Sample Code:

Component:

<aura:component implements="force:appHostable">  
      
    <aura:attribute name="activeSections" type="List" default="['a']" />  
    <aura:attribute name="activeSectionsMessage" type="String" default="" />  
  
    <div class="slds-box slds-theme_default">   
          
        <p>{! v.activeSectionsMessage }</p>  
      
        <lightning:accordion allowMultipleSectionsOpen="true"  
                             onsectiontoggle="{! c.handleSectionToggle }"  
                             activeSectionName="{! v.activeSections }">  
              
            <lightning:accordionSection name="a" label="Section A">  
                  
                <aura:set attribute="body">  
                      
                    This is section A.  
                      
                </aura:set>  
                  
            </lightning:accordionSection>  
              
            <lightning:accordionSection name="b" label="Section B">  
                  
                    This is section B.  
                  
            </lightning:accordionSection>  
              
            <lightning:accordionSection name="c" label="Section C">  
                  
                    This is section C.  
                  
            </lightning:accordionSection>  
              
        </lightning:accordion>  
          
    </div>  
      
</aura:component>  

Component Controller:

({  
      
    handleSectionToggle: function ( cmp, event ) {  
          
        var openSections = event.getParam('openSections');  
  
        if ( openSections.length === 0 )   
            cmp.set('v.activeSectionsMessage', "All sections are closed");  
        else   
            cmp.set('v.activeSectionsMessage', "Open sections: " + openSections.join(', '));          
          
    }  
      
}) 

Output:

All Sections Open:

All Sections Closed: