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 floorstanding 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, divide by 3 to get an appoximation 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 efficencies, and are equidistant from the listening position.
5. Speaker placment. 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.
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 . 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 : (these are A-weighted, most home theater measurements are made with C weighting, which typically gives higer readings for broad spectrum measurements).
Exposure Level (measured in dB SPL)
90 dB SPL
92 dB SPL
95 dB SPL
97 dB SPL
100 dB SPL
102 dB SPL
105 dB SPL
110 dB SPL
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.