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General speaker questions for audio and home theater.

Pheeling Phazy

I’ll take a crack at it (i have a project i need a break from)

Picture a wave like a sine wave, one wave period is one complete cycle, that one cycle period is divided into 360 degrees.

If your amplifier is driving a resistance, one finds the current flowing at any given time is exactly proportional to the Voltage through Ohms law.
Thus, if one viewed the Voltage waveform over time and also the current waveform over time (like an oscilloscope), they have the same shape and same relationship in time.

If one were driving a reactive circuit which had capacitance and / or inductance as well as resistance (like most speakers) then one finds the current waveform can be shifted relative to the Voltage.

If one were driving an Electrostatic system, one finds the speaker can have a significant amount of parallel capacitance and that makes the current waveform lead the voltage as it takes extra current to charge and discharge the capacitance to follow the voltage waveform.

While capacitance acts like “mass” to a changing voltage, inductive reactance acts something like a spring. Driving a woofer where it’s compliance dominates (like a sealed box below cutoff) then the current lags behind the voltage waveform.

Either of these reactance’s cause a large increase in heating in normal amplifiers (not class D), if a reactive load were bad enough the phase shift could approach 90 degrees which for the amplifier output stage, means it will have to deliver max current when the voltage out is crossing zero.

Also, if an amplifiers closed loop gain is used to fight phase, it is not available for fighting distortion.

Amplifier output stages normally have a “safe operating area” a 4 quadrant "bow tie" looking curve beyond which causes smoke and an end to functionality. Because of that, amplifiers also have protection circuitry which hopefully stops the amplifier from trying to do something lethal to itself. For example shorting the output with a big input signal.

Depending how stout the output is and how far the SOA protection allows operation, this determines how the amplifier will respond to a given load at a given level.

While delivering a constant power is fully irrelevant (as one desired a flat frequency response), having a resistive load (little or no reactance’s) helps the most, especially marginal amplifiers work their best.

Fun with signals
Take two voltage sources at the same frequency. Add them together in series and you have 2X the signal. Take one signal, delay or invert it so it is shifted 180 degrees, now add them and the sum is Zero. This is because at 180 degrees, the signals are equal but opposite and cancel out
Take two signals, shift one 120 degrees (1/3 of the way around the clock), add them, the sum has the same voltage of either source but the phase is 60 degrees.

Take two identical loudspeakers producing the same signal, same phase. When they are less than one quarter wavelength apart, the sound is 6dB or four times the strength of a single source. That is because at a very close acoustic distance, the drivers feel each other’s radiation pressure (mutual coupling) and the overall efficiency is doubled as well as power handling (having two). Sadly, this and the low frequency room pressure mode effect are about the only things resembling a “free lunch” in acoustics.
When the spacing is increase to one half wavelength, then one is producing a figure 8 pattern or more correctly sort of a doughnut pattern broadside to the array. If one had two speakers side by side on the ground in this condition, the lobe points forward, up over head and behind. In a concert, subwoofers placed on either side of a stage produce a “power alley” where the bass is very strong. That would be the doughnut which is projected axially forward, overhead and behind.
With a spacing of one wavelength, there is a cloverleaf radiation pattern of lobes and nulls, the lobes usually alternate in polarity and each time the number of wavelengths in spacing is increased, the number of lobes and nulls increases. In the work I do I have come to belive the complexity of this radiation pattern is partly what gives a loudspeaker an identity. That identity is clearly audible when it is easy to identify a single speakers physical depth when standing in front of it with your eyes closed.
A speaker with a very simple radiation pattern is often much harder to “hear” how far away it is when listening to a voice with your eyes closed. When a pair is used in stereo, one normally has a much stronger mono phantom or any other stereo image. You do NOT want the speaker to be saying “hey here I am” you want to preserve what ever was in the recording. Rant over.
Those polar patterns can be measured of course but can also be calculated by adding the two signals at every point in space and then accounting for the phase shift difference between them which governs if they add to something larger or smaller than one source alone or produce an interesting pattern of lobes and nulls that change with frequency.
Take a single speaker, place it on a stand, sit at the other side of the room with a microphone.
Measure your low frequency response, gasp, a large notch in the heart of bass land. Why!!
Well from the woofer to the microphone is th shortest path the sound can take but a short time later, the sound that bounced off the floor also reaches the microphone. Depending what the phase shift difference between the two signals is governs if you add, cancel or somewhere in between. When the path length difference is ½ wavelength, you have near perfect cancellation.
Strangely, this is EXACTLY the kind of problem that you can’t / shouldn’t try to fix with EQ. What you have is a time problem, not something that can be fixed with EQ because any boost you add, adds a phase shift response which does not compliment the notch your trying to fix. Adding energy at that cancellation frequency also adds the same energy to the reflected signal. Anyway, when a speaker is in a room, only eq peaks and shallow depressions, never a sharp / deep notch.
Probably more than you wanted to know but it was all connected in my head.
Best,
Tom Danley


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