Auto/truck tuba designs

[Benflitv30]

Good to know, Bill, about the premium drivers. I did notice that with some of the ‘boutique’ brands I see Parts Express carries. If the cost/benefit is there, I’m all for it otherwise.

Well, after purchasing the plans and following the wise advice of the cardboard mockup...unfortunately I’m more space limited than I originally thought due to the fixed seat back angle and the angled geometry of the hatch.
I have at absolute most:
36” left to right
30” front to back
I mention dimension orientation for the odds of the mouth position in a SUV application making a difference or not. Seems as if it’s a case by case basis with placement and vehicle.

I can build a ‘decent’ width 2x8 cab or a ‘weak’ width 2x10 cab. That being said, while looking at the drivers in the plans, and doing some calculations for different specs, I came across a 10” driver that looks pretty well in spec, that makes me lean to a 1x10 cab, as seen here:
Revc = 2.8 ohms
Fs = 28.37 Hz
Zmax = 66.94
Qes = 0.39
Qms = 9.21
Qts = 0.37
Le = 2.30 mH
Sd = 310 cm^2
Vas = 0.92 ft^3 / 26.12 L
BL = 14.59
Mms = 171.18 g
Cms = 183.88
Sensitivity = 83.6 dB @1W/1m
Sensitivity = 88.3 dB @2.83Vrms/1m
Xmax = 44mm total
Usable excursion = 2.5″ total
Mounting depth of 5.1”

I would think it’s a realistic 20mm Xmax one way driver, similarly to the recommended MCM 8” driver is more like 10mm Xmax one way, based on it’s magnet height.

The kicker is that 10” driver is rated at 750 watt rms and available in dual 1 or dual 2 coils. I already have a 600 watt rms amp (60 amp fused), so no biggie there.

So smaller cab, benefits of a horn, but ‘uses’ more power
or
Bigger cab, benefits of a horn, less power needed?

Huge difference in performance between the two? My novice guess is it will be a slight advantage to the single 10” cab, mainly dependent on which Xmax figures are used for Vd calculation. Perhaps it’s solid performance and can still save some space too.

Last bit of deliberation before building it within a week or so.

[Seth]

In regards to the back seat angle, a thought I had considered for myself if I ever did a 4x super wide truck tuba, I wouldn't have room at the top either. But, at 9" cab width there's a few inches of space left at the bottom of the cab. Figured if I build that panel to match the seat angle that what I would lose on top would closely match what I'd gain on the bottom. Not sure if that makes sense.

Anyway, I just mention it so you can take it into consideration.

In any case, I totally get all the brain work, research, and wanting to do all you can to maximize performance. If you're ever in an unsure state of quandary... take a deep breath and just know, you'll likely be quite satisfied with any configuration, as long as your expectations are reasonable.

You might even consider making a single driver cab with the inexpensive, $35 MCM driver. Just to get your chops up and get a little practice building the design without the pressure of it having to be "perfect". You would also get an idea of the performance capability of the cab and be better able to decide whether it's worth giving up additional space or not for increased output. It would probably only cost you $80, all said and done.

I look forward to hearing about your journey, no matter which direction you choose

[Benflitv30]

Thank you, Seth. Yes, that does make sense about your idea of changing the ‘lower/bottom’ dimension of the box.
I am not sure about just building the single 8” haha, because time is limited, but I definitely get what you mean about the trial and practice.

I just find this stuff fascinating, and the aspect of it being different than what ‘most’ of the car audio fans know of and do. Commonly it’s ported boxes, and the super loud guys are running 4th and 6th order band passes. Not what I’d call musical.

I think the 10” driver I found just might be the ticket if someone wants to go the extra mile with power. In summary between the two drivers:

MCM 8” -
29.1 Fs
0.22 Qts ( 0.08 low, out of spec)
30.43L Vas
16mm Xmax (11mm or less, likely)
255 Sd
11mm Xmax x 255 Sd = 2805 Vd x two drivers = 5610 Vd
16mm Xmax x 255 Sd = 4080 Vd x two drivers = 8160 Vd
120w rms

10” driver -
28 Fs
0.37 Qts
26.12L Vas
22mm Xmax
310 Sd
22mm Xmax x 310 Sd = 6820
750w rms

I know that it is ideal to find a driver that is in the middle of the required specs, and the 10” driver here is borderline on the Fs.
Does Fs have a similar effect in horns as direct radiators, as in that it may potentially help improve lower end response or some other effect realized?

Traditionally, doubling the driver count and proportionate power increases output by 6dB. To make a contrast, would you use the Vd number to specifically determine output between the two drivers/cabs, 2x8 vs 1x10?

Also, consider being able to utilize over double the power to the 10” vs the MCM 8” (x2), which should gain over 3dB.

Looks like it comes down to what the Vd differentiation factor truly is and the benefit of the compliance to the plan’s Qts that the 10” driver offers. Any ideas of what the differences will net? Thank you!

[Bill Fitzmaurice]

Does Fs have a similar effect in horns as direct radiators, as in that it may potentially help improve lower end response or some other effect realized?

It has the opposite effect. That's why the specs list both a minimum and maximum Fs.

Traditionally, doubling the driver count and proportionate power increases output by 6dB.

That's not correct either. Doubling the speaker count gives 6dB. Doubling the driver count without also doubling the size of the enclosure does not.

[Benflitv30]

I appreciate the corrections, Bill. Trying my best to learn, at least enough to have a general understanding. Any bit helps.

Does Fs have a similar effect in horns as direct radiators, as in that it may potentially help improve lower end response or some other effect realized?

It has the opposite effect. That's why the specs list both a minimum and maximum Fs.

Opposite meaning in a properly designed front loaded folded horn, a lower Fs driver will or will not increase the ‘lower’ portion of the response, say 25Hz-45Hz?

Traditionally, doubling the driver count and proportionate power increases output by 6dB.

That's not correct either. Doubling the speaker count gives 6dB. Doubling the driver count without also doubling the size of the enclosure does not.

Double the driver and enclosure size, and the additional power needed for two drivers...or is it strictly about the driver count and enclosure size (halving the power)?
That’s where I seemed to have gotten confused.

Regarding the plan’s driver specs, the MCM’s 8” Qts being lower than recommended, what does that do for response/performance compared to the 10” driver listed that is more in the middle of the range at 0.37?

After doing the cardboard mockup as you wisely suggested, the thought of mixing a horn with a ‘stronger’ overall driver and added power looks more attractive as a compromise of sorts to cab volume. Is that misguided thinking? Hoffman’s law applies, right?

[Bill Fitzmaurice]

The driver specs must match the horn specs. If Fs is too high or too low performance is reduced. If you want lower response you don't get it with a different driver, you get it with a different horn. Two speakers don't 'need' additional power for a 6dB increase in output. It just works out that way, because the voltage swing remains the same into two parallel wired speakers as it does into one, while the halved impedance results in doubled current draw. Constant voltage with doubled current results in twice the power.

[Benflitv30]

Thank you, Bill. That makes perfect sense about the impedance being halved resulting in twice the current draw. My confusion was that I wasn’t assuming that the drivers would be paralleled in that specific example for dab gain equations. I was more so thinking of how much of a role the additional Vd and can size makes.

Do you have any recommendations for a reference book and/or white papers on food horn theory and design? I can read on my own time versus pestering others like yourself with such novice questions.
Perhaps a thread for such material? I’ve already read the ‘FAQ’ sticky threads you have here.

[Bill Fitzmaurice]

There's not that much out there about horns. I designed and built my first in 1969 and I'm still learning. As good a place to start is with Harry Olson http://cyrille.pinton.free.fr/electroac ... /Olson.pdf His math is still the basis of the calculations in the premier design software, HornResp. His work predates Thiele-Small specs, so they're not specifically mentioned, but his math contains the data from which Thiele and Small later calculated the specs.

[Benflitv30]

I appreciate the reference link, Bill, and even more awesome is your humbleness about it all.
I have inquired around to some well known ‘gurus’ since my last post for pure curiosity sake. When mentioning proper folded horns, they all pretty much say the same thing....”requires a lot of calculation and measurement to get it right”, said with a level of high disinterest and a sense of defeat.
I still don’t get it...if folded horns are the holy grail, why not chase it?

I also have not been able to build the AutoTuba yet as my supplier for the subwoofer is awaiting parts. Will update once I receive the driver and build the cab.

[Bill Fitzmaurice]

The reason folded horns are rare is because they're big, and unless you build them yourself they're expensive. Look at the pricing on Danley Sound Labs or Klipsch Heritage series folded horns and you'll see what I mean.

[Benflitv30]

Which of these two 10” drivers would you choose and why if up to you for a Tall Auto Tuba that will be extra braced to handled the extra power. Goal here is best overall high performance. Trying to weed out possible marketing claims etc.

Driver #1 or Driver #2 marked on each slide. Thank you!

[Bill Fitzmaurice]

Probably neither. Horns don't need monster drivers. That's one reason why we use horns.

[Benflitv30]

Probably neither. Horns don't need monster drivers. That's one reason why we use horns.

I apologize, Bill. It’s the same reason my first cab of your designs that I built was a 36” THT. Just wanted the headroom for the times I ‘wanted’ it, but truth be told..I definitely don’t ‘need’ a THT. Not to mention, I rather just build something once and be done. But once, cry once.

Candidly, trying to extract as much performance out of one TaT as possible. Both of these drivers meet the specs within the plans, but there are variances mainly in Vas and Qts. Curious on the impact those would have if tested side by side. The power ratings I figure are likely inflated.
Btw….These days, the two drivers I listed are rather ‘entry level’ in the car audio world. Amp power is incredibly cheap now, which lead to true monster 100+ lb sub designs with 4” and 5” voice coils being much more common place. I’m not going that route, uninterested. Example nonetheless:
https://usercontent.one/wp/www.b2audio. ... 1662639868

[Bill Fitzmaurice]

Drivers with huge xmax and Pe are required to get high output at low frequencies from small sealed cabs. That's of value in a car, where you generally don't have room for a large box. Not only is that unnecessary in a horn, it can be counter productive. http://www.readresearch.co.uk/loudspeak ... icle_1.pdf

[Benflitv30]

Drivers with huge xmax and Pe are required to get high output at low frequencies from small sealed cabs. That's of value in a car, where you generally don't have room for a large box. Not only is that unnecessary in a horn, it can be counter productive. http://www.readresearch.co.uk/loudspeak ... icle_1.pdf

Oh wow, thank you for posting that link. I’ll have to read that a few times I’m sure to have a basic understanding. Ha.
Interesting that it’s from the legacy of Klipsch, I was unaware of that company’s history in regards to it being based on quality science etc.
The takeaways I gathered are:
-Reduced distortion in proper horns
- Even poor horn designs are likely better than a direct radiators
- If a diaphragm moves it creates distortion
- Modulation distortion cannot be fully eliminated
- Higher efficiency promotes lower distortion


This quote of that paper I think sums it up best, to your point:
‘Apparently horn loading remains the best means to reduce diaphragm excursion and increase efficiency so as to minimize modulation distortion.’

My wrong assumption was interpreted from the plans when noting ‘at least’ stated Xmax…as in more would be better.

Does this by chance coincide with Pro Audio drivers traditionally having lower Xmax than car audio drivers, because cab space isn’t as much of an issue vs a car and to reduce distortion at high power levels?