Sonodore Surround Microphone BLM-21

This surround microphone, makes use of the renowned capsule of the RCM-402.
This microphone is suited for placement on the floor or on the wall. Due to the boundary
layer principle, the noise level is even 6dB lower than the RCM-402, and the microphone
has a very distinctive and unique polar pattern.
We found this pattern, which can most easily be described as a large circle laying on the
ground around the microphone, in the shape of a thick 'sausage', to be very suitable for
surround 'back' channels.
The frequency response rolls gradually of from 14kHz, also due to the principle of a
microphone membrane close to a large surface, and the polar pattern of two BLM
channels gives a rather unique, diffuse 'stereo' image, which proves to be ideal for
acoustic surround channels.
Also for placement on stages at live performances the BLM-21 will be very useful.

The result of recording voices and back-ground noises in radio plays is stunningly
transparent and natural. The fact that all sounds from all directions are picked up, and
accumulated at one point on the floor or the wall, and thus no first early delayed reflection
from floor or wall are to apparent like placing the membrane in the air, creates a high
suggestion of actual presence of the sound source.

We believe to have created the best boundary layer microphone to date by using the
un-compromised high quality pressure capsule from the RCM-402 and we also found
that the capabilities of the principle are highly underestimated.

The BLM-21 set consists of a stand and a CCM-65 microphone. 
So the CCM-65 can be used without the BLM stand.


Technical description boundary microphone

A conventional microphone receives the direct sound wave plus a reflected wave from
the floor. It suffers from the comb-filter series of dips in frequency response.
But for  the boundary microphone, the reflections do not interfere  with the direct sound
because it is on the floor or the wall. The result  is a stunningly transparent and
natural sound. A boundary microphone  gives a special sound and It can't be compared
with a conventional microphone.


Rough frequency response


Technical Specifications
Capsule type : prepolarised condenser microphone capsule  
Principle of operation: pressure  
Directional pattern: hemispherical
Frequency response: 5 Hz(-3dB) to 20kHz (-7dB), (14kHz 0dB)
Sensitivity at 1 kHz: 75mV/Pa
Equivalent noise level: A-weighted typically 13 dB
Temperature coefficient: +0.005 dB/ēC
Output impedance: 39 Ohms
Cable drive capability: > 100m
Weight: 650g
Length: 177mm (6.97”)
Capsule diameter: 12.8mm (0.5”)
Dimensions: 18cm x 21cm x 7,2cm (7.1” x 8.3” x 2.8”)
Active  60V model
Maximum sound pressure level: 143 dB SPL peak
Amplifier THD at 130dB SPL: 0,0004% at 1kHz, 0,0013% at 10kHz
Connection: fixed cable (diameter 4mm) with 4-pin XLR male
Powering: via separate power supply (60V)
Phantom 48V model
Maximum sound pressure level: 136 dB SPL peak
Amplifier THD at 124dB SPL: 0,0004% at 1kHz, 0,0013% at 10kHz
Connection: fixed cable (diameter 4mm) with mini 3-pin XLR male
                      and adapter mini 3-pin XLR to 3-pin XLR male
Powering: 48V phantom power, 6mA 



Different BLM-21 models

Active  60V powering in black.
PS-402, MPB-502 or MPS-502 power supply / pre-amplifier is necessary.

Phantom 48V powering in black.
If you want to stay with the phantom standard.

The BLM-21 set consists of a stand and a CCM-65 microphone. 

The stand of the BLM-21 can be ordered separately.


Technical description boundary microphone: Boundary microphone



Power Supply
PS-402                                                   Power Supply battery powered PSB-402


MPS-502                                                  Pre-amplifier battery powered MPB-502


Microphone cables


Sonodore price list with Mogami cable