Speaker Efficiency, Sensitivity and Impedance

 This comes up a lot so worth trying to distill closely related specifications together. 

Speaker efficiency and sensitivity are often treated as the same, but they are different.  Both measure output at 1 meter in decibels, the issue is the input.

Sensitivity:  Output at 2.83V at 1m.  In other words, output vs. VOLTAGE.

Efficiency:  Output at 1W at 1m.  In other words, output vs. POWER.

If you have an 8 ohm speaker, sensitivity and efficiency are equivalent, because 2.83V into 8 Ohms = 1 Watt. 

Here’s a common example: Many mid woofers come in 8 ohm and 4 ohm versions. Nearly identical in all other aspects besides the impedance curves. The 8 Ohm version produces 87 dB at 2.83 V. That is, 87 db @ 1 Watt. The 4 ohm speaker produces 90 dB @ 2.83V, but since it has 2x the current it is 90 dB @ 2 Watts.

We care much more about this difference when dealing with tube or modest output amplifiers. 

If you care about whether a speaker is difficult to drive though, consider the minimum impedance.  Anything below 3.5 Ohms will start to get difficult for most receivers. 

Hypex Filter Designer

 I want to make this quick.  After 2 days wrestling with the Hypex Filter Designer, the tool you must use for Hypex DSP products, I can confidently call it unfriendly. 

I am using a Hypex FA 123, 3 channel DSP + Class D amplifiers on a single plate.  Thank goodness their tech support is so friendly and responsive. When I first got it the firmware was misset and the interface kept imagining it was the 2-channel version, the FA 122.  This left me unable to use the 3rd channel which was intended for the tweeter.  

To make a long story short, after getting the firmware patched up (thanks Jeffrey!) I then had a long slog through the channel configurations and filter settings.  The biggest hassle with the HFD is that nothing is instant.  Experiments take about 10x longer to make than they would with say miniDSP.  You must hit upload, wait 20 seconds, and then fix the bug where it resets all of the mute settings.  

This makes it very hard to ease up into the right filter settings.  Once you even get close you want to stop and leave it alone. 

It is seriously about 10-30x more work to set up a filter here than in miniDSP or Roon by comparison. Otherwise, great hardware, guys!

The Center at Scale

 I have the center on my rack ready to start configuration. 

Despite the size (smaller than many!) this is a deliberately limited-low frequency center.   It was designed to cut off ~ 55 Hz and pretty much does.  With the processor's 80 Hz high pass filter the center cannot produce much bass. 

Lets show how this all integrates.  These are measurements from the listening location, so they SHOULD be dirty.  The red line in the background is the center channel, the blue line is my right speaker.  You can see how well the center is matching my main speakers.  This explains the really great integration.

One thing to note is that DSP processing takes time, so the speaker looks like it is about 10" further back than it really is. The Dayton Audio OmniMic DVD is a great help here.  The short sine sweeps run simultaneously from the left and center speakers make sure you get exactly the right delays. 

Please ignore the poor bass in the chart, I had not yet fixed up the subwoofer phase alignment. 

The Center Assembly

 

Happy to announced that the center channel has been put together!! The four drivers and plate amp have been wired in and assembled into the cabinet.  Drilling was made a lot easier by pre-existing pilot holes from the cabinet maker which was a super nice touch.  Also they went to the trouble of insetting the plate amp giving the rear a very clean and professional look.

Sadly, this speaker still can't make any sound.  Over the next week I'll use the Hypex software to enable each channel in turn and begin the process of building up the Virtual Point Source crossover. 

I don't know why but those who haven't done this part think it's easy or requires no finesse and careful adjustment.  Nothing is further from the truth.  What is easy is we no longer have to fiddle around with individual crossover parts.  No more resistor, capacitor swapping and tinkering here! 

The worst case scenario  I've seen is builders who take the published specifications at face value, throw in some guessed at curves, and then use global EQ to "fix" the speaker. To be clear, you can't achieve anything good that way. Perhaps with horn speakers that are designed for simple crossovers which have long been understood by the users this can yield good results but to achieve the virtual point source effect we are going for here each driver must be individually measured for frequency, phase and delay so the electronic curves are complementary to each driver and work with the driver physics, not against them.  One key ingredient to this is an excellent midrange that can play beyond it's crossover points.

Is This Really a Center Channel?

  Happy to announce that my new speaker cabinet has been finished and I anxiously await delivery.  To be honest, I hesitate calling it a center channel speaker anymore.  In fact this is a high output satellite speaker that could be used in any of the 5 or 7 main speaker positions used in home theater. 

I almost ordered 3 of them but I wanted to take delivery of one first and check that all the dimensions worked as expected.  In addition to being my first speaker build in many years it's the first one that will use what I call Virtual Point Source technology.  It's a combination of driver size selection, crossover slopes and delay setting which will have 95% of the benefits of a single driver, without the usual compromises in compression, distortion and frequency response issues single wide-band drivers have.

This is, arguably, also a counterpoint to the passive D'Appolito alignment.  While those strive for controlled vertical dispersion these speakers absolutely do not.  Go wide or go home.

To summarize, this is a 3-way active center channel with dual 7" woofers which will be a little larger than your average sound bar, but not nearly as large as similar designs from Legacy or ATC.  Originally inspired by Troels Gravesen's center channel monsters.   The three channel plate amplifier by Hypex will have the equivalent single amp output of around 1,000 Watts between the midrange and woofer sections alone.