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From "Physics, Waves, Music": a website about the physics of waves and sound, acoustics of musical instruments, musical scales, harmony and music.

  1. Generalities on waves and sound
    1. What is the difference between sound and noise?
    2. Why do we talk about the principle of wave superposition and not the law of wave superposition?
    3. If two sine sound waves in phase opposition "cancel each other out", then where does the energy of each wave go?
    4. In ripple tank images with two sources, does constructive interference correspond to the bright zones or to the dark ones?
    5. If sound is a wave, is it possible to build acoustic "lenses" or "mirrors"?
    6. Can sound be focused with a "zone plate", as is done with light?
    7. Does sound always propagate in a straight line?
    8. Is it possible to deflect a sound with another sound?
    9. Do sound "mirages", like optical mirages, exist?
    10. Why does lightning create the sound of thunder?
    11. Why is it that when lightning strikes nearby we hear a sharp sound but when it strikes far away, the sound is not just weaker but also longer and blurred?
    12. What would someone hear while moving away from an orchestra faster than sound?
  2. Perception of sound
    1. Why can we hear a sound with one ear (e.g. the left) when it is actually coming from the direction of the other (e.g. the right)?
    2. What is the purpose of having two ears?
    3. How far out of tune must a note be for us to notice?
    4. Why is an E "the same" E even in different octaves?
    5. Why can one word in a recording sound higher than another, even though its fundamental frequency is lower?
    6. Are there "lower harmonics", that is sounds at sub-multiple frequencies, generated by the ear?
    7. Do two instruments playing the same note at different distances from the listener seem to have different pitches?
  3. Unit of measurement
    1. What unit of measurement is the decibel?
    2. Why should we complicate our lives with a logarithmic scale?
    3. Why are decibels more commonly used than Bels?
    4. Why do scales in dB all have a factor of 10 in their definition?
    5. Are there different types of decibel scales?
    6. Why does the sound pressure level (SPL) scale have a factor of 20 instead of 10 in its definition?
    7. Why are there sometimes scales with negative dB on apparatuses?
    8. Why is the level ± 3 dB highlighted on some apparatuses?
    9. Does a 20 dB sound have double the intensity of a 10 dB sound?
    10. Why do we measure the angles in radians?
    11. How do you read a spectrogram?
    12. Why does the spectrum calculated by a program such as Audacity change when I change the size of the window?
    13. What does it mean that the A of the tuning fork corresponds to 440 "double vibrations" per second?
  4. Characteristics of sound
    1. Does an environment with absolutely no sound exist?
    2. What is the loudest possible sound that can be produced?
    3. What is the relationship between the frequency and the pitch of a sound?
    4. If a sound is composed of many harmonic sounds of different frequencies, which harmonic determines the sound pitch that is perceived?
    5. Is it true that sound intensity is proportional to the square of pressure? Why?
    6. Frequency and wavelength are linked: why then, going from air to water, does the wavelength change but not the frequency?
  5. Architectural and environmental acoustics
    1. Why is it that when we put our heads underwater in the bathtub, we hear the noises in our home better?
    2. Why is it that we seem to sing better in the shower?
    3. Is it true that if we place our ear on a train track, we will hear the approaching train sooner than if we were to hear it in the air?
    4. I’ve seen perforated panels hanging in some concert halls. What purpose do they serve?
    5. Can outside noise make a window pane vibrate? And does double glazing help?
    6. Why does an echo repeat only the last syllable of a word?
    7. Do noise barriers with a rounded or shaped top edge reduce diffraction?
  6. Resonance
    1. How does the body of a string instrument make the sound produced by its strings louder?
    2. Is it true that a bridge can collapse after relatively mild stresses due to the phenomenon of resonance?
    3. What are the possible practical applications of the phenomenon of resonance? Are there any devices that exploit this phenomenon?
    4. If resonance amplifies sound, how can a Helmholtz resonator in an exhaust silencer reduce noise?
    5. Is it possible to shatter a glass with only the force of the human voice?
    6. The resonance wavelength of an open tube is equal to twice its length. Is it possible to build a smaller resonator with the same frequency?
  7. Stationary waves
    1. Seeing that a stationary wave does not propagate, how is it a wave?
    2. The frequencies of a string are integer multiples of the fundamental, with no frequencies in between: is this a "quantum" phenomenon?
  8. Beats
    1. How can we listen to the phenomenon of beats in a real instrument?
    2. Why have I never heard the phenomenon of beats between two different instruments?
    3. Why does a slightly mistuned fifth or third produce beats, even though the two notes have very different frequencies? Does it also happen with pure tones?
    4. Are there beats even if the two sounds have different amplitudes?
  9. Technology
    1. I know that two pure sounds with opposite phases cancel each other out; but is it really possible to cancel a sound by adding one with the opposite phase to it?
    2. What are the parameters that determine "good quality" in Hi-Fi system speakers?
    3. What are a "tweeter" and a "woofer" and what are they used for?
    4. Why do home-theatre systems have many small "speakers" instead of one large one in the centre?
    5. What is "Dolby" used for?
    6. Why is it that when I hear my recorded voice it sounds so different than my real voice?
    7. I’ve heard that old Hi-Fi systems with valves are still the best. Is this true?
    8. Can a weak sound, such as a heartbeat, be amplified with a cone or a megaphone?
  10. Musical instruments
    1. The voice
      1. Why do our voices sound so silly after we have inhaled helium from a balloon?
      2. Is it possible to sing two notes simultaneously?
      3. Why is it that when a soprano sings in a high register, it is very difficult to distinguish the vowel sounds being sung?
      4. Why are we able to distinguish vowel sounds if we pronounce them all with the same intonation?
    2. Wind instruments
      1. Is it true that sound also reflects at the open end of a tube and not just at the closed end?
      2. Why do the flute, clarinet and oboe play in completely different registers even though they are almost the exact same length (about 60 cm)?
      3. How important is the material with which a wind instrument is made?
      4. Are the vibrations one feels on the body of a trumpet wasted energy?
      5. Is it possible to play two notes simultaneously with one wind instrument?
      6. If a free reed, like that of an accordion, is fitted in place of the head joint of a recorder, why does the note not change when the holes are opened and closed?
      7. When the oboe gives the A to the orchestra, is it the reed that vibrates at 440 Hz?
      8. Can a saxophonist change the timbre with the mouth and throat?
    3. String instruments
      1. Why do all string instruments have a concave body?
      2. How important is the material with which a string instrument is made?
      3. Why is it that when we rub, pluck or strike the same string, we get different sounds even if it is the same note?
      4. Why does plucking a string at different points give different timbres? Is it true that some harmonics disappear?
      5. The bow excites the string with many frequencies: why do we hear only the harmonics of the string?
      6. If we use harmonics to tune an instrument, will we always get good tuning?
      7. We can create natural and artificial harmonic sounds on a violin and a guitar. How are they different?
      8. Why are the expressions "trumpet in B flat" and "tenor saxophone in E flat" used but not the expression "violin in B flat"?
      9. What is "Tartini's third sound" and how is it obtained?
      10. Why are some guitar strings made from nylon while others are covered with metal?
    4. Percussion instruments
      1. Why don't all percussion instruments produce a precise note the way string instruments do?
      2. Why are some percussion instruments, such as the timpani, tuned while others, such as the cymbals, are not?
      3. In a snare drum, why does the frequency change depending on whether the two heads vibrate in phase or in antiphase?
      4. Why does cutting a tubular bell in half not give the same note an octave higher?
      5. How does a handpan work, and how are its notes tuned?
      6. How are the bars of a vibraphone or a marimba retuned?
      7. Can the frequency of a drum be calculated from its dimensions?
  11. Musical scales
    1. What are the rules for generating the Pythagorean scale? How is the circle of fifths made?
    2. Does the fifth correspond to the ratio 3:2 or 2:3?
    3. Why is it not possible to close the circle of fifths using only natural fifths?
    4. Why is a unclosed circle of fifths a problem?
    5. Can a piano be tuned in equal temperament but with just fifths and fourths?
    6. Can a piano be tuned with A at 432 Hz?
    7. Why does the temperate scale contain 12 notes?
    8. How are octaves numbered in the Italian and the scientific notation?
"Physics, Waves, Music": a website about the physics of waves, acoustics of musical instruments, musical scales, harmony and music.