While putting together the feature on audio myths in the latest edition of E&T, I stumbled across an old Audio Engineering Society paper that tried to deal with a famous piece of audio lore - that it was possible for the opera singer Caruso to shatter a glass by hitting the right note. If nothing else it shows there is, or at least was, some truth in advertising although it does not clear up the original legend.

In between developing the protocols that underpin the internet, technical consultancy Bolt Beranek and Newman (BBN) found itself trying to work out how to make an advertisement work. Forty years ago, Ad agency Leo Burnett was asked by the makers of Memorex cassette tape to demonstrate just how good it was by recreating the Caruso legend.

The BBN guys got straight to the point: "Our immediate response was to suggest a BB gun off-camera. We were told, however, that it had to be a genuine demonstration. The idea was to show a goblet being shattered by a singer, while the singer was simultaneously being recorded on a Memorex cassette. The recording would then be played back to show that it could shatter a second matched goblet."

There was only one problem: no-one was sure Caruso ever achieved this feat. So, the BBN researchers first had to work out whether it was possible at all. It turned out that Corning had been asked to do something similar for a movie and developed a specially shaped and treated goblet that would shatter easily when played the right note. Burnett decided that they needed to use ordinary wine goblets.

So, BBN engineers scoured the department stores looking for goblets with thin glass and a high-Q resonance  - hopefully strong enough to shatter themselves into destruction when shaken with the right note. They then sat the candidate glasses next to a loudspeaker driven by a 50W amplifier, fed by a sinewave generator. Sound levels hit 132dB but none of them broke.

BBN hired a couple of professional singers to try their luck at breaking glass. Even producing 140dB at the lips, they did not succeed.

Hoping that focusing the sound better might demonstrate a result, the team took the speaker horn off and used just the driver. They then moved the glass closer to the output. At about 2cm, "the vibration o fhte rim of the goblet closest to the driver became so violent that it blurred like an image going out of focus, and the goblet suddenly shattered. The sound level meter was placed where the goblet had been, and gave a reading of 136dB".

After some optimisations, they found they could shatter the right goblets more or less at will but with sound pressure levels around 145dB. So, to film the advert, they took singers such as Ella Fitzgerald and amplified their voices to ear-spliting levels before replaying the tape at the same level. Because the glasses don't shatter into lots of pieces, but simply break along the lines of the main vibration modes with an unsatisying 'tink', they overdubbed the sound of breaking glass.

Given that such high sound pressure levels are needed, Peter Tappan from BBN questioned whether the Caruso legend was true: "The human mouth is similar in dimensions to the loudspeaker baffle opening used in the commercials. Some of our goblets shattered with amplified voice levels as low as 141dB. The writer, who is not a singer, is able to produce a voice level of 140dB at his lips. When these facts are put together, it is not difficult to believe that Caruso may indeed have shattered one or more goblets with his voice, if he held them very close to his mouth and used the right goblets. If any reader can document the legend, the writer would appreciate hearing about it."