The voice is the primary means of communicating information to the audience in a technical presentation. The speaker must not rely solely on the slides to convey any critical information.
To ensure this, the speaker must
A speaker who talks quietly will frustrate the audience and will imply a sense of uncertainty or insecurity. Slurred speech will cause the audience to focus on the individual words being spoken and take the focus away from the presented material.
To appear professional, the speaker must use correct grammar and avoid colloquialisms. Some individuals will repeatedly use the same word, such as "like" or "okay". A fifteen minute presentation where the speaker says "okay" sixty times becomes tedious for the audience and some audience members will relieve that tedium by beginning to count how often such words or phrases are said.
It is also common for an novice speaker to abhor silence: rather than say nothing, a novice may make a guttural noise or say "um" or "ah".
The problem with many of the problems mentioned here is that the speaker may be completely unaware that he or she is making these errors. Practicing a speech in front of an audience is very useful to catch such errors.
Finally, the speaker must maintain variation in the voice. There are many ways of changing the characteristics of speech including:
A slow monotone voice will very quickly put the audience to sleep, however a high-intensity talk will quickly tire the audience. Increasing the volume is useful to bring attention to a critical remark. In general, a pace of approximately 160 words per minute is good for a presentation. A slower pace allows the audience to reflect on what is being said, while a faster pace may be useful when relating associate information. As with any such techniques, too much change will annoy the audience while too little change will put the audience to sleep. The voice will set the atmosphere of the presentation: this will require practice.
The best—and most difficult—technique of practicing one's voice technique is to take a video of a presentation and then self critique it. A person is their worst critic, so it is important that another person be present to balance the thoughts of the speaker watching him or herself.