. . . Up to this point, we’ve been considering our ability to hear in noise as if we have only one ear working on this task. However, in most listening conditions, we listen with both ears, binaurally, and our two ears work well together to improve our ability to hear, particularly in noise. Our brain automatically combines and compares the input from both ears which allows us to locate where sounds originate in our environment (“Is the car coming from the left or the right?”) and to help us hear better in noise. One of the ways in which two ears work better than one alone is in the simple detection of sounds. When both ears are presented a sound, as in the real world environment where both ears are listening, our hearing acuity is better; lower sound levels are needed for a listener to just detect the presence of a sound than when only one ear is presented the sound. It’s rare that we actually listen with only one ear. It typically occurs when we’re listening through some sort of listening device, such as during a hearing test where sounds may be presented to the right or left side of headphones alone, or when talking on the telephone and listening with only one ear.

When both ears are presented with the same sound, detection of the sound is not only better, but sounds are also heard as louder. This phenomenon, termed binaural summation, in which sounds are louder when heard with both ears, is another instance where the decibel level of a sound and its loudness are different. As a demonstration that you can try for yourself, you can set the music from an MP3 player or another sound system to a fixed volume level and listen to it with just one earpiece. This can be one side of a headphone or one earbud. When you add the second earpiece, so that you’re listening binaurally, the music will sound louder, even though you haven’t adjusted the volume control. (It’s best to do this in a quiet room, so the background noise isn’t influencing your perception of the loudness of the music. It’s also a fairer demonstration if you can use monaural headphones.)

By comparing the information heard with two ears, the brain can determined where a sound is coming from. We can localize sound because sounds coming from different directions reach the two ears at slightly different times and at slightly different intensities (an exception can occur when sounds come from exactly in front or behind). The auditory system (your ears and brain) is sensitive to very small differences in the timing and intensity of sounds reaching your ears and can use these differences between ears, interaural (between the ears) time and intensity cues, to locate sounds in the horizontal plane. In the situation depicted in Figure 5-6, we can see how the talker’s voice will reach the listener’s left ear first because that ear is nearer to the talker.