Acoustic Terminology

 Decibels (dB): Decibels can be confusing for non-experts, but they are quite familiar to professionals. The decibel is named after the American telephone inventor, Alexander Graham Bell. Since the unit of Bell is too large, the decibel is used to represent a 1/10th of a Bell. The concept of decibels is particularly unique as their calculation is not linear but logarithmic. For example, if two speakers emit sounds at 60 dB each, the combined sound is not 120 dB but 63 dB. If an acoustic material absorbs 80% of sound energy, the reduction in sound is not 0.8 dB or 80 dB but 10lg(1-0.8) = 7 dB. If a partition has a sound insulation rating of 50 dB, the sound passing through it is 0.00001.
The frequency of sound originates from vibration, and vibration corresponds to a frequency (denoted as f), which is the number of vibrations per second, measured in hertz (Hz). Not all frequencies of sound are audible to the human ear; only those with vibration frequencies between 20 Hz (some say 16 Hz) and 20,000 Hz can be perceived as sound. Frequencies below 20 Hz are known as infrasound, and those above 20,000 Hz are ultrasound. Sounds below 20 Hz and above 20,000 Hz are inaudible to the human ear. The ear is most sensitive to frequencies between 100 and 3150 Hz. In architectural acoustics, sounds below 200 Hz or at 200~300 Hz are generally referred to as low-frequency sounds, those between 500 and 1000 Hz as mid-frequency sounds, and those at 2000 Hz or above as high-frequency sounds.
The A-weighting concept can be confusing to the average person. Sound level is the weighted sum of various frequencies' sound (not simply arithmetic addition) to determine the loudness. The A-weighted level is the sum of sound at different frequencies after they pass through an A-weighting network. It reflects the human ear's auditory characteristics of being less sensitive to low and high frequencies. For instance, if the sound pressure level at 100 Hz is 80 dB, it would be weighted down by 50.5 dB in the calculation of the A-weighted level, resulting in a value of 29.5 dB. Conversely, if the sound pressure level at 1 kHz is 80 dB, the weighting is 0 dB, so it is still calculated as 80 dB. The purpose of the A-weighted level is that the higher the A-weighted level, the louder the sound appears. A-weighted decibels are typically denoted as dBA. Many national noise-related standards use the A-weighted level as a benchmark.
Reverberation: The phenomenon of sound persistence after a sound source inside a room stops emitting sound, caused by multiple reflections or scattering of sound waves from room boundaries or obstacles. Reverberation can increase the sound level inside a room by 15dB, while also diminishing the clarity of speech. In spaces designed for musical performances, such as concert halls and theaters, a certain level of reverberation is desired to enhance the soothing and pleasant nature of the music. Conversely, in spaces where speech is the primary focus, like cinemas, classrooms, auditoriums, and recording studios, reverberation needs to be minimized to ensure clearer communication. Therefore, different rooms with varying usage requirements necessitate different levels of reverberation.
SoundBridge: When boards are directly fixed to the joists, the vibration on the sound-receiving side of the board will be transmitted to the other side through the joists, a phenomenon where sound energy is transferred like a bridge, known as a "soundbridge." The more soundbridges, the larger the contact area, and the stronger the rigid connection, the more severe the soundbridge effect becomes, resulting in poorer sound insulation. During sound insulation treatment, soft high-damping materials are used at rigid contact points to block the formation of soundbridges.
Impact Noise: A type of noise generated indoors due to impacts with solid objects. The indicator of the sound insulation performance of impact noise is the impact sound pressure level, which differs from the "number of decibels of sound insulation" expressed by airborne sound insulation, as it represents the loudness of the sound from below when the standard impact hammer (a device capable of generating standard impact energy) strikes the floor. A higher impact sound pressure level indicates poorer transmission and insulation capabilities of the floor against impact noise, whereas a lower level indicates better performance. Generally, when the weighted impact sound level of the floor is below 65 dB, except for knocking and jumping, most sounds are inaudible. At a weighted impact sound level between 75 dB and 85 dB, footsteps, dragging of chairs and tables, and the sensation of children running and jumping are noticeable, with knocking sounds being even more unpleasant.
A standing wave is formed by the interference of identical sound waves with the same frequency, resulting in a fixed spatial distribution of periodic waves.
Reverberation: The phenomenon of sound persistence after a sound source in an enclosed space stops emitting sound, caused by multiple reflections or scattering of sound waves off room boundaries or obstacles within it.
Acoustic Bridge: The connecting material between two layers in a double or multi-layered soundproof structure. Sound energy propagates between the layers in the form of vibrations. Sound Absorption: The effect by which sound energy is converted into heat energy after entering a porous material or causing the vibration of a bendable plate.
Acoustic waves propagate through air and are gradually attenuated as they travel due to the vibration and friction between air particles, converting sound energy into thermal energy—this is known as air absorption. When acoustic waves impinge on porous absorbent materials, a significant portion of the sound energy is converted into thermal energy due to the viscous resistance of the air and the vibration friction against the material's pores—this is referred to as material absorption. Any material possesses some degree of sound absorption capacity; materials with an average absorption coefficient over 0.2 are considered absorbent materials. The frequency characteristics of porous absorbent materials in terms of sound absorption are: higher absorption coefficients at mid-to-high frequencies, and lower absorption coefficients at low frequencies.
Echo generation: The auditory effect caused by an additional sound source, resulting from time delay of a single sound source, due to the path difference between some reflected sounds exceeding 17 meters.
Sound Insulation: The ability of a material to reduce sound transmission. When a building is subjected to external sound fields or vibrates due to impact, sound can pass through the enclosing structure, which is referred to as "sound transmission." Due to the action of the enclosing structure, the incoming sound energy is always reduced, with the degree of reduction depending on the sound insulation performance of the structure. The reduction of sound energy from external sound fields is called "airborne sound insulation"; reducing the sound energy radiated from impact is called "solid sound or impact sound insulation." This differs from the concept of "vibration isolation," which refers to airborne sound reaching the receiver, while the latter refers to the solid vibration felt by the receiver. Vibration isolation measures can reduce the impact of vibration sources or impact sources on the enclosing structure (such as floors), thereby lowering the sound level of the impact noise itself.
The Soundproofing Mechanism: As sound waves sequentially pass through walls with completely different characteristic impedances and air mediums, they undergo multiple reflections, resulting in the attenuation of the sound waves. Additionally, due to the elasticity of the air layer and its supplementary effects, the vibration energy is significantly reduced, thereby achieving soundproofing.
Resonant Frequency: Every partition wall has an inherent resonant frequency. When the frequency of the sound wave matches the wall's resonant frequency, the entire wall resonates, significantly reducing the sound insulation at that frequency.
The Resonance Effect: After sound waves contact the wall panel, the panel not only experiences forced vibrations in the vertical direction but also forced bending vibrations along the surface of the panel. Above a certain frequency, the forced bending vibrations synchronize with the panel's inherent natural bending vibrations. At this point, the panel follows the incident sound's bending very readily, causing a large amount of sound energy to be transmitted to the other side, creating a dip in sound insulation. This phenomenon is known as the resonance effect.
The Masking Effect: The human ear experiences masking, where a sound that is 10dB louder than another is harder to hear and understand. Due to this masking effect, even close conversations can be difficult to hear in environments between 90-100dB.
Noise has two meanings:
① Physically refers to irregular, intermittent, or random sound vibrations.
② Psychologically, any sound that people do not wish to hear. The generation of sound is a physical phenomenon, while noise is a subjective and psychological perception of sound by people. Therefore, any sound that people do not want to hear, which interferes with life, work, and study, is collectively referred to as "noise."
Air Sound and Solid Sound
Sound transmitted through the air medium, such as conversation and music, is referred to as airborne sound; the human ear primarily detects airborne sound.隔音 for residential and office spaces involves blocking airborne sound. Sounds transmitted through floors and pipes, such as machinery vibrations and drainage noises, are known as structure-borne sound. The difference between airborne and structure-borne sound lies in their different pathways of transmission, requiring different methods of treatment.
Acoustic Absorption and Soundproofing
Absorption involves converting a portion of sound energy into heat energy using porous materials. It primarily focuses on absorbing sound energy, such as ultra-fine glass wool and rock wool. It is mainly used to reduce sound reflection.
Soundproofing involves using dense materials to isolate sound within a specific space. The noise reduction capability of soundproofing materials is referred to as soundproofing performance. For instance, if a living room TV is emitting 70dB of sound and the noise level drops to 50dB after passing through a wall to a bedroom, then the wall's soundproofing performance can be said to be 20dB.
Soundproofing and sound absorption are two distinct concepts; while the market commonly offers sound absorption materials, professional soundproofing materials are less common. In the absence of a choice, people often use sound absorption materials in place of soundproofing materials to block sound, which is a misconception with negligible effect. True soundproofing requires genuine soundproofing materials. In practice, to achieve a good soundproofing effect, the "combined use of absorption and soundproofing" method is often employed, with the surface soundproofing layer used for soundproofing, and the cavity behind filled with sound absorption materials. This composite structure can effectively enhance the overall soundproofing performance.
Common Decibel Levels
The human ear's audible threshold is 0dB, and environments below 15dB are extremely quiet, so quiet that they can be disorienting. In rural areas, nights are typically around 25-30dB, with the sounds of flowing water, wind, and small animals being the only disturbances in an otherwise tranquil atmosphere. In urban settings, the noise level can vary greatly depending on the area. In quieter areas, indoor levels are usually around 30-35dB, but if you live in a bustling city center or near a busy thoroughfare, you may have to endure noise levels of 40-50dB (or higher). Normal conversation is around 60-70dB, while shouting can reach 100dB. The upper limit for human hearing is generally 120dB, with sounds above this level causing damage to the auditory organs. At 140dB, hearing can be permanently lost. High-decibel speakers, heavy machinery, and jet engine noise can all produce sounds exceeding 120dB.
Sound Transmission Path of Enclosure Structures
Sound transmission through protective structures is divided into two pathways according to the laws of propagation. One is when vibrations directly impact the protective structure, making it a sound source. The vibrations then propagate along the solid components through the protective structure as the medium, which is generally referred to as solid sound transmission or structural sound transmission. Sound propagation in solid mediums decays more slowly than in air, travels further, and is faster. The other pathway is when a sound source in the air excites the surrounding air vibrations, forming sound waves that propagate to the components (mostly reflected) and excite the vibrations of this medium. This results in a small portion of the sound energy being transmitted through to another space, which is generally called air sound transmission.