1. Propagation of indoor sound

1. Reflection and previous reflection

During the propagation of sound waves, if they encounter the surface of an object that is larger than its wavelength, , reflection will occur. When the reflecting surface is much larger than the wavelength of the sound wave, the reflection law is similar to geometric optics, that is, the reflection angle of the sound ray is equal to the incident angle. At this time, we can use geometric acoustics to study reflections.

We call the reflection that arrives within 50ms after hearing the direct sound the previous reflection or early reflection. Due to the Haas effect, the human ear not only cannot distinguish the previous reflected sound, but also treats it as part of the direct sound, which subjectively increases the loudness of the sound without affecting the clarity. This is why speaking indoors sounds louder than speaking outdoors.

The intensity of the previous reflection in theaters and concert halls is a very important acoustic condition. Some classical concert halls built in Europe in the 18th century are famous for their excellent sound quality and have made many Acousticians and architects were bewildered. However, later research and engineering practice showed that in addition to good sound diffusion and moderate reverberation in some excellent classical concert halls, a very important reason is that the auditoriums of these theaters or concert halls have sufficient front reflections, especially The first reflected sound from the sides and ceiling increases the indoor sound energy density and improves the sense of space and fullness of the music.

2. Reverberation and optimal reverberation time

Reverberation is one of the most important parameters in architectural acoustics. Moderate reverberation can significantly improve the sound quality and change the sound quality. The timbre and style of music.

We already know that indoor sound waves will be reflected when they encounter the surrounding walls, the floor and the ceiling, and this reflection process is repeated many times, thus prolonging the time it takes to reach the listener. If these reflected sounds continue to exist after multiple reflections 50ms after the direct sound reaches the listener, and will not decay and disappear until a period of time later, it will sound like a lingering sound. This process and phenomenon is called reverberation, which means cross-reverberation.

So, how to determine the time from the establishment to the disappearance of reverberation? In other words, how to determine the reverberation time? At the beginning of the last century, acoustician W.C. Sabie proposed after research: When the sound source stops emitting sound, the residual sound energy is reflected back and forth in the room. After absorption and attenuation, the sound energy density drops to one millionth of its original value. The time required for the indoor sound energy density to attenuate by 60dB is called the reverberation time.

There will be a certain difference between the actual measured value and the calculated value of the reverberation time. Generally speaking, the measured value of low-frequency reverberation time is smaller than the calculated value, and the measured value of high-frequency reverberation time is larger than the calculated value. Some corrections should be made based on experience during actual calculations.

The impact of reverberation time on acoustic quality is well known. If it is too long or too short, the audience will feel fatigued. Only with appropriate reverberation time can the audience be in a pleasing artistic enjoyment. At this time, the sound is full and pleasant, the notes are vivid and active, and the language is kind and gentle, giving the audience a strong sense of space and rich color.

So, what is the optimal reverberation time?

In fact, it is difficult to confirm a unified optimal reverberation time standard. There is no definite data and it is largely a range value. Different types, different styles, and different professional theaters have different requirements. Moreover, it is also affected by the national cultural background. Therefore, the data published by each country are different.

A slightly longer reverberation time in the low-frequency band is conducive to the fullness of the music and the warmth of the language, and is suitable for all kinds of music performances, while a longer reverberation time in the high-frequency band will easily express overtones and increase the " Moisture" and freshness.

Different reverberations at different frequencies determine the timbre characteristics of the theater. In the environment of hard decorative materials, the high-frequency reverberation time is long, the timbre is cool, and the timbre effect can imitate caves, cement halls, marble palaces, etc. The soft decorative materials have prominent low-frequency reverberation and warm timbre, which is characteristic of classical concert halls and opera houses.

3. Sound energy ratio and equivalent reverberation

Reverberation is also related to a physical quantity in subjective perception: equivalent reverberation.

The existence of equivalent reverberation time requires us to fully consider its impact during stage amplification and recording. Therefore, we recommend that the stage amplification or recording of large bands and operas should adopt an integral system. For far-field pickup, it is not advisable to use multi-point near-field pickup. The directivity of the microphone should not be too sharp, so as to fully pick up the natural reverberation sound of the theater space and make the music more full and natural.

4. Echo

There is a difference between echo and reverberation.

Reverberation is Reverberation in English. It refers to the gradual attenuation of mixed reflection sounds that arrive at different angles and at different times after multiple back-and-forth diffuse reflections. The listener cannot distinguish any of them. syllable.

The echo is a single fixed reflected sound. Its intensity and time difference are large enough to be distinguished from the direct sound and the syllables can be distinguished. The English name of Echo is Echo.

A certain amount of reverberation is good for the sound quality, but echo can only destroy the sound quality and should be absolutely avoided.

The way to eliminate echo is to perform sound diffusion processing.