First and foremost, achieving a great sound quality starts with creating a superior environment. Proper sound absorption plays a crucial role in acoustic treatment within a listening room. This is directly tied to whether the room’s reverberation time is appropriate. It's essential to recognize that proper reverberation doesn't just enhance the sound but is fundamentally the foundation of high-fidelity playback.
When considering acoustic treatments for a listening room, there's a vast array of considerations. Following this introduction, we'll delve deeper into these aspects. For today's discussion, let's focus on sound absorption from the perspective of tuning. By the time we reach the tuning phase, the home decoration is largely complete. Hard modifications at this stage are impractical, so we must rely on soft decorations to refine the setup.
The concept of reverberation time dates back over a century. The classic definition of reverberation time (T60) refers to the time it takes for the room's sound pressure to decay by 60 dB after the sound source ceases. Based on this definition, the numerical determination of reverberation time can be derived:
\[ T60 = 0.16V/A \, \text{(seconds)} \]
Here, \( V \) represents the room volume (in cubic meters), and \( A \) denotes the total indoor sound absorption (in square meters).
This former definition lays the physical groundwork for determining reverberation time, while the latter offers a basis for controlling it. In essence, for a given room size, controlling the total indoor absorption allows for free adjustment of reverberation time. This key formula is known as the "Sabin Formula," originally established through experiments by Sabin.
Through extensive research and analysis of hall acoustics and their reverberation times, acousticians introduced the concept of the "optimal reverberation time." This highlights that to achieve good sound quality, the reverberation time should neither be excessively long nor overly short, but rather appropriate. This optimal range is termed the "optimal reverberation time."
The optimal reverberation time stands as the first objective parameter in the history of acoustics reflecting a room's sound quality. We often say that "reverberation enhances sound quality," which is the result of "appropriate reverberation." In reality, whether reverberation is appropriate isn’t just about how pleasant the sound feels but directly impacts its truthfulness and naturalness. Subjective listening evaluations like fullness, warmth, clarity, and spatial perception are closely linked to reverberation appropriateness. Thus, the importance of reverberation can be summarized succinctly as:
**Optimal Reverberation Time**
To control reverberation at an appropriate level, it’s first necessary to know what that appropriate reverberation time is and what factors influence it. Acousticians have provided recommended values. The graph above shows the relationship between "optimal reverberation time" and room volume/music/language. While these curves aren’t "absolutely precise," they offer an approximate value for the optimal reverberation time linked to room volume and sound type. There might be a ±10-20% variation depending on the actual situation. Despite this, they give us two critical insights: a fundamental understanding of the optimal reverberation time and factors influencing it and its changing trends. To further comprehend the impact of reverberation on sound, here are some simple guidelines.
Firstly, if the room is large, the "optimal reverberation time" should be longer. This is because larger spaces require more sound to fill the room adequately, ensuring a more satisfying auditory experience. Longer reverberation times allow for greater volume. The concert hall’s reverberation time (around 1.5 seconds) is significantly higher than that of a listening room, which explains this difference.
**Understanding Optimal Reverberation Time**
Secondly, the "optimal reverberation time" for speech is consistently shorter than that required for music. Why is this so? Both speech and music can be viewed as a series of syllables or notes. If the room’s reverberation time is too long, the previous tone hasn’t fully decayed before the next arrives, causing significant overlap. This makes the tones indistinct and muddy. Conversely, if the reverberation time is too short, there’s minimal overlap, making each tone clear but potentially lacking in volume and richness due to insufficient blending of direct sound. Clearly, appropriate reverberation allows for both clarity and loudness. People tend to prioritize comprehension in speech, favoring shorter reverberation times. On the other hand, music listeners often prefer overlapping tones to mask imperfections, enhancing fullness and beauty, hence requiring longer reverberation times.
Lastly, it’s important to note that most home listening rooms are small and primarily used for music appreciation. During design, the initial "optimal reverberation time" should be slightly larger, at least not less than 0.5 seconds. If needed, the reverberation time can be gradually reduced, making it easier to find the "optimal reverberation time" matching the actual situation through listening tests.
**Reverberation Uniformity Requirement**
The "optimal reverberation time" discussed earlier pertains to the mid-frequency range of 500 Hz. Current indoor acoustics typically cover frequencies from 125 Hz to 4000 Hz. Therefore, controlling reverberation time also involves addressing the frequency response issue of "optimal reverberation time." Regarding the frequency response of reverberation time, it’s generally desirable to maintain a flat and uniform response from low to high frequencies. However, a flat high-frequency response benefits some instruments but may seem harsh for others with rich overtones. Considering this, allowing slight reductions in high frequencies is preferable. For low frequencies, moderate enhancement can improve the low-frequency playback effect in small rooms, as seen in Figure 3b. Yet, when low-frequency standing waves in a small room are severe, increasing low-frequency absorption and gradually reducing the low-frequency reverberation time from the mid-frequency range is a common practice, especially when using large speakers in small rooms.
What should be avoided is the fluctuating reverberation characteristic shown in Figure 3d. Here, the reverberation time at frequency \( f_1 \), corresponding to the peak, is long, whereas at \( f_2 \), corresponding to the valley, it is short. \( f_1 \) masks the signal of \( f_2 \), particularly when the amplitude of the \( f_2 \) signal is small, leading to its complete submersion. This results in the loss of subtle musical details, which is detrimental to high-fidelity playback. Poor suppression of standing wave resonances in the room often causes such issues in the low-frequency band, resulting in sound distortion or contamination. Hence, preventing the degeneration of low-frequency resonance frequencies and enhancing sound absorption is particularly vital during listening room design.
In summary, regarding the reverberation characteristics of Figures 3a-3c, it’s challenging to definitively state which is better, as it depends on room standing waves and music type. Broadly speaking, compared to mid-frequencies, the high-frequency reverberation time should be controlled within -10% to 0%, and the low-frequency reverberation time within +50% to -20%, which is generally acceptable.
After initially selecting the "optimal reverberation time" and its frequency response requirements, the next step is to roughly plan the room’s sound absorption.
We can calculate the specific surface area of the home based on the sound absorption coefficient table, compute the total absorption using the Sabin formula and different materials, compare it with the required "optimal reverberation time," and then adjust accordingly based on the actual situation. Some materials are difficult to modify post-renovation, while others are easier to add, such as the third type of material.
The third type of material comprises porous cotton fabrics, with carpets and curtains (i.e., drapes) being representative examples and among the earliest sound-absorbing materials used historically. These materials have high middle and high-frequency sound absorption coefficients but lower low-frequency coefficients, making them suitable as mid-to-high frequency absorbers. Although carpets and curtains are household sound-absorbing materials, their primary absorption is limited to mid-to-high frequencies, often leading to muddled sounds and poor definition. Perhaps for this reason, along with new materials and structures available for sound absorption, using household sound-absorbing materials—especially those with adjustable absorption coefficients within a certain range—is quite convenient. This makes them more suitable for amateur use.
The folding percentage (%) refers to the ratio of the unfolded portion of the curtain hanging suspension to its fully suspended area. Sound absorption during folding suspension improves significantly, mainly due to the increased thickness of the curtain itself, thus raising the sound absorption coefficient. Additionally, when the folding percentage reaches a certain point, the sound absorption coefficient exhibits a distinct peak characteristic. This happens because, at high folding percentages, the air layer behind the curtain thickens, forming a sound-absorbing property akin to a resonant absorptive node. Velvet curtains share similar characteristics. As long as we fully understand these curtain properties and combine them with low-frequency sound-absorbing materials, we can achieve the desired sound absorption across the entire frequency spectrum.
The amount of material required depends on the specific situation and the reverberation of different rooms. Without a reverberation-measuring device, calculating the room's reverberation time may seem impractical for amateurs. In reality, this is highly beneficial. By roughly setting up various room reverberation characteristics, we can find the most satisfying reverberation characteristics through listening and comparison. This somewhat compensates for the lack of reverberation-measuring instruments.
For beginners in "room tuning" with time, energy, and interest, it's recommended to start with a simpler, more flexible, convenient, and cost-effective listening room treatment plan. This helps accumulate experience and ultimately achieve more ideal results.
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