Notes on Binaural Hearing
Hamill and Price (2014) report that to localize is to find the origin of sound around us (in space) whereas to lateralize indicates wearing headphones and determining whether the sound originated in the left or right ear, or inside the head. Binaural summation likely occurs in the brainstem and binaural thresholds are likely 2 to 3 dB better than individual ear thresholds. They report that "a suprathreshold monaural tone must be 3 dB to 6 dB more intense to be judged equally loud…" to a binaural tone of the same frequency. Of note, not all hearing aid fitting systems take binaural summation into consideration, thus, the audiologist may have to lower the gain of a binaural fitting, depending on the system chosen.
The authors note that loud sounds presented binaurally are indeed perceived louder (than monaural presentations) but of note, are better tolerated than loud sounds presented monaurally. Binaural beats occur when two tones are presented simultaneously. For example, if a 1000 Hz is presented to the left and 1004 Hz presented to the right ear, the listener is likely to perceive 1002 Hz that appears to pulse on/off four times per second. Binaural fusion is the idea that although the two ears generally hear very similar things, they are not identical, yet we perceive one fused (or unified) sound image.
Hamill and Price state there are two aspects of temporal (or time) phenomena that play a role in localization—time of arrival and phase differences. Of the two, phase is the more dominant factor with regard to localization, and phase is very important below 1500 Hz. Interaural timing difference (ITD) is the term often used to describe this phenomena whereas interaural loudness difference (ILD) is essentially due to the head shadow effect and becomes more prominent as frequency increases above 1500 Hz. ILDs can be as great as 10 to 20 dB. The authors report that "We tend to perceive sound as coming from the direction of the ear in which the sounds are louder and the ear which the sound reaches first…." The authors note human localization is worst at about 3000 Hz as phase cues have already dropped off and ILDs are not fully effective.
The advantages of binaural hearing includes improved hearing sensitivity, improved tolerance for loud sounds, improved ability to detect differences in intensity and frequency, improved localization, improved use of signal-to-noise ratio at the two ears, and improved listening in noise and more.
For More Information, References and Recommendations
Hamill TA, Price LL. (2014) The Hearing Sciences 2nd ed: Plural Publishing.