Speaker Sensitivity: dB SPL (1 W/1 M)
Amplifier Power: Watts
Distance: feet
No. of Speakers: usually 2, more for multi-channel
Speaker Placement (Choose 1): Away from walls (or do not consider placement)
Near a wall (within 2 to 4 feet)
In a corner (within 18 to 24 inches)
dB Gain from amplifier
dB Loss due to dispersion (distance)
dB Gain from sonic reinforcement (multi speakers)
db Gain from placement (reinforcement from reflected sound)
dB SPL at listening postion
Reference [4}

This calculator will compute the predicted maximum sound pressure level (loudness) at the listening position.  Four pieces of information are required, plus one optional input:

1.  The speaker sensitivity, typically expressed in decibels (dB) with 1 watt (or 2.83 volts across an  8 ohm speaker) measured on-axis one meter away.  Typical values are 85-89 dB for bookshelf speakers, 87 to 92 dB for floor standing models, with high efficiency speakers in the 93 to 100+ dB range.  If you are not sure, try 90 dB as a default. (Pro equipment has sensitivities as high as 111 dB!)

2.  The amplifier rated power per channel, expressed in watts. The calculated SPL will be RMS if amplifier power is RMS (what does this mean?)

3.  The distance from the speaker to the listening position.  Expressed in feet.  If you know the distance in meters, multiply by 3 to get an approximation in feet.  

4.  The number of speakers in the system.  For stereo, use 2.  For multichannel systems, try using 3 (for left,  center, and right).  This calculation assumes that the speakers are each driven by the same power, have similar efficiencies, and are equidistant from the listening position.

5.  Speaker placement.  You may account for "room gain", that is sound reflected from the walls that "reinforces" the sound coming directly from your speakers, by selecting the box that describes your speaker placement.  This effect is most prominent at low (bass) frequencies and accounts for why subwoofers often sound louder when placed in a corner.  This is an optional selection. If you do not have a subwoofer, or full range speakers with good bass extension close to the walls or corners; you will get a more accurate number by leaving the default.

This calculator does not account for room acoustics, amplifier dynamic headroom or off axis listening positions.  For interpretation of the results, see below.

Try it out!
You can download the MS Excel version here.
How Loud is That?

Most home theater speakers and ampliers, even modest ones, are quite capable of producing sound pressure levels in excess of 100 dB.  So how loud is that?  For some analogies, check out this table:
Description Sound Level Example Sound Intensity
barely audible 0 dB Threshold of hearing 0.000000000001 W/m2
  10 dB Rustling leaf 0.00000000001 W/m2
very quiet 20 dB Quiet room 0.0000000001 W/m2
  30 dB Soft whisper 0.000000001 W/m2
quiet 40 dB Quiet library 0.00000001 W/m2
  50 dB Average home 0.0000001 W/m2
moderately loud 60 dB Ordinary conversation, Light traffic 0.000001 W/m2
  70 dB Vacuum cleaner, Heavy traffic 0.00001 W/m2
very loud 80 dB Garbage disposal 0.0001 W/m2
  90 dB Diesel truck (10 m away) 0.001 W/m2
uncomfortably loud 100 dB Newspaper press 0.01 W/m2
  110 dB Jet flyover at 300 m 0.1 W/m2
painful 120 dB Threshold of pain, Thunderclap 1 W/m2
Reference: [1]
OK, So what's this mean for my hearing?

Sounds less than 75 dB have no significant effect on our hearing. 

Exposure to sounds above 85 dB causes short term hearing losses called temporary threshold shifts [2]. If this occurs, your ear becomes less sensitive, and sounds seem quieter than normal. After some time, normal hearing returns. 

Repeated exposure to sounds that cause temporary threshold shifts results in permanent damage to the ear in the form of a permanent threshold shift . The ear loses sensitivity in the frequencies 3,000 Hz through 6,000 Hz, resulting in a "notch" in the hearing range. Time of exposure is important, the louder the sound, the less exposure time before permanent damage sets in.
According to OSHA, the exposure time limits are [3]:  (these are A-weighted, most home theater measurements are made with C weighting, which typically gives higer readings for broad spectrum measurements).
Exposure Time Exposure Level (measured in dB SPL)
8 Hours 90 dB SPL
6 Hours 92 dB SPL
4 Hours 95 dB SPL
3 Hours 97 dB SPL
2 Hours 100 dB SPL
1.5 Hours 102 dB SPL
1 Hours 105 dB SPL
.5 Hours 110 dB SPL
<.25 Hours 115 dB SPL
Get to the point...

If you are like many HT enthusiasts (me included), you like to crank it on up during the "good parts" to really feel the action.  Look at your results from the Peak SPL Calculator compared with the OSHA guidelines.  Most moderate HT systems are capable of  102 to 107 dB peaks at a decent (10+ feet) listening position, at these peak levels the normal dialog is going to be at least 85-90dB, within the range where permanent hearing loss can occur.

Your hearing is precious, take care of it.
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