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In Reply to: RE: Does THX Certified really mean anything? posted by Phil D on March 21, 2009 at 17:36:16
I've heard that it does and that it is an expensive certification to get, therefore many manufacturers do not get it.
It includes some standards like a flat frequency response and an ability to go up to 115db (that is way beyond what makes your head explode) without audible distortion.
As we all care about inaudible levels of distortion, it probably does not mean much to us. But it is nice to know that your amplifier is certified not to have its screws shook loose when it is set to music at 115 db. I
think that's too loud for Sousa. The only thing I could listen to at that level would be polka, as the neighbors might blame it on the alcohol.
Follow Ups:
Why on earth would anyone want to do that? I find a different answer here.
rw
Take a look at this link. It says:
* No audible distortion playing program material at 115dBC
OK. What's a dBC? IDK. A dBA is used for ordinary sound. A dBC is used for very loud sound. It stands for deafening beyond comprehension or death by compression or something like that. THX is a theatre kind of standard. It's for people who want to get shook to death by explosions, earthquakes, jet planes, and car crashes.
Briefly, dBa and dBc involve frequency response weighting or contouring by the sound level measuring device.dBa more-or-less simulates the response of the ear by de-emphasizing the low and high end frequencies and emphasizing the midrange. One use of the dBa weighting was to evaluate the potential for noise induced hearing damage in a working environment because of the way it simulates human hearing (don't know if it's still the standard of the law). dBc is much flatter in response but still has rolloff in the lowest and highest frequencies because it allowed mfg of cost effective sound level meters with reasonably flat freq resp. dBlin is flattest and extends from subsonic frequencies to well above human hearing with little variation. This weighting (lack actually) is expensive to achieve and is usually associated with lab equipment.
Edits: 03/22/09
The idea behind dB A, B, & C weighting was to match the sound level meter response to uman hearing at three different sound pressure levels. The Robinson and Dadson equal loudness contours (replacing the older and less accurate Fletcher-Munson curves) are the inverse of these weighting curves. A weighting is used for low levels where the ear has a lot less sensitivcity in the bass and treble, B at a higher level where the ear's sensitivity to the extremes is better, and C for much higher levels. In my ancient GenRad Handbook, A weighting is intended for use below 55 dB, B for 55-85 dB and C above 85. A weighting is -30 dB at 50 Hz (an indication of just how insensitive the ear is to bass at a low level), B is -12 dB at 50 Hz, and C is -1. All three curves are about -4 dB at 10 kHz and -10 dB at 20 kHz. Oddly, the A curve rises slightly above the others (by a dB or so)at around 4 kHz. 4 kHz is the ear's most sensitive frequency and it explains why we have the "BBC dip".
Clearly, the use of A weighting curves for industrial sound level measurements in the government regulations is a gift as they should be B weighted, not A weighted as specified. The limit for 8 hour exposure used to be 85 dB A weighted - it might still be, but I haven't worked in this area in 15 years.
You might also note that most of the less expensive meters lack the B weighting setting, which is really the right one for most home level measurements. For what it's worth, the preferred level for music listening among recording engineers and musicians is 85-88 dB long term average, which is quite loud.
As to loud: 115 dB is not un-typical for a Disco or club for short term peak levels (fast meter response, no weighting). In doing the engineering for a local Disco, I measured the SPL in several local Discos. The best of them (in terms of sound quality) was a live band and long term average levels were 105 dB (no weighting, slow response) with peaks to 122 dB ( meter set to peak, fast response, no weighting). This was a very large room and the levels were measured 2/3 of the way back, which is where the band's sound man was located. The sound quality was excellent and didn't sound amplified - it was as if they could all play and sing at inhuman levels (the demon band?).
During the band's break, the house system came on and was 108 db average with 118 dB peaks; and sounded like crap.
So I don't find the THX spec of 115 dB to be at all out of line for pro systems.
Jerry
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