Acoustic Characteristics of Japanese Vowels
Tatsuya Hirahara* and Reiko Akahane-Yamada*** NTT Communication Science Labs., NTT Corporation**ATR Human Information Science Labs.
hirahara@idea.brl.ntt.co.jp, yamada@atr.co.jp

Abstract
The purpose of this study was to build a large
database on Japanese vowels and to clarify theiracoustic characteristics. Recordings were made of 256
males and 252 females aged from 6 to 76 years old, forboth Kanto (Tokyo) and Kansai (Osaka) dialects,
producing the vowels /i, e, a, o, u/ in isolation, in /h-Vda/ syllables and in /b-V-ta/ syllables within andwithout a carrier sentence. The five vowels uttered withdifferent notes were also recorded. A total of 154,570
vowel tokens were collected and phoneme labels were
given to each of them. Analysis of the formant data
showed that there was little difference between the two
dialects. Formant frequencies were the highest forchildren's vowels and those of boys and girls werecomparable in distribution. Formant frequencies of men
were the lowest. The mean F0 of men dropped at theage of 12 to 13 due to the voice breaking. Formant
frequencies shifted up when vowels were uttered at
higher musical notes. The rate of the shift due to F0change, however, was smaller than that seen among age
groups. Dynamic features of formants varied withphonemic environment but were not affected by mora
structure and talkers age.

1.

Introduction

The major acoustic correlates of vowel quality areformant frequencies that reflect vocal tract resonances.
Formant frequencies of American English vowels havebeen well studied as represented by Peterson andBarney (1952) and Hillenbrand et al. (1995).
Research on the acoustics of Japanese vowels
started in the 1930's. Obata (1933) applied Fouriertransform to the analysis of sounds including vowels.
Chiba and Kajiyama (1942) carried out a pioneering
work on vowels. Honda (2002) wrote a history of theincunabula. In the late 1950's, Hattori et al. (1957) andTorii (1957) measured formant frequencies of Japanesevowels by reading sonagrams. Suzuki et al. (1963)
analyzed 100 monosyllables for the articulation test
uttered by two male professional announcers by amoment method. Kadokawa and Nakata (1964)
analyzed isolated vowels uttered by five adult males bymeans of an Analysis-by-Synthesis method. Kasuya etal. (1968) investigated changes in F0 and formant
frequencies with age and gender for over 100 talkers

Th.P3.5

aged between 7 and 20 years old using sonagrams. In
the 1970's, digital computers began to be commonlyused in speech studies. Sato (1975) analyzed vowels
uttered by 34 male and 29 female talkers to investigate
acoustic correlates that determine male and female
voice qualities. Keating and Huffman (1984) reported
F1-F2 diagram of Japanese vowels in 30 words forseven young male talkers. Miwa (1991) analyzed
formant frequencies of isolated vowels uttered by 10
male professional announcers using the A-b-S method.
Konno et al. (1994) compared formant frequencies ofvoiced and whispered vowels of three male talkers.
Ueda (1995) analyzed isolated vowels uttered by 6- to
22-year-old talkers of the Kumamoto dialect (83 males
and 125 females) to investigate speech spectral
normalization for a tactile display. Recently, Okuda etal. (2002) analyzed a large-scale Japanese speech
database for automatic speech recognition studies. The
database consists of 177,163 utterances of 3771 talkers
recorded in 18 major prefectural capitals from
Hokkaido to Kyusyu.
Among these past studies, there are no systematicinvestigations of the acoustic characteristics ofJapanese vowels except for those by Kasuya et al. andOkuda et al. Further, there are few acoustic analysis
studies that focussed on dynamic feature of Japanesevowels. Most Japanese vowels ever studied wereuttered in isolation or uncontrolled phonetic
environments. The purpose of the present study,
therefore, is to clarify the acoustic characteristics ofJapanese vowels based on a large-scale database inwhich the uttering conditions are systematically varied.

2. Method
2.1. Talkers
Talkers were 508 native speakers of Japanese
between the ages of 6 and 76. They were categorized
into four groups; AM (adult males), AF (adult females),AD (adolescents), and C (children), as shown in Table 1.
Dialect classification was conducted based on alanguage questionnaire sheet. A talker was classified
as a Kanto/Kansai dialect speaker if he/she had lived in
Kanto/Kansai area from 5 to 18 years old, or had lived
in Kanto/Kansai area more than a half of his/her life. Of
the 508 talkers, 220 were Kanto dialect talkers and 233

IV - 3287

Table 1: Number of talkers for each age group.
NumberAge
NumberGroupof talkers
Rangeof tokens(boys/girls)
AM
19-77
153
46,600(Adult Male)
AF
19-77
151
45,898(Adult Female)83
AD25,28212-18(42 / 41)
(Adolescent)121
C36,7905-11(61 / 60)
(Child)TOTAL
508
154,570

2.4. Acoustic Analysis

were Kansai dialect talkers. The remaining 55 talkers
were neither Kanto nor Kansai dialect talkers.
2.2. Recordings
Talkers read lists containing the five Japanesevowels in several different conditions. (1) Sustained
vowels /i/, /e/, /a/, /o/ and /u/ uttered in isolation (5vowels * 3 times). (2) The five vowels uttered inisolation with different musical notes (5 vowels * 8
notes * 2 times). (3) The five vowels in /h-V-da/ and in/b-V-ta/ syllables within and without a carrier sentence,such as "h-V-da, kore wa h-V-da desu" (35 sentences *3 times). In the ideal case, utterances containing 305
target vowel tokens were recorded from one talker.
Some utterances, however, were discarded because of
inadequate pronunciation, noise interference, and/or
input level overflow during the recordings.
The recordings were made in anechoic rooms at
NTT CS Labs., located in Kanto, and ATR, located in
Kansai, with a digital audio tape recorder (SONY,
PCM7010) and a condenser microphone (B&K, 4003).
Sampling frequency of the digital audio tape recorder
was set at 44.1 kHz and amplitude resolution was 16
bits. To reduce low-frequency fluctuating component, ahigh-pass-filter (NF, DV04), whose cutoff frequency
was set at 73 Hz, was placed in the audio line. The
recorded PCM data were then transferred to a PC via a
digital audio interface (DAT-Link +), and then downsampled to 22.05 kHz by the digital interface.

For the acoustical analysis, speech data were downsampled again to 11.025 kHz. Fundamental frequencies(F0s) were extracted every 1ms by an auto-correlationbased F0 tracker of the ESPS speech analysis package.
Speech data was windowed with 30-ms hanningwindow, and analyzed with 14th-order LPC. F0
tracking errors such as F0 halving or doubling werecorrected by reanalyzing the signal with an adjusted
upper and lower limit on F0.
The first to the fourth formant frequencies (F1-F4)
and their bandwidth (B1-B4) were extracted using
LPC-based formant tracker of the ESPS. The 14-pole
LPC spectra were calculated every 1ms over 25-ms
hanning-windowed segments. When the 14-pole LPC
spectra did not give stable formant frequency contours
over the target vowel tokens or gave excessively wide
formant bandwidths, the analysis was judged to havefailed. In that case, data was reanalyzed with different
orders of LPC analysis in a predetermined sequence, i.e.
14-, 12-, 11-, 16- and 18-pole. The reanalysis was
terminated when satisfactory formant frequency
contours were obtained. This decision was made basedon an examination of extracted formant frequencies
overlaid on a spectrogram, LPC or Fourier spectral
slices, and general knowledge of acoustics phonetics onvowels.
The LPC analysis often gave false formant
frequencies for high F0 data. When satisfactoryformant frequency contours were not obtained with 11to 18-pole LPC analysis, these vowel tokens were notincluded in the analysis.

3. Results and Discussions
3.1. Vowels Uttered in Isolation
Figure 1 shows the mean F1 and F2 frequencies ofisolated vowels for the four talker groups, along with

2.3. Labels
For all the recorded speech data, phoneme labels
were given manually. Labels were assigned to thefollowing points: start and end of a target vowel, startof /h/ and /d/ in /h-V-da/ syllables or /b/ and /t/ in /b-Vta/ syllables, start and end of a carrier sentence and theword containing the target vowel. Four well-trainedlabelers gave the labels to the data by looking at the
waveform, power contour, voiced/unvoiced index,
fundamental frequency contour, formant frequency
contour on a spectrogram using a custom ESPS based
interactive labeling editor and by listening to the sound.

Figure 1. The mean F1 and F2 frequencies and 80% density
ellipses of isolated vowels for the four talker groups. (N=152,
144, 82, and 119 for AM, AF, AD and C, respectively)

IV - 3288

an elliptic fit to each vowel category. The vowels
uttered by adult males (AM) are located in lower F1and F2 regions, those of adult females (AF) andadolescents (AD) sit in similar regions, and those ofchildren (C) are located in higher F1 and F2 regions.
There is no distinct difference between the two talker
dialect groups in the static features of the formant
frequencies, possibly because the talkers read thesentence lists.
In Figure 2, mean F1 and F2 frequencies of isolated
vowels uttered by adult male talkers of this study arecompared with those of previous studies. The mean F1and F2 frequencies for the five vowels bear close
resemblance to previously reported data except for
Okuda et al's and Keating and Huffman's data. Vowels
in these two data sets are found to lie inside the othervowel spaces. It is expected that co-articulation might
have an affect on formant frequencies in these studies:
Okuda et al analyzed the vowels in dialogues and readsentences, and Keating and Huffman analyzed shortvowels in 30 words.
Figure 3 shows mean F0 frequency change alongwith age. While the mean F0 gradually declines as age
increase for females, it drops rapidly for males at theage of 12 to 13 years due to the voice breaking. Thisage period is almost identical to that measured by
Kasuya et al. in 1969.

Figure 2. Comparison of the mean F1 and F2 frequencies ofisolated vowels uttered by adult male talkers. For the present
study, data by AM group are plotted (N=152).

3.2. Vowels uttered with different musical notes

3.3. Vowels uttered in syllables
As the vowel duration differs from token to token,
the frequencies of F0, F1 and F2 were calculated at 11
equally spaced points for each vowel. Namely,
frequencies were obtained at every 10% of the vowel
duration including the starting point and end point.
Figure 5 shows the dynamic feature of mean F1 andF2 frequencies for short isolated vowels uttered inisolation, in /h-V-da/ syllables and in /b-V-ta/ syllables.
F1 and F2 frequencies are almost fixed for isolated
vowels, while they shift both in syllables. F1Figure 3. The mean F0 frequency change along with age groups(N=152 for male, and N=144 for female).

3000
F2 frequency in Hz

Each talker uttered vowels with 8 different musicalnotes; i.e. talkers were asked to sing sol-fa in eachvowel. Some talkers were able to sing with F0 range
close to a full octave; many could not vary their F0s
widely.
Figure 4 shows the mean F1 and F2
frequencies of isolated vowels for the eight notes. The
formant frequency shift due to individual talkers' F0frequency shift was smaller than that seen among age
groups. Note that the number of talkers is 356, i.e.
almost 30% of the talkers were not used to plot thefigure. The LPC analysis method is not versatile. The
weakness of the LPC analysis is that it is hard to extract
formant frequencies of vowels with very high F0
frequency. Many vowel tokens uttered at high notes byfemales and children, thus, remained unanalyzable.

2500

/i/
/e/

2000
1500

/u/

/a/
/o/

1000

high
low

500
200

400
600
800
F1 frequency in Hz
Figure 4. The mean F1 and F2 frequencies isolated vowels
uttered in eight notes (N=356, from all taker groups).

IV - 3289

frequencies shift down almost an octave in /h-V-da/syllables and 2/3 octave in /b-V-ta/ syllables. In /h-Vta/, F2 frequencies of /i/ and /e/ shift down a few
hundred Hertz, those of /a/ remain constant and thoseof /o/ and /u/ shift up about 500 Hz. In /b-V-ta/syllables, F2 frequencies of /i/ and /e/ remain constantand those of /a/, /o/ and /u/ shift upward a few hundred
Hertz.
These dynamic features of F1 and F2 frequencies
did not show substantial difference among different
mora structures (short vowel vs. long vowel vs. shortvowel followed by double consonant), speaking styles(in isolation vs. in carrier sentence), and age and dialectof the talkers.

4. Conclusions
The present study clarified some aspects of theacoustic characteristics of Japanese vowels based on alarge-scale database, in which talker-related attributes,
phonetic environments, and speaking styles weresystematically balanced. F1 and F2 frequencies as well
as F0 of isolated vowels in the present study bear close
resemblance to previously reported data, suggestingthat the present data can be regarded as a standard forthe acoustic characteristics of Japanese vowels. It was
newly found that the formant frequency shift due toindividual talkers' F0 frequency shift was smaller thanthat seen among age groups, implying that the shift of
the formant frequency is affected more by the length ofthe vocal tract than by tension of the vocal folds. It wasalso shown that the phonetic environment significantly
affects the dynamic feature of formant frequencies.
Acknowledgment
Authors are grateful for Rieko Kubo and Naoko Ohtsuki
at ATR for their technical assistance.

5. References[1] Peterson, G.E and Barney, H. L. (1952), "Control
Methods used in a study of the vowels," J. Acoust. Soc.

Am. 24, 175-184.
[2] Hillenbrand, J., Getty, L.A., Clark, M.J., and Wheeler, K.
(1995), "Acoustica characteristics of American English
vowels," J. Acoust. Soc. Am. 97, 3099-3111.
[3] Obata, J. (1933), Jikken-Onkyogaku (Experimental
Acoustics) (Iwanami Shoten, Tokyo, 1933). (in Japanese)[4] Chiba, T. and Kajiyama, M. (1942), The vowel: Its
Nature and Structure, (Tokyo-Kaiseikan, Tokyo, 1942).
[5] Honda, K. (2002) "Evolution of vowel production studies
and observation techniques," Acoust. Sci. & Tech. 23,
189-194[6] Hattori, S., Yamamoto, K., Obashi, Y. and Fujimura, Y.
(1957), "Vowels of Japanese," Bulletin of the Kobayasi
Institute of Physical Research 7, 69-79. (in Japanese)[7] Torii, N. (1957) " Some consideration on Sonagrams ofJapanese," Progress Report of NTT-ECL, No.579, 1-65.
(in Japanese)[8] Suzuki, J., Kadokawa, Y. and Nakata, K. (1963)"Formant-Frequency Extraction by the Method ofMoment Calculations," J.Acoust.Soc.Am. 35, 1345-1353.
[9] Kadokawa, Y. and Nakata, K. (1964) "Extraction ofFormant
Frequencies
byAnalysis-by-Synthesis
Techniques," J. Acoust. Soc. Jpn. 20, 1-13. (in Japanese)[10] Kasuya, H., Suzuki, H. and Kido, K. (1968), "Changes in
Pitch and first Three Formant Frequencies of Five
Japanese Vowels with Age and Sex of Speakers," J.
Acoust. Soc. Jpn. 24, 355-364. (in Japanese)[11] Sato, H. (1974), " Acoustic Cues of Female Voice
Quality," Trans. IECE 57-A, 23-30. (in Japanese)[12] Keating, P.A. and Huffman, M.K. (1984), "Vowel
Variation in Japanese," Phonetica 41, 191-207.
[13] Konno, H., Toyama, J., Shimbo, M. and Murata, K.
(1994), A study on the formant frequency and phonemic
quality of Japanese whispered vowels," J. Acoust. Soc.
Jpn. 50, 623-630. (in Japanese)[14] Miwa, J. (1991) Speech Signal Processing on a PC,(Shokodo, 1991, Tokyo). (in Japanese)[15] Ueda, Y. and Watanabe, A. (1995), "Speech spectral
normalization and its vowel vector representations fortactile display," J. Acoust. Soc. Jpn. 51, 519-528. (inJapanese)[16] Okuda, K., Matsui, T., Naito, M., Sagisaka, Y. andNakamura, S. (2002), "Creation and evaluation of alarge-scale Japanese speech database," J. Acoust. Soc.
Jpn. 58, 569-578. (in Japanese)

Figure 5. F1 and F2 formant frequencies for every 10% of the duration of short vowels in isolation (left), in /h-V-da/ syllables(middle), and in /b-V-ta/ syllables (right). Data by AM talkers are plotted (N=141).

IV - 3290