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Does anyone know why THX wants all systems at 80hz crossover even when a good speaker can handle lower frequencies?


Right now, I'm having a hard time understanding whether I should set mine at 60hz or 80hz. The frequency response on my speakers are:


45Hz – 20kHz

50Hz – 40kHz (– 6dB)


60hz seems to make a lot of sense, does it not? I basically do all my listening tests to Star Wars DVDs, so I can't get that 80hz THX thing out of my head, and which one should sound better.


Also, does it mean that THX material such as Star Wars would sound worse with a 60hz crossover?
 

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Well, the reason THX has a single crossover frequency is that it makes the most practical sense in a standard, such as THX, to do just that: select a single crossover frequency for standardization and facilitation of component compatability.


After you understand that they had to choose some frequency (without making the THX standard and component guidelines much more complex), the question is then why exactly 80Hz?


That answer is pretty simple: 80Hz is a pretty good compromise in that (1) it is high enough to relieve much of the bass requiremet from midbass speakers, resulting in designs for "bookshelf" sized speakers that are a good match with commonly intrenched opinions about aesthetically acceptable speakers sizes; (2) is not so high that typical bass drivers have a response problem due to inductance; and (3) is low enough that the sub is still difficult to localize.


If you have a flexible crossover point, you are well advised to make an intelligent choice that best matches your mains and sub. That isn't always "lower is better" by the way, but in some cases that may well be true.
 

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Megalith: I'm in the exact same boat as you (60hz or 80hz). I found that 60hz sounds "best" to me, on my system, in my house.


My speakers can handle bass clean to 60hz. When I set it to 60hz, it seems like the sub and the receiver/mains are working better as a team, and my bass response gets much smoother. With no "boominess"


BUT!!, I keep wondering if 80hz is better, because THX says it is.


Anyway, I'm at 60hz for now.......


Darryl
 

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80Hz is were most of us can no longer localize the source. That is why you can put a sub anywhere in the room. Plenty of folks with full range speakers and strong amps pick a low crossover.
 

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Most people cannot localize bass below 100hz. With the crossover at 80Hz, it ensures that most everyone will not be able to hear where the low bass is originating from. A higher crossover will compromise the realism for many people.

Many speakers are not flat much below 80Hz, even if they do extend below that point, so for the average mid-fi system, a lower crossover does not make much sense.


It takes a lot more power to produce a 50hz tone than a 100hz tone, so why not let the sub do what it is designed to do? This will minimize the chance of clipping at reference levels, not to mention that the bass that the speakers are producing may sound tighter when the woofers do not have to work so hard.

Of course if you have ample power and ALL of your speakers are flat to 50hz, it certainly won't hurt to set your crossover at 60hz.


The THX police aren't going to knock down your door if you set it below 80hz :)
 

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Quote:
Originally posted by Karp
The THX police aren't going to knock down your door if you set it below 80hz :)
No they're not. And how could they? I can't imagine them crossing all those B&W monsters at the Skywalker ranch at 80Hz either. Filtering all the sub 80Hz material to a sub in the front corner of the studio probably wouldn't have the same effect!


That's why I don't think anybody with reasonable speakers and power should use an 80Hz crossover. I think there is more immersion into the soundtrack when you can feel the lower frequencies coming from the direction they are intended. They say 80Hz bass cannot be localized but when the sub is at the front of the room say 12' away, I can tell that a "boom" from the surrounds is NOT coming from the back of the room. It's coming from the front.


I think for the 80Hz crossover to work effectively, there should be a sub in the front of the room for mains and centre, and another in the rear of the room playing the sub 80Hz material of the surrounds only. This is more realistic.


In my case I've got Monitor 9's for mains, CC270 and Focus surrounds. I've got a Pioneer 56TXi crossed over at 50Hz. I never would have thought of trying this due to the threat of the THX police but after a strong suggestion of my dealer (audio), it makes a big difference in the depth of the soundtracks. He told me to give it try and change it if you hear any signs of compression or clipping - which I never have.


I will say that this WON'T work if you are playing movies at reference level. I don't have an SPL meter, but I basically watch movies as loud as I feel necessary, usually around the -10 - 15 mark on the 56.


You may not agree, but do give it a shot you might be pleasantly surprised.
 

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Quote:
Originally posted by Mattsushiba
That's why I don't think anybody with reasonable speakers and power should use an 80Hz crossover. I think there is more immersion into the soundtrack when you can feel the lower frequencies coming from the direction they are intended. They say 80Hz bass cannot be localized but when the sub is at the front of the room say 12' away, I can tell that a "boom" from the surrounds is NOT coming from the back of the room. It's coming from the front.
Then there is something wrong with your system or your subwoofer.


BTW, the 80 Hz crossover value was decided on a whim, not on some advanced research evidence -- Tom Holman has publicly stated as much. Crossover your speakers where ever they sound best to you without distorting them.
 

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Quote:
Originally posted by Mattsushiba
No they're not. And how could they? I can't imagine them crossing all those B&W monsters at the Skywalker ranch at 80Hz either. Filtering all the sub 80Hz material to a sub in the front corner of the studio probably wouldn't have the same effect!


That's why I don't think anybody with reasonable speakers and power should use an 80Hz crossover.
Kevin Voecks at Revel Speakers recommends an 80 hz crossover with the Revel floorstanders. From the UltimateAV review of the Performa system:

Quote:
Although the F32 has the bass capacity to extend down to 30hz with little rolloff, Voecks configured the system with the standard THX handoff to the subwoofer at 80hz. He feels this delivers the smoothest frequency response, as well as the least restricted dynamic capabilities.
 

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There is a lot of information here. Unfortunately, most of it is not correct. I've taken many classes on acoustics and here are some facts. Tom Holman did not make up 80hz on a whim. The reality is the crossover should be set to give the smoothest response between the mains and the sub. This varies based of the physics of your room and the placement of seating, speakers and subs within in this acoustical space. Tom knows from research that most small rooms (ie not auditoriums or other large acoustical spaces) found in homes the ideal crossover usually will fall somewhere between 60hz-100hz. To simplify things (there are other factors involved) Tom did pick 80hz as a good average and works in most rooms. Obviously, he can't measure everyone's room. If this is considered a whim so be it.


We know from room acoustics that the ideal placement for low frequencies isn't the same for high frequencies. This is why we have a separate subwoofer system. One of the reasons for this is standing waves and modal response. Another is SBIR. When a speaker is near a wall it creates SBIR (Speaker Boundary Interference Response) which causes cancellations and a notch at a certain frequency. When you move a speaker closer to the wall the notch becomes higher in frequency and lower in frequency as you move it away from the walls. This is why the sub is often placed near a wall and the speakers moved further into the room to move this notch in frequency above or below the crossover point. Also, bigger towers cause quite a lobing effect which makes their placement much more difficult to place.


Frequencies can't be localized until ~200hz or greater. The best measured response of someone being able to localize bass is a little over 160hz. This is for someone with exceptional hearing, most of us it is probably near or slightly >200hz. If you can localize a sub it is usually because of other resonances the subs vibrations are creating. Also, remeber the crossover point is not a cutoff point. Frequencies do slope off above and below their respective crossovers.


Using 2 or more subs can help smooth bass for all listening positions in your rooms. That is why 2 or more subs is often recommended. Proper set up is required.


The crossover point as stated above should be set based off of the room's acoustics and not the capabilites of the speakers. Some very small satellites excluded. Setting the croosover should provide a smooth frequency response between the mains and subs. All because you have monster mains that go flat to 40hz doesn't mean 40hz is the best crossover point for your room and placements. I will leave you with a quote from Tom:


"If you aren't measuring, you are guessing!"


Just my .02.


Hope this helps.


Bob
 

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BobL:


Good stuff. "The x-over point should be set based on the room's acoustics and not the capabilities of the speakers."


That makes sense. Thats why I'm getting a smoother bass response with the 60hz x-over vs the 80hz. It's my room that "needs" 60hz, not my speakers.


Based on your post, even the same speakers, with the same receiver would require different x-over settings in different rooms.


So, THX's 80hz sounds like it's just a good compromise for "most folks" aka: (NOT the ones that are on this forum!!)



Darryl
 

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You also have to remember the THX crossover is set at 80hz because they recommend that the sub be placed in the FRONT with your mains. If you put your sub close to the listening area (example, behind your couch) then you will be more likely to localize 80hz.
 

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I used to run my Adire 281's full-range (!!!) during movies. It was cool to begin with but something just wasn't right. Then I tried them at 60hz and thought that that was the best compromise. It sounded smooth with just the right amount of mid-bass coming from the left and right.


Months later and a BFD upgrade have left me using an 80hz crossover. Huge difference even before I got my BFD. Now I don't use multi-hundred watt amps, sorry. My HK525 is "just" 70 watts per channel.


The difference in sound quality in the midrange was what was most noticable when using an 80hz crossover. All the sound just seemed to sound "clearer". It was because my left and right don't have to work so hard down low. I didn't think that I would end up with 80hz crossovers all around but I did and I love it! Especially since the 281's can easily put out decent energy in-room to the 40's and extension down to at least 25hz, in--room.


Unless you have six-foot tall speakers with a couple of twelves or larger drivers I would keep a 80hz crossover always, for movies.


Music is another story.
 

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Really the only true way to do this would be to measure your mains on an FFT then measure the sub and see where each roll off. Then place the lo pass on the sub and the hi pass on the mains to the correct level, then make sure the phase trace is lined up correctly. The 2 filters should come together to create a smooth response in the crossover region.
 

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Setting your A/V preamp processor, or receiver to small for the satellites and sub on is best in most situations. With Bass Management the sub is implemented in mono and all the satellites in theory play a 20Hz-to-20KHz bandwidth, so 80Hz is a good crossover point.
 

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One fact that hasn't been mentioned yet is that the crossover is typically (maybe always) applied to the LFE channel. In theory, the LFE extends up to 120 Hz. So the lower the crossover frequency, the more LFE content you are removing.


Ed
 

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BobL,


Thanks for the post. A lot of us have posted subsets of what you wrote, but rarely as complete and succinct - a very difficult combination - as what you provided.



Cheers,

Bill
 

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Quote:
Originally posted by ekb
One fact that hasn't been mentioned yet is that the crossover is typically (maybe always) applied to the LFE channel. In theory, the LFE extends up to 120 Hz. So the lower the crossover frequency, the more LFE content you are removing.


Ed
Hi Ed,


Do you have any documentation on this? I don't believe it is true. The Dolby spec certainly does not call for it.


Joe
 

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Quote:
Originally posted by JoePerri
Hi Ed,


Do you have any documentation on this? I don't believe it is true. The Dolby spec certainly does not call for it.


Joe
This sort of info has been discussed many times in these forums. One reference is the "DD Professional Encoding Guidelines". Sorry I don't have a link since I have the PDF on my disk. It was posted in another thread. Maybe you can find it at the DD site. Also this information can be found at the "Secrets" web site - again, sorry I don't have a link - maybe someone has it? But what exactly don't you believe?


Ed
 

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Just to add to my post - the reason for low pass filtering the LFE when there are small speaker channels is to avoid phase cancellation. If you just low passed the small channels and added that to the LFE, you'd have phase problems in the crossover region. Dolby's solution is to add the full frequency signal from all small channels to the LFE channel and then just low pass it all at once.


Ed
 
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