Where can i find the THX requirements for a 5.1 Home Theater?

This is a good question.

First you should know that THX requirements are not just for sound, they focus on video performance and room impact as well to obtain the full cinema like experience.

THX requirements are in constant change due to the introduction of new audio and video technologies. On the audio side THX focuses now on 7.1 surround sound with its THX surround EX. Of course there was a time when it was focused on 5.1 surround sound.

THX requirements for home theater were posted some time ago on the Technical Library Section at THX website. For some reason they're not available anymore. Maybe they are going to be released with new requirements as surround technologies are in change.

On 2004 I downloaded the THX standards for home theater directly from THX website and I think all specs can be applied to 5.1 systems as well, I hope this could help:

THX Standards:

Picture

• 16:9, ≥ 36°horizontal viewing angle.

• Native HD capable.

• Screen luminance ≥16ft lamberts.

• THX Certified, acoustically transparent

screen

Sound

• Mid-band pink noise ≤±3dBC between any two

seating positions.

• No audible distortion from subwoofer playing

program material at 115dBC.

• No audible distortion from satellites playing

program material at 108dBC.

• Background noise ≤ NC22

• At least four surround speakers, two side and

two rear.

User Interface

• Remote controls with a logical user

interface.

• All basic functions available with a single

key stroke.

Measurement

• Video performance measurements taken

using Sencore CP5000 quality gear or equal.

• Audio performance measurements taken

using 1/12 octave RTA with four

microphone spatial/temporal averaging or

better.

That's the specific info (unedited) that I downloaded. As you can see they're all performance focused.

I can add other facts I know about:

• All equipment used must be THX certified including the cables used for connecting them. Right now no THX certified Displays are available only THX certified screens to be used with front projectors. THX certification now involves room size and viewing distance so equipment must be carefully selected (Only THX Select/Select2 and THX Ultra/Ultra 2 equipment are certified for dedicated home theaters).

• Only front projectors will work here because front and center speakers must go behind an acoustically transparent screen just like in the cinema. So forget about plasmas or lcd's.

• Projector's keystone correction is not allowed to be used so proper alignment of projector to screen is mandatory.

• True HD native resolutions are only 1280x720 (720p) and 1920x1080 (1080p). The use of different native screen resolution will require the display to perform some scaling with corresponding degradation of picture quality.

• You will need 30" of screen diagonal minimum for each meter of viewing distance to the screen to achieve the minimum 36° viewing angle (Example: from 2.5 m away you will need 75" diagonal screen as minimum)

• Calibration disc with video calibration patterns will be needed to properly calibrate screen performance.

• For 5.1 surround you will only need two side surround speakers and they must be dipole ones.

• In the receiver's set-up menu all speakers must be set to SMALL and bass set to LFE (or SUBWOOFER) when speakers are built with 8" or less woofer cones.

• Subwoofer crossover must be set to 80Hz

• THX recommends the use of two subwoofers but multiple subwoofers could be used to achieve optimal bass experience.

• Only active subwoofers should be used (self-powered with bulit-in amplifier)

• A SPL meter must be used (in C scale, slow mode) to calibrate the output sound levels of each channel (front, center, surround and subwoofers) to 85 dB. Calibration disc with pink noise test signal played directly from the source should provide better results that when using receiver's test tones.

• Dedicated room acustic isolation will be needed to control external background noise. Other background noise could come from installed equipment like projector's and receiver's built-in cooling fans or some air conditioning system and special measures should be taken to overcome these.

Only THX qualified designer/installer companies can test your dedicated home theater and guarantee it meets all THX performance requirements. Only that way you will get a THX Certified Home Theater plaque on the wall.

ADDITIONAL INFO:

THX standards above where taken from CHTechnicalOverview.PDF document I downloaded directly from THX website on november 2004. It's just 1 page long. It has some other info that I didn't post but those are related to the Home Theater Certification Program itself. Altough there's something I think it's important to post:

"Requires measured proof of audio and video performance using THX specified measurement techniques."

This techniques are not included within the document so there's no way I can't tell how they measure those specs.

The additional facts info I posted is what I remember of what I've read since the 90´s until now about THX. And I really begin understanding THX concept since 1996. So feel free to belive it or not.

Henry, are you sure all of those specs are right? I've heard of a lot of those before, but 115 db from the sub with no audible distortion? First off, how many subs can even do that? And at what frequency? Secondly, that's a subjective requirement ("audible distortion"--differs between people). And I'm pretty sure that the db spec for THX has always been 105 db <1% total harmonic distortion (or is that .1%?--can't remember), not 108 or 115.

Maybe there's a reason it's not on the site anymore... maybe it was bogus.

Thx Certification

THX is nothing special.

For home theater equipment to have the THX label, the company pays THX a huge amount of money to be certified / licensed to wear the THX logo.

A perfect example is the Onkyo 990 HTIB (home theater in a box).

Sure, it sounds okay, but for near $1,000 there are many better sounding and higher quality alternatives.

Thx Jeff for capturing this, as I asked THX what objective test stds do their Home Theater have to meet and got a std "we currently do not publish our specs for achieving THX Certification on screening rooms. If you want more information, or have a client that's interested, email design@thx.com" .

Red flag, if a company does not want to publish it's objective test std how can you truly trust it??

Izzy S hit the nail on the head.

THX IS NOT 5.1 thx is only 7 chanels or higher, in surround sound 5.1 is basic. thx 7.1 or higher is certified thx. and in order to get true thx sound it has to be certified equipment.

Check out this link. It shows the correct speaker layout and display setup etc..

http://www.thx.com/home/setup/index.html