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Hello all,I am currently building (actually still designing, though I have the parts) my first pair of towers (maybe they won't be towers in the end). I am having a few problems though during this phase.
First of all, what wiring configuration is ideal for a pair of woofers and 1 tweeter? I have found that mathimatically wiring the woofers in series and then wiring that netwok to the tweeter in parallel gives me an impedence around 5.5oHMs. Is this ideal though for the sound quality? Does it matter?
Secondly, how do you calculate the effeciency of the system (1 speaker) based on the effeciency of each component? Does the crossover effect this?
Any help you guys could give me would be much appreciated. Thanks.
Follow Ups:
In addition to the excellent advice you receive here you may want to pick up a copy of Vance Dickason's Loudspeaker Design Cookbook. If you will be continuing this great fun hobby you won't regret the purchase! Also check out the forum at Madisound.
Nate -Congrats on your foray into the wonderful world of speakerbuilding!
As far as how to wire the woofers, you want to match up the sensitivities of the woofers and tweeter. And if you can't match the sensitivities, it's better to pad the tweeter down to match the woofer than vice versa. Note that sensitivity and efficiency are not the same thing - sensitivity is dB per voltage (usually 2.83 volts, which is 1 watt into 8 ohms), while efficiency is dB per watt. The reason we want to match sensitivity (and not efficiency) is so that the drivers will all be at the correct loudness relative to one another, regardless of how many or how few watts a given driver is drawing at that particular volume control setting.
You see, amplifiers actually put out volts, not watts - although they are rated in watts. For instance, at a given volume setting, let's say an amplifier is putting out 2.83 volts. IF the speakers are an 8 ohm load, then the amp is putting out 1 watt. If they are a 4 ohm load, then the amp is putting out 2 watts. And if the speakers are a 16 ohm load, then the amp is putting out 1/2 watt. So, unless the drivers are all the same impedance, matching efficiencies won't match up their relative volume levels - but matching sensitivities will, regardless of their impedances.
For example, let's say your drivers are all rated at 8 ohms, and the woofers are rated at 85 dB/watt efficiency and the tweeter at 91 dB/watt. The sensitivity of each woofers is 85 dB/2.83 volts, and the sensitivity of the tweeter is 91 dB/2.83 volts. Wiring the woofers in parallel will bring their efficiency up to 88 dB because of the doubling of woofer area, but will bring their sensitivity up to 91 dB because they'll now be a 4-ohm load, and drawing 2 watts instead of 1 watt for the same amplifier output setting (2.83 volts). In this case the "nomimal impedance" would be 4 ohms, the woofers' impedance; the system sensitivity would be 91 dB/2.83 volts; and the efficiency would most be conservatively and correctly given as woofers' 88 dB/watt, but might be optimistically given as the tweeter's 91 dB/watt.
Let's look at another example. Say we have the same tweeter, but now the woofers' efficiency (and, since they're 8-ohm drivers, sensitivity) is 88 dB/watt. If we parallel the woofers, the sensitivity will be 94 dB, which is too much of a discrepancy to work well with a 91 dB tweeter. So, let's wire the woofers in series. Now the woofer section efficiency goes up to 91 dB, but remember we have to match sensitivity - and doubling the impedance of the load halves the wattage drawn, so the +3 dB from the doubled cone area is negated by the -3 dB from the halved power, and the sensitivity of our woofer section remains at 88 dB. So, we'll need to pad down the tweeter by 3 dB to match the sensitivity of the woofer. In this case the "nominal impedance" would probably be given as somewhere between 8 and 16 ohms; the system sensitivity would be 88 dB/2.83 volts; and the efficiency is once again a bit harder to pin down, but might optimistically be given as 91 dB/watt (the woofers will do 91 dB with a 1 watt input, but at the same volume setting the tweeter would actually be drawing more than 1 watt - how much more depends on whether we padded them down with series resistance or with an L-pad circuit).
A significant difference between these two cases is the impedance in the bass region. For the first example, it's 4 ohms. For the second, it's 16 ohms. Most solid state amps will deliver two to four times as much power into a 4 ohm load as into a 16 ohm load, so parallel connection will give a higher maxium volume level. On the other hand, many tube amps (OTL's and SET's in particular) deliver more power into a 16 ohm load than into a 4 ohm load. I think most amps sound better driving a high impedance rather than a low impedance load, but evidently many speaker designers disagree with me because speakers are routinely designed as 4-ohm loads or less, while few are designed as 16-ohm loads.
Note that in these examples I've ignored crossover losses and response-shaping circuitry. Crossover losses might nibble a dB or so off of the figures given. Note also that many designers, (including my amateur self), prefer a gently downward-sloping response to a "flat" response - so having the tweeter a dB or so less sensitive than the woofers would be okay.
To get back to your questions, as you can see the ideal wiring configuration depends on the characteristics of the drivers, and to a certain extent perhaps upon the characterisics of your amplifier.
If I you would like, Nate, let me know the impedances and efficiencies (or sensitivities) of your drivers, and I'll take a shot at suggesting a configuration.
Best wishes,
tweeter:Specifications: * Power handling: 45 watts RMS/60 watts max * Nominal impedance: 8 ohms * DC resistance: 7.6 ohms * Ribbon radiating area: 120 mm x 27 mm * Frequency range: 2,000-26,000 Hz * SPL: 92 dB 1W/1m
woofer:
Specifications: *Power handling: 60 watts RMS/85 watts max * Voice coil diameter: 1-3/8" * Voice coil inductance: 1.60 mH * Impedance: 8 ohms * DC resistance: 6.7 ohms * Frequency response: 29-3,000 Hz * Magnet weight: 15 oz. * Fs: 29 Hz * SPL: 88 dB 1W/1m * Vas: 2.04 cu. ft. * Qms: 3.23 * Qes: .44 * Qts: .38 * Xmax: 4.5mm
I appreciate your kind words.Okay, first of all, amplifiers usually have a more difficult time with a low-impedance load than with a high-impedance load, so you needn't worry about the high-impedance load stressing your amp. Think of a couple of extreme cases - a dead short, and an open circuit. The dead short is extremely low impedance and hopefully your amp will blow a fuse or shut down before it self-destructs. An open circuit is an infinitely high impedance, and poses absolutely no threat to the amplifier.
Whether the voice coils of the two identical woofers are hooked up in series or in parallel has no practical effect on their power handling.
If you wire the woofers in parallel, that will give you a sensitivity of 94 dB for 2.83 volts, while your tweeter is 92 dB sensitive. This might work - you see, in general a gently downward-sloping response curve sounds better than a "flat" one. You'd have to do a little juggling of component values to get a good blend - for example, on the low-pass filter, you might want to use an inductor 1.5 to 2 times as large as the equations predict, and a correspondingly smaller capacitor (assuming a 2nd-order filter).
If you wire the woofers in series the sensitivity will remain at 88 dB, so you'd need to "pad" the tweeter down. I'd probably try a resistor in series, before the crossover filter. Unless you really need to squeeze as much power as possible out of your amps (I'm assuming you have solid state amps), I'd go with the series woofer configuration (unless, of course, it turns out that parallel sounds better).
Best of luck to you!
Duke
duke, i am very impressed with the detail of your post. it is very imforative and quite a great help. I have included my drivers specs below for your consideration. i was very interested in your mention of a high resistance system as opposed to the traditional 8ohm or lower. does this have a negative effect on the drivers coil though (heat)? does the amp feel any stress with higher resistance?
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