Fundamentals of Sound PDF

1 - Fundamentals of Sound Sound Design - Games and Multimedia ©2024 Miguel Negrão CC BY-NC-ND 4.0 Sound What is sound ???  What is sound ? Sound is a pressure wave travelling through the air or other elastic media. Animation courtesy of Dr. Dan Russel...

1 - Fundamentals of Sound Sound Design - Games and Multimedia ©2024 Miguel Negrão CC BY-NC-ND 4.0 Sound What is sound ???  What is sound ? Sound is a pressure wave travelling through the air or other elastic media. Animation courtesy of Dr. Dan Russell, Grad. Prog. Acoustics, Penn State  Longitudinal wave vs Traverse wave L T Animation courtesy of Dr. Dan Russell, Grad. Prog. Acoustics, Penn State Physical properties of sound Pressure Amplitude Pressure amplitude The pressure amplitude is the difference between the (air) pressure at rest and the (air) pressure at compression. Pressure amplitude Atmospheric pressure at sea level = 101325 Pa Typical average pressure amplitude of the sound of a TV = 0,02 Pa Sound creates very tiny changes in pressure !!  Sound - Pressure amplitude Higher pressure amplitude means the sound is perceived as being louder.  P Frequency t Frequency Any phenomena which repeats itself in time, and always takes the same time between repetitions, is called periodic. Period: how many seconds does it take until the next repetition starts ? (s) Frequency: how many times does it repeat in one second ? (Hz) Frequency Example: If I clap every two seconds: Period ? Frequency ? Go to menti.com and enter 4993 5063 0s 1s 2s 3s 4s |----|----|----|----| |---------|---------| clap clap clap Frequency Example: If I clap every two seconds: Period: Frequency: Frequency Example: If a wheel spins 4 times every second Period: ? Frequency: ? 0s 1s 2s |-------------------|-------------------| |----|----|----|----|----|----|----|----| Go to menti.com and enter 4993 5063 Frequency Example: If a wheel spins 4 times every second Period: Frequency: Frequency Period: how many seconds does it take to repeat itself ? Frequency: how many times does it repeat per second ? Units: Frequency → Hertz or Hz or. Period → Frequency and Period Frequency Period 0,1Hz 10s 1Hz 1s 10Hz 0,1s 100Hz 10ms 1000Hz 1ms 10.000Hz 0.1ms What is the relation betwen sound and frequency ? What is the thing in sound which repeats n times per second ? How does it feel if that something repeats more or less times per second ? Sound and Frequency 1. Sound are variations of pressure. 2. Variations can be periodic or non-periodic. 3. If they are periodic they have a period: the time it takes for the waveform to repeat.  Sine wave: represents circular motion. It is the most fundamental waveform. Animation courtesy of Dr. Dan Russell, Grad. Prog. Acoustics, Penn State Phase Phase: time-shifting of the wave form P t P out of phase t Changing the phase corresponds to "delaying" the start of the wave Or moving the same waveform to left or to the right on the graph. Phase: time-shifting of the wave form P t P out of phase t The sum of two time-shifted sine waves is always another sine-wave with same frequency, but different amplitude and phase try it !: https://www.geogebra.org/m/km9rku34 (click on red button "show/hide y1 + y2" and move the slider for " ") Complex wave: that which is not a sine wave.  Every periodic wave can be decomposed into a sum of sine waves with different frequencies (and phases). ( Fourier analysis) These individual sine waves are called the harmonics of the fundamental frequency. Harmonics - multiples of a fundamental frequency 1st harmonic (x1) = 200Hz 2nd harmonic (x2) = 400Hz 3rd hamonic (x3) = 600 Hz...  Square Wave - sum of hamornics 1,3,5,... Animation courtesy of Dr. Dan Russell, Grad. Prog. Acoustics, Penn State Non-periodic waves: noise. Recap: types of waves Periodic Sine wave: one single frequency Complex wave: sum of harmonics of fundamental frequency. Non-periodic: noise Waveform and Spectrum Waveform: the shape of the wave. Waveform: time-domain analysis Spectrum: frequency-domain analysis Waveform: time-domain analysis Spectrum: frequency-domain analysis animation: Spectrum: frequency-domain analysis Amp 100Hz 200Hz 300Hz 400Hz 500Hz freq Periodic sounds only have frequencies which are multiples of the fundamental frequency (thanks Fourier !). Real-world sounds have some degree of noisyness (non-perioic wave), therefore they have a continuum of frequencies from lows to mids to highs. Other properties of sound: Timbre - that which makes a violin sound different from a guitar Duration Envelope Pitched - noisy. Harmonic - inharmonic. Correlation Intuitively measures the "similarity" of two signals. A sine wave and a copy slightly delayed are entirely correlated. A person singing and a vacuum cleaner are decorrelated. Decorrelation is very relevant for stereo. Stereo recordings of ambient sound have a lot decorrelation which gives a sense of space.  Lets get back to physics: what is the relatio between frequency and wave propagation ? Animation ISVR, University of Southampton Wave propagation Wave speed (v): the speed at which the pattern of the wave moves in space. It is the speed of sound. Wave length ( ): distance between two repetitions of the wave in space.  Speed of sound (in air) Depends mostly on temperature, but also altitude and humidity. Temperature Speed 0º C 331,4 m/s 15º C 340,5 m/s 30º C 349,6 m/s Wavelengths frequency wavelength 20Hz 17m 200Hz 1.7m 2000Hz 17cm 20.000Hz 1.7cm Doppler effect - moving source In the direction the source is going wave fronts are "compressed". In the opposite direction they "spread out". Animation courtesy of Dr. Dan Russell, Grad. Prog. Acoustics, Penn State Doppler effect - moving source Experiment with a car: https://www.youtube.com/watch?v=p-hBCcmCUPg Let's recap Video - Animagraffs How Speakers Make Sound https://www.youtube.com/watch?v=RxdFP31QYAg&t=4s Psychoacoustics  Psychoacoustics: How do humans hear ?  Psychoacoustics: Studies the relation between the characteristics of sound that enters the ears and the sensations that they produce. Limits of human hearing Frequency What is the highest sound humans can hear ? What is the lowest sound humans can hear ? Frequency range: Humans can hear sounds between 20Hz and 20.000Hz Other animals can hear in other ranges, for instance dogs can hear up to 45Khz. Frequency range: If we can move air in a cyclic way fast enough we will hear a sound. If we could move our hand back and forth faster than 20 times per second we could hear a sound. An insect beats the wings 600 times per second therefore we can hear the pressure wave created. Pressure differences due to weather changes are very slow, therefore we don't hear them.  Loudness What is the loudest sound humans can hear (before damage to the ears) ? What is the softest sound humans can hear ? Pressure amplitude range: Loudness is related with the physical variable "pressure amplitude". 1. The maximum pressure amplitude humans can hear is 2. The minimum is 20 µPa (micro) =. 3. - That is a very big range ! 4. 20Pa is 1.000.000x (million) bigger than 20µPa !!! Sound Level We can think of: A physical measurement. We can measure the pressure amplitude. A sensation. "Does this sound feel louder than that sound ?" Logarithms Smallest amplitude we can hear: 0,00002Pa. To avoid having to deal with numbers across such a big range we instead use the logarithm and the dB scale linear dB 1 0 0,5 -6 0,25 -12 0,12 -18 0,062 -24 0,031 -30 0,015 -36 0,0078 -42 0,0039 -48  Linear scale: every unit of distance corresponds to adding by the same amount (+ 1). Logarithmic scale: every unit of length corresponds to multiplying the previous value by the same amount (x 10). 0.4 0.6 0.7 0.9 40 60 70 90 5 7 9 0.2 0.3 0.5 0.8 2 3 4 6 8 20 30 50 80 0.1 1 10 100 Logarithms quick recap: What is the value of ? It's Logarithms quick recap: What is the value of ? It's if then Logarithms - properties Sound Pressure Level - SPL Measured in decibel - a dimensionless unit. Decibel always compares two values. SPL: compares the average pressure amplitude with the smallest average pressure amplitude we can hear. If you multiply a sound by a constant (i.e. amplify it), then the SPL will increase by summing a value: double the amplitude (x 2) → +6dB SPL dB values are summed. If the amplitude is first increased by 3dB and then by 5 dB, in total there was an amplitude increase of 8dB. If the amplitude is increased by 3dB and then _ decreased by 5dB (-5dB), in total there was an amplitude decrease of 2 dB. dB is also used for digital signals Sound pressure level can be measured with a device: a sound level meter Image: CC BY-SA 3.0 Wikipedia  Sound pressure Levels source dB SPL Jet engine 150 Threshold of pain 130 - 140 Risk of instantaneous noise-induced hearing loss 120 Music festival 100 - 120 Traffic on a busy roadway 80 - 90 TV (set at home level) 60 Normal conversation 40 -60 calm breathing 20 - 30 Auditory threshold at 1 kHz 0 Anechoic chamber, Microsoft -20,35 Courtesy of wikipedia under CC BY-SA 3.0 license. Humans can hear from 0db SPL to around 140 dB SPL perceiving loudness 1 dB is the smallest perceptible difference in loudness (in perfect acoustic conditions) In most situations and for most people 3 dB is the smallest perceptible difference in loudness. "Turn the music down, it's too loud !" What is loudness ? Loudness “ Loudness is defined as that attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud. “ An introduction to the psychology of hearing, Brian Moore. Sound pressure level is a physical attribute (measured with sound level meter). Loudness is a subjective perception. What is the relation between the two ?  Equal-loudness contours The perception of loudness depends on frequency First study: Fletcher and Munson (1933). 130 (estimated) 120 110 100 phon 100 Sound Pressure Level (dB SPL) 90 80 80 70 60 60 50 40 40 30 20 20 10 (threshold) 0 -10 10 100 1000 10k 100k Equal-loudness contours (red) (from ISO 226:2003 revision) Original ISO standard shown (blue) for 40-phons Percepetion of loudness As a simplifaction it can be said that a 10dB increase in SPL causes the perception that the loudness has doubled. (+10dB → x3.16) Perception of pitch Think of the piano keys on the right have a higher pitch. keys on the left have a lower pitch. Perception of pitch Pitch may be defined as “ that attribute of auditory sensation in terms of which sounds may be ordered on a scale extending from low to high. “ (ANSI, I 994b) Perception of pitch If a sound is mostly noise a pitch is not perceived. If a sound is mostly periodic a pitch is perceived. We can distinguish two notes which have different pitches. A C or a D in a piano sound different. Harmonic sounds Multilpe Sine tones which are multiples of a fundamental frequency are heard as a single sound. We are incapable of hearing the each harmonic separately. Animation courtesy of Dr. Dan Russell, Grad. Prog. Acoustics, Penn State  Masking: When one sounds influences the perception of another sound.  Simultaneous masking If a sine-tone is played together with a noise signal, if the noise signal is loud enough the sine tone is not heard. Summary Sound is a pressure wave moving through the air (or water, etc). Waves are characterized by their amplitude. Periodic waves have a fundamental frequency and harmonics(2x, 3x,...). - The most important periodic wave is the sine wave. Waves with some noisiness have a continuum of frequencies from lows to mids to highs. A sound with higher pressure amplitude is perceived as louder. Frequency is related with pitch (notes Dó, Ré, Mi). Standard A (Lá) is 440Hz. A standard A in a guitar or piano will have many frequencies besides 440Hz which determine its timbre. Resources for review: Online: Digital Sound and Music - 2.1 Digital Sound and Music - 4.1 (pdfs available in moodle) Resources for review: Paper: Português: "Introdução à Engenharia de Som"; Nuno Fonseca; FCA; 2012 - Capítulo 1 English: "Modern Recording Techniques"; David Miles Huber, Robert E. Runstein; Focal Press; 8th edition; 2013 - Chapter 2