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Today’s Show:

Crossover Frequency

We got a Twitter question from a listener about how to set the Crossover Frequency for his speakers and subwoofer, so we thought it might be worth while to explain the concept for everyone. For some this may be review, but if you’ve struggled with crossover, or have just left it set at the default because you didn’t know what to do with it, this is for you.

For a really simplified definition, you can think of Crossover Frequency as a cutoff point. Anything below the crossover is sent to the subwoofer, anything above the crossover is sent to your speakers. This makes sense because your sub is intended to handle all the really low stuff and your speakers are meant to get the middle and high range sounds.

How to Set It

Setting the right crossover requires a little bit of research.  The THX standard for Select and Ultra2 systems is 80Hz, and this tends to be the default for a lot of processors and receivers.  But just because it’s the THX standard or the default doesn’t make it the best for your setup.  After all, the first thing you do with a new TV is change the default picture mode away from vivid or dynamic, right?

Your research involves finding the specs on your main left and right speakers in your theater.  The manufacturer should tell you how low they can actually reproduce sound.  Anything below that can be sent to them by your receiver, but they won’t reproduce it, so you won’t hear it.  If your speakers only go down to 100Hz, leaving the crossover at 80Hz means that you’re missing the entire 80-100Hz frequency range whenever you watch a movie, watch TV or listen to music.  It’s like having a black dot somewhere on your screen – a part of the picture you just can’t see.

Get it Just Right – For You

If all you get in your spec sheet is the lower limit of the range, you can assume the speaker will go down that far, but probably not very efficiently.  At some point above the lower limit the sound will begin to roll off, so you actually want to set the crossover above that bottom point.  We tend to recommend setting it 10Hz above the lower limit of the speaker.  Of course that’s your starting point.  Use your ears to adjust from there.

Some manufacturers will list a different value, a effective lower limit value.  For example, Polk lists the “Lower -3dB Limit” for their speakers.  They’ve taken the guess work out of finding the roll off point.  That value is essentially saying that although the speaker can go lower, below this point you’ll start to have diminishing returns.  When you see a value like that in the spec sheet, use it as your starting crossover point.

A Real World Example

Our Twitter listener’s speakers were the Polk RM 75’s.  They have a frequency response range from 95Hz to 24kHz.  In most cases you’d start with the crossover at 105Hz (if your processor allows that, you might need to compromise and use 100Hz instead).  However, Polk went the extra step of listing the Lower -3dB Limit, which is actually all the way up at 130Hz.  So in this case, if your receiver will let you, you want to set the crossover at 130Hz.  Otherwise you’ll get that big hole in the sound from 80Hz or 100Hz up to about 130Hz

Other Things to Ponder

You’ll want to keep in mind that the higher the frequencies go on your sub, the more directional the sound gets.  Instead of just being the bass that rumbles in your chest, the subwoofer actually becomes like a speaker.  This can produce sound issues you haven’t had before and may require you move the sub to find a better spot.  In some cases the issue introduced, like standing waves and reflections, can be worse than missing some frequencies in the 100Hz/110Hz area.  Again, let your ears decide.

Also, when they master content for 5.1 they tend to assume you’ll be using 5 satellite speakers and a subwoofer.  In most cases they actually master the LFE content at 120Hz.  Some receivers may have a setting for the LFE Filter or LPF (Low Pass Filter).  This is not the same as the Crossover frequency and in most cases you’ll want to leave this at 120Hz.  Since LFE content is never sent to the speakers, you won’t get a double bump on this stuff, it will only ever be played back on the sub, regardless of where your speakers crossover.

 

Download Episode #472


Posted by The HT Guys, March 31, 2011 11:48 PM

About The HT Guys

The HT Guys, Ara Derderian and Braden Russell, are Engineers who formerly worked for the Advanced Digital Systems Group (ADSG) of Sony Pictures Entertainment. ADSG was the R&D unit of the sound department producing products for movie theaters and movie studios.

Two of the products they worked on include the DCP-1000 and DADR-5000. The DCP is a digital cinema processor used in movie theaters around the world. The DADR-5000 is a disk-based audio dubber used on Hollywood sound stages.

ADSG was awarded a Technical Academy Award by the Academy of Motion Picture Arts and Sciences in 2000 for the development of the DADR-5000. Ara holds three patents for his development work in Digital Cinema and Digital Audio Recording.

Every week they put together a podcast about High Definition TV and Home Theater. Each episode brings news from the A/V world, helpful product reviews and insights and help in demystifying and simplifying HDTV and home theater.