Is it a yowl or a yip, a shriek or a squeal, a yelp or a screech, we can't exactly tell. Except to say that when Carolina Marin wins a point she sounds like a cat on whose tail someone has accidentally dropped a suitcase. Often she shrieks twice, as if to emphasise her intensity.
It is an awful, inharmonious sound and yet Marin makes you smile for she is a character. When she plays badminton, boredom heads for the exit. Problem is yesterday she was playing Tai Tzu-ying and when she plays badminton these days there is no way out. After all, the Chinese Taipei player has won 24 of her last 25 matches.
Other men and women, in singles and doubles, compete in the Indoor Stadium yesterday, but with due respect they are appetisers. This, Asia v Europe, reigning All England champion v Olympic champion, No. 1 v No. 2, is the main course.
Marin, who is shadow playing before she even arrives on court and is constantly conversing with her coach, is a force of nature and noise. She used to dance the flamenco as a child and Wikipedia says the word could be a derivative of "fire". Nice, except Tai's parents are firefighters.
Both women are different in size - the Spaniard nine centimetres taller - and sizeably different in style. You can hear Marin pounding the court while Tai has the soft swiftness of clouded leopards that once could be found in her nation.
Marin is the leftie but like Rafael Nadal v Roger Federer she is unusually the less artistic one. She is dynamic, Tai is dazzling. The Chinese Taipei player hits drops shots as soft as raindrops and has more deceit in her wrist than may be legal. At one point she hesitates, as if she has time to debate what shot she should hit, and then flicks a backhand down the line. It is astonishing in its audacity and Marin can only look.
To look at these two women play badminton with such graceful aggression is to wonder at their relative anonymity in world sport.
Marin, left leg taped, leads 10-9 in the first game and never leads again. She rushes, Tai, her right wrist taped, is patient. Marin's coach gives her what sounds like a short sermon, but nothing can lift her out of her negativity. From 14-14, Tai wins seven of the next eight points to take the game.
Later, I ask Tai if she watches tennis. A little, she says. Have you seen Roger Federer? Yes, she says. She describes his play as "unflustered and cool" and she could be talking about herself. When asked how she came to play such a beautiful game, she says that instead of conventional practice she often played with aunties and uncles and thus learnt an unorthodox game.
To look at these two women play badminton with such graceful aggression is to wonder at their relative anonymity in world sport. John Dryden, the English poet, wrote that "dancing is the poetry of the foot" but the phrase could equally fit badminton. These players find angles at speed and move in all directions with such fine balance that tennis players might applaud.
I mention tennis only because it is where badminton wants to be - a global game of celebrated athletes earning a fine living. One way to get there would be to vigorously market rivalries like Marin v Tai, who are mostly unrecognised and yet belong with Simone Biles, Katie Ledecky and Lydia Ko in any list of great female athletes.
Meanwhile, on went the match, smashes hissing at over 300kmh and yet being returned by players with such fast hands they could probably catch a bird in flight. The second game seems close, 8-8 at one point, but Marin is as errant as Tai is composed. Now the Spaniard shrieks less and the Chinese Taipei player pumps her fist. The end is inevitable, 21-15, 21-15.
Tai speaks softly and without any hubris. She is a lyrical talent, the Spaniard is a tough one. "Some day I will be be No. 1 again," Marin says. It hasn't been a classic match but certainly a visually splendid one. If Tai has given us the shots of the night, Marin gives us one final moment.
The match done, Tai stands under the lights on centre court and smiles for the cameras. In the corner of the stadium, Marin stands in the shadows, alone, sweaty, beaten, fixing her hair. She is looking at Tai and you can almost hear her thinking:
See you next week.