Objective:

• Understand the use of decibels (dB) as a measure of loudness
• Measure the limit of human hearing vs. frequency

Resources

• • Decibel Meter
  • Computer and onlinetonegenerator.com
  • Speakers
  • Headphones

Background

The “loudness” of a sound can be thought of as the energy the sound wave deposits in a given area, or its intensity. Intensity is measured in Watts per meter squared. Human ears can detect sound over a wide range of intensity. The logarithmic decibel scale, referenced to the quietest sound detectable by human ears at I_0~=~1~\times~10^{-12}~\mathrm{Watts~m^{-2}}, provides a clearer representation of the relative “loudness” of a sound of intensity I.

SIL_{[\mathrm{dBm}]}=10 \log \frac{I}{I_0}.

Human hearing responsiveness varies by frequency, as can be seen in the diagram below. The contours show sounds of equal apparent loudness across frequency.

Using Sound Meters

Acoustical engineers organize the audio spectrum into sound octave bands. Sound level meters usually have different settings that add or subtract different values to the octave band components. The “A” weighted sound level discriminates against low frequencies, in a manner similar to the response of the ear. In this setting, the meter primarily measures in the 500 to 10,000 Hz range. It is the weighting scale most commonly used for OSHA and DEQ regulatory measurements. The “C” weighted sound level does not discriminate against low frequencies and measures uniformly over the frequency range of 30 to 10,000 Hz. This weighting scale measures the sound intensity level without the influence of the human auditory system. The sound levels measured with these two weightings have units of “dBA” and “dBC”, respectively.