The critics
There is a photocopied poster in Bristol's Fleece & Firkin advertising forthcoming attractions. The name of each artiste is followed by a handy indication of their musical genre - from These Animal Men (Indie Rock) to Baby Bird (Pop). Under the names Polly Jean Harvey and John Parish, however, the classification is replaced with the words "All Sold Out".

This makes sense for two reasons. The first is that the week-long residency is indeed "all sold out" - Harvey's only shows of the year are in a venue patently too small for a star of her magnitude, and are, I suspect, as much for the press as they are for the fans. The second is that no one in their right mind would attempt a three-word precis of the sounds made on stage on Tuesday.

After a fine support slot from dEUS ("Belgian art-rock", as the poster might put it), Harvey, Parish and their besuited band played selections from Dance Hall at Louse Point (Island). This new album was flagged as a "dance project", but anyone who expected PJ Goes Disco (ie me) was not left in their misapprehension for long. This is dance as in contemporary choreography, and the songs - frightening music by Parish, even more frightening words by Harvey - are febrile jazz fragments, consisting of Harvey whispering and wailing over Parish's dissonant wall of guitar. Her transformation into Patti Smith is close to completion.

With her emaciated, exaggerated beauty, though, and dressed in a charming bra-and-curtain-wrapped-round-the-body ensemble, she is perhaps less Patti Smith than Morticia Addams. Either way, she has a control and an enigmatic dignity that make for a riveting show whatever the music. And this is not easy listening. It is, not unusually for Harvey, fraught with madness and tragedy, and the songs dodge from section to jittery section instead of flowing from verse to chorus as we like our pop songs to do. It is a theatrical work, even without the dance troupe - who needs one when you've got PJ? - and as such it makes more sense live than it does on the record. And if we're no closer to formulating what the Fleece & Firkin should have put on their poster, at least we've established that they were right not to call it "pop".

No such difficulties in classifying Metallica, though. James Hetfield's announcement at the Newcastle Arena on Monday that "we're going to play some heavy stuff" must rank as the most redundant piece of banter in the history of showbiz. Miles Davis saying that he's going to play some trumpet? Celine Dion declaring that she's going to warble a bombastic ballad in a cloyingly overdramatic voice? Gob-smacking news compared to Metallica playing heavy stuff.

The band have sold 50 million albums by being darkly, seriously, heavily heavy. They have sneaked the odd blues-rocker on to their new album, Load (Vertigo), and compounded this betrayal by cutting their long hair. But they are still the very essence of heaviosity: furious snarling; thick, stuttering chords; twiddly guitar solos; and two bass drums, because one is just not heavy enough. The visuals are similarly subtle. As Hetfield and his black-clad chums stalk around the two connected stages in the middle of the arena, they have to dodge flame throwers, explosions and hydraulic lighting rigs in the shape of robotic arms. Oh, and Metallica's final pyrotechnic is an audacious example of heaviness which I won't reveal so as not to spoil it for those of you who will be rushing out to buy a ticket. If all of the above suggests that Hetfield's statement was a teeny bit tongue-in-cheek in its obviousness, then I think that it probably should. He does seem to have a sense of humour. Unfortunately, it didn't manifest itself for the rest of the show, and it was the overarching, frowning grimness of the music - basically prog rock, at times - that put me off. That and Hetfield's walrus moustache, anyway.

Recently, an NME journalist asked the eponymous leader of the Jon Spencer Blues Explosion whether he worried about being dismissed as someone who did nothing but roll around the stage. "Well, what's better than that?" came the heroic reply. Hear, hear. There is not enough rolling around the stage in this world. We should take with a cellar of salt the claims made for Jon Spencer's ability to save music as we know it (and, more importantly, as we don't yet know it), but there is no denying his proficiency when it comes to jumping on and off the bass drum like a maniac.

The Blues Explosion were at the London Astoria this week, supported by their buddy, RL Burnside. The presence of this veteran corroborates their professed love of the Blues, but you wouldn't always notice it. Russell Simins either plays funky hip-hop beats or else pummels the snare a savage eight times a bar. Spencer and Judah Bauer both tear at tinny guitars (they do without a bass player), and the sonic anarchy is exacerbated by Spencer's grunting in an affected Elvis accent, which the micro- phone distorts to a degree that would do a railway station Tannoy proud. It's great fun.

The group pull an impressive array of sounds from their modest collection of instruments, but the hyperactive psychobilly anthems barrel into one another so relentlessly that they blur into one long thrash. It's hard to imagine that there were people who left with a specific song in their heads, and almost as hard to imagine that the Blues Explosion arrived with a specific song in theirs. And by the arrival of the unnecessary encore, this perpetual noise had become a little boring. Better to see the Blues Explosion supporting the Beastie Boys for 20 blinding minutes, as they did last year, than to have the spell wear off over an hour and a quarter. After all, how long is an explosion supposed to last?

Metallica: Cardiff International Arena (01222 224488), Mon; Manchester Arena (0161 930 8000), Tues; Sheffield Arena (0114 256 5656), Wed.