Notes 4: Introduction to Sound Chapter 11 Section 2 Page 327

What You’ll Learn Recognize how amplitude, intensity, and loudness are related. Explain the relationship between frequency and pitch.

Why It’s Important Each property of a sound wave affects how things sound to you - from your blaring MP3 players to someone’s whisper.

Review Vocabulary Compressional Wave - a wave for which the matter in the medium moves back and forth along the direction that the wave travels. Amplitude - a measure of energy carried by a wave. Frequency - the number of wavelengths that pass a fixed point each second; is expressed in hertz.

Sound Waves Sound waves are compressional waves. When an object like a speaker vibrates, it collides with nearby molecules in the air, transferring some of its energy to them. These molecules then collide with other molecules in the air and pass the energy on to them.

Speed of Sound in Different Mediums Sound travels pretty fast. In fact while I am speaking to you, the sound wave originating from my mouth is travelling around 347 m/s (or 776 mph!) However, sound waves travel at different speeds in different mediums: Speed of Sound in Different Mediums Medium Speed of Sound in m/s Air 347 Cork 500 Water 1,498 Brick 3,650 Aluminum 4,877

Intensity and Loudness Imagine you are sitting in the passenger seat of a car and the radio is turned up pretty loud to a good song. Now imagine you need to tell the driver to take a turn. In order to make sure the driver hears you, the radio is turned down. What did you do to the sound waves? The song on the radio sounds the same, but now it is softer. We say that you have changed the intensity or the loudness of the sound wave. Intensity and loudness are represented by the amplitude of a wave.

Intensity Intensity - amount of energy that flows through a certain area in a specific amount of time. Intensity determines how far away a sound can be heard. When you whisper a conversation, sound waves have low intensity and do not travel far. When you shout, sound waves have high intensity and travel further. Intensity decreases with distance.

Loudness Loudness - human perception of sound intensity. Loudness can vary from one person to another, but the higher the intensity is the louder the sound is. Decibel - unit for sound intensity; abbreviated dB.

Pitch If you have ever taken a musical class, you are familiar with the musical scale: Pitch - how high or low a sound seems; related to the frequency of the sound waves. Pitch is represented by the frequency of a wave.

Musical Notes Every note has a different frequency, which gives it a distinct pitch.

Summary Sound waves are compressional waves. Tight, dense compressions in a sound wave mean high intensity, loudness, and more energy. Sound intensity is measured in decibels. High frequency sound waves have closer compressions and rarefactions and higher pitch than those of low frequency.