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Unit 5: Speech Perception

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Western University
Psychology 2134A/B
Marc Joanisse

Unit 5 Speech Perception Overview y Perceiving auditory speech o Physics of sounds o Pectrograms y Acoustic cues of speech y Recognizing auditory words o Models theories Recognizing Spoken Words Humans are very good at recognizing speech We can understand all the speakers of our language even people weve never heard before We do this in a way that is quite effortless and online that is rapidly and without overtly controlling how quickly the speaker is producing speech What is interesting is that scientists have had a hard time programming computers to do the same task Even with very fast computers and over 60 years of engineering we still dont have computers that can understand natural speech in the same way humans do Instead computer speech systems are only good at recognizing a few words at a time and are poor at compensating for different accents and noisy environments What makes speech such a complicated task The Nature of Sound Waves To answer this we must first explore how sound works We can describe sounds as vibration traveling through the air The air is being compressed and rarefied very rapidly and our ears pick this up as sound Sound can be described as a wave or ripple that is traveling through the air If we analyze a simple tone on a computer display we see something like thisHere the X axis is time and the Y axis is the amount of energy being transmitted The waveform oscillates up and down over time The rate with which this oscillation happens is called the frequency The amount of energy that is being transmitted roughly how much the air is being compressed by this wave is the amplitude We perceive the amplitude as the volume of a sound we perceive the frequency as the pitch of a sound Frequency is measured in Hz cycles per second Higher frequencieshigher pitchAmplitude Increasing the amplitude of a sound wave increases its volume or loudnessAcoustics of Speech The examples above are of simple tones Speech waveforms turn out to be much more complex For instance here is the waveform for the word dogInstead we prefer to analyze the acoustic properties of speech signals by looking at a spectrogram This shows us the amplitude of a waveform at different frequencies The image below depicts a sound on a spectrogram The X axis is time The Y axis is frequency So we are plotting the frequency of a sound that changes over time first it increases then it decreasesLearning check If you click the picture above you can hear the sound that is being depicted in this spectrogram But before you click on it think about what this should sound likeNow what about speech Ive illustrated a speech spectrogram belowAs you can see it is quite a bit more complex instead of having single acoustic components you will see that at any given time there are different frequencies that are active These areas of concentrated acoustic
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