High Efficiency Speakers

[AudioFreak]

Originally posted by PassFan
If you double the drivers without the power then you half the power to each speaker which causes a 3db loss per speaker. Naturally if I am driving my speakers to their max eff. then if I double my power I must add more drivers. My example was not a typical case of driving 100 watt speaker with a SE tube amp. I'm assuming a perfect output match; amplifier to driver.

if you have 2 x 90db/1W/1M drivers and you connect them is paralell and give it a total of 1W the you'll get 93dB ... same power in gives an extra 3dB because the radiating area is doubled.

its the same with crossovers if both drivers are down 3dB @ the crossover frequency there is a 3dB peak on axis in the frequency response.

[PassFan]

Well help me then with the math; if twice the power is a 3 db gain then half the power would be a 3db loss. Are you saying that a speaker at 90db 1w/1m will not change its output spl if you drop it to a half watt. It would then be 90 db at 1/2w/1m. You have to double your spl to gain 6db, but I don't understand were your increase is coming from. If you cut half the power don't you get half the movement of the cone. If thats the case; if I half the movement of one cone and add another to it then havn't I gone back to were I started as it relates to air being moved.

Aren't there other forces in play at the x-over point.

[AudioFreak]

Originally posted by PassFan
Well help me then with the math;

1.if twice the power is a 3 db gain then half the power would be a 3db loss.

2.Are you saying that a speaker at 90db 1w/1m will not change its output spl if you drop it to a half watt. It would then be 90 db at 1/2w/1m.

3.You have to double your spl to gain 6db, but I don't understand were your increase is coming from. If you cut half the power don't you get half the movement of the cone. If thats the case; if I half the movement of one cone and add another to it then havn't I gone back to were I started as it relates to air being moved.

4.Aren't there other forces in play at the x-over point.

1.This is correct.

2. no the individual driver will decrease it's output according to it's efficiency which does not change.

3.its got to do with how the acoustic waves add together when on axis.

4.as far as this is concerned, no there are not.

[PassFan]

Well I couldn't put this one to bed without knowing what is going on here, so I went off to read and what I read confused me even more until I thought about it. The book stated exactly as I told it here in my original post. Also it stated that two sine waves exactly in phase with each other when added will double the size of the wave. Elswhere in the book it stated when the angles of the acoustic throws converge and cover over each other that anywhere from 3db to 6db of increase will be realized. Well how can the same book give and take away at the same time. It dawned on me while getting into the shower. It has to do with the axis. Large concert sound systems are designed for maximum coverage of large areas (which we don't concern ourselves with in our listeneing rooms). They target their maximum db level in the dead zones. Their use of multiple arrays with precise off axis angles are designed for coverage not convergance. They don't worry about a 6db gain at the convergance only the db level in the dead zone. We at home are only concerned with one axis, as it relates to the listening position. That being, then our drivers are mostly all convergence as close as 1 meter away from the speaker. That is how Klipsch gets their eff.. What do you think audiofreak.

[AudioFreak]

yep i agree with that ... as you stated the gain is upto 6dB if you are in the right spot .... well 2 x 90db/1W/1M drivers each fed 0.5W and producing a signal that is perfectly in phase with the other .... each individual driver will be down to 87dB but then you add the 6dB of gain which will bring it back to 93dB so as i there you have it .... we were both right about different things.

[jteef]

I am sorry, you guys are correct on the calculations of the math.

I had written 104dB and changed it for reasons that elude me right now.

in any case, the principle is still valid.

SPL is equal to 20 log pRMS / 2x10^-5

one equation I have relating RMS air pressure to a moving coil loudspeaker is:

pRMS = p0/(2*pi) * BL*EgRMS / (Sd*RE*MAS) *G(s) * Tu1(s)

BL is the BL product, RE is voice coil resistance, Egrms is the generator/input voltage, Sd is cone area, MAS is acoustical mass of diaphram and air load in infinite baffle, G(s) and T(s) represent the 2nd order high pass and 1st order low pass transfer functions and dont affect pRMS in the midband.

If you double EgRMS, pRMS doubles, giving rise to a 6dB increase in SPL.

however, when you double the power, EgRMS only increases by sqrt(2) = 1.41, and thus pRMS increases by 1.41 and SPL increases by 3dB. 20 * log(1.41) = ~3dB vs 20 * log(2) = 6dB.

You can also use the pressure equation above to explain why a doubling of the cone area will produce a 6dB increase in SPL.

MAS = Mass of Diaphram / (Sd^2) + 2*MA1

Since MAS is inversely related to the square of Cone Area, we end up with Sd also being directly related to Prms. MA1 is the mass of the air load impedance on the piston.

When you connect two drivers you are doubling the cone area which calls for a 6dB increase, and dividing the power between them in half, which only calls for a 3dB decrease.

Suppose you had two 92dB/1w/1m 8 ohm speakers with the amp I referred to above ( capable of 11.32V(16W) into 8 ohms). If you wire the coils in series, you'll be left with a 16 ohm load for the amp. 5.66V will appear across the coil of each speaker which is one doubling of 2.83V, a 6dB increase.

92+6dB = 98dB per speaker. Adding the 6dB for the increase in cone area brings us to 104dB. Note that the amplifier is only supplying 8 watts total or 4 watts to each speaker.

I will redo the math for the next one since i wrote the wrong numbers down in my other post. 2 speakers each on their own 8W amplifier(8Vrms output)

The voltage is doubled from 2.83 to 5.66 and then 8/5.66 = 1.41

2*1.41 = 2.83...20 * Log(2.83) = 9dB

so you get 92+9+6 = 107dB

a 3dB increase in efficiency compared to one speaker with 16 watts input at 104 dB.


Now consider two 8 ohm speakers with their coils wired in parallel. Here each speaker will be receiving 16 watts each.

The voltage doubles twice from 2.83V to 11.32V. 20*log(4) = 12dB + 6 more because there are two of them.

92+12+6 = 110dB assuming your amplifier is capable of delivering the extra current required.

Sorry if i made this a lot more confusing than it really is, I hope I'm never a teacher...

jt

[markkanof]

Wow, I think I understand this right now. You guys said a mouthful, I am just trying to obsorb it all.

These are the speakers I was talking about in my original post:

http://www.klipsch.com/products/comp...=floorstanding

I don't mean to be rude and restate my original question, because I really do appreciate all the information that I am getting from your debate. However what amazes me about these speakers is thier efficiency with only two mid/bass drivers and a tweeter. It doesn't seem that from your talks I could achieve this with only two drivers, especially not any that I have seen. I beleive about 95 db/1w/1m woudl be the best I could do because I have been unable to find any drivers with spl above 92 db/1w/1m. So now that you have worked out the theory I am wondering if there is a way to bring it to a real enclosure.

I do realize that the increase in efficiency from 95 db which I am saying I could probobly get to 98 or 100 db which the klipch speakers achieve will really only seem like a small increase in volume. However I am just interested in the concept of creating the most efficient speaker possible. Any other advice would be appreciated.

Mark

[AudioFreak]

Ok, well odds are that klipsch measures the efficiency @ just the right point in order to get such high numbers.... either that or they just simulate it on computer. with 2 drivers covering the same passband the efficiency /1W/1M that you wish to achieve must be 3dB greater than the efficiency of a single driver.... so if you want 100dB/1W/1M you'll need to find drivers that have an efficiency of 97dB/1W/1M .... now here comes the killer......... baffle step contributes a 6dB loss in the lower octaves and is dependant on the width of the enclosure so now the 2 mid/bass drivers must be about 103dB/1W/1M and then you must equalize the response in the crossover