There is really no standard distance from the sound source where the sound pressure level (SPL) is measured.
Sound is measured in decibels
Loudness of sound is measured in decibels (dB).
intensity
The loudness of sound is measured using the "decibel" or "bel" unit.
Sound is normally measured is naturally measured in decibels (dB) which is a logarithmic scale.
decibels
The loudness of a sound depends on the amount of energy it takes to make the sound and the distance from the source of the sound.
Units used to measure loudness are Sone (loudness N) and Phon (loudness level L). Scroll down to related links and look at "Loudness - Wikipedia" and "Sound level change and the change of the respective factor of volume and loudness"
Feelings of loudness are different from person to person. So don't talk of "measuring" the "loudness".For psycho acoustic values we have units to express loudness:Sone (loudness N) and Phon (loudness level L).You can really measure the sound pressure with a sound pressure level meter. That is measured in dB SPL. Loudness is a subjective measure but sound pressure is an objective measure. But that is different. Don't mix it up. Scroll down to related links and look for more on decreasing of sound pressure with distance.
Feelings of loudness are different from person to person. So don't talk of "measuring" the "loudness".For psycho acoustic values we have units to express loudness:Sone (loudness N) and Phon (loudness level L).You can really measure the sound pressure with a sound pressure level meter. That is measured in dB SPL. Loudness is a subjective measure but sound pressure is an objective measure. But that is different. Don't mix it up. Scroll down to related links and look for more on decreasing of sound pressure with distance.
The loudness of a sound depends on the amount of energy it takes to make the sound and the distance from the source of the sound
Just like any other substance in existence, the magnitude of sound or loudness can also be measured. In this case, the standard unit of measurement for loudness is called decibels.