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Re: dual woofer

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,

Duke


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  • Re: dual woofer - Duke 12:41:20 07/22/02 (3)


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