Psychology of brand names Marketers use the fact that different letters are associated with different mental images and emotions when they create brand names.

Studies show that letters and their combinations produce different connotations in terms of size, shape, brightness and overall impression of an object. The letters &lt;a&gt; and &lt;o&gt; symbolize depth and size, while &lt;i&gt; evokes the mental image of something small. Among consonants, soft, voiced sounds such as &lt;d&gt;, &lt;l&gt;, &lt;m&gt;, &lt;n&gt;, &lt;w&gt; can be distinguised from hard, voiceless sound such as &lt;f&gt;, &lt;k&gt;, &lt;r&gt;, &lt;t&gt;, &lt;p&gt;. The soft consonants evoke associations of harmony, softness, gentleness and femininity, while the hard cononants are connected to hardness, dynamism, vigor, strength and masculinty.
(translated from http://comrecon.wordpress.com/2010/02/26/namen-sind-wegweiser-bei-konsumentscheidungen-naming-branding/)

On some other brand name blog I read that the letter &lt;b&gt; is associated with trustworthiness.
What are those studies, and what else has been found regarding the psychology of letters (graphemes) and sounds (phonemes)?
What you are looking for is usually considered within synesthesia, specifically auditory-visual synesthesia.

Usually these topics are developed in "commercial or industrial psychology" in a general way and more specifically by psychologists incorporated in marketing, these last studies are often not published, are made for the best initial orientation of the product with a line of consumers.

Here is a general book:
https://books.google.es/books?id=ESH7AAAAQBAJ&amp;pg=PA812&amp;lpg=PA812&amp;dq=psychology+synesthesia&amp;source=bl&amp;ots=zC3KmJPT2h&amp;sig=pGlZ9qPf1d14T7i96AL8OmMvVSo&amp;hl=es&amp;sa=X&amp;ved=0ahUKEwjmnYvRyanVAhUNfFAKHbX-CV84ChDoAQhWMAY#v=onepage&amp;q&amp;f=false

It seems that what ultimately interests most is why (cerebrally) and what repercussions (especially in clinical) has this phenomenon.

You will not find it difficult to find end-of-grade papers, there are also theses.