Sensitivity and impedance for multi-driver systems

antonymity

Member
2013-04-22 3:02 am
I'm fairly new to DIY audio in general, so I have some questions. I've exhausted my resources in finding a simple explanation for these points. Everything I find winds up going into complex math that I don't understand.

I'm not sure the easiest way to ask this, so I'll explain what I'm trying to do. I want to build a pair of partial line arrays with 8 small full-range drivers and one subwoofer. Everything I've done before has stayed inside the safe zone (and simple zone) when it comes to impedance, so I'm lost here.

So. According to the specs of the full-ranges, they have a sensitivity of 83db and are 8 ohms each. According to the subwoofer's specs, it is 96db when the two voice coils at 8 ohms each are wired in parallel.

First question is: When it comes to impedance, does the system impedance affect the amplifier as a whole or does the way each driver is wired affect it?

Example: Say I wired four in parallel, four in parallel, (2 ohms each set), then the two sets of four in parallel with each other and in series with the subwoofer (DVC wired in parallel for 4 ohms), it would be a system impedance of 5 ohms according to an impedance calculator I hope I'm not using improperly. The amplifier I have in mind (a T-amp) would have no problem with 5 ohms, but would the sets wired into two ohms each strain the amp at certain frequencies, damaging everything involved? Or does the rest of the wiring which makes the system impedance a total of 5 ohms negate the low impedance of the other components?

I'm sorry that my question sounds so convoluted. Wasn't sure how to word that.

Also, how do you calculate the overall sensitivity of that many drivers wired in such a way? I know two identical drivers wired in parallel will yield about +6db over a single driver, but I'm not sure how to go about approximating this many. I would like them to be in the range of the subwoofer.

The whole idea behind this is to create simple but decent-sounding and quite sensitive speakers to reach higher SPLs powered by a small 20W T-amp.

I may wind up putting the subwoofer in a bandpass box if the full range array winds up being too sensitive.
 
I want to build a pair of partial line arrays with 8 small full-range drivers and one subwoofer.
:confused: Can you clarify this a bit. Do you want to build a pair of speakers for stereo, or a single speaker (mono). Since you've only got one subwoofer, maybe you want to make a stereo pair of full-range arrays and share the sub with both channels?

First question is: When it comes to impedance, does the system impedance affect the amplifier as a whole or does the way each driver is wired affect it?
The amp cares about the total impedance of the speaker system, which depends on how the drivers are wired up.

Or does the rest of the wiring which makes the system impedance a total of 5 ohms negate the low impedance of the other components?
Yes the amp will only care about the 5 Ohms total. BTW, you got the math right in your example, but that wouldn't be my first choice of how to wire them up.

Sounds like you're not planning to use any kind of crossover, right?
 

antonymity

Member
2013-04-22 3:02 am
Sorry for not being clear. I want to build a pair of speakers for stereo with 8 full range drivers and one sub in each speaker. 16 FR and two subs total.

If I wired them to be a 4 ohm load and then wired them in series with the sub for a system impedance of 8, would that decrease the sensitivity by much?

I will probably have to use some sort of crossover. I haven't worked it out yet, and my crossover will probably consist of the components, wire nuts, and glue, since I have no other tools now. I'm pretty bare bones at the moment. Anyhow, the "sub's" spec sheet claims it is fairly flat up to 800hz (and useable up to 1500) and the FR down to 150. So 200 or 300 hz seemed right to me, since it's supposedly lower than the hyper-sensitive section of human hearing.
 
For starters, you seem to have missed that the "nominal" impedance of the drivers is a pretty useless spec. You need to know what the impedance curves of the system look like over the frequency range to determine the impact on the amplifier.

The impedance curve will also be affected by the crossover. It isn't simle and it takes some math. A crossover simulation program will handle the math.

You've also picked a fairly challenging project for a beginner.
 
Hi,

I hope not to cloud the issue but there are somethings you should have in your arsenal before making a decision.

Think of the line array and the woofer as separate circuits. You won't be combining them in a series or parallel, you are dividing the source and send part of it to the woofer and part to the tweeter. If the drivers are not carrying the same frequencies, the amp does not see it as a raising or lowering of impedance.

The line array can be wired as 4Ω and should have at least a cap as a high pass filter. 200 hz is a good starting point unless the woofer is not designed to go that high. Please post the woofer model so we can help judge a good cut off point. The woofer has to have a low pass filter. If you are cutting it off quite low, you might even want to consider a plate amp for the woofer as the cost of a good low pass filter can be almost as much as a plate amp. The plate amp will give your system balance and versatility in one package, whereas a low pass filter can be a headache. For a few more dollars, you get peace of mind and a sound that is easy adjusted to your taste.
 

antonymity

Member
2013-04-22 3:02 am
Thank-you Cal. Price is a big part of my decision making. I'm trying to stay as cheap as I can. Do you think a bandpass box for the woofer would make a good enough replacement for an electronic lowpass? From what I've read, it might raise the sensitivity too much. I guess I could wire the DVC in series to make up for that. I need some box software to figure that out.

I'll post the model # when I get home from work if that would help.
 

Pano

Administrator
Paid Member
2004-10-07 6:05 am
Panama
I'll parrot what's been said above
  • This is a big project.
  • You need a crossover of some sort.
  • Wire the line array drivers up in series/parallel to get 4 ohms (or 8)
  • You can use a plate amp with a built in low pass if the line array will play down low enough (probably will)
  • If you use a plate amp, the subwoofer impedance isn't seen my your main amp, it's taken care of by the plate amp.
  • You'll want a high pass filter, even as simple as a single cap, to keep the bass out of the line array.

The main challenge outside the actual building will be your crossover points and levels between line array and sub. We can help with that. Go for it!
 
Sorry for not being clear. I want to build a pair of speakers for stereo with 8 full range drivers and one sub in each speaker. 16 FR and two subs total.

If I wired them to be a 4 ohm load and then wired them in series with the sub for a system impedance of 8, would that decrease the sensitivity by much?
Each time drivers (or cone area) is doubled there is an increase in sensitivity of 3 dB, so 1 driver at 83 dB, 2=86,4=89, 8=102 dB if the drivers together could be wired to 8 ohms. This sensitivity applies to the low frequency range of the full range drivers, at high frequencies the sensitivity will probably be less due to comb filtering etc.

Assuming your amp is a perfect voltage source, it produces double or half the power each time impedance is halved or doubled, in other words, an amp with 10 watts at 8 ohms, would put out 5 watts at 16 ohms and 20 watts at four ohms.

With eight 8 ohm speakers, the nominal impedance for all in series would be 64 ohms, series parallel can yield 16 or 4 ohms, parallel 1 ohm.

Assuming the perfect voltage source amp putting out 2.83 volts, the 64 ohm load would end up at 83 dB sensitivity, each 3 dB gain for the increase in cone area is negated by the 3 dB power loss due to the impedance rise.

The 16 ohm load would be 99 dB sensitivity.
The four ohm load would be 105 dB sensitivity.

Unless you wire all the full range speakers in series, they will be more sensitive than the 96 dB four ohm woofer, unless the woofer is put in a BP or TH which could raise sensitivity as much as 6 dB.

Anyways, that's all the easy stuff, hard part is making a decent passive crossover that fits the impedance curves of the drivers ;).

Art
 
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Thanks for the explanation in layman's terms. Makes more sense now.

That plate amp idea sounds better with each post. But I'm wondering if it'll sound good. Would a plate amp sound as good as if the whole thing was being pushed by a t-amp? It would certainly simplify the design, but after it's built that would be two more cords to plug in, two more wires to hide.
 

Pano

Administrator
Paid Member
2004-10-07 6:05 am
Panama
That plate amp idea sounds better with each post. But I'm wondering if it'll sound good. Would a plate amp sound as good as if the whole thing was being pushed by a t-amp?
Yes, for most plate amps. I've done it plenty of times. You do need to be careful with the smaller (bridge mode) T-Amps as they can not have the negative speaker post connected to ground. Sometimes that a problem when connecting to a plate amp at speaker level. There are ways around that, tho.

The plate amp makes your crossover so much simpler. You don't have to go that way, it's just easier and more likely to get you what you want without much hassle.

Your subwoofer driver is dual coil, right?
 
Bandpass speakers bass rings the most. That is, after the electric signal stops, the cone keeps one moving back and forth, like a mass hanging on a spring. All speakers do that, but to wildly generalize, sealed do it the least, vented more (and more, depending on the tuning) and bandpass the most. That is why many folks think bandpass speakers sound very boomy and muddy and yukky.

The acoustic bandpass will NOT prevent the amp from being loaded too much.

Passive crossovers at low frequencies SUCK. I had to design a new model of one of those, and I was horribly shocked at how non-ideal it was. The problem is the big impedance peak at resonance (or peaks of ported/bandpass boxes) "defeats" the passive crossover. Even 200 Hz is just maybe 2 octaves above those resonances-and upper impedance peaks of a bandpass could be 100, 150 Hz.

So forget the passive crossover unless you shove it up to, I dunno, 500 Hz (which maybe your "sub" is flat enough for).

Anyway, I think we really need better information to give good advice. 83 dB drivers and 96 dB "subs" just don't match at all. If you give us some specific model numbers or attach data sheets it would help.
 
Anyway, I think we really need better information to give good advice. 83 dB drivers and 96 dB "subs" just don't match at all. If you give us some specific model numbers or attach data sheets it would help.
I think this not a problem. Separate amps driving would then be able to match the lower level of 92dB while adding 4dB of headroom on the sub. I'd give that a thumbs up