General introduction 23 levels receiving more reduced amplification (Levitt, 2001). Compression can improve the intelligibility of soft speech, by increasing the sound level. It can also make loud sounds more comfortable, by decreasing the sound level. A disadvantage of compression is that it may increase the level of soft background noise. Hearing aids are able to reduce the effects of noise to some degree (Brons et al., 2013; van den Tillaart-Haverkate et al., 2017). The aim of noise reduction programs in hearing aids is to increase listening comfort in noisy environments by amplifying the speech signal more than the noise. Therefore, the program needs to recognize and analyze the speech and noise separately which can be accomplished since the spectrum of noise differs from the spectrum of speech. In modern hearing aids, directional microphones are used. These microphones are more sensitive to sounds coming frontally than to sounds coming from other directions (Dillon, 2008). Especially in noisy environments with a close, frontal talker, the use of directional microphones can improve speech understanding (Boymans et al., 2008). Disadvantages of directional microphones can be that wanted sounds from other directions may receive insufficient amplifications and that increased internal noise can be experienced in quiet places. Also, a directional microphone is only effective for sounds that are relatively close to the person who is wearing the hearing aids. Many hearing aid features, such as compression or directionality, are useful in some, but not in all situations. The hearing aid fitting can be optimized by using multiple hearing aid programs that are tailored to specific situations. This allows for optimizing the hearing aid settings for different situations, such as situations with or without background noise. Alternative devices The aim of alternative listening devices is to improve hearing and communication outcomes in individuals with hearing loss by amplifying sounds (Maidment et al., 2016; Maidment et al., 2018). These devices can be either stand-alone products that amplify sounds – such as smartphone hearing aid applications – or assistive listening devices that provide additional features to conventional hearing aids. For example, an external microphone or a table microphone may facilitate speech understanding during meetings and wireless products may enhance making a phone call by
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