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¹The popularity of headphones has rapidly increased in recent decades with the rise in portable audio devices, such as MP3 players and mobile phones.

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²When listening to music or radio broadcasts with headphones, high degrees of isolation are often desired in order to reduce background noise.

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³However, high isolation can be seen as a disadvantage, since it necessitates the removal of the headphones when engaging in conversation with other people.

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⁴Recently, hear-through headphones have been developed in order to allow the user to better perceive ambient sounds and the surrounding environment while using headphones [1]. ⁵These headphones use microphones to capture and reproduce ambient sounds in the ear canal. ⁶Hear-through headphones are designed to provide an acoustically transparent experience. ⁷In other words, the listening experience should be identical to that without the headphone. ⁸Thus, a hear-through headphone enables the speech of others to be heard clearly and without attenuation. ⁹Hear-through headphones can also improve situational awareness and thus help to avoid potentially hazardous situations especially in busy environments, such as city streets.

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¹⁰However, a problem occurs when a person speaks while wearing these highly isolating headphones, the user’s perception of their own voice is altered due to the blocking of the ear canals by the headphones.

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¹¹The perception of one’s own voice involves two components: air conduction and bone conduction of sound. ¹²Air-conduction sound is what most people imagine sound to be; that is, sound waves propagating through the air. ¹³However, when a person speaks, vibrations of the vocal chords are also transmitted by bone conduction to the ear. ¹⁴This can only be heard by the speaker, whereas the air-conduction sound can be heard by others as well. ¹⁵This is also the reason that recordings of people’s voices sound odd to them, as the recordings do not include the bone-conducted part of their voice. ¹⁶In a blocked ear canal, the low frequency components of the bone-conducted sound are amplified, leading to the altered perception of one’s own voice. ¹⁷This amplification of the low-frequency sound in a blocked ear canal is known as the occlusion effect. ¹⁸The occlusion effect has been a major issue for the hearing aid industry, as it causes some people to reject hearing aids [2]. ¹⁹Headphone users have not been affected by the occlusion effect to the same extent, since the primary application of headphones has been listening to music or radio broadcasts. ²⁰These are activities during which talking is not required.

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²¹However, for hear-through headphones, the occlusion effect presents a problem, as communication is an important application for these devices.

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²²The occlusion effect has been reduced using acoustic vents in hearing aids [2]. ²³Acoustic Vents allow the sound energy to radiate out of the ear, thus reducing the sound pressure in the ear canal.

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²⁴However, this solution is not sufficient for hear-through headphones, as their sound quality is deteriorated by leakage occurring through the vents [3]. ²⁵Moreover, acoustic vents would allow feedback to occur in the hear-through loop.

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²⁶Recently, active noise cancellation (ANC) has been proposed as a solution to the occlusion effect in hearing aids and headphones [4],[5].

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²⁷However, no work has yet attempted to apply ANC to hear-through devices.

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²⁸Therefore, the aim of this thesis is to evaluate analog ANC as a solution for reducing the occlusion effect in hear-through headphones. ²⁹For this purpose, the thesis will answer two questions:

1. What is the range of frequencies that need to be attenuated?
2. What is the degree of attenuation required to remove the occlusion effect at these frequencies?

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³⁰The rest of this thesis is organized as follows. ³¹Chapter 2 briefly reviews the human auditory system and the basic properties of headphones. ³²The chapter also presents the occlusion effect. ³³Chapter 3 reviews hear-through equalization and active noise control techniques and presents previous solutions to the occlusion effect. ³⁴Chapter 4 describes the measurement methodology and presents measurement results. ³⁵Chapter 5 discusses the obtained results. ³⁶Finally, the conclusions are presented in Chapter 6.