## 3 Passively Augmenting Mobile Devices
### Leveraging Built-In Sensors and Elements
#### 3.2.2 Speaker  
Even though their quality and power has significantly increased during the last decade, mobile device built-in speakers are generally only good for speech, not music. This is mostly due to their small size and the lack of a proper resonance chamber to boost bass, resulting in a very curvy frequency response and a lack of power.  
There exists a wide range of passive amplifiers on the market to boost the sound generated by the built-in speakers of mobile devices, also attempting to flatten their frequency response (see Section 1.4). These passive amplifiers can be seen as resonators driven by the speaker. In this section, we present various kinds of resonators that can be connected to the built-in speaker of mobile devices to amplify and/or modify their sound.  
Mobile3D contains multiple passive amplifiers of various kinds that can be used to boost the loudness of the built-in speaker of mobile devices (e.g., see Figure 4). Some of them were designed to maximize their effect on the generated sound [54]. Their shape can vary greatly and will usually be determined by the type of the instrument. For example, if the instrument requires the performer to make fast movements, a small passive amplifier will be preferred to a large one, etc. Similarly, the orientation of the output of the amplifier will often be determined by the way the performer holds the instrument, etc. These are design decisions that are left up to the instrument designer.  
3D printed musical instrument resonators (e.g., guitar body, etc.) can be seen as a special case of passive amplifiers. Mobile3D contains a few examples of such resonators that can be driven by the device's built-in speakers. While they don't offer any significant advantage over \"standard\" passive amplifiers like the one presented in the previous paragraph, they are aesthetically interesting and perfectly translate the idea of hybrid instrument developed in Section 5.  
Another way to use the signal generated by the built-in speakers of mobile devices is to modify it using dynamic resonators. For example, in the instrument presented in Figure 6, the performer's hand can filter the generated sound to create a _wah_ effect. This can be very expressive, especially if the signal has a dense spectral content. This instrument is featured in the teaser video ([[https://www.youtube.com/watch?v=dGBDrmvG4](https://www.youtube.com/watch?v=dGBDrmvG4) Yk]([https://www.youtube.com/watch?v=dGBDrmvG4](https://www.youtube.com/watch?v=dGBDrmvG4) Yk)) of the workshop presented in Section 3.4.  
Figure 6: Hand resonator (**on the left**) and mouth resonator (**on the right**) for mobile device built-in speaker.  
Similarly, the sound generated by the built-in speaker is sent to the mouth of the performer in the instrument presented in Figure 6. The sound is therefore both modulated acoustically and through the embedded synthesis and touch-screen. The same result can obviously be achieved by directly applying the mouth of the performer to the speaker, but the augmentation presented in Figure 6 increases the effect of the oral cavity on the sound through a passive wave guide.