ohm pose a problem for many amps and even the ones which can handle it often don't sound that great because of the diminished damping factor. Therefore only a serial wiring can be realized and then you lower the sensitivity compared to the other drivers by far. What's even worse, the inductance is doubled, having a much greater impact on the sound.

The moral of the story is, you cannot chose a mid-woofer from the parameters alone. It does not say one is better than the other except how deep it will go.
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Old 12th July 2019, 01:52 PM   #10
adason is offline adason  United States
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Quote:
Originally Posted by ICG View Post
The distortion of the amp is laughably low compared to the distortion of the speakers. On class A the impedance doesn't matter as long as it can deliver the voltage and/or the current. The impedance of the speakers on class D is a completely different matter. Most of these amps are optimized for either 4 or 8 Ohm and get a bump or dip on high frequencies. Since practically none are built for 16 Ohm I'd avoid high impedances for tweeters and also for the midrange if they are crossed over high or they have a higher inductance which increases the impedance at their upper end often a lot.
It depends...some classA amps, while subjectively excellent sounding, can have few % distortion, way above the good hifi speakers.
ACA for example (I built four pairs and I like it). Or Son of Zen. I have few great sounding tube amps with 2-5% distortion. This may be of topic, but if it is predominantly second harmonic, it may be pleasing.
However, all amps distort more with lower load, I repeat and I stand behind this claim. Plus, even if the amp is for 4 ohm speaker, and you select 4 ohm speaker, it does not mean its impedance stays above 4 ohms across its operating range. It may dip well below.

I got quite a few transconductance amps, and they all like higher impedance too. Have you ever inserted 10-30 ohm resistor between amp and speaker? Just for fun of it?
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