Volt BM165 6.5 inch midwoofer

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It is often overlooked because it doesn't follow anything consistently. It's not outstanding in any regard, it got a lot of comprimises. Here's the simulation in 22l and 35Hz tuning:

661796d1518352021-volt-bm165-6-5-inch-midwoofer-volt_br-jpg


I'm sorry but that's not very convincing for the price. The problem is, they want to do everything and in the end they got a driver that's not very good at anything.

Don't bash something you have not heard. I also use Audio Technology 18H52, and Scan drive units and like them a lot, but the Volt sound exceptionally good,
so much so that I prefer them over the AT and S/P. They just sound right!
 

ICG

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My 12" woofers have a Xmax of +-5.2mm with Xdam of 30mm but the most extreme is their 10" PA mid with an Xmax of 0.44mm and Xdam of 20mm. Both have double spiders.

This is a 6,5 inch bass-mid driver and got 6,3mm each side. That's a mismatch. The PA mid got an Xdam of 20mm? It would be much better to get the driver a progressive suspension instead.

Always good to have more than sufficient headroom

No, that's not always a good idea. For PA speakers i.e. that's a major problem. If a big excursion driver gets high power but not enough excursion, the ends of the VC burn to a crisp.

but double spiders also make it easier to achieve a symmetric suspension and they are part of the cooling system.

I don't know how you came to that misconception but it's wrong. The main advantage of double suspension is a reduced tumble movement at huge excursions. A 2nd spider does not provide more cooling because it does not help to move air, unless you use it as a kind of air pump. But that increases the losses and compression as well and introduces asymmetrical load.

All in all Volt drivers are very, very good. Among the very best available.

I haven't seen measurements yet, I can't tell how good they are. The concept of a mirrored second spider can at huge excursions but that's something which does not help a mid-bass. It makes the assembly much more expensive and introduces more points where tolerances are critical. The concept is very good in theory but it's not very good in every use and driver. Compared to a car, you don't have all wheel drive on every car because - even if it's very good in some situations - it got a lot drawbacks too.
 

ICG

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Attached is the simulated MLTL response using the BM165 from the MJK worksheet.
Cabinet size is 200x206x840mm (WxDxH), 18mm wall thickness
Port is 66mm dia, 127mm long, near the bottom
Damping is 0.5lb/cu ft, for the top 2/3rds of the cabinet

Indicates -3dB at ~45Hz, -6dB at ~40Hz.

I can see it got a bump of ~2dB at 55Hz. Did you calculate with a serial resistance of the xo coil? If not, that bump will increase further. That's not exactly what I expect from a 130€ driver.
 

ICG

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Don't bash something you have not heard

I agree, a lot of things have to be heard to allow a judgement. But: The simulations in the bass are very accurate, you don't need to listen to it before you know what's going on. And intermodulation distortion is directly dependent on the excursion, that's physics, you don't need to listen to that either to know it.
 
I can see it got a bump of ~2dB at 55Hz. Did you calculate with a serial resistance of the xo coil? If not, that bump will increase further. That's not exactly what I expect from a 130€ driver.

The Qts of 0.24 is a bit on the low side, so there is a broad dip (~0.8dB) centred on 100Hz. Fortuitously, the 2nd order filter inductor combined with the capacitive driver impedance above resonance gives a similar broad peak at the same freq, resulting in a flat response overall.
 

ICG

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The Qts of 0.24 is a bit on the low side, so there is a broad dip (~0.8dB) centred on 100Hz. Fortuitously, the 2nd order filter inductor combined with the capacitive driver impedance above resonance gives a similar broad peak at the same freq, resulting in a flat response overall.

I haven't simulated it. I think you are right but in the most cases that's not what will happen. And yes, you are right, that's a benefit, I apologize for not simulating it myself.
 
It increases the Qt and that increases the bump in the most cases of a non-clean port tuning.

Yes, the Qts does increase with a series resistor, but you then need to re-optimise the port tuning appropriately. The attached plot shows the result of using a 1 ohm series R, and adjusting the port. The response is now flatter, but with lower sensitivity.
 

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  • BM165 MLTL Rs=1ohm.PNG
    BM165 MLTL Rs=1ohm.PNG
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No evidence of that on the response plot. Fairly flat up to 2kHz, rises a few dB, then rolls off smoothly above 3kHz. Pretty classic behaviour for a poly-cone woofer.
That is exactly what we are referring to - the fact that in the FR plot there is no evidence of a rolloff until above 3K, however when if you inspect the impedance the rolloff due to the voicecoil (inductance and series resistance which results in an LR lowpass filter) should start at about 1K. Therefore there must be significant breakup from the cone itself which is pushing the response back up to near-flat in the 1K-3K region. If the cone was perfectly well behaved, perhaps the response would look closer to the red line like this:
8ku5gTZ.png


The implication is that any non-linear distortion components landing in the 1K+ breakup region will be 'amplified'. To avoid this means to cross it low like in a 3-way so that all non-linear components are <1kHz, or have a motor which is super linear such that very little non-linear distortion is produced (this remains to be proven). Ultimately this means using a lowpass filter with a corner frequency perhaps 500-1Khz depending on how aggressively you want to avoid the problem.

Example of a driver which seems to be flat-ish out to 9kHz
http://www.zaphaudio.com/6.5test/AC180F1-FR.gif

but when you inspect the harmonic distortion you find that it is rising steeply (blue and grey traces) with increasing frequency
http://www.zaphaudio.com/6.5test/AC180F1-HD.gif

This is because the voice coil is rolling off the response while the cone breakup is simultaneously making it rise. Hence you end up with something that is near-flat up to a higher frequency but all the non-linear distortion is higher (compared to a driver with low inductance coil and well behaved cone) since it is not rolled off by the voicecoil inductance like the fundamental is.
 
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if you inspect the impedance the rolloff due to the voicecoil (inductance and series resistance which results in an LR lowpass filter) should start at about 1K.

Not really.

The voice coil looks like 5.5 ohms + 1.2mH, so if it was fed from a series inductor, you would get a slight shelf function, dropping down a few dB, not a LPF as you have drawn. Actually, such a shelving function is very commonly used to implement the baffle step with many drivers.

Volt drivers are already widely used in 2-way designs (even the 8 and 10 inch versions), so they DO work well in practice.
 
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On edit of my post above (crappy forum software won't let me edit again):

Replying to TMM:

Yes, I see what you are getting at - the internal response will roll-off as you describe. But nearly all 6.5 inch woofers will not be pistonic much above 2kHz, they will all exhibit breakup, but a soft cone will do so in a much more benign manner.

It could be that the Volt drivers are intrinsically more linear to begin with (maybe due to the double spiders and very large Xmech). They certainly have a strong reputation for use in studios, where they sound natural and effortless even at high volume. Whether the 6.5 inch unit has the same traits is another matter.
 
On edit of my post above (crappy forum software won't let me edit again):

Replying to TMM:

Yes, I see what you are getting at - the internal response will roll-off as you describe. But nearly all 6.5 inch woofers will not be pistonic much above 2kHz, they will all exhibit breakup, but a soft cone will do so in a much more benign manner.

It could be that the Volt drivers are intrinsically more linear to begin with (maybe due to the double spiders and very large Xmech). They certainly have a strong reputation for use in studios, where they sound natural and effortless even at high volume. Whether the 6.5 inch unit has the same traits is another matter.

:up:
 

ICG

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Hi, yes it was a two way crossed over at 2.2k
I have several of these speakers in studios in Sandton and Foghound Studio's
in Midrand. The engineers preferred them to the Adam monitors used before!

Ah, well, that's a commercial production studio. Almost all commercial studios use more ..customer oriented.. speakers because they want to hear it like it would for a typical target audience member. And commercials are usually compressed (listened to some of them on the Foghound Studios page), isn't that the opposite of an endorsement for hifi?
 
Not getting the break up mode argument

Hi,

I don't really follow the red line chart that shows voice coil inductance would produce a roll off and therefore the flat response must be because of cone break up.

I'm asking because I'm interested in trying these drivers out as I had very good luck with the BM2500.4 in the past.

Here's the what I don't get:

If inductance is really producing a roll off like the red line that looks like LPF, wouldn't the break up have to be precisely the inverse of that in amplitude for most every frequency to level the response off?

Normally when we see a serious cone break up the response starts to go all over the place because the break up is very erratic.

I know this is an old thread, but hoping someone can help me understand.

Thanks!
 
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