Power required to produce THX reference levels - AVS Forum
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post #1 of 10 Old 08-21-2013, 12:06 PM - Thread Starter
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It seems as though one would require a power amp with significant capability ( >>> 500 watts/channel) or speakers that are very sensitive ( >95db) to produce THX reference levels of 85db with 20db headroom ( definition of reference levels on THX website).

The 20 dbs of headroom is the "key" here....most receivers/ amps can probably drive all average-sensitive, 6 or 8 ohm speakers to a level of 85db at 3-4 meters listening position, but to get to the +20db headroom, they need 100x more power.

Does this mean that these receivers/ amps would clip trying to produce such dynamic peaks in HT/music sources if playing at a reference level? Would most amps just drop out because of sensor-induced overload at that point?

I don't come close to listening to anything near reference level at my ~3 meter listening position, probably at 75db or so......given this, should my 110 watt/channel AVR be able to handle the dynamic peaks using average-sensitive speakers ( 86.5db/W/m in this case)?

Here's my math............

86.5 db/W/m

Tripling distance decreases intensity by 10 db, so at 3 m, level would be at 76.5 using 1 watt of power.

Increasing intensity by 20db requires 100x the power, so the level would be 96.5 ( the headroom) using 100watts.


(Do I pass??)
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post #2 of 10 Old 08-22-2013, 08:33 AM - Thread Starter
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Bump for ANY response.............
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post #3 of 10 Old 08-22-2013, 08:49 AM
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My normal listening volume for BD movies is @ -20db level (calibrated AVR, 0 db means reference level). My AVR goes to about +12db in max volume level. So there is about 32db head room. More than enough. 0 db reference is very very loud. If you need to listen at that level, something is wrong either with you or your setup.

The power requirement depends a lot of room size, speakers and other environmental factors. But most powers are consumed by subwoofers. So make sure your subs have a large enough power amp.
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post #4 of 10 Old 08-22-2013, 12:54 PM - Thread Starter
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Thanks, Foxbat.
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post #5 of 10 Old 08-22-2013, 01:07 PM
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Quote:
Originally Posted by padgman1Β View Post

It seems as though one would require a power amp with significant capability ( >>> 500 watts/channel) or speakers that are very sensitive ( >95db) to produce THX reference levels of 85db with 20db headroom ( definition of reference levels on THX website).

The 20 dbs of headroom is the "key" here....most receivers/ amps can probably drive all average-sensitive, 6 or 8 ohm speakers to a level of 85db at 3-4 meters listening position, but to get to the +20db headroom, they need 100x more power.

Does this mean that these receivers/ amps would clip trying to produce such dynamic peaks in HT/music sources if playing at a reference level? Would most amps just drop out because of sensor-induced overload at that point?

I don't come close to listening to anything near reference level at my ~3 meter listening position, probably at 75db or so......given this, should my 110 watt/channel AVR be able to handle the dynamic peaks using average-sensitive speakers ( 86.5db/W/m in this case)?

Here's my math............

86.5 db/W/m

Tripling distance decreases intensity by 10 db, so at 3 m, level would be at 76.5 using 1 watt of power.

Increasing intensity by 20db requires 100x the power, so the level would be 96.5 ( the headroom) using 100watts.


(Do I pass??)

I think your math is a little off. You lose 3dBs for every doubling of distance. So a speaker with an 86dB sensitivity rating at 1m would drop to 83 dB at 2m and 81db at 4m. Based on those numbers you would need 256W to drive the speakers to reference levels that includes the 20dB peaks at 4m. A little less at 3m.

Those are absolute numbers though. It doesn't take into consideration that "most" HT setups run the speakers set to small and frequencies below the crossover point are sent to the sub. That being the case, most setups will only require a small fraction of those wattage numbers since lower frequencies are what consumes the most power. That's why subwoofer amps are usually much more powerful than amps for the satellite speakers. This is the reason why receivers with 100W/CH are able to drive speakers to reference levels.
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post #6 of 10 Old 08-22-2013, 01:50 PM
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No, here is the correct math.
Quote:
When referring to measurements of power or intensity, a ratio can be expressed in decibels by evaluating ten times the base-10 logarithm of the ratio of the measured quantity to the reference level.
http://en.wikipedia.org/wiki/Decibel

Thus, to express a ratio of two intensities I2 and I1 as a difference in decibels, the equation you need is

L (dB) = 10 * log10 {I2 / I1}

where L is the difference in decibel units (like "+ 3.7 dB" or "-25.1 dB"), and log10 is the logarithm to the base 10 function. (In many places, such as the Microsoft Windows built-in calculator, the logarithm to the base 10 function is just labeled "log", and the logarithm to the base e or natural logarithm function, which we don't want here, is labeled "ln".)

We are considering a "model" where the sound intensity (power per unit area) varies as the inverse square of the distance from the source. This is the correct model for far-field measurements (distance large compared to physical dimensions of the speaker) in a space with no reflected sound, only direct sound from the source, i.e. "anechoic chamber".

For this model, we have (I1 = P0 / R1^2) and (I2 = P0 / R2^2), where the intensity I1 is measured at a distance of R1 from the source, and the intensity I2 is measured at a distance R2 from the source. So the equation becomes

L (dB) = -20 * log10 {R1 / R2}

Thus, for "doubling of distance", R1/R2=2, and the decrease of the sound intensity level (or sound pressure level (SPL)) in dB is -6.0 dB.

For tripling of distance, such as going from the 1m distance at which the loudspeaker sensitivity is specified to 3 m (9 ft 10 in), R1/R2=3, and the decrease of the SPL is -9.5 dB, which is pretty close to -10 dB, as stated by the OP.

There's a big HOWEVER, however wink.gif Your home theater or audio system isn't in an anechoic chamber, it's in some kind of "normal" room with a lot of reflected sound, so you need to account for "room gain" (assuming that the specified loudspeaker sensitivity is "anechoic"). That makes the final result "mushy" because you don't know exactly how much the room gain increases the SPL at "main listener position" in your room.

I've read opinions that typical room gain, averaged within the audio frequency range, is +2 dB. If you believe that your room gain is the typical +2 dB, then you should use the conversion factor -7.5 dB (= -9.5 dB + 2.0 dB) to convert from loudspeaker efficiency at 1 m, anechoic, to loudspeaker efficiency at 3 m, in your room with room gain.
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post #7 of 10 Old 08-22-2013, 02:05 PM
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Still another complication is that your receiver/amp will only ever need to hit the absolute peak level of 105 dB per channel for very short periods of time, because the movie soundtracks you are playing are made like that (if there is any content at all at the 105 dB level, it will only be brief transients). And your receiver or amp can output brief transients at significantly higher power levels (without clipping) than the average power at clipping. I believe this has to do with power supply capacitors charging and discharging - I don't have the expertise to give more details - a few AVS regulars know much more about this.
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post #8 of 10 Old 08-22-2013, 03:37 PM
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We need more info on the system loudspeakers..
1. Brand/model
2. Full-range?
3. Any powered subwoofer? If yes, brand/model
4. Identify the source stream? Digital? Lossy? Lossless?

Just my $0.02... πŸ‘πŸ˜‰
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post #9 of 10 Old 08-22-2013, 07:32 PM - Thread Starter
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Quote:
Originally Posted by M CodeΒ View Post

We need more info on the system loudspeakers..
1. Brand/model
2. Full-range?
3. Any powered subwoofer? If yes, brand/model
4. Identify the source stream? Digital? Lossy? Lossless?

Just my $0.02... πŸ‘πŸ˜‰

1. Arx A1b speakers for L/R 85db sensitivity or Vapor Audio Breeze for L/R 86.5db sensitivity ( I go back and forth with them for listening)

2. Both are monitor-type ( bookshelf) speakers

3. Subwoofer is PSA XV-15

4. Blu-ray/CD player is a Panasonic 220.
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post #10 of 10 Old 08-22-2013, 07:35 PM - Thread Starter
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Very informative posts, duc135 and Sonic Icons..........thanks:)
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