Who says miracles don't happen? Twenty-one years after the Aotea Centre was opened, the ASB Theatre has finally been transformed into the fine-sounding performance space we were promised all those years ago.

Last Thursday, I turned up to the opening night of New Zealand Opera's Rigoletto, expecting once again to hear the singers and orchestra struggling to fill the dull, unresponsive hall with sound.

But from the opening rumbles of the orchestra it was obvious that something magical had happened during the interior upgrading over the summer.

Instead of the underwhelming, "mono LP" sound, there was volume, bloom and the stereo-like envelopment you get in a great lyric theatre.

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At the first interval, a group of venue-weary opera buffs grumpily decided the management must have committed the ultimate sin and installed a grunty amplification system. Not true. The only microphones on stage were those recording the performance for Radio New Zealand.

Acoustics consultant Chris Day says much of the credit for the transformation goes to the large wooden reflector panels installed along the room's sides and overhead. That and the replacement of the old high-frequency absorbing carpets and cloth covered stall seating with parquet floor and wooden-backed seats.

Mr Day says: "The key acoustical deficiencies of the hall have historically been a lack of reverberance and a lack of early reflections."

Reverberance is what makes a note bloom, extending it a fraction before it dies. Ideally it should last around 2.2 seconds for symphony concerts and around 1.5 seconds for opera. Until now, the ASB Theatre has been a dead 1.2. Removing the carpets and seats instantly increased the reverberance and brightness of the sound.

The new side reflectors provide the "stereo" effect, in Mr Day's words, providing "the feeling of spaciousness and of envelopment." Without it, "all the sound is frontal".

Aotea Centre director Robbie Macrae says that sitting four rows back in the circle, he almost felt it was amplified as well - except he knew better.

"The opera always used to do a very subtle stage microphone reinforcement, but for the first time in years there was none."

What seems particularly promising is that the $11.9 million renovations are yet to be finished. Next Christmas, more wooden reflectors will be added to the front of the circle and balcony, and the carpet and seating in the circle will be replaced.

What I've left until last is the possibly controversial installation of the Meyer Constellation electronic assisted reverberation system, created by New Zealand acoustic scientist Dr Mark Poletti.

It had its first public outing last Thursday and those involved would have preferred I not mention it until next year, by which time audiences would have become enamoured of the improved sound.

There is concern purists would write off the whole transformation as "amplification".

A primitive assisted reverberation system was part of the original ASB Theatre plan, but it was quickly rejected by orchestras and, eventually, management.

Mr Day was on the acoustics team that resigned from the original project when the decision was made to go down the "electronic" rather than "natural" acoustic trail.

But short of lifting the roof 5m to create enough volume for sufficient "natural" reverberation, which isn't going to happen, Mr Day says the Poletti system is they way to go.

Far from being a public address system, this set-up has little microphones in the hall walls which pick up the residual sound from the stage as it hits the walls, then, acting as an electronic extension of the wall, bounces the captured sound back into the room through myriad in-wall speakers. It adds a modest - say 15 per cent - boost to the reverberation.

When the Aotea Centre opened in 1990, I was so critical of the bad acoustics that the management appointed me to a special "hearing panel" of experts (plus me) to try to shut me up. I got to attend a lot of free shows, pinpoint where the less bad seats were, and confirm my fears that acoustically it was a dog.

To my delight, this is no longer true. If this has required a little electronic wizardry, so what?