I want to check my calculations. Please critique.

[tspence73]

Hi guys,

Yeah yeah. It's me. AGAIN. Mr. numbersboner. Deep breath. Exhale. Keep it clean and don't be willy nillies or jerks. Serious question and since it's audio related, I'm trying to be precise here and draw up my theory on setting the system starting with the subwoofers.

I will have two RE SE 12's coming tomorrow. I will set them up this weekend with my Cadence A7HC amp. I believe I have my math right, but you know me and details. We don't always mix, so here it goes.

I will be wiring the subs in parallel as a 1-ohm load. The Cadence amp will put out 1,200 watts RMS. I will be using a 0db 50Hz tone and my DMM to set the amp to 35 volts (square root of 1,200) to simulate a PEAK output signal to the amp. The subs will be mounted in a dual 1.0 cuft chamber which will yield a rather flat frequency response (or as close as I can get with this enclosure on this sub).

As you all know, my goal is to have as distortion-free an output as possible. By setting my amp gain to the RMS wattage, it will allow for plenty of headroom and close to distortion-free peaks as possible.

****Now, I will be assuming my SPL numbers WITHOUT factoring cabin gain.

1. The sensitivity of each sub is 86db 1w/1m.
2. Each sub is dual voice coil. That means 4 coils.
3. The amp will be feeding each coil 300-watts PEAK (when music on a CD hits 0db max).

At PEAK output, here is how I figure the SPL using this site's calculator:
SPL Calculator

I start with a single coil at 300-watts:
1 watt = 86db
11 watts = 96db
111 watts = 106db
300 watts = 110db (according to the calculator in the link)

Add the 2nd coil @ 300-watts = 113db
Add the 3rd coil @ 300-watts = 116db
Add the 4th coil @ 300-watts = 119db

So, at peak output the SPL should be able to reach 119db. Is this correct?

Now, I'm not sure what the rules are for losses of SPL due to each coil sharing a woofer cone. So, that's the varible I'm looking for. Remember, this is when NOT factoring in cabin gain. Cabin gain will be addressed AFTER I've figured what the theoretical output is first. I intend to use equalization to minimize cabin gain when tweaking at the next step.

The reason I'm setting the sub stage first is that it's usually the most distorted stage of a system and using it's initial setting as a way of tweaking the sound output of the other speakers should help keep audible distortion down.

So, the question is, are my calculations for SPL correct without figuring in cabin gain?

[mikey7182]

Correct me if I'm wrong, but isn't it theoretically about a 3db gain every time you double wattage? If so, I think you're good up until you add the 3rd VC. At that point, I would think it should go from a 300w gain to a 600w gain for a 3db gain, right? So it should go 2nd VC- 300w: 113db, then 3rd/4th VC- 600w: 116db. Or, are you compensating for the 2nd woofer's cone area? I don't know that much about it... everything else sounds good. That was the only thing that sounded off to me.

[tspence73]

Originally Posted by mikey7182

Correct me if I'm wrong, but isn't it theoretically about a 3db gain every time you double wattage? If so, I think you're good up until you add the 3rd VC. At that point, I would think it should go from a 300w gain to a 600w gain for a 3db gain, right? So it should go 2nd VC- 300w: 113db, then 3rd/4th VC- 600w: 116db. Or, are you compensating for the 2nd woofer's cone area? I don't know that much about it... everything else sounds good. That was the only thing that sounded off to me.

I'm waiting for the more engineer types to chime in. I've gotten clarification in the past that each coil does theoretically add another 3db, even if it's occupying the same driver. I just don't know what math to use to find the limitations of that stat. You simply add the same power to an additional speaker to get 3db. It's still "double the power". Each voice coil is technically "another speaker". I've thought about this extensively and just need a finer point of guidance on this. Chad or someone like him could help here.

[bikinpunk]

look into SPL loss due to transfer through solids. That'll really mess with your head...

[mikey7182]

Originally Posted by bikinpunk

look into SPL loss due to transfer through solids. That'll really mess with your head...

Sounds like the farty Taco Bell dump I took earlier.

[bikinpunk]

it's real. but, man, I can't remember much about it. I'll see if I can find any technical papers on the ASME site tomorrow...

In addition, I know I read/heard once that a panel flexing even in millimeters would result in SPL loss.

[ca90ss]

You don't gain 3db for the 2nd vc in a dvc sub.

[tspence73]

Originally Posted by ca90ss

You don't gain 3db for the 2nd vc in a dvc sub.

Care to elaborate or show linky? Maybe something more than just sayin' it's so?

[chad]

Hoestly I would run the amplifier at an impedance higher than one ohm in lieu of going for max power.

BUT the drivers have been purchased, oh well.

[tspence73]

Originally Posted by chad

Hoestly I would run the amplifier at an impedance higher than one ohm in lieu of going for max power.

BUT the drivers have been purchased, oh well.

It's going to be a 1-ohm load. Two DVC 4-ohm wired in parallel. I've spoken with a Cadence rep already and he indicated the subs should work fine with the amp at 1-ohm.

[ca90ss]

The gain is from the extra power not the extra vc. If you give an 86db/1w dvc sub 1w per coil it's the same as giving a svc 86db/1w sub 2w of power. Same sensitivity and same total power applied whether through single or mulitple vc's gives the same output.

[chad]

Originally Posted by tspence73

It's going to be a 1-ohm load. Two DVC 4-ohm wired in parallel. I've spoken with a Cadence rep already and he indicated the subs should work fine with the amp at 1-ohm.

Rep does not equal engineer. Remember how we were talking about an amp NOT ACTUALLY putting out the same RMS voltage at low impedances as at high impedances? 1 ohm is a VERY low impedance (shuddup SPL guys) and I can certainly assure you that if you check the clip point of the amp unloaded it will clip MUCH sooner while loaded at one ohm. Find a scope and check it, I'm not shitting you.

[tspence73]

Originally Posted by ca90ss

The gain is from the extra power not the extra vc. If you give an 86db/1w dvc sub 1w per coil it's the same as giving a svc 86db/1w sub 2w of power. Same sensitivity and same total power applied whether through single or mulitple vc's gives the same output.

Ok, so in principle either way it's the same result. On paper then it looks to be as I'm intending. Next we have the cabin gain to talk about. What will the SPL look like when figuring cabin gain? Will it be likely that it will look like what Andy Wehmeyer posted in that past thread? If so, then I think I have a solid plan mapped out to work with it, instead of against it by adding the midbass. I definitely want to HEAR the subs set the way I'm thinking before making any decisions.

I'll actually heed you guys' advice to do more listening to the system before putting my ducks in a row on this one. I might end up not having nearly the bass I'm expecting by setting the amp so conservatively but then again that is one reason I went with larger subs and more power this time just to make sure I can get the best peak output with the least chance of it being too wimpy.

[tspence73]

Originally Posted by chad

Rep does not equal engineer. Remember how we were talking about an amp NOT ACTUALLY putting out the same RMS voltage at low impedances as at high impedances? 1 ohm is a VERY low impedance (shuddup SPL guys) and I can certainly assure you that if you check the clip point of the amp unloaded it will clip MUCH sooner while loaded at one ohm. Find a scope and check it, I'm not shitting you.

Hmm. For the record Cadence is not trying to boast the amp will double it's power at half the ohm load. In fact they have put rather realistic specs out for it. 500-watts 4-ohm / 800-watts 2-ohm / 1,200-watts 1-ohm. That's pretty realistic. The video Cadence published of the new A7 amp is proof it very likely can do it's rated output. Cadence stated that the old A7HC amp will do pretty much the same power as the new A7. Do you still think I need a scope to verify this? I was planning on waiting and buying a scope in January. I couldn't wait to buy the subs. I jumped the gun. The 10" Pioneer sub just isn't going to cut it for me. I want more clean SPL!

[chad]

those indeed are VERY realistic figures and Kudos to Cadence for that. BUT if you run it hard it's gonna get HOT, watch that at first.

My motto, "The number one issue for longevity of electronics is heat cycling, followed closely by idiots with screwdrivers "

[tspence73]

Originally Posted by chad

those indeed are VERY realistic figures and Kudos to Cadence for that. BUT if you run it hard it's gonna get HOT, watch that at first.

My motto, "The number one issue for longevity of electronics is heat cycling, followed closely by idiots with screwdrivers "

I plan to set it in a conservative way, using it's 1200-watt RMS as the peak. The amp has enough ventilation too. It's under the passanger seat but gets a full 4.5 inches of clearance to get air. I don't plan on pushing it harder than it's rated output. Most of the time it will likely only be at half it's RMS or lower during playback.

Now back to cabin gain! I'm excited to finally be able to test out my kooky ideas. One day when I get a sound processor like the MS-8 watch out. Tspence will be rolling with smooth thunder.

[shadowfactory]

Originally Posted by tspence73

I'm excited to finally be able to test out my kooky ideas.

Two RE 12's are a kooky idea? Kooky = two Strokers

[06BLMUSTANGGT]

FYI: If you want to know the true SPL of your system, I am sure that some shop in Vegas would be glad to test your car with a Termlab for a nominal fee.

Secondly, running a Class D amp @ 1 ohm in a daily setup gets a big thumbs down from me. Class D amps run daily at 1 ohm have a shorter than usual life span. But hey, if you don't mind replacing your amp every few years, then go for it! Most 1 ohm setups I have heard also sound like garbage and I can't quite put my finger on if it is install dependent or if there is some truth to the fact that "barely above single digit damping factor sounds like poo" rumor.

[tspence73]

Originally Posted by 06BLMUSTANGGT

FYI: If you want to know the true SPL of your system, I am sure that some shop in Vegas would be glad to test your car with a Termlab for a nominal fee.

Secondly, running a Class D amp @ 1 ohm in a daily setup gets a big thumbs down from me. Class D amps run daily at 1 ohm have a shorter than usual life span. But hey, if you don't mind replacing your amp every few years, then go for it! Most 1 ohm setups I have heard also sound like garbage and I can't quite put my finger on if it is install dependent or if there is some truth to the fact that "barely above single digit damping factor sounds like poo" rumor.

Perhaps one day I'll buy an even more insane amp for the subs and wire it 4-ohm series-parallel with 1,200-watts RMS. The A7HC amp is pretty much a solid design and is commonly run @ 1-ohm. It has another year of warranty if it comes down to it. For now it should serve it's purpose.

The midbasses I plan on running will be powered off my Planet Audio amp @ 4-ohms and it has >400 damping factor.

BTW, what is up with damping factor lately? I read somewhere that >50 damping factor would be inaudible in a blind test between two amps. Is that wrong info? Is there a subjective study/test paper showing that's not the case?

[06BLMUSTANGGT]

Originally Posted by tspence73

BTW, what is up with damping factor lately? I read somewhere that >50 damping factor would be inaudible in a blind test between two amps. Is that wrong info? Is there a subjective study/test paper showing that's not the case?

It depends! I have yet to be impressed with a 1 ohm Class D setup and I was always told it is the crappy damping factor. When a friend comes to the house and says "You gotta hear my new system", I pop in Raining Blood by Slayer to point out how inadequate their sub stage is! Then I take them to either one of my cars and show them how it is supposed to sound.

I think the lack of fast bass drum reproduction is either a bad setup (box, sub placement, inadequate electrical, etc.), a bad tune (installer over zealous with gain/bass boost etc.), running the class d amp at ETA: 1 ohm mono (already hot enough to cook hot dogs by the time they arrive at my house), or a combination of all those previously mentioned factors.

Bottom line, if you like it and it sounds good to you, that is all that really matters in the end!