ACOUSTIC CHARACTERISTICS OF
NON-AUDIBLE MURMUR
Tatsuya Hirahara, Shota Shimizu and Makoto Otani
Faculty of Engineering, Toyama Prefectural University

ABSTRACT
Acoustic characteristics of Non-Audible Murmur (NAM) are clarified. NAM isa very weak whispered voice which can be detected by a NAM sensor attachedto the surface of the skin close behind the ear. NAM is inaudible
body-conducted sound whereas normal voice is audible air-conducted sound.
NAM signals are recorded from 6 male and 7 female speakers using asoft-silicon type NAM microphone. The signal-to-noise ratio of the NAM signal
is around 15 dB. The bandwidth of the NAM signal is 3 kHz. Long-term
averaged spectra (LTAS) of the NAM signal show energy concentration around
500 to 800 Hz and a -17 dB/octave roll off. The calibration of NAM
microphones reveals that a soft-silicon type NAM microphone has a low
sensitivity above 3 kHz, whereas a urethane-elastomer type NAM microphone
has a good sensitivity up to 10 kHz. After compensating the NAM microphone
response, the LTAS of the raw NAM signal show energy concentration around
200 to 300 Hz and a -23 dB/octave roll off, which is roughly parallel to thenumerical simulation of transfer characteristics of sound in head and neck.

INTRODUCTION
The ordinary voice is the air-conducted sound, which is the vocal-tract resonance ofthe vocal cords vibration due to the air stream from the lungs. Other than the ordinary
voice, we are using diverse types of voice in daily life such as singing voice, shouting
voice, laughing voice, crying voice, small voice, whispered voice, and so on. Further,
we hear our own voice through bone conduction, but others' voices conducted throughthe air. This is why our voice sounds different when it is played back from a recording.
These diverse types of voices can be classified based on two axes (Fig. 1).
One axis is the phonation type of the voice sound source. The sound source is either
periodic glottal flow or random noise flow provided by the air stream passing throughthe glottis. Stronger air stream is needed to shout and to sing, while weaker air stream isenough to whisper. The sound source is then filtered by the vocal-tract configuration
and radiated into the air from the mouth. This axis also represents the energy of voice.
Another axis is the conduction type of the voice sound. Typically, the voice soundwe use is air-conducted sound, which is small and fast vibration of the air. Meanwhile,the small and fast vibration of the air in the vocal-tract vibrates the vocal-tract wall.
Some sound energy passed through tissues and bones of the neck. This bone-conductedvoice can be picked up with a bone-conduction microphone. As is the bone-conduction,
the body-conducted voice, that is the voice running through neck muscles, can besensed with a special microphone invented by Nakajima [1-3]. He also found that weakmurmured voice, which is usually unheard by people nearby, can be detected by theBone
Bone-conducted
ordinary voice

Conduction typeC = 4000
Z = 7.8×105

Body
C = 1500
Z = 1.5×105

ＮＡＭ

Air

Body-conducted
ordinary voice

WhisperC = 340 [m/s]Z = 400 [kg/m2s]

Singing voice

Ordinary voice

Crying/Shouting

Phonation type / Energy

Fig. 1.

Diverse types of voices.

special microphone attached to the surface of the skin close behind the ear. Namely,
voice sounds propagate through not only the air but also the bone and the body. Suchbody-conducted weak murmured voice sounds are named non-audible murmur (NAM).
These new voices extend the horizon of the voice fields. The use of thebody-conducted voice, in particular, expands the capacity of voice technologies. As the
NAM sounds are unheard, the NAM can be applied for automatic speech recognition(ASR) system and telephone system that require privacy. As the air conducted noisesounds are not mixed with sounds picked up by the NAM microphone, the device caneliminate noise problems of the ASR system. The NAM microphone also will be able torevive the speech communication of those with vocal cord problems caused by larynx
cancer, nerve disorders and muscle diseases.
The nature of NAM, however, is less well understood because it is only recent that
the NAM has ever recorded. Not only the acoustic characteristics of the NAM signal but
also the frequency response of the NAM microphone has yet to be revealed. This paper
clarifies acoustic characteristics of the NAM signal by analyzing large amount of data,
and also characterizes the frequency response of NAM microphones.

CALIBRATION OF NAM MICROPHONE
NAM microphones are calibratedusing an audio analyzer (Brüel & Kjær:PULSE 3560C), an accelerometer (OnoSokki: NR-3211), a bone conduction
weight 1kg
accelerometer
NAM
vibrator (Rion: BR-41) and an urethaneNP-3211
mic
B&K
elastomer cylinder of 75 mm in diameter
urethane eastomer
and 50 mm in height.
3560C
cylindersilicon cover
75 mmφ×50mmH
The vibrator is placed beneath thecylinder. A NAM microphone and an
vibrator
accelerometer are placed on the top of theBR-41
cylinder. The accelerometer is covered
with silicone cover so as to align theheight to the NAM microphone. A weightof 1 kg is added to hold them down (Fig. 2).
Fig.2. Microphone calibration system.

Figure 3 show the soft-silicon type NAM microphone made by Mitsumi Electric
Corp. and the home-brew urethane-elastomer type NAM microphone. The signal sensed
by the accelerometer was amplified by a sensor amplifier (Ono Sokki: SR-220), whose
frequency response is flat. The signal sensed by a NAM microphone was amplified by a
NAM microphone amplifier (Mitsumi: NAM-01A), whose frequency response is broadband-pass characteristics as shown in Figure 4.
Let measured frequency response of the accelerometer be A(ω), then A(ω)/ω2 gives
the frequency response for the displacement. The accelerometer has flat response foracceleration below 10 kHz. As NAM microphones are displacement sensors, comparinga measured frequency response of a NAM microphone D(ω) with A(ω)/ω2 gives thecalibrated frequency response of the NAM microphone. The frequency response of theNAM microphone amplifier was compensated.
Their frequency responses are shown in Figure 5. Both NAM microphones have
similar response below 1.5 kHz, but they differ in higher frequency region. The
response of the urethane-elastomer type NAM microphone is fairly flat up to 10 kHz,
while that of the soft-silicon type NAM microphone gradually rolls off at a -17
dB/octave between 2 kHz and 9 kHz. Sensitivities of the two types of NAM microphone
at 159.2 Hz are 15 dB higher than that of the accelerometer.

Fig. 3. A soft-silicon type (left) and a urethane-elastomer type (right) NAM microphones.
40
Relative Gain [dB]Gain [dB]

20
0
- 20
- 40
0.1

0.5

1.0

5.0

10

20

urethane elastomer0- 40
soft silicon
- 80
0.1

Frequency [kHz]Fig. 4. Response of the NAM microphone amplifier.

0.5

1.0

5.0

10

20

Frequency [kHz]Fig. 5. Responses of the two types of
NAM microphone.

NAM SIGNAL RECORDING
NAM signals were recorded in a sound proof room. 6 male and 7 female adultsubjects read 50 ATR phoneme balanced Japanese sentences in NAM style. A
soft-silicon type NAM microphone and the NAM microphone amplifier were used in
the recording. The microphone was attached to the surface of the skin close behind theear and fixed with a neck band. The setting position of the NAM microphone waschosen carefully so as to detect good NAM signal. The NAM signal were recorded by asolid state recorder (Marantz: PMD670) with sampling rate of 48 kHz and 16 bits
quantization. During the recording, the NAM phonation style and signal level weremonitored by an expert to keep the recording signal quality. When large noise wasinvolved, talkers were asked to reread the sentence. In total, 80 minutes NAM signals of
640 sentences were recorded.

ACOUSTICAL CHARACTERISTICS OF NAM SIGNAL

Frequency in kHz

Waveform and spectrogram of an NAM signal sensed by a soft-silicon type NAM
microphone are shown in Figure 6. Speech period is identifiable on the signal waveform.
But background noise level is high and a number of impulsive pop noises areoverlapped with NAM signal, suggesting that the signal-to-noise ratio (SNR) of theNAM signal is low. The spectrogram also suggests that the SNR of the signal is low andthat the bandwidth of the NAM signal is narrow.
The pop noises are generated either by a rustle of the NAM microphone and the skinor by articulatory organs' contacts when producing plosives. Detailed waveforms ofthese pop noises are depicted in Figure 7. Some of the noise can be reduced by athreshold processing, as the amplitude of the noise is usually larger than that of theNAM signals. After reducing some pop noises, speech periods of the NAM signals aredetected using their power and delta cepstrum coefficients.
Mean signal-to-noise ratio (SNR) is 15 dB for the NAM signals of 48 kHz samplingrate. Lowering the sampling rate improves the SNR and it reaches 21 dB in maximum atsampling rate of 2 kHz. The SNR of the NAM signals, however, is much worse thanthat of ordinary speech signal sensed by a normal microphone.

8
6
4
2
0
0

1

4

3

2
Time in seconds

Fig. 6. Waveform and spectrogram of a NAM signal sensed by a soft-silicon type
NAM microphone.
0.6

0.6

0.4
Amplitude

Amplitude

0.4
0.2
0
-0.2
-0.4
-0.6
0

0.2
0
-0.2
-0.4

400

800

1200

Number of data [samples]

1600

-0.6
0

400

800

1200

1600

Number of data [samples]

Fig.7. Waveform of the pop noise caused by an articulatory organs' contact (left) andby a rustle of the NAM microphone and the skin (right) with a magnified time axis.

20
10
0

Relative Spectrum Level [dB]

Relative Spectrum Level [dB]Long-term averaged spectra (LTAS) of the NAM signals resampled at 16 kHz are
shown in Figure 8: blue and red solid lines indicate mean LTAS of 6 male and 7 female
NAM signals and dotted line shows the noise floor. LTAS show energy concentration
around 500 to 800 Hz both for male and female NAM signal. Their spectrum roll-off
above 800 Hz is at a -17 dB/octave and that below 500 Hz is at a 10 dB/octave. The
LTAS also show the bandwidth of the NAM signal is 3 kHz. Noise floor level of therecording system and the NAM signal strength limit the bandwidth.
The frequency responses of the NAM microphone as well as the NAM microphone
amplifier are involved in the NAM signals to calculate the LTAS. The LTAS of the raw
NAM signal, thus, can be obtained by subtracting those responses from the LTAS.
Figure 9 shows the LTAS of the raw NAM signal. The LTAS show energy concentration
around 200 to 300 Hz. Their spectrum roll-off above 300 Hz is at a -23 dB/octave.
According to the source-filter theory of speech production, the LTAS of the ordinary
voice has an approximately -6 dB/octave roll-off. As the radiation characteristics fromthe lips is +6 dB/octave, spectral tilt is estimated -12 dB/octave for the ordinary voice inthe vocal tract. Although the spectral tilt of whispered voice is less sloped than that ofordinary voice, the spectral tilt of raw NAM signal is far less sloped than that of
air-conducted whispered voice and ordinary voice.
Numerical simulation of transfer characteristics of sound in head and neck [4]suggests that body conducted sound has fewer higher-frequency components than anair- conducted sound and that the decay of the transfer characteristics is approximately-10 dB/octave in the audible range. Adding this decay to the -12 dB/octave is -22 dB/
octave, which is roughly parallel to the spectral characteristics of the raw NAM signal.
Male
Female

-10
-20
-30
-40
-50

Noise floor

-60
0.1

0.4

1
Frequency [kHz]Fig.8. LTAS of the NAM signals.

4

8

20
10
0

Female

-10
-20
-30
-40
-23 dB/ oct.

-50
-60
0.1

0.4
1
Frequency [kHz]

Male
4

8

Fig.9. LTAS of the raw NAM signals.

DISCUSSION
Two types NAM microphones show quite different frequency responses. Existing
soft-silicon type NAM microphone senses traditional telephone band signals. Lately
developed urethane-elastomer type NAM microphone can sense broader signals.
Urethane elastomer is an industrial material having both the hardness of plastic and theelasticity of rubber. As the material can be poured, it is easy to fill the material into thecondenser microphone. In addition to that, urethane elastomer has impact absorbing
qualities with glutinosity, and is resistant to oil and chemicals. An urethane-elastomer
type NAM microphone can, thus, be attached to neck without using a neck band. As it
sticks to the neck skin, rustle of the NAM microphone and the skin decreases, reducing
the pop noise generation. Waveform and spectrogram of a NAM signal sensed by aurethane-elastomer type NAM microphone are shown in Figure 10. As a talker and aFrequency in kHz

8
6
4
2
0

0

1

2
3
Time in seconds

4

Fig. 10. Waveform and spectrogram of a NAM signal sensed by a urethane-elastomer type
NAM microphone.

NAM microphone amplifier used are different, direct comparison is not fair. The NAM
signal, however, apparently has better SNR and broader bandwidth. Fewer impulsive
pop noises are observed in the waveform in Figure 10 than that in Figure 6. The
urethane-elastomer type NAM microphone is promising.

CONCLUSIONS
Two types of NAM microphone were calibrated. A soft-silicon type NAM
microphone only senses signals below 3 kHz. Meanwhile, a urethane-elastomer type
NAM microphone can sense broader signals up to 10 kHz. Sensitivities of the two typesof NAM microphone are 15 dB higher than that of the accelerometer.
Acoustic characteristics of NAM signals sensed by a soft-silicone NAM microphone
were clarified by analyzing large amount of data. The mean SNR of the NAM signal is
15 dB and the bandwidth of that is 3 kHz. The LTAS of the NAM signal, including
frequency responses of the NAM microphone as well as the NAM microphone amplifier,show energy concentration for frequencies between 500 to 800 Hz and a -17 dB/octaveroll off. Meanwhile, after compensating the NAM microphone response, the LTAS of
the raw NAM signal show energy concentration around 200 to 300 Hz and a -23
dB/octave roll off. This is roughly parallel to the numerical simulation of transfer
characteristics of sound in head and neck.
This work is supported by SCOPE-S of the Ministry of Internal Affairs and Communications
Japan.
REFERENCES[1] Nakajima Y. et al., “Non-Audible Murmur Recognition Input Interface Using Stethoscopic
Microphone Attached to the Skin, ” Proc. ICASSP, 708-711. (2003).
[2] Nakajima Y., “Development and Evaluation of Soft Silicone NAM Microphone,” Technical
Report of IEICE Speech, SP2005-7, 7-12. (2005)[3] Nakajima Y., Kashioka H., Campbell N. and Shikano K., “Non-Audible Murmur (NAM)
Recognition,” IEICE Trans. on information and systems, Vol.E89-D, No.1, 1-8. (2006)[4] Otani M. and Hirahara T. and Adachi S., “Numerical simulation of sound originated from thevocal tract in soft neck tissues,” J.Acoust.Soc.Am., Vol.120, No.5, 3352. (2006)