During normal conversation sound heard at 1 meter distance is-
**Core Concept**
The intensity of sound perceived by a person is directly related to the sound pressure level, which is measured in decibels (dB). The sound pressure level is the logarithmic ratio of the sound pressure to a reference pressure.
**Why the Correct Answer is Right**
The correct answer is 20 dB SPL (Sound Pressure Level) because it is the threshold of hearing for a normal adult at a distance of 1 meter. This is based on the concept of the sound pressure level, which is the logarithmic ratio of the sound pressure to a reference pressure. The sound pressure level of 20 dB SPL is the minimum sound pressure that can be perceived by a normal adult ear. This is also known as the absolute threshold of hearing.
**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because the sound pressure level of 20 dB SPL is the minimum sound pressure that can be perceived by a normal adult ear, not the maximum sound pressure level.
**Option B:** This is incorrect because the sound pressure level of 60 dB SPL is the level at which a person can just begin to feel the effects of noise, such as a slight annoyance, but it is not the threshold of hearing.
**Option C:** This is incorrect because the sound pressure level of 80 dB SPL is the level at which a person can feel a significant effect of noise, such as a moderate level of annoyance, but it is not the threshold of hearing.
**Option D:** This is incorrect because there is no specific sound pressure level for this option, and it is not a valid option.
**Clinical Pearl / High-Yield Fact**
The threshold of hearing is the minimum sound pressure level that can be perceived by a normal adult ear, and it is typically around 20 dB SPL at a distance of 1 meter. This is an important concept to remember in the field of audiology and hearing science.
**Correct Answer:** D. 20 dB SPL