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Various portable devices such as mobile phones, personal navigation devices, portable media players, notebook PCs and portable wireless speakers have flooded the market today with exquisite, appealing packaging. However, it does not suffice to have good "looks" alone! End users want good communication features in the device and the need for clear and loud audio is only growing.
A number of issues drive the need for louder volume from speakers used in portable applications. Designers of these portable audio systems need advanced device solutions to address design challenges including the following:
- Provide high output volume at low battery levels
- Optimize loudness while maintaining audio clarity
- Protect speakers from "high-power damage"
This article discusses the need for louder volume from speakers and some techniques that portable audio designers can use to achieve louder sound.
Need for loud volume
Different portable applications have different speaker sizes and ratings, but they all have a common need to sound louder. This louder sound is needed along with improved voice and audio recognition.
Many users play voice/audio from their portable device in noisy environments such as personal navigation devices in cars amidst road noise. The distance of the equipment from the person aggravates the issue even more.
Commonly used speakers are rated anywhere between 0.5 W to 2 W, but future speaker trends are moving towards supporting higher power ratings to enable greater loudness. The use of ceramic elements is also targeted to obtain higher sound pressure levels (SPL) with minimal power consumption.
Conventional speaker amplifiers
While designing a speaker amplifier, key parameters to consider are:
- output power: Typically < 3W for portable applications
- battery efficiency
- supply voltage range
- signal-to-noise ratio (SNR)
- total harmonic distortion (THD)
- power supply rejection ratio (PSRR)
- start-up time
- number and size of external components
In a conventional speaker amplifier, a fixed input amplitude results in a fixed output amplitude. This output amplitude is determined by the fixed gain of the system, typically at +6dB. The output is linearly related to the input signal within the audio frequency band and amplifier operating thresholds.
The output power will cross maximum speaker threshold ratings, if the input levels get very high. The results are distortion of the output audio signal and possible damage to the speaker.
Class-D architectures have become very popular in battery-operated devices due to their high efficiency as compared to class-A or class-AB amplifiers at high output power levels. However, the amplifier output power typically is limited by the battery level. For instance, when the battery level in a portable device discharges with usage, the output volume or loudness from the speakers also reduces.
There is a great need for automatically optimizing the loudness while maintaining SNR.
To optimize loudness, the gain can be controlled to amplify/attenuate the input signal. Users want the maximum output power to be limited and stay within the maximum power rating of the speaker used. Additionally, the amplifier needs to provide high-output volume at low battery levels.
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