CHAPTER 4 COMPLEX STIMULI
CHAPTER 4
COMPLEX STIMULI
Types of Sounds
• So far we’ve talked a lot about sine waves
= periodic
= energy at one frequency
• But, not all sounds are like that
Periodic/Aperiodic Sounds
• Periodic -- Repeating regular pattern with a constant period
• Aperiodic-- no consistent pattern repeated.
Simple/Complex Sounds
• Simple -- Having energy at only one frequency
• have a sinusoidal waveform
• Complex -- Having energy at more than one frequency
• may be periodic or aperiodic
A Complex Sound
Looking at a Waveform
• You may not be able to tell much about frequencies present in the sound
• Another way of displaying sound energy is more valuable:
AMPLITUDE SPECTRUM--display of amplitude (y-axis) as a function of frequency (x-axis)
Waveform and Spectra
Harmonic Series
• When energy is present at multiples of some frequency
• Lowest frequency = FUNDAMENTAL FREQ
• Multiples of fundamental = HARMONICS
Transients
• Brief acoustic signals or “clicks”
Envelope--The Outline of the Waveform
One Interesting Envelope
• Amplitude Modulated (AM) Tone
• Tone whose energy is varied is called CARRIER
• You can also talk about the FREQUENCY OF MODULATION--How many times a second does amplitude cycle up and down and back again.
AM Tone: Waveform & Spectrum
Spectrum of an AM tone:
• Has Energy at 3 frequencies:
1. at the frequency of the CARRIER
2. at Carrier freq PLUS Modulation freq.
3. at Carrier freq MINUS Modulation freq.
Square Wave
• Similar to transient spectrum
• Fundamental frequency at reciprocal of period
• Can also be a modulator
• All harmonics are at a 90 degrees phase angle
Square Wave
Frequency Modulation (FM)
• Waveform shape changes like a spring
• Looking at TIME domain
Not Everything is so Regular
• Aperiodic sounds vary randomly
• = NOISE
• Waveforms may look wild
• EXAMPLE:
• White Gaussian Noise = equal energy at all frequencies
Gaussian Noise Waveform
Amp. Spectra: White & Pink Noise
Levels of a Band of Noise
• Overall Level = SPL (Total Power)
• Spectrum Level = Ls level at one frequency
• Bandwidth Level = Lbw freq width (in dB)
Lbw = 10 log (bandwidth (in Hz)/ 1 Hz)
• SPL = Ls + Lbw
Overall Level Equals Spectrum Level Plus Bandwidth Level
Lbw
Ls
SPL
Example of Deriving Ls
• Given SPL = 80 dB
• and Bandwidth = 1000 Hz
• Lbw = 10 log (1000Hz / 1Hz) = 30 dB
• SPL = Ls + Lbw
• 80 dB = Ls + 30 dB
• 50 dB = Ls
Combining Sound Sources
• Adding additional (identical) sources produces summing of intensities
• e.g., adding a second speaker playing the same signal
• If one produced 60 dB IL, what would two produce?
Working out the example:
• one produces 60 dB IL
• 60 = 10 log (Im/10-16 W/cm2)
• 6 = log (Im/10-16 W/cm2)
• 106 = Im/ 10-16 W/cm2
• 10 6 + (-16) = Im
• 10 -10 = Im
• 2 x 10 -10 = Intensity of two sources
• New IL = 10 log (2 x 10 -10 /10-16 W/cm2)
Working it out (cont’d)
• New IL = 10 log (2 x 10 -10 - (-16) )
• = 10 log (2 x 10 6)
• = 10 (6.3010)
• = 63 dB IL
How About a SHORT CUT?
• New IL = IL of OLD # + 10 log (new #/old #)
• = 60 + 10 log (2/1)
• = 60 + 3
• = 63 dB IL