Consider the general purpose of the speech: is it instructing, informing, persuading, or entertaining?
From there, incorporate the topic into the purpose. Is the speech instructing, informing, persuading, or entertaining about X?
Going from the general to the specific is all about refinement.
Crafting the speech is a balance of reinforcing the general purpose while being specific enough to make a case.
As previously stated, think of a speech as an inverted pyramid. As the speaker refines his or her purpose, the speech begins to narrow to its ultimate point. The widest part represents the topic, followed by the general purpose (instructing, informing, persuading, or entertaining).
From there, the next most-refined level is the specific purpose, which fuses the topic and general purpose. For example, if the topic is social media and the speaker's intention is to inform, the specific purpose would be to inform your audience about social media. The speaker might get more specific by focusing on a narrower subject within your topic, such as Twitter. In this case, the more specific purpose might be to inform the audience about the evolution of Twitter as a social media platform.
Going from the general to the specific is all about refinement. If the speech is too broad, the audience is left confused or unclear about what the speaker is saying or trying to achieve with the speech. At the same time, the speaker must temper just how specific to get in relation to the audience. How much do they already know about the subject? How might their demographics such as age, gender, culture, and education levels already inform that knowledge base?
By using the inverted pyramid model to outline exactly how to arrive at the speech's most specific, narrowest point, the speaker should avoid losing the audience by getting too specific at the wrong time.
But what if the speech has more than one purpose? As previously discussed, not all speeches conform strictly to the four general purposes for speaking. Some persuasive speeches may contain elements of informative or entertainment speeches. If this is the case, first identify the most important purpose of the speech. At the end of the day, what exactly is the speech trying to achieve? From there, subordinate the other, more specific purposes.
For example, when giving a persuasive speech about the rise of Twitter as a dominant form of social media, the speaker's general purpose is to persuade, and the specific purpose is to persuade about the notion that Twitter is a dominant form of social media. But the speaker may have other purposes, to show the "lighter side" about Twitter by talking about how some fake and parody accounts carry more weight than their official counterparts (such as the BP Oil magnate and the fake @BPGlobalPR account in the wake of the Gulf Oil Spill in 2010).
At the end of the day, the speaker is still trying to achieve the specific purpose to persuade your audience to believe that Twitter is a dominant social media platform. Using entertaining anecdotes as one part of your strategy would fall under that purpose, not alongside or above it.
Just keep picturing the inverted pyramid, getting closer and closer to the most specific points to assist in the refinement process of honing a topic into a specific purpose and a solid thesis with substantive evidence to make a case.
Source: Source: Boundless. "Specific Purpose of a Speech." Boundless Communications Boundless, 3 Mar. 2017. Retrieved 29 Jun. 2017 from https://www.boundless.com/communications/textbooks/boundless-communications-textbook/choosing-a-topic-6/establishing-a-purpose-and-a-thesis-36/specific-purpose-of-a-speech-159-5419/
a result that is desired; an intention
explicit or definite