TEST: VOLT VM527, 2" midrange dome
Copyright 2026 © Troels Gravesen

Working on a new construction from an 8" midbass and 1" dome I just couldn't tune it to something satisfactory. Sometimes this approach works, e.g. SEAS-22, sometimes it just doesn't. I know a commercial manufacturer who did a 2-way from these two drivers and I seriously doubt the performance. I know the two - quite expensive - drivers intimately from precious constructions and no matter what the crossover, point of crossover and slopes, they make a perfect match, technically, but fail in overall synergy. This despite both drivers working very well way above and below target frequency range, So, what to do?
Maybe introduce a midrange driver to better link the two.

I like midrange domes. I have a friend having vintage Acoustic Research AR11 speakers, featuring a decent 12" bass, a 2" dome and a 3/4" dome and there are a lot of things this speaker doesn't do well. It lacks deep bass, it lacks bass dynamics and the tweeter is really not the best of domes to put it mildly. However, you can listen to this speaker for hours without any listening fatigue.
What mid-domes can do is bridge the bass and treble without interfering in the critical upper-mid/lower-treble range, right where the ear is most sensitive. We're talking the ~600-4000 Hz range.

Mid-domes appear to fighting their way back into hifi. SBAcoustics has launched one; I used it my in SBA-7MD construction, and I like it a lot! SEAS has recently launched a 2½" mid-dome too, although the frequency response doesn't look nice. ATC (UK) unfortunately removed their excellent 3" domes from DIY marketing and ScanSpeak now makes an OEM 2" dome for Wilson Audio - and sadly not a version for us DIY'ers. Link to you tube video here, after 49 minutes. Very sad.

VOLT, UK, makes 2" and a 3" domes for the DIY market and why not take a closer look at the 2" domes, possibly suitable for my project.

VOLT VM527, 2" dome, 8 Ohm impedance.
Very nice build.
The mid-dome must be mounted from behind, not a big issue here.
Download specs here.

Mounting the VM527 on my "infinite" baffle, 75 x 115 cm, more than enough for a 2" dome.

Frequency response with the above baffle from unit 1 and 2.
This looks nice!
Very smooth response from 500-6000 Hz - and a smooth roll-off. What happens from 12 kHz and up doesn't matter.

Impedance of the two units. A little below 500 Hz.
One thing we have to do when modeling a mid-dome is linearise the impedance.
Here an LCR circuit of 6R8+40uF+2.7mH does the trick. 

Horizontal dispersion of VM527. NICE!
Very even distribution all the way to 6 kHz.

CSD, cumulative spectral decay.
Now, this looks nice until 7.5 kHz where a massive resonance sets in.
WHAT is going on here???

Let's dismantle the dome see what's going on. Just loosen the four front screws and it all comes apart.
Hmm... A center pole piece totally un-damped. Can this be the cause?
The magnet gab is quite wide. Tighter tolerances might have increased sensitivity - but reduce low-end extension. There's a price for everything.

Four dots of Superfix or two strips of double adhesive tape - and -

 
- a damping layer of felt. 41 mm diameter.
I just happen to have a dinking die of 41 mm diameter. It can be cut by hand if you don't have such.
(Is "dinking die" really the English word for a hole punching tool?)

Frequency response of the two units after adding the felt pad. NICE!
An even smoother roll-off.

CSD of unit 1.
7.5 kHz resonance is completely gone!

CSD of unit 2. Resonance gone.

It's hard to believe that VOLT didn't take the time to add some sort of damping material behind the dome. I mean, this is 70'ies technology. An un-damped pole piece behind a large dome. Today we would take the time to make a center pole piece with some size boring leading to a damped rear chamber. The possibilities here are numerous in terms of dimensions and damping material in order to eliminate rear reflections. That said, it may be enough to add a piece of felt like seen here. The frequency response looks better than any other 2-2½" dome on the market.

The final test, distortion. Here at a level equal to around 96 dB/1m.
Green = 2nd harm. Orange = 3rd harm.
Well, not much to say here. Even with a 2nd order high-pass filter at 500 Hz, this dome should do well.

Smaller baffle. What happens?

My basic mounting panel is ~21x36 cm.
Measuring the unit of the basic baffle (green) lifts the response at the low end.
Adding a piece of wood to make 25 cm width with the dome now off-centered, makes a nice smooth response. This dome may like a bit of off-setting, but all depends on actual baffle and placement.

Now, there are many positives about this dome - and one negative. The latter seems to easily be overcome.

I set up the VM257 in a 3-way with an 8" bass driver and a 1" dome tweeter with points of crossover at around 700 and 3500 Hz. Boy does this sounds good. On a few records with a lot of high-frequency -  trumpets, violins, flamenco guitar - the best I've had. Honestly!

The above well in accordance with two reviews:
Klang&Ton: http:/www.troelsgravesen.dk/VM527/VM527-review.png
Dickason: https://audioxpress.com/article/test-bench-volt-loudspeakers-vm527-midrange

The units I have appear to have a wider frequency range compared to Dickason's units.