The term stiff voice describes the pronunciation of consonants or vowels with a glottal opening narrower, and the vocal cords stiffer, than what occurs in modal voice

Modal voice

Modal voice is the vocal register used most frequently in speech and singing in most languages. It is also the term used in linguistics for the most common phonation of vowels...

. Although there is no specific IPA diacritic

Diacritic

A diacritic is a glyph added to a letter, or basic glyph. The term derives from the Greek διακριτικός . Diacritic is both an adjective and a noun, whereas diacritical is only an adjective. Some diacritical marks, such as the acute and grave are often called accents...

 for stiff voice, the voicing diacritic (a subscript wedge) may be used in conjunction with the symbol for a voiced consonant. In Bru

Bru language

Bruu is a Mon–Khmer dialect continuum spoken by the Bru people of mainland Southeast Asia. There are varieties called Bruu, Western Bru in Thailand and Eastern Bruu in Laos and Vietname, which are partially intelligible...

, for example, stiff-voiced vowels have tenseness in the glottis and pharynx without going so far as to be creaky voice

Creaky voice

In linguistics, creaky voice , is a special kind of phonation in which the arytenoid cartilages in the larynx are drawn together; as a result, the vocal folds are compressed rather tightly, becoming relatively slack and compact...

d, whereas slack-voiced vowels are lax in the glottis without going so far as to be breathy voice

Breathy voice

Breathy voice is a phonation in which the vocal cords vibrate, as they do in normal voicing, but are held further apart, so that a larger volume of air escapes between them. This produces an audible noise...

.

One language with stiff voice is Thai

Thai language

Thai , also known as Central Thai and Siamese, is the national and official language of Thailand and the native language of the Thai people, Thailand's dominant ethnic group. Thai is a member of the Tai group of the Tai–Kadai language family. Historical linguists have been unable to definitively...

 

phonation	Thai Thai alphabet Thai script , is used to write the Thai language and other, minority, languages in Thailand. It has forty-four consonants , fifteen vowel symbols that combine into at least twenty-eight vowel forms, and four tone marks ....	IPA	trans.	Thai	IPA	translation
stiff voice	บ้า	[b̬âː]	crazy	ด่า	[d̪̬àː]	curse, scold
tenuis Tenuis consonant In linguistics, a tenuis consonant is a stop or affricate which is unvoiced, unaspirated, and unglottalized. That is, it has a "plain" phonation like , with a voice onset time close to zero, as in Spanish p, t, ch, k, or as in English p, t, k after s .In transcription, tenuis consonants are not...	ป้า	[pâː]	aunt	ตา	[t̪āː]	eye
aspirated Aspiration (phonetics) In phonetics, aspiration is the strong burst of air that accompanies either the release or, in the case of preaspiration, the closure of some obstruents. To feel or see the difference between aspirated and unaspirated sounds, one can put a hand or a lit candle in front of one's mouth, and say pin ...	ผ้า	[pʰâː]	cloth	ท่า	[t̪ʰâː]	landing place

Javanese

Javanese language

Javanese language is the language of the Javanese people from the central and eastern parts of the island of Java, in Indonesia. In addition, there are also some pockets of Javanese speakers in the northern coast of western Java...

 contrasts stiff and slack voice

Slack voice

The term slack voice describes the pronunciation of consonant or vowels with a glottal opening slightly wider than that occurring in modal voice. Such sounds are often referred to informally as lenis or half-voiced in the case of consonants...

d bilabial, dental, retroflex, and velar stops

phonation	IPA	trans.	IPA	trans.	IPA	translation	IPA	trans.	IPA	trans.
stiff voice	[b̬aku]	nail	[d̬amu]	guest	[d̬z̬ariʔ]	sheet (of paper)	[ɖ̬iɖ̬iʔ]	little	[ɡ̌ali]	river
slack voice	[b̥aku]	standard	[d̥amu]	blow	[d̥z̥arit]	(type of women's clothing)	[ɖ̥isiʔ]	first	[ɡ̊ali]	dig

Mpi

Mpi language

-Phonology:Mpi has six tones and two phonations in its vowels, modal voice and stiff voice:...

 (Loloish

Loloish languages

The Loloish languages, also known as Ngwi or in China as Yi, are a family of fifty to a hundred languages of the Tibeto-Burman language family. They are most closely related to Burmese and its relatives. Both the Loloish and Burmish branches are well defined, as is their superior node, Lolo–Burmese...

) contrasts modals and stiff voice in its vowels. This is not register: for each of the six Mpi tones, a word may have either a modal or stiff-voiced vowel. For example, low tone contrasts /sì/ 'blood' and /sì̬/ 'seven'.