《奥运年，新空间，新搞作》——肥仔的毒气房。 [复制链接]

8.1 Standing Waves

The parallel surfaces of most listening rooms can lead to a potential problem in the low frequencies. A sound wave can be repeatedly reflected from opposing surfaces, back and forth. If the distance between the surfaces is an integral multiple of one-half the sound wavelength, a standing wave will be set up. This means that the incident and reflected waves combine with each other so that a stationary pattern of high and low sound pressures is established in the room. This irregular distribution of sound level is caused by cancellation and reinforcement between the reflected and direct sound waves.

At high frequencies, this pattern of high and low sound pressure levels within the room becomes too finely spaced to be discerned. However, when the dimensions of the room are comparable to the wavelengths of the musical notes, there will be obvious changes in the intensity of certain bass notes in different locations within the room. Additionally, the existence of the standing wave implies a resonant condition where acoustic energy is stored in the room. This energy storage can result in "heavy", "muddy", or "slow" bass.

Since the presence of standing waves is caused by parallel reflective surfaces, practically every listening room suffers from this problem to some degree. However, several factors are working in our favor here. First, as the room size increases, the affected frequencies become lower and thereby less audibly apparent. Second, the presence of shelving or furniture against the walls will break up the large surfaces, reducing the magnitude of the problem. Third, upholstered furniture can absorb a significant amount of bass, diminishing the build-up of resonant energy. Fourth, typical wall construction is not completely reflective at low frequencies.

However, in some cases audibly objectionable standing waves will still be present in the listening room. This can be note  by large variations of the intensity of certain bass notes in different areas of the room. Another indicator is an unevenness of loudness of different bass notes. (This is sometimes what is actually on the recording, so be sure that this is consistently a problem on a variety of recordings.)

If you wish to reduce or eliminate standing waves that may exist in your room, it will be necessary to reduce the low-frequency reflectiveness of at least one of the parallel opposing surfaces. The most effective method is to use DAAD’s, available from Acustica Applicata. This is the only commercially available sound treatment that absorbs significant amounts of energy below 400 Hz. Experimentation will be needed to determine the optimal locations.

8.2 Flutter Echo

These same parallel, reflective surfaces can also produce a different audible problem. If there is little absorption at higher frequencies, a musical transient containing high frequencies, such as a hand clap or the strike of a percussion instrument, can be heard bouncing repeatedly between the surfaces. Called flutter echo (or slap echo), these multiple reflections can obscure musical detail. The situation is analogous to standing between two parallel mirrors, when the outline of your reflection becomes more difficult to discern, due to the additional reflected images present.

Again, it is only necessary to reduce the reflectiveness of one of the surfaces in each pair of surfaces to eliminate flutter echo. Since we are concerned with the high frequencies, any soft material is appropriate. Drapery or fabric wall hangings are quite effective on the walls. Bookshelves also work well by breaking up the flat surfaces. Carpeting should eliminate potential problems between the floor and ceiling.

8.3 Early Reflections

Another situation that can reduce the subjective quality of reproduced sound is the presence of early reflections. By early reflections, we are referring to reflected sound waves that reach the listener within 10 to 20 milliseconds of the direct signal from the loudspeaker.

When a reflected sound reaches the listener more than 40 milliseconds later than the direct sound, the reflection is heard as a discrete echo. However, if the reflected sound arrives within around 20 milliseconds of the direct sound, the ear/brain system integrates the two sounds so that only one sound is heard. This integration is done in such a way that spatial information is preserved, providing an acoustical "picture" of the physical space that created the reflections.

However, the source recording also contains ambient information that portrays the original recording site. Early reflections in the listening room will tend to obscure the ambient information in the recording, leading to a loss of dimensionality or spaciousness. Secondary arrivals within the first 10 milliseconds are especially problematic, becoming less troublesome as the arrival time lengthens to 20 milliseconds or so.

Avoiding Early Reflections

The speed of sound is approximately one foot (30 cm) per millisecond.Therefore, to preserve the natural soundstage on your recordings, there should be no reflected sounds arriving at the listening position with a path length less than ten feet (3 meters) longer than the direct path from speaker to listener (see Figure 8.1). This means that if the speaker or listener is placed closer than about five feet to a wall or other surface, that surface should be covered with sonically absorbent material.

Since the floor is within five feet of the speaker, it is best to have a carpeted floor to absorb floor reflections. A thick, dense carpet and pad will absorb lower frequencies more effectively than a thin one. Due to their complex structure, carpets and pads of natural materials, such as wool and jute, will exhibit a more uniform absorption over the frequency spectrum than synthetic materials will.

It is not necessary to acoustically treat the entire room to achieve good results. Strategic treatment of specific locations can realize considerable benefits. Remember that when sound waves reflect from a flat surface, the angle of reflection is equal to the angle of incidence, just as a mirror reflects light waves. Therefore, the most important location for sound absorbing material is the point where the sound waves reflect to the listener (see Figure 8.2).

8.4 Bass reinforcement

By bass reinforcement, we mean the effect of the room boundaries on the propagation of sound. It is widely known that speaker placement relative to the floor and walls can affect the relative amount of bass that the system produces.

To make this interaction more clear, let us refer to the optical analogy of the candle. Similar to the way that the mirror reflected the light of the candle, so can the surfaces near the loudspeaker reflect the sound waves back into the listening room. However, when the path length difference of the reflected wave is short relative to the wavelength of the sound, the reflected wave is substantially in-phase with the original wave. When this condition is met, the coupling coefficient between the speaker diaphragm and the air increases,and the speaker efficiency increases. This changes the actual frequency response of the speaker, and is not attributable to standing waves or other room resonances.

As frequency increases and wavelength becomes more similar to the distance to the boundary, the phase difference between original and reflected waves increases, and the air coupling effect is diminished. In particular, when the wavelength equals about four times the distance to the boundary, the reflected wave is antiphase to the original wave, resulting in a cancellation (dip) in the output. At frequencies above this level, the effect becomes less significant and creates similar but smaller variations in output.Figures 8.3 and 8.4 illustrate these concepts.

There are typically three reflective surfaces near each speaker: the floor, the rear wall, and the side wall. Each of these surfaces produces its own reflection, and hence its own cancellation and reinforcement. By properly selecting the distances to each surface, we can provide a uniform and extended bass response. Conversely, improper placement of the loudspeakers can result in uneven frequency response, resulting in diminished bass quality.

In order to take full advantage of bass reinforcement and to provide the most uniform and extended bass response, it is recommended that the loudspeaker be placed between three and seven feet from one of the walls (side or rear), and between five and fifteen feet from the other wall. The measurements are made from the wall to the center of the 9" woofer cone. The exact distances are not overly critical, although the two distances should not be within about 33% of each other.

A good rule of thumb in establishing the lateral position of any quality transducer is to apply the ratio 4 : 10 : 4. In other words, the distance to the side wall is 4/18 (or about 22.2%) of the room width, and the distance from speaker to speaker is 10/18 (or about 55.5%) of room width.

8.5 Summary of Recommendations

Now that we have looked at some of the common problems of listening rooms, as well as their remedies, let us summarize our findings and recommendations.

Flutter Echo and Standing Waves

These situations are the result of the room having parallel, reflective surfaces.The potential problems are independent of the audio system, and need to be addressed at the source. This means that at least one surface in an opposing pair of surfaces needs to be made less reflective and/or non-parallel.

• High Frequency Absorption and Room Symmetry

Since flutter echo is a high-frequency effect, it becomes much easier to manage potential problems in this area. Almost any item attached to the walls will be less reflective at high frequencies than the bare walls themselves. Draperies, wall hangings, paintings, bookshelves and other items will normally be present in the room, and will usually eliminate any possible problems. If flutter echo is still audible, a fabric wall hanging provides an effective and attractive cure.

Additionally, it is desirable to maintain a degree of left/right symmetry in the room to preserve a balanced acoustic "space". For example, if your listening room has full length draperies along the right wall, and the left wall is bare, slap echo will not be a problem. Nonetheless, the sound-stage may be somewhat distorted, and it could be beneficial to place a fabric hanging or tapestry on the wall opposite the draperies.

• Parallel Surfaces

Although it is not generally possible to make the walls non-parallel, the same effect is achieved by breaking up the large, flat surfaces with furniture and shelving.

Speaker Placement

Although your Avalon Professional Products loudspeakers may be placed in a wide variety of positions relative to the walls of the room, it is still wise to experiment a bit to achieve optimal results. The suggested minimum distances for the Mixing Monitor are two feet from one wall (side or rear), and four feet from the other (all distances are measured to the center of the woofer cone).This will provide the proper degree of bass reinforcement, as well as minimize early reflections.

The suggested maximum distances for the Mixing Monitor are five feet from one wall (side or rear), and ten feet from the other. As the distance from the speaker to the nearby walls increases, early reflections become less of a problem, and the sound-stage becomes more spacious. However, regardless of the absolute numbers used, the most even bass response will be attained if the distances from the side wall and the rear wall are not overly similar.

Early Reflections

When arranging the furnishings in your listening room, remember that reflective objects should not be within a five foot radius of either the speaker or listener to avoid early reflections. This suggests the possibility of a dual-purpose room, with one end devoted to music reproduction, and the other end for another use, such as a study or office. In this way, the area behind the listener will contain items that will reduce problems with standing waves and/or flutter echo, while the zone around the speakers remains relatively free from reflective objects.

If you wish to achieve an even more spacious sound-stage, it may be useful to place a sonically absorbent material on the side and rear walls near the speakers. This can be particularly effective at the points where the sound wave is directly reflected to the listening position (a mirror can be used to determine these points, as illustrated in Fig. 8.2). As the distance to the wall becomes smaller, the suppression of these reflections becomes more important.

Early reflections will tend to diminish the soundstage in the direction of the reflections, i.e. early reflections from the side walls tend to reduce sound-stage width, while early reflections from the back wall will reduce image depth. We have found that a strong sense of depth enhances the feeling of involvement when listening, due to the three-dimensional solidity of images. Therefore, it is more important to have a greater distance from the speakers to the rear wall than to the sides walls. Typically, this is easier to achieve if the speakers are placed along the short wall of the listening room.

原帖由 ivangogo 于 2008-4-29 13:21:00 发表 不怕宝宝，经常多教育，我的小孩也3岁了，我从小就跟她说。她会自己去开音响，但不敢碰音箱。

你BB真乖！
我那件一摆好就话，好丑样啊，做么是黑色咯，看下啦。
我的CD全是他的手指印，抢着帮我换碟。

原帖由 乐林8 于 2008-4-29 12:01:00 发表 是一对难煲的音箱，只要在中低频单元之上;P 朋友买的新箱（之后报导）。

乐林兄，你朋友用么来推啊？
有空要多交流心得哦！

原帖由 dhft 于 2008-4-29 14:05:00 发表 也许稍微片面，但可以很肯定的说：空间决定一切．．．

如果将CAV放到一个好空间会不会好好声呢！
以前FM方讲过一句话：“假设同舒马赫赛车，他开大巴，我开法拉利，一起步就抛离他一截，点比啊！”。

器材一千万元如果空间不正确处理，声音和声场是绝对不完美也是不耐听的！！！这是颠覆不破的道理吧。

蒋兄，您指点下我要如何正确处理空间。谢谢！
按Avalon的方法可行吗。

东莞兄，指点不敢当呀！如果你觉得你的声音已蛮满意那是最好不过了。如果你希望还想有所提升，那么我们是否可以交流一下（电话）。
短信怎么发？

哪有满意的啊，只有更满意的声音！

你发个短信到我手机吧，我打给您。139-268-08048。

考虑上几个DAAD，有没有在用的前辈啊，请教！

以前香港有几位玩家用过，效果不错，但是现在已经没有代理。

有啊，先声还在做，就是要订货。国际、天下的试音室好像也有摆。

周公，RealTraps出手了吗？我搭单入几只MONDOTRAPS。

我打算用DAAD解决Standing waves，用MONDOTRAPS解决Early reflections。

原帖由 东莞肥仔 于 2008-5-1 0:59:00 发表 有啊，先声还在做，就是要订货。国际、天下的试音室好像也有摆。 周公，RealTraps出手了吗？我搭单入几只MONDOTRAPS。

我曾经打电话给他们，他们说没有，没有提供订货的选择。

我还没有进Realtrap，希望五月中有空请小方拿四块过来测试。

如果你进货，可以考虑Corner Mondo 在角落，处理驻波是最有效。
了可以用四块Mini 处理四个角落，运费较低。第一反射的处理，选择很多。

5寸的陶瓷中低音还会出驻波，不会吧？？

原帖由 recorders 于 2008-5-1 9:08:00 发表 5寸的陶瓷中低音还会出驻波，不会吧？？

7" concave ceramic woofer

原帖由 newbjman 于 2008-5-1 5:29:00 发表 [我曾经打电话给他们，他们说没有，没有提供订货的选择。 我还没有进Realtrap，希望五月中有空请小方拿四块过来测试。 如果你进货，可以考虑Corner Mondo 在角落，处理驻波是最有效。 了可以用四块Mini 处理四个角落，运费较低。第一反射的处理，选择很多。

这边代理好像说可以订、、、？？？

谢谢推荐，我再研究下Realtrap的产品，英文差，看着费劲。唉、、、