SPL: Non-THX vs THX Speakers - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 5 Old 03-10-2000, 06:13 PM - Thread Starter
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Hi,

I have an article from Audio Magazine (November 1994 issue - note a correction to the graphs appear in the March 19995 issue). The article is called " Planning a Home Theater System", the correction appears in "signals and noise".

THX Ultra certified speakers are supposed to be able to achieve 105 db peaks in a 3000 cubic foot room. THX Select is 105 db in a 2000 cubic foot room.

The article explains how to determine the power in watts to each channel to achieve 112db on peaks. The article explains that most commercial theaters target 107 db on peaks, therefore a crest factor of 5 db is included in the calculated power per channel bringing the total to 112 db on peaks.

First a power ratio number is read off of a graph and it is based on the sensitivity of the speakers.
Then a power multiplier is read off of another graph and it is based on both the volume of the room and if the room is "Live", "Dead" or "Average" in terms of sound absorption.
Next, a third graph is read to determine the power per channel necessary to achieve the 112 db peaks. The power ratio and power multiplier are both utilized in this graph to determine the power.
Also, the article further explains that if you are redirecting the bass (using a subwoofer) then you only need one half of the calculated power per channel.

OK - NOW ON TO MY QUESTION:
The M&K 750 THX Select can only achieve 105 db SPL on peaks in a maximum of 2000 cubic feet. I ran the numbers through this procedure for my room (2700 cubic feet)and it came out with 175 W/channel - half of that if I use a subwoofer! So, my question is why can't this speaker reach 105 db in a larger room than 2000 cubic feet.

Oh, I just thought of something as I'm writing this. Maybe it can't do it in a 3000 cubic foot room, but maybe it can in a 2700 cubic foot room. THX had to pick a nice round number - 2000 cubic feet!

HOWEVER, IF THIS ARTICLE IS ACCURATE THE SUITABILITY OF ANY SPEAKER SYSTEM IN TERMS OF SPL ONLY SHOULD BE ABLE TO BE DETERMINED - THX CERTIFIED OR NOT!! I know that their are a lot of other parameters that are measured for THX certification - directivity, minimum impedance levels, frequency response, etc. But, in terms of hitting a THX SPL limit, if this procedure works then GREAT.
At first I was giving the M&K S-125 a good look at, but then it wouldn't hit the THX reference level of 105 db in my 2700 cubic foot room - but, this article says it can. As long as the speaker can handle the calculated wattage necessary, then it should work.

NOW, THE BIG QUESTION:

IS THIS ARTICLE FOR REAL - CAN I NOW JUDGE NON-THX CERTIFIED SPEAKERS AGAINST THE REFERENCE SPL LEVEL?

I would appreciate any and all advice/comments.

Thanks,
Greg
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post #2 of 5 Old 03-10-2000, 06:40 PM
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Judging a non-THX speaker against it's SPL (calculated), is rather like judging a used car by the feel of the door handle. That's just one of many (and in some cases, much more important) criteria.

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post #3 of 5 Old 03-11-2000, 04:34 AM
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I have Atlantic Technology THX ultra speakers and I love them, but I would not take those THX requirements seriously while judging a speakers performance.
I have 3000 cubic foot room that supposedly needs a second sub for thx-ultra cerification. Without it it is only select certified.
If I bought another sub, the condo asssociation would try to kick me out of here. I think that unless you are building a public theatre and want George's logo on it, I would just listen and buy what your ears tell you.

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post #4 of 5 Old 03-12-2000, 12:02 PM
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Quote:
would not take those THX requirements seriously
My general advice is you take them very seriously. SPL is not the only criteria of THX certification (nor is THX the only sign of excellent speakers). Unless you know the speaker's horizontal, vertical dispersion patterns, off axis response, response linearity, and lobing characteristics, you could be a sad lad. I know of one $8K center channel speaker whose lobing is soooo bad, that unless you have your head in exactly the right location (and never move it), your sensibilities will be sorely wounded. In fairness, I also know of some very nice non-THX speaker systems.

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[This message has been edited by Dennis Erskine (edited March 12, 2000).]

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post #5 of 5 Old 03-12-2000, 12:06 PM
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Oh, BTW, using the math, you can reasonably predict the SPL of a speaker (the standard is at 1 meter). It becomes more difficult to predict that across its entire operating range and just how it will behave in a room (meaning your room).

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[This message has been edited by Dennis Erskine (edited March 12, 2000).]

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