I got a question that's confusing me.
Say if I have 2 speakers, one is active, and one is passive. Let's pretend this is a 2 way speaker and the tweeter and woofer have the exact same sensitivity of 90dB.
On passive, this speaker would be 90dB sensitive. So one watt will produce 90dB of sound over the entire bandwidth of the speaker.
Now, on active, the speaker needs two amps. Since the woofer is 90dB sensitive, it needs 1 watt to produce 90dB for its bandwidth up to the crossover point, say 50-2000Hz. Now the tweeter will need 1 watt to produce 90dB for its bandwidth from 2000-20000Hz. So the active speakers needs 2 watts to make 90dB of sound? Where is my error?
Say if I have 2 speakers, one is active, and one is passive. Let's pretend this is a 2 way speaker and the tweeter and woofer have the exact same sensitivity of 90dB.
On passive, this speaker would be 90dB sensitive. So one watt will produce 90dB of sound over the entire bandwidth of the speaker.
Now, on active, the speaker needs two amps. Since the woofer is 90dB sensitive, it needs 1 watt to produce 90dB for its bandwidth up to the crossover point, say 50-2000Hz. Now the tweeter will need 1 watt to produce 90dB for its bandwidth from 2000-20000Hz. So the active speakers needs 2 watts to make 90dB of sound? Where is my error?
Nope. After crossover and boxed, the woofer would show about -3 dB.
The sensitivity data is taken on the bare driver, by positioning a microphone in front of it and sending it 2,83 V ( for 8 Ω, 2 V for 4 Ω ) signal.
The driver sits on a baffle.
You didn't account for Re of the lowpass inductor and the loss of energy when boxed ( going from 4 pi to 2 pi shows the directivity)
That's why for PA active is used, the long distance of the amplifiers to the speakers dictates for low losses.
1W of 50-20kHz does not equal 1W of 50-2kHz + 1W of 2k-20kHz.
The active speaker will be louder than 90dB.
The active speaker will be louder than 90dB.
If you had one watt going to your woofer and one watt going to your tweeter you'd get an SPL of 93dB, not 90. But when dealing with complex audio signals, one watt plus one watt does not add up to two watts. If both were reproduced by the same amplifier (and split later in a passive crossover), it requires FOUR watts, not just two. Remember, power is proportional to voltage sqaured, so the combined signal requires four times as much power as each one individually to reproduce without clipping. A bi-amped system with the same total power will always sound louder and cleaner.
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Are you saying even if there were no resistors padding down the tweeter, a passive crossover still requires 2x more power than an active crossover for the same volume?
It depends on the parts count inside and how it's structured.
The passive crossover performs also the equalization of the single drivers to obtain a final balance. You can see designs that require lots of power.
The passive crossover performs also the equalization of the single drivers to obtain a final balance. You can see designs that require lots of power.
It needs 2.83 Vrms to produce 90 dB SPL.
Sensitivity is expressed for a given voltage across the voice-coil of the driver.
Not for the power dissipated which is not directly measurable and is variable with
the impedance, not constant with frequency.
For drivers, the conversion from Volts to Watts is often fallacious.
For the same undistorted volume yes. The average power requirement is 2X, but the peak id 4X. Since amplifiers are voltage devices, you're stuck needing 4x. Each amp feeding the pink and green waveforms may be 1X watts. An amp at 2X the power would put out 1.41 (normalized) volts. Clearly, the blue waveform will clip. Real audio signals are more complex than this, so the situation is worse. And before you go off thinking that the tweeter power is lower (sensitivities being equal), it is only so on average. The peak voltages required are actually pretty close (there is a thread in the solid state section on this). A higher sensitivity tweeter that you can drive directly without padding if using an active crossover makes the active one even more favorable. If the tweeter is 6dB more sensitive, you can use 6dB less power in the tweeter amp. 106dB/W compression drivers have a real advantage over 96dB/W woofers this way, and is the only reason you can get away with a 100W tweeter amp while using a kW on the woofer. The same system passively crossed (with the CD padded) would need 4kW to play as loud without clipping.
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