Practical techniques for bass box

Reproducing bass is a very tricky business, one that causes a lot of trouble for speaker designers and audiophiles alike. But it is a challenge, and the potential rewards more than justify the effort to improve bass reproduction.

The core of the problem is that loudspeakers do not operate in an isolated environment, but many loudspeaker designers and Hi-Fi audio enthusiasts often ignore this point. Designers always use anechoic chambers isolated from the actual environment to develop their products. Practice has proved that there is a very close mutual influence between the sound quality of loudspeakers and the actual listening room, especially in the bass band. As we all know, the frequency range of human hearing is about from 20Hz to 20,000Hz, which spans about 10 octaves. The middle C of the piano is located in the middle of the fourth octave from the lowest note. Therefore, the bass area itself is almost confined to the lowest octave (20Hz~40Hz) and the second octave above it (40~80Hz). Although pure bass is not heard too much in real life, it plays an important role in creating the realism and strength of music.

If you don't use a subwoofer and rely entirely on ordinary speakers to create bass, such speakers will not only be large in size, but also very heavy. Subwoofers have many advantages, such as small size, unnoticeable placement in the room, and no additional requirements for the system because they have their own amplifiers and filters installed in the box. With a subwoofer dedicated to reproducing bass in the system, other channels can use smaller speakers, making the configuration and use of the entire system more flexible and convenient. Generally speaking, creating bass requires a larger speaker, but just a pair of ordinary small speakers placed against the wall is enough to make the reproduced sound dive into the mid-bass range. This is partly because placing it against the wall helps to enhance the bass from 50Hz to 100Hz, and partly because the "standing wave resonance" caused by the shape and size of the room provides a "room gain effect". The common problem of most commercially available subwoofers is that the low-end roll-off frequency cannot dive below 50Hz, and its frequency response cannot achieve an ideal match with the ordinary speakers of other channels, and their partial frequency responses often constitute harmful overlap.

The influence of standing waves

Every room, whether it is a concert hall or an ordinary family's living room, has standing waves, which is the main source of bass playback problems. The standing wave frequency is closely related to the size of the room. In a large indoor concert hall, the standing wave frequency occurs in the deepest bass and infrasound area, which is lower than the lowest frequency of bass instruments, so it will not have a bad effect on bass playback. But in a small living room of an ordinary family, due to the small size and space of the room, the standing wave frequency will be higher and enter the bass area of ​​the music, and overlap with the latter, making the sound quality worse. While providing "room gain", standing waves will also make the frequency response of the bass area very uneven, that is, it will increase the response of some frequencies and offset the response of some other frequencies. In addition, different rooms have different standing wave patterns, and it is related to many factors in the room, such as wall materials, furniture, listening position and speaker placement, so it is difficult to accurately predict it.

● Create bass in the room

As we all know, in order for a speaker to make sound, it is necessary to use the vibration of the cone to move air molecules and excite sound waves in the air with the same frequency as the cone vibration frequency. Since the frequency of bass is very low, the vibration speed of the bass speaker cone is also very slow. Therefore, in order to produce strong bass in the room, the cone must move a large number of air molecules per unit time. To achieve this goal, subwoofers usually use large diaphragm, small stroke cone or small diaphragm, large stroke cone. The latter has a small cone diameter, which is conducive to reducing the volume of the entire subwoofer, and has the advantages of high power and high loudness, and has become the mainstream of modern subwoofers.

However, although installing a subwoofer in a large speaker can reproduce some shocking explosions and landslides in movies, it is unpleasant to reproduce a lot of high-intensity bass when listening to music. In addition, research and practical results have proved that it does not make much sense to make the subwoofer louder than other speakers in the system. These are the reasons why modern subwoofers are made smaller.

● Subwoofer placement and connection

The inherent variability and unpredictability of indoor standing waves leave people with no choice but to tolerate them. Although some computer software provides some interesting methods for controlling standing waves, they cannot completely solve the problem of standing waves. Fortunately, the wavelength of bass is much larger than the size of an ordinary room, and bass is actually non-directional. Using this feature, ordinary users can gain almost unlimited freedom to experiment with the best placement of the bass speaker in the room by using a separate subwoofer. Although its placement is not as important as that of the main speaker, different placements still have different sound effects. The following introduces three ways to place the subwoofer, each of which has its own advantages and disadvantages. Readers can choose according to the experimental results.

Place the subwoofer in a corner (Figure 1). This helps avoid acoustic reflections in the room, making the reproduced sound clearer and more distinct. However, the room mode is more strongly excited, which can make the bass response less flat. Since the subwoofer is the farthest from the listening position in the entire system, the bass you hear will be slightly delayed from the main speakers. When placing the subwoofer, keep it at least 10 cm away from the wall.

From left to right: Figure 1- Subwoofer placed in the corner, Figure 2- Subwoofer and main speakers placed in a line, Figure 3- Subwoofer placed in the listening area

Place the subwoofer and main speakers on the same line (Figure 2). The distances between the subwoofer and main speakers and the listening position are equal, so the direct sound from the three speakers reaches the listener's ears almost at the same time, without any time difference. The disadvantage of this placement is that the room reflections increase, and these delayed reflections will reduce the clarity of the reproduced sound.

Place the subwoofer close to the listening area (Figure 3). This approach is worth trying, especially in some rooms that are difficult to get the best acoustics from, because it minimizes the excitation of room modes and greatly shortens the transfer path of the direct sound and the time it takes for the bass to reach the listener's ears, but it also makes the room reflections more obscured.

Another less scientific method is to place the subwoofer at the seat where you usually listen and let it play music, then move slowly around it and listen carefully to the music until you find the point where the bass sound quality is the best. This point is the best place to place the subwoofer, and finally place it there.

In addition to two sets of high-level (speaker level) output terminals for connecting the left and right main channel speakers, modern home theater AV amplifiers also have a low-level (line level) output terminal, and the active subwoofer has a corresponding input terminal. In order to make the system have the best sound effects when playing music and movie programs, in addition to connecting the two sets of speaker level input terminals of the subwoofer to the corresponding output terminals of the AV amplifier, the line level terminals of the two should also be connected accordingly, as shown in Figure 4. The advantage of this connection is that the bass can be effectively enhanced when the system plays two-channel stereo music and 5.1-channel movies.

● Some controversy about bass

At present, there is still some controversy about the role and effect of bass. One of the easier to understand viewpoints is that "subwoofers can enhance the bass of the sound system." However, many people believe that "adding extra bass can also improve the characteristics and sound quality of the midrange and even the treble bands", "using subwoofers can improve the depth of the stereo sound field", "adding bass extension can disrupt the original role of the main speakers, and if the relative levels of bass and treble are deliberately chosen, they can also cancel each other out on both sides of the midrange band"... etc. These views are quite difficult to understand. Although it is difficult to put forward a scientific explanation for these views at present, there is a lot of "evidence" circulating widely. Some psychoacoustic phenomena are often difficult to understand, and history shows that in the development of high-fidelity technology, certain subjective views are often more authoritative and influential than theoretical assumptions. From this point of view, many problems in the field of audio are still waiting for people to conduct more in-depth exploration.

Figure 4: Subwoofer connection