An acoustic coupler
<a href="//commons.wikimedia.org/wiki/User:Lorax" title="User:Lorax">G. Edward Johnson</a>
The original uploader was <a href="https://en.wikipedia.org/wiki/User:Lorax" class="extiw" title="wikipedia:User:Lorax">Lorax</a> at <a href="https://en.wikipedia.org/wiki/" class="extiw" title="wikipedia:">English Wikipedia</a>..