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/ W/cm 2 ) 6 = log (Im/ W/cm 2 ) 10 6 = Im/ W/cm (-16) = Im = Im 2 x = Intensity of two sources New IL = 10 log (2 x / W/cm 2 )

Working it out (cont’d) New IL = 10 log (2 x (-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 #) = log (2/1) = = 63 dB IL