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Schoeps话筒麦克风 BLM3g 话筒  Schoeps麦克风 BLM 3G 界面话筒

界面话筒 BLM 3g

专用于在声学界面上使用的话筒拾音头（压力传声器）

当拾音角度变化时，声音的“染色”不会改变

出色的低频再现能力

更适合于管弦乐队的录音

界面录音可以提供不引人注意的话筒摆放方式；无需使用立式或悬吊的固定设备。如果使用压力传声器，它具有与频率无关的半球形方向特性；因此无需对准话筒，即使拾音角度改变，本质上音质仍是相同的。

该技术以物理效应为基础，即：沿着声反射面的声音，其声压将加倍。如果压力传声器直接放置于声音反射面或内部，如BLM 3，其对于直达声的灵敏度将是同等传声器在自由声场内的2倍。它对于自由声场的响应不会改变，但对于在给定距离时的话筒拾音，其清晰度却可以得到显著的提升。

理论上讲，该类型理想的传声器应当具有一个无穷小尺寸的振膜，并被安置在一个无限大的、完美的声音反射表面上（内）。实际中，安置小型传声器BLM 3g的金属底板只反射高频和中频。要反射低频（更长的波长）声音，界面的尺寸相对来说就要足够大。因此话筒要被放置在地板上、墙面或天花板，或安置在其它足够大的表面上，这样才能反射所需要的最低频率。

由于BLM 3g的金属底板只有5mm厚，传声器不能安放在它的中心，而金属板的边缘对于声音的拾取影响非常小。BLM 3g不会妨碍自然声场；偏离轴向（off-axis）上极低频率的声音也能辨别出来。即使移动声源，声染也很低，这是因为，本质上在半指向拾取模式内所有角度拾取的声音其频响都是一样的。

用于靠近声音反射表面的话筒通常会产生梳状滤波效果。但这不会在BLM 3g上发生，这是因为在安置传声器的分界处，直达声及其反射声没有相位差别。

对于高频处的直达声和反射声，通常，压力传声器在灵敏度上会呈现出差异。无论如何放置界面话筒，这个差异在所有频率处都会发生。直达声的声压在声音分界线处是双倍的（增加6dB），由于扩散场反射声相位的关系导致界面话筒的灵敏度只增大3dB。因此界面话筒将会抑制扩散声，即便它不是指向性的。它将拾取少量的“环境声”，它的录音效果比全向话筒听起来“更干涩”。

拾音头本身是一个经典的静电压力传声器，它能在拥有高信噪比的同时，还可以确保低频的正确再现以及典型专业电容话筒的稳定性能。

该类型传声器对风和颤动不敏感，BLM 3g的录音效果比较重，这是因为它仅拾取非常小的固载噪声。举例说，当录制脚步声时，绝大部分到达话筒的声音都来自空气而不是地板。

典型的应用包括：舞台对话的拾音，乐池内拾音，作为环境话筒和特殊的拾音。由于话筒通常放在地板上，就存在被踩踏而被破坏的可能性，通过精心的设计，这类破坏的可能性已经大为降低。一般来说，普通的鞋子并不能损伤拾音头。

与其它Colette系列话筒拾音头类似，BLM 03g（界面话筒拾音头）也需要话筒放大器（如CMC、CMD 2U或M 222），从而组成一只完整的话筒。

还可以增加衰减器（DZC）和有源附件，如可变低频切除滤波器（CUT1或CUT2）或是附加有源线缆（KC）。

界面话筒十分适合A/B（间隔话筒）摆放方式的拾音，也可以用于M/S方式，但需要使用特殊的MSBLM附件。

尺寸：金属底板： 200mm × 200mm × 5mm

话筒放大器连接器净高：21mm

重量：570g（不含话筒放大器）

表面工艺：磨砂灰

附件（包括）： 抛光木盒

BLM 3 BOUNDARY LAYER CAPSULE

PROPERTIES

plate with 200 mm edge length

preferred use: as a room microphone

DESCRIPTION

Boundary-layer recording offers unobtrusive microphone placement possibilities; no stands or hanging fixtures are required.

When pressure transducers are used, the result is a hemispherical directional characteristic that is independent of frequency; thus it is not necessary to aim the microphones, and if the angle of sound pickup changes, the sound quality remains essentially the same.

The technique is based on an effect in physics by which sound pressure doubles along a sound-reflecting surface. If a transducer is placed directly on such a surface, or is built within it as with the BLM 3, its sensitivity to direct sound will be twice that of an equivalent transducer in a free sound field. Its response to diffuse sound remains unchanged, however, causing a notable increase in the clarity of pickup for a given miking distance.

In theory an ideal transducer of this type would have a membrane of infinitesimally small size, and would be mounted flush with an infiniby large, perfectly sound-reflecting surface. In practice, the plate in which the small transducer of the BLM 3 is mounted assures reflection of only the high and medium frequencies.

To reflect sound at lower frequencies (greater wavelengths), the dimensions of the surface must be correspondingly larger. So the microphone should be placed on a floor, wall or ceiling, or mounted on another surface large enough to reflect the lowest frequencies of interest.

Since the plate of the BLM 3 is only 5 mm thick and the transducer is not mounted at its center, the edges of the plate have very little effect upon the sound pickup. The BLM 3g does not interfere with the natural sound field; off-axis sounds thus encounter very little frequency discrimination. Sound coloration is low, even for moving sound sources, because the frequency response is essentially the same for all angles within the hemispherical pickup pattern.

Microphones used near sound-reflecting surfaces ordinarily produce comb-filter effects. This does not occur with the BLM 3, since at the boundary in which the transducer is mounted there can be no phase difference between the direct sound and its reflection.

Ordinary pressure transducers show a difference in sensitivity between direct and diffuse sound at high frequencies. In a properly placed boundary-layer microphone, however, this difference occurs at all frequencies. The sound pressure for direct sound is doubled at an acoustic boundary (6 dB increase), while the essentially random phase relationships of reflections in the diffuse sound field cause the boundary-layer microphone's sensitivity to increase by only 3 dB. Thus a boundary-layer microphone will suppress diffuse sound somewhat, even though it is not directional. It will pick up less “room sound“ and the recording will be slightly “drier“ than it would be with an omnidirectional microphone.

The capsule itself is a classic electrostatic pressure transducer, which assures accurate reproduction of even the lowest frequencies along with the high signal-to-noise ratio and stable perbance typical of professional condenser microphones. This type of transducer is inherently insensitive to wind and vibration and the BLM 3 is comparatively heavy, so it picks up very little solid-borne noise. For example, when footsteps are recorded, most of the sound reaching the microphone will be conducted through the air rather than the floor.

PRACTICE

Typical applications include pickup of stage dialogue, or in orchestra pits, as an ambience microphone and for special public address applications. Since the microphone may well be used on the floor, the likelihood of damage from its being stepped on has been reduced by careful design and construction. As a rule, the capsule will not be harmed by normal footwear.

The BLM 3 (boundary-layer microphone capsule) resembles any other microphone capsule of the Colette series in that it needs a microphone amplifier CMC to b a complete microphone. This arrangement allows Active Accessories such as the low-cut filter CUT 60 or the Active Cable (KC) to be added as well.

Boundary layer microphones are particularly useful for unobtrusive recording with A/B (spaced microphone) bs, but M/S recording is also possible, using the special MS-BLM accessory.

TECHNICAL SPECIFICATIONS

Pickup pattern

Half-space

Frequency range ：20 Hz - 26 kHz with CMC 6 / CMR,20 Hz - 43 kHz with CMC 6xt

Sensitivity ：-36 dB (V/Pa), 16 mV/Pa with CMC 6

Equivalent noise level (A-weighted) ：12 dB with CMC 6

Equivalent noise level (CCIR) ：23 dB with CMC 6

Signal-to-noise ratio (A-weighted) ：82 dB with CMC 6

Maximum sound pressure level (THD < 0.5 %) ：130 dB-SPL with CMC 6

Length ：200 mm

Width ：200 mm

Height ：5 mm

Weight ：570 g

Surface finish ：matte gray