14-2-sound-intensity-and-sound-level_summary

In this section, students will learn about sound intensity, sound level, and how they relate to loudness and energy of a sound wave. The decibel scale for measuring sound intensity and solving problems involving the intensity of a sound wave are discussed. Students will also learn about how humans produce and hear sounds. The section includes various learning objectives related to the characteristics and behaviors of waves, and its role in medical and industrial applications. Key terms: Sound, properties of sound waves, characteristics of sound waves, amplitude, loudness, energy of a sound wave, decibel scale, sound intensity level, pressure amplitude, power, density, pitch, human hearing. Misconception Alert: Students may be confused between amplitude and sound intensity. While sound intensity is proportional to amplitude, they are different physical quantities. The intensity of a sound depends upon its pressure amplitude and the density of the medium through which it travels. The pressure variation (Δp) is proportional to the amplitude of the oscillation, and the intensity of a sound wave I is proportional to (Δp)². The decibel scale (β) for measuring sound intensity level is more relevant for how humans perceive sounds. The decibel level of a sound is defined as: 14.7 where I is sound intensity in watts per meter squared, and I0 = 10^(-12) W/m^2 is a reference intensity. I0 is chosen as the reference point because it is the lowest intensity of sound a person with normal hearing can perceive. Each factor of 10 in intensity corresponds to 10 dB. So, a 90 dB sound compared with a 60 dB sound is 30 dB greater, or three times as intense. In Snap Lab "Feeling Sound," students can explore how a sound wave's intensity affects the vibrations they feel when the volume of music is increased. They can also solve problems involving sound wave intensity in a worked example. Humans produce sounds by pushing air up through their lungs and through elastic folds in the throat called vocal cords, which produce a pressure buildup. As air travels past the vocal cords, it causes them to vibrate, and the vibration escapes the mouth along with puffs of air as sound. A voice changes in pitch when the muscles of the larynx relax or tighten, changing the tension on the vocal cords. A voice becomes louder when air flow from the lungs increases, making the amplitude of the sound pressure wave greater. Hearing is the perception of sound and can give us information about pitch, loudness, and direction. Humans can normally hear frequencies ranging from approximately 20 to 20,000 Hz. Sounds below 20 Hz are called infrasound, and those above 20,000 Hz are ultrasound. The perception of frequency is called pitch, and the perception of intensity is called loudness. The way we hear involves the ear converting sound waves into electrical nerve impulses, similar to a microphone. The ear divides into three parts: the outer ear or ear canal, the middle ear, which runs from the eardrum to the cochlea, and the inner ear, which is the cochlea itself.