loudspeaker
Sensitivity:
Nominal speaker impedance:
Lowest speaker impedance:

Lowest supported frequency (at +/- 3dB):
Highest supported frequency (at +/- 3dB):
Maximum output volume:

listener
Distance to listener:
receiver
Desired average volume:
soundwaves
Expected peak dynamic range:
Sensitivity is the sound pressure level produced at 1 meter from the speaker, for a 1Khz frequency, at 1W of power. This is normally found at the specifications page for your speaker. Example: the Kef R3 meta has a sound pressure level (SPL) of 87db for a voltage of 2.83V. (be careful that if the impedance is not 8 Ohm, such as with the Kef R3, that sensitivity is still listed at 2.83V and not 2V)
The nominal impedance is normally found at the specifications page for your speaker. Example: the Kef R3 meta has a standard impedance of 4 Ohm. Note: the given impedance may be incorrect, due to manufacturers not obeying the standard IEC 60268-5 spec.
Minimum impedance is normally found at the specifications page for your speaker. The lowest speaker impedance (rounded down) should be at worst 80% of the nominal impedance, per the standard IEC 60268-5. If it is lower, your nominal impedance is probably rated too high. Example: the Kef R3 meta has a minimum impedance of 3.2 Ohm, which is 80% of 4 Ohm.
The frequency range is normally found at the specifications page for your speaker. Example: the Kef R3 meta has a frequency range at +/- 3dB of 58Hz - 28000Hz according to the vendor. However, in testing the lower range is more like 40 Hz. You should go with 40 Hz in that case.
The frequency range is normally found at the specifications page for your speaker. Example: the Kef R3 meta has a frequency range at +/- 3dB of 58Hz - 28000Hz according to the vendor, so please enter 28000.
This is the distance from the speaker to the listening position in meters. For a distance of 1 meter, the sound pressure will be equal to the sensitivity. Every time the distance from the speaker doubles, the sound intensity drops by 6 dB and the sound pressure lowers by 3 dB. This doubles the required Wattage to keep the same SPL.
The average volume at which you listen to music *as heard at the listening position*. If you are unsure, listen to music, and use a decibel meter app. Note that "sounds at or below 70 A-weighted decibels (dBA) are generally safe. Long or repeated exposure to sounds at or above 85 dBA can cause hearing loss." (CDC). For iOS devices there is an official app of the US CDC.
This indicates the expected difference in dB between the highest (peak) and the average volume for the music you like to listen to. This factor determines the maximum power required. Extreme DR can be found in experimental modern music, but virtually all actual music is under 24 dB dynamic range. Because most music lives at 15 DR at most, this is the default value.

Example DR for specific albums:
Queen - Bohemian Rhapsody12 dB
The Prodify - Music For The Jilted Generation13 dB
Pink Floyd - Delicate Sound Of Thunder (Disc 2)14 dB
J.S. Bach: Matthäuspassion (Velthoven)15 dB
John Cage: Concert For Piano And Orchestra / Atlas Eclipticalis24 dB
The maximum output volume is the limit it will accept. Example: the Kef R3 meta has maximum output volume of 110 dB SPL. Combined with the lowest impedance, this determines the maximum power it can draw from the amplifier without damage. For the Kef R3, this is 110dB at 3.2 Ohm (25% worse than 4 Ohm). Given its sensitivity it has a powerdraw of 256W@111dB, which translates to 256W * (1/1.26) * 1,25 for 254W @ 110dB worst case. However, this is at 1Khz. It can sustain this volume over the frequency range, so the worst case is at the bottom of the supported frequency range which in our case is 58Hz.
The loudness is color coded for the danger it represents. It can be flat, or weighted differently for each frequency according to the ITU-R 468 noise weighting function. This corrects for perceived loudness to mirror the way the ears get damaged by different volume for different frequencies.
 
Show loudness contour colours: