His girlfriend liked to party, so young Indeep Bakshi wrote a fun-filled song for her birthday called Saturday Saturday. He sang it for his friends too, who quite liked it. A recording was soon uploaded on YouTube.

Bakshi, 25, never imagined that the song would be among the biggest hits of 2014 in Bollywood. But the peppy number from Humpty Sharma Ki Dulhania is a rage among partygoers - and has given its composer-singer a break in Hindi cinema.

It happened after the film's producers, Dharma Productions, saw the video clip and got in touch with him. "I never thought that one day I would get a call from the producers and that they'd want me to record the song for them," an elated Bakshi says.

Bakshi is among the many young men and women who are the voices of Bollywood today. Like the architect, who is now a full-time Bollywood singer, a host of others are playback singing in Hindi films. Take Monali Thakur, who won hearts with the melodic Sawaar loon from Lootera (2013), and Mamata Sharma, who sang the raunchy Munni badnaam hui from Dabangg (2010). Then there is Arijit Singh, the voice behind the romantic Sun raha hai from Aashiqui 2 (2013). There are many others - such as Akriti Kakkar, Sona Mohapatra, Kanika Kapur and Toshi Sabri.

The advent of new singers underlines the changes that have taken place in the Hindi film industry. New Hindi songs are danced to in parties and at discos, so there is an emphasis on foot-tapping music. And young voices are just right for such songs, Bakshi points out.

Films with various kinds of themes also need different voices. "Listeners want a variety of voices. And there are a plenty of voices in the market, so why not use them," says Sourabh J. Sarkar, founder, Indian Idol Academy, which grooms musical talent across India.

Singer Mamta Sharma agrees that there is an emphasis on change. "I believe there is an earthiness and 'Indianness' in my voice which many music directors are looking for," she says.

The new singers are also being given a boost by Bollywood's new music composers, who often prefer fresh voices to established singers.

"Music composition is all about presentation and packaging these days. Our challenge is how well to present these voices," says music composer-cum-singer Ankit Tiwari, whose hits include Sun raha hai and Gaaliyan, teri gaaliyan.

Many of the new singers have been "discovered" by television reality shows. Monali Thakur, for instance, made a mark in the second edition of Sony's reality show Indian Idol in 2006. She got her first break for the song Kubool kar le from the film Jaan-e-Mann the same year from music director Anu Malik, who was one of the judges.

Arijit Singh, too, rose to fame with Sony's reality show Fame Gurukul. He lost the finals in 2005, but his versatile voice amassed fans. He went on to sing Phir mohabbat from the 2011 film Murder 2 and is now one of the most promising singers of Bollywood. Singer Neeti Mohan, too, was first discovered in a Channel V talent hunt, Popstars, where she was a part of the band, Aasma.

"I don't want to limit myself to a particular genre of music," Mohan says. "I can sing the high pitched Tune maari entriyaan and the light romantic Ishqwala love with the same ease."

Music directors and talent scouts say there are three things that they look for in a singer - a different voice, the ability to sing according to the demands of the market, and basic training in singing.

Not all, however, are trained. Mohan is among those to have studied music. The Delhi girl received formal training from the age of six at her boarding school in Rajasthan and later at the Shriram Bharatiya Kala Kendra in Delhi as a teenager.

But Sharma, who started singing on stage at 11 at night-long religious meetings in her hometown, Bhopal, learnt how to sing by listening to playback singers. She performed at concerts, and after finishing school moved to Mumbai to study fine arts and do stage shows. At one such show, Lalit - one half of music composer duo Jatin-Lalit - heard her and gave her a break. That was the Munni song, which became a huge hit.

Her voice is a reflection of the changing times - and the change in the timbres and tones that have swayed the industry over the years. The nasal style of K.L. Saigal, for instance, was followed by the versatile Mohammed Rafi, the classically trained Manna Dey and the romantic Kishore Kumar. They made way for another generation - Udit Narayan, Kumar Sanu, Abhijeet and Sonu Nigam, to name some.

When it came to female voices, the industry for long years was dominated by Lata Mangeshkar and Asha Bhonsle, though Shamshad Begum and Geeta Dutt also left an indelible mark. Later, singers such as Alka Yagnik and Kavita Krishnamurthy took over. The new millennium saw the rise of Sunidhi Chauhan, who signalled the change that audiences were looking for. Her voice was strong and bouncy. The sweet, however, had its place - as represented by Shreya Ghosal.

"But now cinema is changing," Sharma says. "As the portrayal of characters change, new actors and actresses are being launched every day. Hence the demand for new voices for playback singing is also increasing."

Music composer Malik stresses the need for a singer to modify his or her voice to suit a particular song. "I don't choose singers by their popularity but go by the narrative of the film, the situation and the character," the composer says.

The problem with popularity, veteran musicians stress, is that it is here today, gone tomorrow. "Giving one hit number in two or three years is different from sustaining oneself in the industry," composer Bappi Lahiri says. "If they can do it for 10 years in a row, I will call them successful."

Tiwari, on the other hand, blames the system, which seeks to replicate successes. "We have limited scope of creativity. When my songs Gaaliyan and Sun raha hai became hits, film directors started asking me for similar numbers. We have to go by popular demands," Tiwari reasons.

But will any of the newcomers be as fondly remembered as Rafi or Mangeshkar? Composer-guitarist Susmit Sen is most sceptical about the new crop. "They all sound the same. Where is the creativity," he asks.

But the old order does change. And while the masters are still the rulers, the young ones are humming their way in.