The Crossover - Power Handling

In my previous post I wrote about how the crossover contributes to distortion and dispersion. Here we focus on the crossover's effect on power handling as well as dynamic range.  While the previous article focused on the midrange here we'll use the tweeter to start the discussion.  

Frequency vs. Excursion

There are some rules of physics we just can't avoid.  For any given driver it takes more excursion (movement) to achieve the same volume or loudness as the frequency drops.  But a driver's maximum excursion is not infinite.  Every driver has a range of movement within which they are linear.  Movement beyond these limits will cause distortion at first and then driver failure.  By reducing the amount of low frequency energy in each driver we also increase how loudly the driver can play without distortion.  In other words a good crossover improves dynamic range. 

We start the discussion by examining the center channel tweeter, the ScanSpeak Illumintor D3004-66200 and the high pass filter choices made for  it. 

The black line represents the high pass filter point.  This particular tweeter performs really well all the way down to 1 kHz, but like with the other drivers distortion starts to rise at the low end.  By limiting this tweeter to 3 kHz we not only eliminate the low-end distortion but also minimize the music power it will have to respond to.  

Picking 3 kHz instead of 2 kHz as our cut off increases the maximum output by about 7 dB.

The Slope

A high-pass or low-pass filter has a slope associated with it.  For each "order" we add 6 dB/octave.  In the drawing, above, we drew a first order filter in green (6dB/octave) and a fourth order (24 dB/octave) filter in orange. 

Using a steeper filter reduces the amount of power the tweeter will be subject to, which in turn reduces the excursion (movement) of the dome.  At 1 kHz the difference between these two filters is about 30 dB! 

This difference explains why some speakers famous for using first order crossovers exclusively also suffer from premature driver failure.   The lower a crossover frequency you want to use the steeper a filter you need to prevent damage. 

Where does the Power Go? 

So if the tweeter isn't handling this musical power where does it go? It goes to the midrange which has about eight times the surface area of the tweeter, which allows the driver to move about 20x less at 3kHz. 

 

Frequency vs. Dispersion

If you read the previous article you might be thinking that power handling and dispersion are at odds with each other, and you'd be right.  For the sake of power handling we want to raise the crossover point, but for the sake of dispersion we want to lower it. Congratulations, you now finally understand why there are so many 3-way speakers out there. 

The Woofer

The same principles and problems the tweeter has are true for the 7" woofers as well.  Here I'm going to digress a little and talk about a major difference between the center channel I made vs. other similar designs by big names.  I found several very expensive, and very large center channel speakers with a similar layout:  Center channel 3-way with 7" woofers in a ported enclosure. 

Ugh.  They were all physically huge and in my mind had the wrong priorities.  They prioritized bass extension instead of output volume and cabinet size.  Given my reliance on a subwoofer I knew I did not need much output below 80 Hz, and by using a sealed enclosure my center was significantly smaller than several $15k-$20k center channels out there and most of the DIY designs from the esteemed Mr. Troels Gravesen.   Let's look at the distortion curves for the woofer. It's actually a midwoofer, and an excellent choice for 2-way designs as well,  but we limit the output to 350Hz.

I've drawn a black line at 80 Hz, the intended high pass filter position.   Like with the other two drivers, we've limited the distortion and excursion by our choice of filter.  This position also happens to be the THX standard.  To be clear we achieve a 4th order response in this case as a combination of the acoustical response (12 dB/octave) plus the external 2nd order high pass provided by the home theater processor.  

Another "trick" here is that we are using two woofers, which cuts the excursion per driver in half. 

The end result is that the woofers can put out 110 dB @ 80 Hz before reaching their maximum linear excursion of 6.5mm.  However, if we tried to push these drivers to 45 Hz we'd be limited to about 100 dB.   10 dB is huge, by the way.  It does mean 10x the power but also twice the audible volume. 

Conclusion

With the crossover points and slopes given the maximum low frequency output for the tweeter, midrange and woofer output is 123 dB, 118 dB and 110 dB respectively, which is about double the audible output possible with lower crossover points using a sealed and relatively small cabinet. 

 

We've shown how picking a crossover point and slope affect power handling in general and also demonstrated how the crossover and speaker design have resulted in a speaker capable of playing twice as loud as it otherwise would be able to while keeping distortion to absolute minimums.