主题：[求助]关于开孔式音箱的近场测试

今天彭老师讲到用近场测试开孔式音箱时，喇叭及风管分别测试，然后把测试结果做相减处理，为什么会是相减，而不是相加呢？不管风管在前面、侧面还是后面都一样相减吗？为什么？对于无源辐射器音箱也是一样吗？谢谢！

ERIC:

你今天也去听课啦?

个人理解应是相加,虽然倒相管所起的作用是倒响,但它是将低音扬声器背向辐射的声音反了一个相位,对正面而言是负负得正.

至于倒相管在箱体侧面或后面,需加一个距离修正(即相位修正),全部换算到远场测试,先相加再与远场测试的曲线拼接.MLSSA测试就是如此处理的,含无源辐射器.

不知LMS是否测试信号和规定不同?可实际验证一下,哪种说法更准确.

水仙前辈，谢谢！对啊，我有参加这次的培训。

我想不管风管（或者假喇叭）在哪一个面，都应该把相位考虑进去吧，可是相位又该怎么测量呢，用LMS里的数据处理功能也取得吗？谢谢！

有没有考虑面积因数？

水仙老师：我个人认为倒相管并不是把声音反相出来，而是在较低的频率时强大的声压让倒相管产生谐振而出的声音，从倒相管的声阻峰可以看出。可能这种说法没有一个人赞同，但是我说出来，是想您能给我解答疑惑。

仅倒相管是不会产生谐振的，很简单就好象但有一个电感，或但有一个电容的电路不会产生谐振一样！倒相管（等效为质量元件）和箱体内的空气（等效为顺性元件）一起才组成一个谐振系统！在阻抗型类比线路中类比为并联电感和电容，流过它们的是体积速度！在谐振频率点前后体积速度有个反相的过程，这就是倒相管的名称由来！

如果是LMS的话，那么就是相减！首先LMS测试说明书上有做出了解释，而且实际上也做了很多的验证的。不过减之间是必须要做相位修正的。

应该是相减

看这个帖子，看的晕头转向，说出结果的不说原因，为什么是减？

个人觉得，无论是否倒相孔或者多个单元，在低频的时候，由于波长已远远大于它们的尺寸甚至它们之间的距离，因此，对多个辐射源来说，我们可分别测量他们的瞬时声压，然后按其辐射面积换算，再矢量叠加（更准确的可考虑多辐射源之间的距离）便可以得到他们对应于1米或N米的低频特性（仅指安装在无限大的障板上的）。

这种叠加方法对于能测量实际相频特性的测量系统是完全没有问题的，比如MLLSA，但对于LMS这样的测量系统来说，（由于我没有使用过LMS系统，以下所有都是道听途说回来的，错了的话饶了我吧）由于LMS不能得到实际的相频特性，仅根据其幅频响应而通过希伯特转换得到该幅频响应对应的所谓的最小相频特性。所以。。。

按道理来说无论扬声器振膜和倒相孔在近场测试的时候都可以视作最小相位系统，故此上面的转换出来的相位曲线并无技术上会引起变化的问题（指针对叠加来说），那为什么需要对某一声源相位进行反相再进行叠加呢？估计还是LMS对相频的转换问题引起的。

以上乱批一通，希望大家给我解惑

这么多人回答了问题, 没一个回答对的.初哥的迂腐分析尤其可笑

"彭老师"自己都不是很懂,所以没办法告诉你了.我在课堂上当他面敢这么评论他,你信不信?

既然大家都沒有比較滿意的結果,E文好的弟兄發個郵件到LINEARX請教一下吧.

木源在哪里高就啊

远近场转换当然要考虑辐射面积,上次赵老师上课已讲得很清楚并给了公式.

相位的概念,上次赵老师上课也讲得很清楚,既与测试距离有关,又与时间有关,简单表达如下:

φ=ωt-kr+φ_o

看来上面的分歧来源于不同的测试系统,请参照各自的说明书!

我记得是LMS还是LEAP的说明书上有提过那么一句,但是没有讲得很详细,看来木原的注意不错,直接问LINEARX好了.呵呵

根据本人以前的实际操作经验，应该是相加。但我的操作不是在LMS中操作的，因为我记得我用过的LMS测频响时不包含相位，现在不用LMS了，不知是否改进了。

本人理解倒相管是和箱体组成一个谐振系统。通过谐振来改变声波的相位的，否则就箱体内的那点传播距离不可能让低频声波反相。所以倒相管放前面还是后面对低频部分的影响不是太大——当然有影响而且出来的声音不一样。个人认为要得到一个比较真实的结果，应该把声波相位换算到同一点后再相加。所以如果是后倒相，应该在倒想管的结果上再加上从倒相管口到前面喇叭附近的这段传播距离发生的相移。

不知各位高手有何高见。

倒相箱某点的声压决定于喇叭正反俩面在此处的体积速度，只是反面的经过箱体和孔的作用改变了相位。正面的体积速度为Usp,反面的体积速度为Up,那结果应该为Usp-Up, 而Up经过箱体和孔后发生了相位改变，而此相位改变是包含在Up以内的，简单讲如果Up变成了负数，实际上就是相加了--这正是倒相的原理。

此主题相关图片如下：

MLSSA:

Center the microphone and position it as close to the
diaphragm or dust cap as possible without touching. In the case of open ports, position
the microphone's front face flush with the enclosure surface.

Make the first measurement by executing a Go Once command. Use FFT Execute to
compute the near-field transfer function of the port or driver in question. Save the
transfer function to an FRQ data file by executing Transfer Save from the frequency
domain. Use Go Once from the frequency domain to repeat this procedure for all ports,
drivers or passive radiators. Each measurement must be saved to a separate FRQ file.
A good idea is to use filenames that start with the same prefix and use a suffix that
clearly identifies the port or driver in question later on when it is time to combine the
files.
Measure the diameter of each port, driver or passive radiator and record these data on
a piece of paper for future reference. The diameters of the drivers should be measured
to include about 1/3 of the surround on each side.
If any port or radiator is not circular, you can alternatively use the square root of its area
to compute its effective diameter. For example, assume a loudspeaker having an
elliptical 6" by 9" passive radiator. These are nominal dimensions but in practice, you
should actually measure the diameters involved including about 1/3 of the surround on
each side. The area of an ellipse is (πab)/4 where a and b are the minor and major
diameter respectively. This area of this passive radiator is then 42.41 sq inches and
thus its effective diameter is √(42.41) = 6.51 inches.
When radiators or ports are mounted in vastly different planes further corrections are
required as explained below.

To properly combine all the near-field measurements execute Library Operations
Frequency-files Average Weighted Complex. Enter the directory where the near-field
measurement files reside followed by the file prefix you selected or just the file prefix if
they reside in the current directory. MLSSA will list all the FRQ files beginning with the
specified prefix. Use Spacebar or the left mouse button to mark all the near-field FRQ
files you just measured. Press the Ctrl-Enter key combination or the right mouse button
to exit the filebox. MLSSA will next display each filename in turn and prompt you to
enter the weighting factor for each. Enter these as the diameters you had previously
recorded making sure each corresponds to the correct filename. Finally, enter an
output filename to save the composite near-field low frequency response.
The resulting anechoic low frequency response can either be spliced to a far-field
Transfer-Function or Sensitivity measurement using the Library Operations Frequencyfiles
spLice command or, displayed as an offset overlay on top of the far-field anechoic
response.
The Library Operations Frequency-files spLice command will prompt you for the splice
frequency in Hz. If you enter nothing and press the Enter key, MLSSA will select a
splice frequency determined by the true resolution of the high frequency far-field
measurement file. This command adjusts the decibel level and phase delay of the low
frequency near-field file to smoothly connect it to the high frequency far-field file. The
output file contains the spliced measurements, which can be loaded into the frequency
domain and displayed through Transfer Load.
In case a port or woofer is mounted in a vastly different plane than the others, you need
to account for the acoustical delay between it and them. This is most easily
accomplished in the frequency domain through the View Phase Delay command.
Suppose a passive radiator is mounted at the rear of the enclosure with the woofer in
front and, suppose further that the enclosure is 2 feet deep. This requires that a delay
of about 2 ms be added to the passive radiator output before combining it with the other
radiators. To do this, load the rear port's FRQ file and execute View Phase Delay and
enter -2 ms. Next save the result to the original file and answer 'yes' when asked to
apply the phase delay correction to the saved data. You can now combine the FRQ
files according to the procedure given above.
Here are some points to remember when making near-field measurements. Near-field
loudspeaker measurements correctly predict the true anechoic far-field response only in
the driver's piston range of operation below roughly f = 4290/d Hz where d is largest
dimension of the largest diaphragm or port in inches.
Another limitation of the near-field method is that it does not account for sound
diffracted from the enclosure edges. Diffraction will affect the desired far-field response
but will not affect the measured near-field response. While this is a theoretical
limitation, in practice, diffraction effects from typical enclosures are small below about
250 Hz because the wavelengths involved at such low frequencies are typically much
larger than the enclosure dimensions.

昨晚做梦，被驴踢了一脚，踢在脸上，火辣辣的痛，天亮起来发现没什么不正常的，到坛子上一看，明白了，原来在这里被“菜鸟”踢了一下，哈哈！

有的人原来还能发表一些自己的“错误”见解，不管怎么样还比较实在，被批了几次以后，就剩下说空话，白话了！看来大家以后还是少批为好！呵呵！！。。。。。。。

问题无高深和简单之分！一些刚入行的朋友，基本概念还没完全搞清楚，很多问题可能很简单，难入一些高手的法眼，愿答你就答，不愿答，也没人说你“藏私”，吃“独食”，不要等人家答了，然后你再来PK一顿，这是什么？小人！

知识是慢慢积累的，不是说所有人都能搞清楚“多普勒效应”与“成功学”的关系的，呵呵！我就搞不懂！