SPHSC 461
Beats, Lateralization, and Masking Level Difference
I. Purpose
The purpose of this lab is to demonstrate effects that can occur when two tones are heard simultaneously, to demonstrate the auditory system’s use of interaural time and intensity differences in localization, and to demonstrate masking level difference.
II. Equipment
CD player, headphones, and Auditory Demonstrations CD:
Demonstration 32: Primary and Secondary Beats (tracks 62 and 63)
Demonstration 36: Binaural Beats (track 71)
Demonstration 37: Binaural Lateralization (tracks 72, 73, and 74)
Demonstration 38: Masking Level Difference (tracks 75, 76, 77, 78, and 79)
III. Orientation Points
A. Beats
are an important contributor to the sensation of dissonance in music, and
form an invaluable perceptual tool for the tuning of musical instruments.
Primary beats: If two pure tones have slightly different frequencies f1 and f2, where f2 = f1 + delta-f, the phase difference, p1-p2, changes continuously with time. The amplitude of the resultant tone varies between A1+ A2 and A1 - A2, where A1 and A2 are the individual amplitudes. These slow periodic variations in amplitude at frequency delta-f are called primary beats. Beats are easily heard when delta-f is less that 10 Hz, and may be perceived up to about 15 Hz.
Second-order beats: A sensation of beats also occurs when the frequencies of two tones f1 and f2 are nearly, but not quite, in a simple ratio. If f2 = 2f1 + x (mistuned octave), beats are heard at a frequency x. In general, when f2 = (n/m)f1 + x, mx beats occur each second. These are called second-order beats or beats of mistuned consonances because the relationship f2 = (n/m)f1, where n and m are integers, defines consonant musical intervals, such as a perfect fifth (3/2), a perfect fourth (4/3), a major third (5/4), etc.
Binaural beats: An important issue in the study of sound localization concerns the ear’s ability to process phase differences at the two ears. One way to study this phenomenon is to present two sinusoids of slightly different frequencies, one to each ear. At low frequencies the sound may appear to fluctuate or beat slowly at a rate equal to the frequency difference between the two tones.
Note that binaural beats are unlike the physical beats that can be heard by a single ear (as discussed above). There, the small difference in the two frequencies causes the physical stimulus to wax and wane in intensity. When the fluctuation is slow enough, the listener will experience a beating sensation. With binaural beats, the interaction between the two tones occurs because of some kind of interaction in the nervous system of the inputs from each ear. The best binaural beats occur at frequency separations of about 30 Hz near 400 Hz and much smaller frequency separations at the higher frequencies. No binaural beats are evident above about 1500 Hz.
B. Localization and lateralization — the most important benefit we derive from binaural hearing is the sense of localization of the sound source. Although some degree of localization is possible in monaural listening, binaural listening greatly enhances our ability to sense the direction of the sound source. Localization includes up-down and front-back discrimination, but most attention is focused on side-to-side discrimination or lateralization.
When we listen with headphones, we lose front-back information, so that lateralization becomes exaggerated; the image of the source appears to switch from one side of the head to the other by moving "through the head", or the sound source appears to be "in the head."
Low frequency sounds are lateralized mainly on the basis of interaural time difference, whereas high frequency sounds are lateralized mainly on the basis of interaural intensity differences.
C. Masking level difference: Any decrease (improvement) in the masked threshold obtained when two ears are used instead of one. Besides enabling us to localize sound sources, binaural hearing helps us to receive auditory signals in noisy environments.
IV. Lab Assignment
A. Demonstration 32: Primary and Secondary Beats (tracks 62 and 63)
1. Two tones having frequencies of 1000 and 1004 Hz are presented separately and then together.
2. When the tones were presented together did you hear beating? What was its frequency?
3. Next, pairs of pure tones are presented having intervals slightly greater than an octave, a fifth, and a fourth, respectively. The mistunings are such that the beat frequency is always 4 Hz when the tones are played together.
4. What did you hear?
B. Demonstration 36: Binaural Beats (track 71)
1. A 250 Hz tone is presented to the left ear while a 251 Hz tone is presented to the right ear.
2. What did you hear?
C. Demonstration 37: Binaural Lateralization (tracks 72, 73, and 74)
1. Listen to tones of 500 Hz and then 2000 Hz with alternating interaural phases of plus and minus 45 degrees.
2. What did you experience with the 500 Hz tone? And with the 2000 Hz tone? What does this indicate about the auditory system’s use of interaural time differences?
3. Next, listen to a 100 µs pulse (click) presented with an interaural time difference that cycles from 5 ms to - 5 ms.
4. What did you experience?
5. Finally, listen to tones of 250 Hz and 4000 Hz with varied interaural intensity differences.
6. What did you experience? What does this indicate about the auditory system’s use of interaural intensity differences?
D. Demonstration 38: Masking Level Difference (tracks 75, 76, 77, 78, and 79)
1. Condition 1: Listen to a stepwise decreasing 500 Hz tone in the left ear. Staircases are presented twice. Count the number of steps you hear.
2. Condition 2: Now listen to the signal masked with noise. Count the number of steps you hear.
3. Condition 3: Now listen to the tone masked with noise in both ears. Count the number of steps you hear.
4. Condition 4: Next, listen to the tone and noise applied to both ears. Count the number of steps you hear.
5. Condition 5: Finally, listen to the tone and noise applied to both ears, but with the signal phase reversed in one of the ears. Count the number of steps you hear.
6. In what conditions did you do the best? Why do you think you did better in these conditions?