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Continuing from the previous article, let's look at some basic acoustics knowledge:
93. The main solution to the problem of front-seat audience members hearing echoes in large theaters is to enhance sound absorption on the back wall of the audience seating area.
94. The correct way to write decibels is dB.
95. In musical notation, the distance between 1 and i is one octave.
96. In musical notation, the interval between 1 and 2 is 1 degree.
97. The relationship between the speed of sound C, the frequency of sound f, and the wavelength of sound λ is C = fλ.
98. The relationship between the sound wave frequency ƒ and the sound wave period Τ is f=1/T.
99. The conditions for the formation of standing waves are reverse propagation, the same amplitude, equal frequency, and a phase difference of 0 or constant.
100. In the effects pedal, CHROUS means chorus.
101. The change in local pressure of a medium caused by the vibration of sound waves is called sound pressure.
102. The unit of sound pressure level is dB.
103. The unit of sound pressure is Pa.
104. The unit of sound intensity is w/m².
105. The sound pressure level of the threshold of hearing is 2×10-5 Pa.
106. The sound pressure level of the pain threshold is 2×10 Pa.
107. The sound pressure level of the pain threshold is approximately 120 dB.
108. The sound pressure level of the hearing threshold is approximately 0 dB.
109. When a concave curved surface forms a concentrated reflection of sound waves, the sound energy is concentrated at a certain point or in a certain area; this is called sound focusing.
110. A convex curved surface reflects sound waves, causing sound energy to diffuse.
111. The minimum time interval between two sounds that the human ear can distinguish is 50ms.
112. Melodies in music include vocal melodies and instrumental melodies.
113. In musical notation, 1 to i is called an octave.
114. Indoor reverberation is caused by reflected sound.
115. Raising a basic note by a semitone is called a sharp, which is indicated by the # symbol.
116. Lowering the basic note by a semitone is called a flat, and it is represented by the symbol b.
117. To restore a raised or lowered note to its original pitch is called naturalization, indicated by the symbol ㄆ (pronounced "p").
118. MIDI stands for Musical Instrument Digital Interface.
119. When a sound source is reflected between two parallel surfaces at a distance greater than a certain value, forming a series of echoes, this is called a flutter echo.
120. The ratio of sound pressure to reference sound pressure (2×10-5 Pa), multiplied by 20 with the logarithm to the base 10, is called the sound pressure level.
121. Most of the timbre in music is a complex tone.
122. Early indoor reflections refer to reflected sounds that enter the ear after only one reflection.
123. There are 7 basic notes in music.
124. Two commonly used sound-absorbing materials: porous materials, thin plates with cavities behind them.
125. Not a sound insulation structure: perforated steel plate.
126. Belongs to the category of sound insulation structure: double-layer brick wall.
127. Not a porous sound-absorbing material: gypsum board.
128. Rock wool is a porous sound-absorbing material.
129. The characteristic of thin plate resonant structures is that they have low-frequency sound absorption properties and also help the diffusion of sound waves.
130. By fixing the wooden board to the frame and leaving a certain air layer behind the board, a thin-plate resonant sound-absorbing structure can be formed.
131. The sound engineer recorded the sound of birds in the tree at 0.01 Pa and the sound of bugle playing at 1 Pa, with a difference of 40 dB between the two sounds.
132. Reverberation can prolong the duration of a sound and enhance its fullness.
133. When two wave sources have the same or similar frequencies, and their waves meet and overlap, wave interference may occur.
134. If two waves propagating in opposite directions along the same straight line have the same amplitude and frequency, a standing wave will appear on the line connecting the two wave sources.
135. The first-level indicator for the total noise level of a hall used for both speech and music is NR30.
136. The first-level standard for the total noise level in a karaoke room is 40 dB [A].
137. The sound focusing generated indoors has a non-uniform effect on the indoor sound field because there are concave reflecting surfaces indoors.
138. Dead spots in indoor sound are caused by interference from indoor sound sources or the formation of standing waves.
139. The sound shadow zone refers to the area indoors where no direct sound can be heard.
140. The sound insulation R of an object is related to the object's thickness, as well as its surface structure and density.
141. Laying sufficient sound-absorbing material on a concave surface can solve the problem of sound focusing.
142. Adjusting the speaker position or assuming that a supplementary speaker can resolve the defects in the sound shadow area.
143. Adding strong sound-absorbing materials or convex diffusers to the back wall can mitigate the defect of long-delayed echoes.
144. Adding diffusers to the surfaces of two parallel walls or changing the parallel angle can solve the problem of flutter echoes.
145. A condenser microphone has a maximum sound pressure level of 126dB, an equivalent noise level of 20dB, and a dynamic range of 106dB.
146. The mid-to-high frequency range of audio determines the brightness and clarity of the sound.
147. The high-frequency range of audio determines the timbre of the sound.
148. The low-frequency range in audio determines the richness and fullness of the sound.
149. The mid-to-low frequency range of an audio signal determines whether the sound is solid and powerful.
150. A glass line refers to the direction of wave propagation.
151. An echo is caused by the reflection of sound.
152. When designing an indoor sound field, the stronger the sound absorption performance of the sound-absorbing materials used on the room walls, the smaller the amplitude of early reflected sound and the shorter the reverberation time.
153. The smaller the sound absorption coefficient α of an object, the stronger its reflected sound. The larger the sound absorption coefficient of an object, the weaker its emitted sound.
154. The effect of early reflected sound is to give people a sense of intimacy.
155. If the natural reverberation time T60 is too long after the interior decoration is completed, it can be improved by adding heavy fabric curtains to the doors and windows.
156. In the design of large halls, near-secondary sound reflections should be fully utilized.
157. The combination of reverberation and early reflections makes the sound sound fuller.
158. Sound pressure level and sound intensity level are numerically the same.
159. The phenomenon of acoustic staining has an adverse effect on diffusers.
160. The indoor sound frequency transmission characteristics are related to the sound absorption coefficient of surrounding objects.
161. Pitch is directly related to sound frequency.
162. The room equilibrium compensation curves are different in different rooms.
163. The sound intensity of a point sound source is inversely proportional to the square of its distance from the source.
164. The sampling frequency must be at least twice the highest frequency of the sampled signal.
165. The lower the frequency of a wave, the stronger its diffraction effect.
166. The unit of sound power is W.
167. The unit of sound intensity is watts per square meter.
168. When a sound source and a listener are in motion, the listener will perceive a change in the frequency emitted by the sound source. This phenomenon is called the Doppler effect.
169. When the direct sound is delayed and phase-inverted by 180°, it is superimposed on the direct sound, creating a spatial impression in the human ear. This is called the Laue effect.
170. The subjective feeling that people can distinguish between two different sounds with the same frequency and amplitude is called timbre.
171. Of the three elements of sound, the element that is mainly related to the frequency of sound is called pitch.
172. The distance between the pitches of two sounds is called the interval.
173. Arranging sounds according to certain intervals is called a musical scale.
174. A signal whose instantaneous voltage changes sinusoidally with time is called a pure tone signal.
175. A pulse signal consisting of a series of intermittent sine waves with a certain duration and each wave containing a certain number of sine waves is called a burst sound.
176. A noise signal containing various frequency components from 20Hz to 20kHz, and whose energy distribution at each frequency is uniform, is called white noise.
177. A noise signal containing various frequency components from 20Hz to 20kHz, and whose power spectral density is inversely proportional to the frequency, is called pink noise. Pink noise can be obtained by adding an attenuation filter that reduces the frequency by 3dB per octave to white noise.
178. The frequency component power of brown noise is mainly concentrated in the low-frequency range. Its energy decrease curve is 1/f^2, and its waveform is highly self-similar. Overall, it is somewhat similar to the background noise of a factory.
179. Two cardioid or omnidirectional microphones, spaced apart at the distance between a person's ears, are used to pick up sound. This is called the A/B stereo system.
180. Two microphones are combined into one unit: one is a figure-eight directional microphone pointing mainly to the left; the other is a cardioid or omnidirectional microphone pointing directly forward. The signals from the two microphones are input into a matrix for sum-difference transformation before output, which is called the M/S stereo system.
181. Two directional cardioid or figure-eight shaped microphone tips are installed one above the other inside the same microphone housing, with the angle between their main axes varying within 0 to 360 degrees. This is called the X/S stereo system.
182. When a sound is heard at a certain point in a room, the order in which it reaches the ear is: direct sound, near reflection sound, and reverberation.