Viridian Ensemble, who play Bristol New Music festival on May 7

The first time I saw Viridian Ensemble live was at London's Cafe OTO in 2019, supporting Áine O'Dwyer. Their show featured a growling double bass; the trilling of flute motifs; loops of operatic glossolalia; cymbals like the bursting of molten subterranean bubbles, and a swelling rhythm that provokes a trance-like space, with rattling projector and vividly coloured, flickering film.

Afterwards at the bar, I got chatting to Laura Phillips, the group's founder (but emphatically not their leader). Their album Trotula had just come out, which took as its title and thematic source the Trotula texts – medieval writings on women's medicine that includes guidance on fertility, childbirth, abortion, menstruation, and how to deal with cracked lips from kissing too much.

I asked Phillips what she was working on solo, not expecting the answer: she was fascinated, she explained, by the shrimps that thrive in a North Bristol underground cave system called Pen Park Hole that was discovered accidentally in the 17th Century. There's shrimp in the West Country!? I hadn't expected to be chatting about urban marine invertebrates at an improvised music gig. It signalled to me a group decidedly more interesting and idiosyncratic than most. If I liked Viridian Ensemble before, now I loved them.

The work in question was performed by Phillips in 2020 with Joe Kelly at Spike Island, and Trotula has since gone into a second pressing with new artwork and packaging. Now, Viridian Ensemble are preparing for a show at Bristol New Music (which also includes workshops and solo performances by their members) where they'll play with a new film that they have all contributed to.

I manage to get four of them on a video call together ahead of the May festival – Dali de Saint Paul, Tina Hitchens, Esme Betamax and Phillips. The group as a whole is somewhat shifting, but currently includes: Hitchens on Flute, de Saint Paul on vocals; Betamax on percussion, Phillips on 16mm film and waterphone (a hand-held instrument made from a resonating bowl and brass rods), Liz Muir on cello and Caitlin Alais Callahan on double bass (although she is currently studying in Cairo). However, they have no hard edges and are a deeply fluid operation that weave in contributions from across Bristol's fecund underground scenes, touching jazz and improv scenes, and that city's current crop of noisy, feral electronic musicians.

At their forthcoming Bristol New Music Show Joseph Kelly (aka Wendy Miasma) will play double bass, and they have played with or collaborated on projects with sound artist and cellist Shirley Pegna; vocalist Ellen Southern; harpist Rhodri Davies and Guest. All members have other projects too, highlights of which include but are not limited to Phillips' solo artwork involving hand-developing film; Esme Betamax's queercore punk group The Perverts; Hitchen's deep listening workshops (with Dan Johnson); de Saint Paul's Harrga, a duo with Miguel Prado, and EP/64, a sprawling live improvising and recording project that will cease this May after 64 performances.

However, their film for Bristol New Music does something that's a first for all of them, as it includes footage of themselves, unobscured for the most part. The film takes its cues from conversations around domesticity, femininity and shame – what is permitted; what is denounced. It was prompted in part by a news story local to Caitlin in Cairo, about a teacher who was fired because a video of her belly dancing at a private party was posted to social media, but it also references the banging of pots and pans for the pandemic – 'clap for carers' – and includes a cut-up made from a news story that forms a sort of visual poem. The film is used not as a score but as a prompt – it guides and contributes to the playing in mood or pace – and the link between music and image is not one way: the sound of the projector is often included in recordings, and de Saint Paul often takes emotional prompts from the images.

Members of Viridian Ensemble met through the Brunswick Club (a Bristol collective of collectives that was for years resident in a former working men's club), and Phillips suggested they play some music together. While other members finger Phillips as the founder, she is reluctant to claim any leading role, and reiterates the group's non-hierarchical ways of working, the inspiration she takes from structures in the women's movement and projects like Feminist Improvising Group: "I hate being the mouthpiece for the band," she says. "I'm constantly saying, 'Play without me!'"

The three of them began talking about making a space for improvisation in what at that time they felt was a bit of a lad-centric music scene. "I came later to the group," explains Hitchens. "As a flute player, big improv groups can be really difficult. What we all had in common is that we'd had experiences of not feeling as though there was a place for us in some improv settings we'd found ourselves in. And this felt more open."

After a handful of shows, their first major gig was at Supernormal 2017, where de Saint Paul says they "made children cry and run away in the middle of the woods!" The playing is best described as a dialogue: "We are talking," says de Saint Paul. "And that potentially includes all the things that dialogue includes," follows Hitchens. "So, some people see it as a battle; there might be conflict; there might be cooperation. Someone might support somebody else for a bit. Or they might also say no."

"It's also down to where we are physically when we're performing," explains Betamax. "We're in separate zones – Laura is with the projector, and we all have to be on either side of the screen. Any combination, something new comes from it. I might have Dali opposite me and we're egging each other on, or if Caitlin's next to me, we'll often get some really low beats and sounds."

Structurally, traditional elements such as flute and percussion do not perform traditional roles. The drums are not a rhythm section; the flute does not sit decorous on top. It is noisy music but not noise music, which permits a broad range of dynamics and sharp, pleasing details. "What we have in common is that we like being at the edges of our instruments in some way," says Tina. It's also not a fight for space, or a mush of sound – there is light and shade drawn in a moment's play between strings and wind; which erupts in great clouds of chaotic noise; a squall of vocal exclamations and throbbing pulses. "The rhythm for me, it's just always a heartbeat," explains Esme. "[When] things get exciting and my heart's racing, those are the rhythms that are coming out."

They are happily untethered to genre, and unwilling to be pinned down: not particularly attached to jazz or free improv and not quite noise. It's experimental music, which is not a particularly useful descriptor of an improvised, many-pronged project like this, although this group are actually experimenting. They came across the Trotula texts when chatting to a medievalist (two of them work in an academic library). Phillips became interested in what it said about the creation of knowledge and early notions of empowering women. "[Viridian Ensemble] was very much about creating a space, a place and a practice for women-identifying musicians," she explains. "It seemed really integral then, to talk back to that history in regards to the Trotula texts."

Their work is never linear or permitting of one theme. Partly due to how long it takes Laura to create films, stuff is plucked magpie-like from historical events, folklore, esoteric figures, and contemporary stories. De Saint Paul remembers an interest in Bristol chemist Humphry Davy's discovery of nitrous oxide, and Phillips explains how she has done some contact printing of the body of a fish onto film. What also came into Trotula – and what has also played into the new film for Bristol New Music – are themes around horror and the home. Phillips mentions 2016 British comedy slasher Prevenge, in which a pregnant woman goes on a killing rampage. De Saint Paul took her vocal – verbal and non-verbal – for 'Home' on Trotula from the emotions of a woman trapped in a house, and her escape. "I really want to be totally ready to respond to people," she explains, and so doesn't often prepare text for Viridian Ensemble.

While they have contributed to a clutch of compilations, Trotula is their only full-length release (there is also a download of their live show available through Cafe Oto), and their online existence is patchy (their Discogs page is woefully incomplete). This is partly down to the audiovisual nature of the shows – there is a symbiotic relationship between moving image and music (Philips elsewhere describes her work as being "visual music"). There is also a fondness for the unrecorded, the communal, and – like much of the current Bristol scene around the fragmented Brunswick club and Avon Terror scene – concerned with contributing to, shaping and developing a collaborative local scene first. Tangents and experiments also exist off-stage – Betamax reminds Philips that they buried a reel of film in her allotment about 10 months ago and have yet to exhume it. They're also thinking about incorporating ecological ideas – specifically weeds. For the moment, they remain a group whose life has mostly been lived live and in collective spaces, and remain an ensemble that's as much about ideas as it is about improvisation. "I'm all for not documenting it," says Phillips. "You just have to be there."

Bristol New Music festival runs May 5 – 8