Sitting in a padded room where you can’t hear the sound of your own voice could make you feel a little crazy, but it’s a crucial part of the job for sound engineer Oscar Lopez.
Lopez worked with a series of “rapid prototypes” for the Nokia Lumia 900, before attaching it to a human-sized dummy in an anechoic (echo-free) chamber, specially designed to exclude outside noise, and stop sound reflections. The results give the most accurate reflection of a phone’s acoustics.
After that he sends the phones out on an epic global journey, to hear how they sound in the real world, in environments as varied as a California freeway, a Lagos market, or an Asian village.
“Achieving that final level of quality audio depends on several design factors, and they have to be adjusted for each model of handset,” says Lopez.
The Audio Lab team is responsible for designing the best earpiece and microphone for each phone, as well as working to reduce “Rub n’ buzz” which is distortion caused when the speaker moves and hits against the back cavity.
“The Lumia 900 was a step forward in several ways,” Lopez says. “It has a bigger earpiece and a different microphone with better noise sensitivity. The speaker handles more power, so it’s also louder.”
Each phone needs a customized acoustic design because size, form and materials can change the sound. Sound will react differently with glass than it does with polycarbonate, for example.
The microphone itself is “really simple,” according to Lopez, but the teams uses an integrated combination of microphones, precisely placed, to build the best sound.
“Our work has really progressed in this area. Looking back, the microphone for the Nokia N-900 was a big step forward for us, because it was digital and much less noisy.”
With the Lumia 900, Lopez says it was about analyzing “how many holes are in the polycarbonate, where they are placed, and what shape they are in. Having the microphone right in the front, close to the mouth is the best placement.”
“One hole for the microphone is usually the best, but the form factor of the phone has in impact. Sometimes we need to balance the phone, sometimes the holes need to be small so we have to integrate a few together to get the same performance.”
And you can do a lot more if you use more than one microphone:
“With two microphones you can do noise suppression. When you are in a noisy place, like a restaurant or a mall you only want to hear the person’s voice. Nokia has an algorithm that picks up your speech, using the microphone close to your mouth, and another that picks up noise from far away from mouth. They get combined and the strong signal gets let through.”
Lopez says the Lumia 710 has a secondary microphone on the back, next to the flash, while the Lumia 800 has an additional microphone right above flash. On the Lumia 900 the second microphone is next to the headset jack. Future phones might add additional microphones to improve noise suppression even further.
You can’t get good acoustics without a well-designed earpiece, and the Lumia 900 has the biggest and best. The earpiece is always on the top side of your phone, because that’s what you press to your ear when you use it. The speaker itself needs a certain amount of space as it has to allow the diaphragm of the microphone to move. This translates to voltage and frequency that gets sent to the earpiece, which also has a diaphragm that converts from voltage to sound.
The Lumia 900 speakerphone uses the newest technology to increase wattage and get the loudest sound, and it’s great at withholding drops and stress. The speakerphone’s usually placed at bottom of the phone where there is more space.
Reducing ‘Rub n Buzz’
Good audio quality isn’t just about loudness. Designing the acoustics of a phone’s “back cavity” means the speaker can move up and down without any distortion, or “rub n buzz” as sound engineers call it.
Distortion happens when the speaker reaches the maximum of how much the diaphragm can move. The speaker moves more when it’s turned up louder and the frequencies are lower. Lopez says, the result is distortion that sounds “like paper crumpling”.
“We control it by giving it enough back volume of air to make sure that the speaker doesn’t overshoot. The speaker diaphragm acts like a spring, and the back volume needs to be vacuum sealed so that the diaphragm doesn’t move more than the air in the space allows.”
When you answer a call on a Nokia Lumia 900 you’ll never notice that its acoustics are the result of being strapped to a dense rubber head in an anechoic chamber, or that it’s been tested in the furthest corners of the earth by a team of sound engineers who’ve precisely integrated two microphones and tuned up the wattage on the speakerphone. Oscar Lopez hopes that you’ll just think that it sounds good.