Explain Me This - Page 3 - Home Theater Forum and Systems - HomeTheaterShack.com

Old 03-09-09, 08:32 PM
Senior Shackster

Brent

Join Date: Feb 2007
Location: Wilmington, NC
Posts: 407
Re: Explain Me This

Let's try to say this another way for you.

THX/Dolby Reference is really calibrating so that a 0dBFS (that's the loudest a signal can be digitally recorded) will produce 105dB at the seats from each main channel. When you calibrate to Reference level by measuring 75dB using a test tone, that signal should have been recorded at -30dBFS, giving you 30dB of dynamic head room. If your test tones said measure 85dB, the source signal was -20dBFS, leaving 20dB of dynamic range. Both will produce 105dB at 0dBFS, assuming the amp, speakers, and listener are up for it.

The LFE channel is decoded and +10dB of boost added after the DAC....can't add it before the DAC because you can't go into the DAC above 0dBFS without causing nasty noises. This allows the LFE channel to call for 115dB from a 0dBFS signal when calibrated even with the main channels.

And, yes, bass redirection will call for even more output from the sub channel, I think 121dB is the theoretical max.

In the real world, you're measuring output from all speakers combined, not just one. I find I don't like to listen at a level where peaks are more than 95-100dB. That typically works out to -15dB below Reference on most DVD/Blu-ray tracks. Hearing damage doesn't care if you're listening to one speaker at Reference, or 7.1 at 15 below...get much above 100dB for too long in a session and it's going to happen.

Trying to guesstimate amp power using ideal math is a no win exercise. The ideal math assumes no boundary gain and a loss of -6dB for every doubling of distance from the source. However, typical room acoustics more than offsets the loss due to distance, IME. When you consider that the typical THX speaker will produce 89dB/1w/1m under ideal conditions and 100 watts into 8 ohms is enough to reach 105dB at 12' (Ultra cert) in a typical room, you can see how much the room counts. Under anechoic conditions, that 89dB/1w/1m speaker needs 512 watts to hit 104dB at 4m!

-Brent
brent_s is offline

Old 03-10-09, 09:15 AM Thread Starter
Senior Shackster
George

Join Date: Jan 2009
Location: Lafayette, LA
Posts: 152
Re: Explain Me This

Thank you, Brent. I think the "playing field" is well defined by your explanation.

You said "Trying to guesstimate amp power using ideal math is a no win exercise." Does this mean that attempting to predict clipping, at least, in a narrow range, is a no win exercise, also?
joorge is offline
Old 03-11-09, 04:58 PM
Senior Shackster

Brent

Join Date: Feb 2007
Location: Wilmington, NC
Posts: 407
Re: Explain Me This

Quote:
joorge wrote: View Post
Thank you, Brent. I think the "playing field" is well defined by your explanation.

You said "Trying to guesstimate amp power using ideal math is a no win exercise." Does this mean that attempting to predict clipping, at least, in a narrow range, is a no win exercise, also?
Yes. The ideal theoretical model only works when applied to anechoic conditions. You can't predict power needed for a given SPL in a room without knowing how the room acoustics are modifying the theoretical response. With bass in room bass peaks and nulls, the exercise will be even more frustrating.

The good news with subwoofer output is you get the benefit of both boundary gain across all frequencies and pressure (room) gain below the corner frequency of the room. Boundary gain can be worth up to 6dB per surface (wall, floor, ceiling) across the freuqency spectrum. Pressure/room gain is as much as 6-8dB per octave below the room's corner frequency...generally the frequency with a half wavelength equal to the longest dimension of the room with parallel walls.

If you're just worried about not clipping your amp, you should notice the increased audible distortion before you actually hit clipping. Assuming you don't press play with the volume know already turned to "11". :-)

I'm still not sure you grasp how loud 100+dB is. Real world example My mains amp is rated at 130 watts per channel and my speakers are rated at 89dB/1w/1m. Conveniently, the amp has LED indicators that indicate when the amp is approaching 1% total distortion in any given channel. Just for curiousity's sake, I've turned up the volume using two channel music to the point the LEDs were just barely flickering on bass/kick drum peaks...SPL reading, a rather painful (IMO) 110ish dBs. The manual suggests the amp can actually be pushed hard enough to keep the LEDs lit solidy without driving the amp into shutdown...I can't begin to imagine where that limit is. BTW, the OSHA limit for 110dB exposure is 30 minutes before permanent hearing damage is expected.

-Brent
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Old 03-12-09, 12:56 PM Thread Starter
Senior Shackster
George

Join Date: Jan 2009
Location: Lafayette, LA
Posts: 152
Re: Explain Me This

Thanks brent. Got the info I was after. Again, I am not worried about my woofer. I was trying to determine if the propensity to clip could be accurately incorporated in the design process. I may want to try something more borderline in the future.

Believe it or not, I do have an idea what 110 dB is like. I am not one of them autopyles with magic ears, but I have a sound meter and use it. The proposition was never to intentionally play at 110 dB. Just trying to predict if those 30+dB excursions that are beyond my control would be harmful to a particular amp/speaker combo.

No worries, as there is significant info on this forum concerning particular combos.
joorge is offline

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