I'm moving my massive 18" Dayton UmaxII subwoofer out of the dining room hi-fi system to the downstairs home theater. I'm always moving speakers around. My previous subwoofers used two towers each with 2 x 7" and 2 x 10"PR, Now I'm spoiled by the low-end extension of the 18" umax so I've drawn up plans of new towers using 4x8" per tower (for a total of 8x8") or 2x12" per tower (for a total of 4x12") in sealed enclosures. Using the Dayton umax drivers, the 4x12" has more air displacement and SPL below 30Hz, but I won't be pushing these to max SPL. So take SPL off the table. Instead, I'm looking for listening impressions down under 30Hz. They're being matched to Triangle Acoustics or CSS 3TD-X. Both mains have hi-fi analytical presentation. X-over will be around 80 to 100Hz minimum 12dB using DSP, depending on what I hear. Since they are towers, they will be situated nearby the mains.
My experience is if the drivers have the same linear displacement (Sd x Xmax) capability and are equalized to the same target response in the same room position, they sound the same.
That said, it takes more than 2x8" to equal the displacement of 1x12" if their Xmax is the same.
That said, it takes more than 2x8" to equal the displacement of 1x12" if their Xmax is the same.
If you compare the volume of displacement, you might find examples fitting that "rule of thumb", though not often.
Here is a comparison of Dayton subwoofers Vd (Volume of Displacement, Sd x Xmax) in cubic centimeters (Cm3):
8" Vd 188.2 (Classic)
10" Vd 287.5 (Classic)
12" Vd 468.8 (Classic)
15" Vd 688.3 (Classic)
18" Vd 1092 (Apollo 18N)
Then compare the OP's 18" sub, which has three times the displacement of the previous 18":
18" Vd 3315 (Ultimax II)
The 8" Ultimax II has a Vd of 360, considerably more than the Classic 10".
With variations in "subwoofer" linear displacement (Xmax) going from under 3mm to 30mm (or more), and the amount of Sd lost to surrounds, "rules of thumb" don't work very well for comparison.
Art
If you replace a large sub with multiple small ones, summed up, be prepared to need much more amplifier power. Today this can even be an advantage, if you use multiple smaller amps instead of a single large one. For me the sweet spot for home HIFI and HT are 12"-15" sub drivers, maybe 10" for smaller rooms. You have quite a lot to choose from in that range. As soon as you go for specal, extraordinary drivers, the price goes up. Smaller ones don't like it that deep and larger ones get just too large in volume.
Too much linear displacement is not really helping with sound, as there is no linear suspension.
Too much linear displacement is not really helping with sound, as there is no linear suspension.
If you're not driving them to their designed capability, you have some headroom for bass boost using equalisation. That could give you some additional extension at the lower end.
Multiple smaller diameter drivers will almost always have worse distortion performance than fewer larger diameter drivers, even when overall Sd*Xmax is exactly the same between the two systems. But if you want to know for sure you need to measure each one.
@CharlieLaub Based on what do you make that claim? Can you show some articles describing it? I would thing it is the other way round, but I am guessing only. I would like to see some experimental results at least.
Just go to one of those web sites where distortion data is posted and compare drivers of different sizes below 100Hz. The SPL level is usually standardized, so this is a good way to make such a comparison.
This statement captures the gist of it: every driver is different even when comparing drivers of the same nominal diameter. Also, TS parameters are just not a reliable way to compare drivers...
For example the distortion performance of a Purifi 8" and a GRS 8" driver, even though they have similar Xmax and Sd, will be very different:
Purifi 8" ~9.3 mm Xmax, Sd = 235 cm^2, costs $695 each: https://ptt.purifi-audio.com/shop/ptt8-0x08-nab-01-ptt8-0x08-nab-01-1962
GRS 8" ~10.8 mm Xmax, Sd = 227 cm^2, costs $40 each: https://www.parts-express.com/GRS-8...rround-High-Excursion-Subwoofer-4-Ohm-292-816
As with many aspects of loudspeaker building, there are many variables, some co-dependent, that are involved in a design. Also there is size (of the box) and cost to be considered. Some may not care too much about harmonic distortion performance and only want maximum volumetric displacement capability to "play loud", etc.
I can't believe someone would pay $695 for an 8" driver.
For that price, that bad boy better move a couple inches.
For that price, that bad boy better move a couple inches.
Well, in the world of audio, sky is not the limit. There are people that pay more for a piece of wire.
I'd agree but for the fact that Purifi has systematically gone through and attempted to address/minimize each source of drive unit nonlinearity and distortion, and documented this in their white papers and patents. The driver prices need to pay for the R&D, and no doubt a premium to reflect the novelty of the work. I wouldn't buy them as they're well beyond my audio budget, but I can certainly respect the engineering.
If a driver moves, then it distorts. 9.3mm Xmax is not a lot of movement for $695. For that price, I need ZERO distortion above 25.4mm Xmax.
An easy explanation would be: more smaller drivers have a lot more surround surface and length at the same Sd. That (usually) makes distortion go up.
Are you sure? Can you please give more examples? If I compare Purifi to another Purifi, the difference is not that large:
6.5 https://hificompass.com/en/speakers/measurements/purifi/purifi-ptt65x04-naa-08a
8 https://hificompass.com/en/speakers/measurements/purifi/purifi-ptt80x04-nab-02
10 https://hificompass.com/en/speakers/measurements/purifi/purifi-ptt100x04-nab-02
The biggest differences are in the lowest frequencies, I'm not sure how radical that would going to be.
I'm genuinely interested in this subject because I was thinking of building new subs. 18" reflex is very large and heavy, so I thought to myself why not double amount of 12", they would be much easier to manage. I even found a driver that simulates like a dream into 20 Hz tuned relatively small cabinet.
With quads I get to my SPL target and at low volumes xmax is very small, thanks to summed Sd of many drivers. The price for drivers I eyed on is roughly the same, 4x12" vs 2x18". But pulling the trigger on them feels risky if the simulation parameters are off or simply doesn't sound good.
Again, you are making up this explanation. Surroundings are inaudible.
I think the source of distortions is the fact that cone excursion is not linearly dependant on the applied voltage because of different forces in cone suspension at different excursion levels. But this is only an explanation that I am making up.
In any case I personally avoid having multiple drivers producing sound above about 500Hz - 1k because multiple sound sources will interfere with each other rpoducing uneaven sound pressure (having a xo is fine, I mean only having for example two mid drivers connected in parallel or series).
No, surrounds do induce distorsion, because they do not deform perfectly linearly and they modulate Sd (except maybe purifi with their mountain ridge surround).
Whether this distorsion (I suppose mostly h2) is audible and if so if it degrades subjective sound impression is another question.