When computers speak, how human should they sound?

This was a question that a team of six IBM[1] linguists, engineers and marketers faced in 2009, when they began designing a function that turned text into speech for Watson, the company's "Jeopardy!"-playing artificial intelligence program.

Eighteen months later, a carefully crafted voice — sounding not quite human but also not quite like HAL 9000 from the movie "2001: A Space Odyssey" — expressed Watson's synthetic character in a highly publicized match in which the program defeated two of the best human "Jeopardy!" players.

The challenge of creating a computer "personality" is now one that a growing number of software designers are grappling with as computers become portable and users with busy hands and eyes increasingly use voice interaction.

Machines are listening, understanding and speaking, and not just computers and smartphones. Voices have been added to a wide range of everyday objects like cars and toys, as well as household information "appliances" like the home-companion robots Pepper and Jibo, and Alexa, the voice of the Amazon Echo speaker device.

A new design science is emerging in the pursuit of building what are called "conversational agents," software programs that understand natural language and speech and can respond to human voice commands.

However, the creation of such systems, led by researchers in a field known as human-computer interaction design, is still as much an art as it is a science.

It is not yet possible to create a computerized voice that is indistinguishable from a human one for anything longer than short phrases that might be used for weather forecasts or communicating driving directions.

Most software designers acknowledge that they are still faced with crossing the "uncanny valley," in which voices that are almost human-sounding are actually disturbing or jarring. The phrase was coined by the Japanese roboticist Masahiro Mori in 1970. He observed that as graphical animations became more humanlike, there was a point at which they would become creepy and weird before improving to become indistinguishable from videos of humans.

The same is true for speech.

"Jarring is the way I would put it," said Brian Langner, senior speech scientist at ToyTalk, a technology firm in San Francisco that creates digital speech for things like the Barbie doll. "When the machine gets some of those things correct, people tend to expect that it will get everything correct."

Beyond correct pronunciation, there is the even larger challenge of correctly placing human qualities like inflection and emotion into speech. Linguists call this "prosody," the ability to add correct stress, intonation or sentiment to spoken language.

Today, even with all the progress, it is not possible to completely represent rich emotions in human speech via artificial intelligence. The first experimental-research results — gained from employing machine-learning algorithms and huge databases of human emotions embedded in speech — are just becoming available to speech scientists.

Audio | IBM's Search for Watson's Voice In 2009, a team of six from IBM began searching for a voice for Watson, the company's A.I. program. Many months later, a carefully crafted voice expressed Watson in a highly publicized game against two of the best human "Jeopardy!" players. Watson won.

Audio | "Watson Finalist B" "Watson Finalist B" had much internal support but was not chosen due to "Finalist A's" strong test response. It is now available for use by developers and IBM's partners via Watson services.

Audio | "Watson Finalist C" "Watson Finalist C" was also popular with the IBM researchers and marketers, but testing suggested it might be better suited for automated help desk and customer support applications.

Audio | "Watson Exploratory D" The IBM researchers explored voices that had varying levels of expressiveness and intonation. "Watson Exploratory D" was designed to sound childlike but was rejected when evaluators said it sounded "creepy."

Synthesized speech is created in a variety of ways. The highest-quality techniques for natural-sounding speech begin with a human voice that is used to generate a database of parts and even subparts of speech spoken in many different ways. A human voice actor may spend from 10 hours to hundreds of hours, if not more, recording for each database.

The importance and difficulty of adding an intangible emotional quality can be seen in the 2013 science fiction movie "Her," in which a lonely office worker played by Joaquin Phoenix falls in love with Samantha, the synthetic voice of an advanced computer operating system.

That voice was ultimately portrayed by Scarlett Johansson, after the film's director Spike Jonze decided the voice of the original actress did not convey the romantic relationship between human and machine that he was trying to portray.

The roots of modern speech synthesis technology lie in the early work of the Scottish computer scientist Alan Black, who is now a professor at the Language Technologies Institute at Carnegie Mellon University.

Mr. Black acknowledges that even though major progress has been made, speech synthesis systems do not yet achieve humanlike perfection. "The problem is we don't have good controls over how we say to these synthesizers, 'Say this with feeling,' " he said.

For those like the developers at ToyTalk who design entertainment characters, errors may not be fatal, since the goal is to entertain or even to make their audience laugh. However, for programs that are intended to collaborate with humans in commercial situations or to become companions, the challenges are more subtle.

These designers often say they do not want to try to fool the humans that the machines are communicating with, but they still want to create a humanlike relationship between the user and the machine.

IBM, for example, recently ran a television ad featuring a conversation between the influential singer-songwriter Bob Dylan and the Watson program in which Mr. Dylan abruptly leaves the stage when the program tries to sing. Watson, as it happens, is a terrible singer.

The advertisement does a good job of expressing IBM's goal of conveying a not-quite-human savant. They wanted a voice that was not too humanlike and by extension not creepy.

"Jeopardy!" was a particularly challenging speech synthesis problem for IBM's researchers because although the answers were short, there were a vast number of possible mispronunciation pitfalls.

"The error rate, in just correctly pronouncing a word, was our biggest problem," said Andy Aaron, a researcher in the Cognitive Environments Laboratory at IBM Research.

Several members of the team spent more than a year creating a giant database of correct pronunciations to cut the errors to as close to zero as possible. Phrases like brut Champagne, carpe diem and sotto voce presented potential minefields of errors, making it impossible to follow pronunciation guidelines blindly.

The researchers interviewed 25 voice actors, looking for a particular human sound from which to build the Watson voice. Narrowing it down to the voice they liked best, they then played with it in various ways, at one point even frequency-shifting it so that it sounded like a child.

"This type of persona was strongly rejected by just about everyone," said Michael Picheny, a senior manager at the Watson Multimodal Lab for IBM Research. "We didn't want the voice to sound hyper-enthusiastic."

The researchers looked for a machine voice that was slow, steady and most importantly "pleasant." And in the end, they, acting more as artists than engineers, fine-tuned the program. The voice they arrived at is clearly a computer, but it sounds optimistic, even a bit peppy.

"A good computer-machine interface is a piece of art and should be treated as such," Mr. Picheny said.

As speech technology continues to improve, there will be new, compelling and possibly perturbing applications.

Imperson, a software firm based in Israel that develops conversational characters for entertainment, is now considering going into politics. Imperson's idea is that during a campaign, a politician would be able to deploy an avatar on a social media platform that could engage voters. A plausible-sounding Ted Cruz or Donald Trump could articulate the candidate's positions on any possible subject.

"The audience wants to have an interactive conversation with a candidate," said Eyal Pfeifel, co-founder and chief technology officer of Imperson. "People will understand, and there will be no uncanny-valley problem."

Correction: February 14, 2016

An earlier version of this article misspelled the surname of an IBM researcher. He is Andy Aaron, not Aarons.