Throughout the 19th century, and presumably back to the time of the TRAP-BATH split, the timbre of the BATH vowel in Kent had been a bright [aː]-like quality, roughly in the vacant central open position on an IPA vowel diagram (vacant because no phonetic character is assigned to it). Anyone needing a phonetic character for a vacant vowel timbre has to borrow one, so I’ve borrowed [a] from nearby cardinal 4 (see also Barry, W J & Trouvain J, 2008, Do we need a symbol for a central open vowel?  Journal of the IPA 38:349-357). The second formant (F2) of this type of vowel would be centred about 1300-1400Hz for someone with a longer vocal tract, a little higher for a shorter vocal tract. Note that what is said about BATH also applies to START and PALM.

The TRAP-BATH split probably occurred no earlier than the late 17th century in rural Kent. This can be inferred from the migration of parties of puritans to Massachusetts from the Weald in the middle of the 17th century (Nora Groce, 1985, Everyone here Spoke Sign Language – Hereditary Deafness on Martha’s Vineyard, Harvard University Press), taking with them a gene for deafness, the FOOT-STRUT split, but not the TRAP-BATH split, hitherto unknown in the American colonies and and apparently still in Kent too. When the TRAP-BATH split happened in southern England, TRAP landed with an [æ]-like timbre close to DRESS (Henry Wylde, 1936, A History of Modern Colloquial English, Blackwell, 196-205). And there they stayed for perhaps 200 years in Kent, until about 1870-1890 when TRAP was being shifted away from DRESS towards cardinal 4 (see Kent accent in the 19th century: TRAP on this website), followed by BATH being darkened to [ɑː] during the first decades of the 20th century. The timbre shift from [a] to [ɑ] would be associated with a lower F2 that’s no higher than about 1100Hz, done by narrowing the lower pharyngeal constriction from about 1.5 sq.cm. to about 0.5 sq.cm.

The timbre of BATH was studied in the speech of eight informants,  all born between 1865 and 1895, for any signs that this sound change had already started. Seven of them were recorded by the Survey Of English Dialects in the 1950s (see Kent accent in the 19th century on this website), and the eighth, author H G Wells from Bromley (see Halfway to Estuary English-H G Wells on this website), was recorded during a BBC radio broadcast in 1931. The result is shown in Figure 1.

Figure 1. The distribution of BATH, START and PALM pronunciations by the eight informants (each denoted by the place and year of birth). All had the earlier [aː]-like pronunciation, none had the new [ɑː]-like pronunciation.

Figure 1 shows that this sound change hadn’t started spreading from the estuary towns to rural Kent during 1870-1890. It possibly hadn’t even appeared in the estuary towns, although time was short, it must have been developing there by the 1890s (I have examples who grew up in the first decade of the 20th century with the darker [ɑː]-like BATH).

	Warren Street 1894 (left) with the highest F2 for BATH, about 1500Hz
	Appledore 1880 (above right) and Staple 18xx (left), with the lowest F2 for BATH, about 1200Hz Figure 2. F1/F2 diagrams for all the informants, arranged from the highest F2 for BATH (top) to the lowest (bottom)

Figure 2 shows F1/F2 diagrams for all the informants, ordered from the highest F2 in BATH (about 1500Hz) to the lowest (about 1200Hz).

This variation seems to be random, although there might be a tendency for rhotic speakers to have lower F2 in BATH. In part it might be related to vocal tract length, raising or lowering F2 individually. It might also mean that some informants had begun to darken this vowel transitionally but without going all the way to [ɑ].

What is most apparent is that this was a very dynamic period for the vowels with low pharyngeal constrictions (with F1 higher than about 600-650Hz): TRAP is being shifted away from DRESS, LOT is shifting from [ɑ] to [ɔ], STRUT might be shifting from [ɑ] or [ʌ] to [a], and BATH is going to shift from [aː] to [ɑː]. All these informants have different mixes of unchanged and changed vowels, with different arrangements (from highest F2 for TRAP to the left, lowest F2 on the right):

TRAP STRUT+BATH LOT (4)
TRAP BATH LOT STRUT
TRAP BATH+LOT STRUT
TRAP STRUT BATH LOT
TRAP STRUT LOT BATH

Listen to examples of BATH by these informants, ordered from the highest F2 to the lowest (rhoticity is also indicated by bold type in the word lists):

Warren Street 1894 (F2 around 1500Hz for BATH):
father, ya(r)d, ba(r)n, pa(r)snips, ca(r)t

H G Wells, Bromley 1866 (F2 around 1400Hz for BATH)
Stalin, task, vast

Goudhurst 1881 (F2 around 1300Hz for BATH)
(h)alf-past, (h)arness, chaff

Denton 1888 (F2 around 1300Hz BATH)
father, farmers, market

Stoke 1868 (F2 around 1300Hz BATH)
grass, large, can’t

Appledore 1880 F2 around 1200Hz for BATH)
past, farm, market

Staple 18xx (F2 around 1200Hz for BATH)
(h)ard, dark, bars

For comparison, here is an example of a darker [ɑː]-like BATH acquired after 1900. This is Dr William Penney (1909-1991), who grew up in Sheerness (Figure 1). Lord Penney was Professor of Mathematics at Imperial College (London), head of UK nuclear research and finally Rector of Imperial College. His Estuary English is very obvious, despite modest adaptations towards RP, especially the HAPPY vowel. Most vowels and diphthongs are unmodified, particularly LOT, TAUGHT, FOOT, GOOSE, MOUTH and GOAT. Yod coalescence, preglottalized voiceless stops, and vocalized /l/ are unrestricted, all shunned by his contemporary RP speakers.

Listen to examples of dark [ɑː]-like BATH by William Penney:
department, asked, Harwell, after

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©Sidney Wood and SWPhonetics, 1994-2015

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