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Enhance audio quality of mobile-device loudspeakers

Posted: 04 Jul 2011 ?? ?Print Version ?Bookmark and Share

Keywords:audio quality? speakers? cellphone?

Over the past few years, there has been astounding growth in the number of portable devices that integrate miniature speakers!cellphones, MP3 players, GPS systems, laptops and notebook computers, tablet computers, gaming devices, toys, and so forth. As consumer tastes become more refined, the demand for ever-higher quality sound reproduction in these devices is also growing.

This presents product manufacturers with ever increasing challenges in coaxing louder, higher-quality sound out of these small, lightweight, inexpensive speakers. This paper addresses some of the methods portable electronics makers can and do employ to achieve these goals.

The ideal loudspeaker (of any size) would have a "flat" frequency response!that is to say, it could transmit sound to the air around it with the same volume level at all frequencies from 20Hz to 20kHz, without peaks or dips in the amplitude of the spectrum. In practice, no speaker can in fact do this, and the greater the effort made to flatten a speaker's response the greater the cost and complexity involved.

Since the speakers employed in portable devices are both low in cost and simple in construction, they therefore cannot contain the sophistication necessary for truly flat response, and inherently exhibit fairly significant variations in their output level across the audio spectrum!most notably at frequencies below a few hundred Hz.

To compensate for unevenness, a speaker's spectral response can be measured and characterized, and then compensated for via the use of equalization or filtering circuitry whose frequency response complements the unevenness of the speaker response. That is, at frequencies where the speaker attenuates the audio signal, equalization circuitry can be made to proportionately boost the signal. Commensurately, in areas of audible peaks in the speaker's response, equalization circuitry can soften the signal. The result is greater flatness in the speaker's perceived output.

There are at least two drawbacks to equalization. First, it adds complexity to the system. The more uneven the speaker's response, the more involved the equalization scheme. DSP can be effectively used to implement the equalization curve, but this comes with at least some cost in silicon area and power consumption.

Second, physical limitations in the speaker prevent flatness from being possible across the entire audio spectrum even with equalization added. A single element transducer such as a cellphone speaker whose effective driver-element diameter is often one inch or less is not capable of delivering useful or audible energy across the full audio band (which is why two-way and three-way speakers are common in applications such as home and automotive stereo gear and in public address systems).

This is particularly obvious in the low-frequency area. A diaphragm as small as this simply cannot couple low-frequency energy to the air effectively, and attempts to boost the amplitude of the signal at low frequencies to compensate would push the speaker past its physical and thermal limits. Thus, even in the presence of equalization, low frequency!or bass!response in portable electronics is generally lacking.

Synthetic bass enhancement
The tiny size of speakers in handheld devices, as described above, compromises their ability to deliver low frequencies, and thus the bass portion of the audio program material suffers. Methods do exist, however, to synthetically introduce elements into the sound that make the bass frequencies seem to be present.

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