# Speech Enhancement Algorithms: A Systematic Literature Review
## 3 Method
### Search Strategy Based on Search Strings and Online Electronic Databases  
After developing the research questions related to the SEA topic, the search strategy was performed by identifying specific categories and keywords that are commonly used in the related studies. The search process was then conducted across five electronic databases. The first step in formulating a search strategy is to take the topic and break its terminology into discrete concepts (categories) and keywords. The following four concepts were formalized based on the literature review: Concept 1 is associated with the signal type (speech, noise, voice, and audio); Concept 2 is associated with the algorithm name (enhancement, improvement, noise reduction, and denoising); Concept 3 is associated with algorithm types (Wiener filtering, MMSE estimation, deep learning, spectral subtraction, and noise estimation); finally, Concept 4 is associated with the approach (system, technology, technique, method, and algorithm). Then, a search string is formed by identifying primary keywords for each concept. Table 1 outlines the defined concepts along with their corresponding keywords. These keywords are commonly used in research studies on SEA and are aligned with the research questions. The categories and their corresponding keywords were selected based on an extensive literature review. As shown in the table, different keywords were defined for the four categories, which are expressed using similar or related terms. A thorough search was formalized based on the identification of alternative synonyms, acronyms, and spellings which include variations of the major term. The defined keywords were then organized under the four distinct concepts and combined using the Boolean operator OR. Subsequently, the Boolean operator AND was applied to incorporate keywords across categories. As a result of the above procedure, search strings were created from the keywords within each concept and Boolean operator. Boolean operators have the ability to link together two or more search conditions, allowing more complex search logics to be specified. The implemented procedure was as follows:  
* **Concept 1: \"Speech\" OR \"Voice\" OR \"Noise\" OR \"audio\"**
* **Concept 2: \"Enhancement\" OR \"Improvement\" OR \"Noise Reduction\" OR \"Denoising\"**
* **Concept 3: \"Wiener Filtering\" OR \"MMSE Estimation\" OR \"Deep Learning\" OR \"Spectral subtraction or Noise estimation\"**
* **Concept 4: \"System\" OR \"Method\" OR \"Technology or Technique\" OR \"Algorithm\". The resulting string was structured as (Concept 1) AND (Concept 2) AND (Concept 3) AND (Concept 4). The search results were subsequently retrieved and stored using Mendeley Reference Management software. Then, the search string including the keywords based on the categories and Boolean operators (logical operations) was entered into the search box of the selected electronic library database or search engine. Afterwards, the search process was conducted by retrieving articles from the five electronic databases used for data collection. The five sources cover many aspects of the reviewed research area and provide effective search engines that are easy to use and appropriate for automatic search [46]. The list of libraries included SpringerLink, IEEE Xplore, ScienceDirect, ACM Digital Library, and Google Scholar. The aforementioned combinations of keywords based on Boolean expressions were used in the search process. The search engines mostly support logical operators and permit the use of parentheses to link multiple search conditions and define more complex logics. Parentheses are significant because they separate the specific elements of the search string in order to confirm that they are regarded as a group. However, the details of the search step based on the defined search string sometimes need to be carefully checked to ensure the best results depending on the keyword combinations and the database being used.