resonance, revisited march 4, 2013 leading off… project report #3 is due! course project #4...
TRANSCRIPT
Leading Off…• Project report #3 is due!
• Course Project #4 guidelines to hand out.
• Today: Resonance
• Before we get into that: dogs talking!
• And cats, too!
Where were we?• Shorter windows give us:
• Better temporal resolution
• Worse frequency resolution
• = wide-band spectrograms
• Longer windows give us:
• Better frequency resolution
• Worse temporal resolution
• = narrow-band spectrograms
• Higher sampling rates give us...
• A higher limit on frequencies to consider.
“Band”?• Way back when, we discussed low-pass filters:
• This filter passes frequencies below 250 Hz.
• High-pass filters are also possible.
Band-Pass Filters• A band-pass filter combines both high- and low-pass filters.
• It passes a “band” of frequencies around a center frequency.
Band-Pass Filtering
• Basic idea: components of the input spectrum have to conform to the shape of the band-pass filter.
Bandwidth• Bandwidth is the range of frequencies over which a filter will respond at .707 of its maximum output.
bandwidth• Half of the acoustic energy passed through the filter fits within the bandwidth.
• Bandwidth is measured in Hertz.
Your Grandma’s Spectrograph
• Originally, spectrographic analyzing filters were constructed to have either wide or narrow bandwidths.
Narrow-Band Advantages• Narrow-band spectrograms give us a good view of the harmonics in a complex wave…
• because of their better frequency resolution.
modal voicing EGG waveform
Narrow-Band Advantages• Narrow-band spectrograms give us a good view of the harmonics in a complex wave…
• because of their better frequency resolution.
tense voicing EGG waveform
Comparison• Remember that modal and tense voice can be distinguished from each other by their respective amount of spectral tilt.
modal voice tense voice
The Other Half• Answer: we filter the harmonics by taking advantage of the phenomenon of resonance.
• Resonance effectively creates a series of band-pass filters in our mouths.
+ =
• Wide-band spectrograms help us see properties of the vocal tract filter.
Formants• Rather than filters, though, we may consider the vocal tract to consist of a series of “resonators”…
• with center frequencies,
• and particular bandwidths.
• The characteristic resonant frequencies of a particular articulatory configuration are called formants.
Wide Band Spectrogram• Formants appear as dark horizontal bars in a wide band spectrogram.
• Each formant has both a center frequency and a bandwidth.
formants
F1
F2
F3
Narrow-Band Spectrogram• A “narrow-band spectrogram” clearly shows the harmonics of speech sounds.
• …but the formants are less distinct.
harmonics
Questions1. How does resonance occur?
• And how does it occur in our vocal tracts?
2. Why do sounds resonate at particular frequencies?
3. How can we change the resonant frequencies of the vocal tract? (spectral changes)
Some Answers• Resonance:
• when one physical object is set in motion by the vibrations of another object.
• Generally: a resonating object reinforces (sound) waves at particular frequencies
• …by vibrating at those frequencies itself
• …in response to the pressures exerted on it by the (sound) waves.
• In the case of speech:
• The mouth (and sometimes, the nose) resonates in response to the complex waves created by voicing.
Traveling Waves• Resonance occurs because of the reflection of sound waves.
• Normally, a wave will travel through a medium indefinitely
• Such waves are known as traveling waves.
Check out the tsunami!
Reflected Waves• If a wave encounters resistance, however, it will be reflected.
• What happens to the wave then depends on what kind of resistance it encounters…
• If the wave meets a hard surface, it will get a true “bounce”
• Compressions (areas of high pressure) come back as compressions
• Rarefactions (areas of low pressure) come back as rarefactions
Wave in a closed tube• With only one pressure pulse from the loudspeaker, the wave will eventually dampen and die out
• What happens when:
• another pressure pulse is sent through the tube right when the initial pressure pulse gets back to the loudspeaker?
Standing Waves• The initial pressure peak will be reinforced
• The whole pattern will repeat itself
• Alternation between high and low pressure will continue
• ...as long as we keep sending in pulses at the right time
• This creates what is known as a standing wave.
Lenticular Clouds• An interesting example of standing waves that we can often see around Calgary is called a lenticular cloud.
• These are formed by air (wind) bouncing in waves over the mountains.
• When the air reaches a certain height, it will condense into clouds. After it drops back down, the condensation will disappear.
• The clouds stay in one place while the wind passes through them! (check out http://www.atoptics.co.uk)