CHAPTER ONE
Hearing and Hearing Loss: Basics of Noise
and its Measurement
Marshall Chasin, Au.D., M.Sc., Aud(C), Reg. CASLPO and Alberto Behar, P.Eng., CIH

Introduction

Sound is a marvelous energy. It can convey signals that warn like a horn, or that bring great joy or comfort like gentle words spoken from the heart. Sound is a vibration in any elastic medium (such as through air, water or steel). The beauty of sound is both in its simplicity and complexity. The simplest sound is a pure tone single frequency sine wave (~). Its complexity evolves when many sounds are mixed together, like the image on the cover of this book, or like the blending of many musical instruments in an orchestra. The instruments generate a magnificent array of frequencies that bring great pleasure to our ears, from subtle tones to loud crescendos. Conversely, imagine that an entire orchestra played completely off key. This would resonate through our ears and be perceived in our brains as a terrible noise (defined as unwanted sound), perhaps no different from a screen door in the wind banging against the house. It’s all noise.

Sound travels through air at 1,125 feet a second (which equates to about 768 miles an hour). In water it travels at approximately 4,400 feet a second depending on the water temperature (for example, 4,700 feet a second if the water temperature is 59° F). In the Old West, remember the movies where a character put his ear to the railroad track? He could hear the vibration of the train coming (16,000 feet a second in steel) long before he could see it or hear the sound traveling through air.

Noise is always sound, but sound is not always noise. Noise is a subjective interpretation of sound. Sound comes from many different sources and is comprised of many different elements, most of it adding richness to life, like speech and music. However, noise has progressively become a part of our lives in ways we would often like to avoid. Typically, noise carries no information. That is, there’s nothing meaningful about it. This helps to distinguish it from speech and music, yet really doesn’t explain why noise can be so bothersome and most forms of music are pleasant. For most of us, the sound of a rattle in the car is noise. A neighbor’s barking dog is noise. The racket of a cocktail party if you’re not part of it is noise. These are “random phase” events, a fancy way of saying there’s nothing meaningful about the vibration. When scientists at SETI (Search for Extraterrestrial Intelligence) aim their radio signal dishes up into the sky, they’re receiving many random phase signals that they ignore. In fact, a non-random signal might cause alarms to go off!

This characteristic of noise (random phase) allows us to focus on speech or music in the presence of background noise. We will leave the specifics about how this is accomplished to later chapters, but suffice it to say that the human brain is an amazing computer that allows us to understand speech in the presence of noise—to a point. We’ve all been in a noisy social gathering and regardless of our hearing ability, just could not distinguish what was being said. Now let’s turn to the receivers of the noise—our ears and brain—and the process of how we hear and interpret sound.