Popular Science | 182 Acoustics Facts You Should Know (Part 1)

Sound is everywhere, yet it's easily overlooked. Do you know what sound coloration is? Do you know what the binaural effect, Haas effect, de Boer effect, and masking effect are? Do you know what a dead spot in a hall is? Do you know what...

These basic acoustic facts aren't complicated, but they haven't been widely disseminated. Many acoustic professionals may not even know them. If you want to expand your knowledge, take a look at these basic acoustic facts.

1. The frequency range that the human ear can hear is 20-20 kHz.

2. Devices that convert sound energy into electrical energy, such as microphones.

3. A device that converts electrical energy into sound energy is called a loudspeaker, such as a horn.

4. If the audio system exhibits acoustic feedback howling, adjust the equalizer as needed.

5. If the room reverberation time is too long, the sound will become muddy.

6. If the room reverberation time is too short, the sound will sound dry.

7. If your voice sounds too dry when singing, you should adjust the reverb.

8. If your voice sounds muffled when you speak, it may be because a reverb effect has been added.

9. The three elements of sound are intensity, pitch, and timbre.

10. The objective evaluation scale corresponding to sound intensity is amplitude.

11. The objective evaluation metric corresponding to pitch is frequency.

12. The objective evaluation scale corresponding to timbre is the spectrum.

13. The loudness of a sound stimulus perceived by the human ear is related to the frequency of the sound vibration.

14. The loudness perceived by the human ear for high sound pressure levels is not significantly related to frequency.

15. The human ear is most sensitive to mid-frequency sounds.

16. The human ear is less sensitive to high-frequency and low-frequency sounds.

17. The loudness perceived by the human ear for low sound pressure levels is greatly related to frequency.

18. In equal loudness curves, each curve shows a different sound pressure level at different frequencies, but the loudness perceived by the human ear is the same.

19. In equal-loudness curves, the numbers marked on each curve represent the loudness level.

20. The voltage gain of an amplifier, expressed in decibels, is 20lg(output voltage/input voltage).

21. The unit of loudness level is phon.

22. The dB value measured by the sound level meter represents the weighted sound pressure level.

23. Timbre is determined by the waveform of the sound produced.

24. The time required for a sound signal to drop from a steady state to 60 dB is called the reverberation time.

25. The basic elements of musical sound are melody, rhythm, and harmony.

26. The maximum instantaneous moment of a sound wave is called amplitude.

27. The number of vibrations per second is called frequency.

28. If a sound is as loud as a selected 1kHz pure tone, the sound pressure level of the 1kHz pure tone is defined as the loudness of the sound being measured.

29. The human ear is most sensitive to sounds in the 1-3kHz range.

30. The human ear is less sensitive to sounds below 100Hz and above 8kHz.

31. Early reflected sound from both sides of the stage amplifies and thickens the original sound, which is a beneficial effect of reflected sound.

32. The reflected sound from the back of the audience seats creates an echo effect on the original sound source, which is a harmful emission effect.

33. The speed of sound in the air is 340 m/s.

34. To ensure that spectators approximately 34m away from the main speaker in the stadium cannot hear the two sounds, a 0.1s delay should be added to the supplementary speakers near the spectators.

35. Materials with a low reflection coefficient are called sound-absorbing materials.

36. Materials with low transmission coefficients are called sound insulation materials.

37. Materials with a high transmission coefficient are called sound-transmitting materials.

38. Fully sound-absorbing materials refer to materials with a sound absorption coefficient α=1.

39. Total internal reflection material refers to material with a sound absorption coefficient α=0.

40. Materials such as rock wool and glass wool mainly absorb high and medium frequencies.

41. Polyurethane sound-absorbing foam mainly absorbs high and medium frequencies.

42. Thin plates with cavities mainly absorb low frequencies.

43. Thin boards nailed directly to the wall have very poor sound absorption.

44. Curtain fabrics mainly absorb high and medium frequencies.

45. Rough cement walls have poor sound absorption.

46. ​​The human ear can determine the spatial location of a sound source by the difference in intensity and time between the sound source signals; this is called the binaural effect.

47. When two sounds arrive at the human ear one after the other, with a difference of 5ms to 50ms, the human ear perceives the sound as coming from the direction of the sound source that arrived first. This is called the Haas effect.

48. When there is a sound intensity level difference of more than 15 dB between two sound sources, the listener will perceive the sound source as being in the direction of the source with the higher sound intensity level. This is called the de Boer effect.

49. The hearing threshold of a sound must be raised due to the presence of other sounds.

50. In certain locations within a hall, sound interference causes certain frequencies to cancel each other out, resulting in a significant reduction in sound pressure level; these are called dead spots.

51. When sound encounters a concave reflecting surface, the sound pressure level in one area is much higher than in other areas; this is called sound focusing.

52. When sound reflects back and forth between two parallel walls in a room, producing multiple identical sounds, this is called a flutter echo.

53. Due to reflection, the reflected sound differs from the direct sound by more than 50ms, resulting in an echo.

54. When a room is excited by external sound vibrations, it vibrates according to its own natural frequency, which is called room resonance.

55. The phenomenon of several overlapping resonant frequencies in a room is called resonant frequency degeneracy.

56. When the spectrum of the original sound signal is altered due to degeneracy or other reasons, an additional timbre is added, resulting in distortion, which is called sound coloration.

57. The distance between the point in a sound field where the direct sound energy density is equal to the reverberant sound energy density and the sound source is called the reverberation radius.

58. When the listening point is within the reverberation radius, direct sound plays a major role.

59. When the listening point is outside the reverberation radius, the reverberation plays a major role.

60. The vibration of a sound source causes additional alternating pressure in the air, which is called a sound wave.

61. When the direction of particle vibration is parallel to the direction of wave propagation, it is called a longitudinal wave.

62. When the direction of particle vibration is perpendicular to the direction of wave propagation, it is called a transverse wave.

63. Sound waves radiated by a point sound source in space are generally spherical waves.

64. Sound waves travel the fastest through different materials, especially metals.

65. Sound waves travel the slowest through different substances, especially air.

66. The speed of sound waves in different substances, in descending order, is: metal > wood > water > air.

67. An echo is produced because the reflected sound and the direct sound differ by more than 50ms.

68. Flutter echoes are produced by sound reflecting back and forth between two parallel walls of light.

69. Sound focusing occurs when sound encounters a concave reflecting surface.

70. Sound diffusion occurs when sound encounters a convex reflecting surface.

71. If you hear two repeated voices from the stage while sitting in a seat in the auditorium, the possible reason is that the transmitted sound and the direct sound are more than 50ms apart.

72. The human ear is most sensitive to mid-range frequencies, followed by high frequencies, and less sensitive to lower frequencies.

73. The directional characteristics of sound waves of different frequencies are that high frequencies are highly directional and low frequencies are weakly directional.

74. The diffraction ability of sound waves of different frequencies is that low frequencies are easier to diffract, while high frequencies are not.

75. The usual practice for speaker placement is to hang the tweeter high and adjust the angle; and place the woofer close to the ground.

76. If the low-frequency reverberation in the hall is too long, a more effective measure is to decorate the wall with thin panels with cavities.

77. The best sound insulation material is a double-layered brick wall with an air layer in between.

78. The fourth harmonic of a 50Hz non-sinusoidal periodic signal is 200Hz.

79. The third harmonic of a 100Hz non-sinusoidal periodic signal is 300Hz.

The fifth harmonic of a non-sinusoidal periodic signal of 80 Hz is 400 Hz.

81. The fifth harmonic of a 300Hz non-sinusoidal periodic signal is 1500Hz.

82. To ensure that spectators approximately 17 meters from the main speaker in the stadium cannot hear the two sounds, a 50ms delay should be added to the supplementary speakers near the spectators.

83. The equalizer is divided into frequency bands of 63, 125, 250, 500, 1K, 2K, 4K, 8K, and 16K, which is a 1/1 octave band division.

84. The equalizer is divided into frequency bands of 50, 200, 800, 3.2K, and 12K, which is a 4-octave division.

85. The equalizer divides the frequency bands into 40, 50, 63, 80, 100, 125, 160, 200, 250, 315, 400...20kHz, which is a 1/3 octave band division.

86. The venue with the longest optimal reverberation time is a concert hall.

87. The best place to choose the shortest reverberation time is a multi-track, staged recording studio.

88. Multifunctional halls are suitable venues for designing reverberation time that can be adjusted.

89. The Sabine formula is applicable to calculating the reverberation time of rooms with a low sound absorption coefficient.

The reverberation time is the time required for the sound pressure level to decrease by 60 dB after the sound source stops, so it is represented by the symbol T60.

Formula: T60 = 0.161V/A = 0.161V/αS

Where A is the total sound absorption, α is the sound absorption coefficient, S is the sample area, and V is the reverberation chamber volume.

90. Airun's formula is applicable to calculating the reverberation time of various types of rooms.

91. To reduce room degeneracy and avoid sound coloration, the optimal room dimensions are length:width:height = 2:3:5.

92. In a large theater, the seats where you are most likely to hear an echo are the front seats. Note: This article is compiled from online sources.

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