CA918 Free Field Standard Source
Usually, CA918 for microphone sensitivity comparison method determined in the free field, its principle is: the first to use the reference microphone position measurement of sound pressure level in the free field, and then replaced with the microphone under test. If two microphone acoustic center located in the same point in the sound field, and the point of the acoustic field remains the same, depending on the microphone to be tested with the ratio of the output voltage of the reference microphone and the free field reference microphone sensitivity, can determine the free field of the measured microphone sensitivity. This test method is not only requires a high quality full anechoic chamber or a semi-anechoic chamber, also requires a high quality source.
Choose what kind of speaker as the sound source, frequency range of the free field test and measurement uncertainty has an important influence. Ideally, the speaker should be small enough to meet the characteristics of noise source, and in the frequency range are required to keep all the directivity, the sensitivity should be high enough to produce enough in the measurement location of sound pressure level, the output should be stable over time, and frequency response should be flat as far as possible in the frequency range.
However, the speaker's actual design seldom meet all these requirements, must make a compromise.
In addition, because of the voice from the loudspeaker radiation in all directions, the speaker shell or the edge of the installation can be secondary radiation position, making it impossible to keep the plane wave sound field. Therefore, the speaker enclosure must be designed to minimize the effects of.
For this, as the sound source, in the free field use CA918 chose 3.5 inch high sensitivity speakers unit, mechanical Q (Qms) as low as 3.2, at the same time the total Q value (for Qts signs) as low as 0.29, such a low Q value makes the speaker has a wider range of frequencies, can be in 125 Hz ~ 20 kHz frequency range meets the requirement. The sensitivity of up to 89.8 dB in the reach regulation sound pressure level remains low distortion, 4? The nominal impedance can achieve maximum 30 W power output, and reduce the demand for power amplifier output voltage. In addition, the design for spherical shell of the sound source, and makes the edge of the speaker and the spherical tangent, also for the spherical shell caused by secondary radiation, avoid a directivity.
Anyhow, CA918 is a perfect design, excellent standard of free field source, is the ideal choice of the dead room source.
3.5 inch high sensitivity speakers unit
Frequency range of 125 Hz ~ 20 kHz (plus or minus 7 dB)
Low distortion, high Q value
Spherical shell is designed to reduce the effects of secondary radiation
Scope of application: