Test Methods
30092212
FIGURE 2. FFNSE, NFSLE, SNRIE Test Circuit
FAR FIELD NOISE SUPPRESSION (FFNSE)
two microphones (see Figure 9). In this configuration the
speech signal at the microphone closest to the sound source
will have greater amplitude than the microphone further away.
Additionally the signal at microphone further away will expe-
rience a phase lag when compared with the closer micro-
phone. To simulate this, phase delay as well as amplitude
shift was added to the NFSLE test. The schematic from Figure
3 is used with the following procedure to measure the NF-
SLE.
For optimum noise suppression the far field noise should be
in a broadside array configuration from the two microphones
(see Figure 8). Which means the far field sound source is
equidistance from the two microphones. This configuration
allows the amplitude of the far field signal to be equal at the
two microphone inputs, however a slight phase difference
may still exist. To simulate a real world application a slight
phase delay was added to the FFNSE test. The block diagram
from Figure 3 is used with the following procedure to measure
the FFNSE.
1. A 25mVP-P and 17.25mVP-P (0.69*25mVP-P) sine wave is
applied to Mic1 and Mic2 respectively. Once again, a
signal generator is used to delay the phase of Mic2 by
15.9° when compared with Mic1.
1. A sine wave with equal frequency and amplitude
(25mVP-P) is applied to Mic1 and Mic2. Using a signal
generator, the phase of Mic 2 is delayed by 1.1° when
compared with Mic1.
2. Measure the output level in dBV (X)
3. Mute the signal from Mic2
4. Measure the output level in dBV (Y)
5. NFSLE = Y - X dB
2. Measure the output level in dBV (X)
3. Mute the signal from Mic2
4. Measure the output level in dBV (Y)
5. FFNSE = Y - X dB
SIGNAL TO NOISE RATIO IMPROVEMENT ELECTRICAL
(SNRIE)
NEAR FIELD SPEECH LOSS (NFSLE)
The SNRIE is the ratio of FFNSE to NFSLE and is defined as:
SNRIE = FFNSE - NFSLE
For optimum near field speech preservation, the sound
source should be in an endfire array configuration from the
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