Volume is the loudness of the speaker. It is the psychological characteristic of physical strength (amplitude). It is perceived as auditory sensation by the listener which can be ordered on a scale from quiet to loud.
Loudness is then a subjective measure of the listener, which is often confused with objective measures of sound strength such as sound pressure level (in decibels), sound intensity, or sound power.
Amplitude is the strength or power of the wave signal. Higher amplitudes on the wave graph are interpreted as a higher volume, hence the name "amplifier" for a device that increases amplitude.
When speaking naturally without any amplification you need to keep in mind the distance to be covered by your voice.
IN CONTEXT
You might compare speaking with the act of throwing a ball. Consider how much more muscular effort is needed to throw the ball a long distance than is required to throw it a short one. As you speak, think of your words as balls and mentally watch them covering the space between you and your audience. Notice how you unconsciously lengthen the voice.
If speaking in a face-to-face group without a microphone ask yourself if you are loud enough to be heard by audience members in the last row. Speaking to a large group will require more energy to breathe and control your sound production.
To become a better speaker with your "naked" speaking voice, try the following techniques:
In large rooms or when using videoconferencing equipment, you will use a microphone to convert sound into electrical signals for amplification.
The signal may then be sent to an amplifier. The electronic amplifier increases the power of a signal. It does this by taking energy from a power supply and controlling the output to match the input signal shape but with a larger amplitude. A loudspeaker or headset receives the input to produce the amplified sound.
There are several different types of microphones that the speaker might commonly use in different situations, such as:
Hand-held mics: High quality mics usually attempt to isolate the diaphragm from vibrations using foam padding, suspension, or some other method. Low quality mics tend to transfer vibrations from the casing right into the diaphragm, resulting in a terrible noise.
Lavalier mics: These are attached by a small clip to the clothing of the speaker, usually to collars or ties. The cord may be hidden by clothes and either run to a radio frequency transmitter or a digital audio recorder kept in a pocket or clipped to a belt (for mobile work), or directly to the mixer. These usually do not have protection from handling noise.
Stationary mics: These are permanently attached to a podium. With an attached microphone, you are limited to the space immediately in front of the podium. Some stationary mics are in a holder on the podium, which can be removed to allow you to move around at least the length of the connecting cable.
Microphones can be placed in several different arrangements in the room for recording or videoconferencing, such as:
In close: The microphone is placed relatively close, within three to twelve inches, which reduces extraneous noise.
In distant or ambient miking: The microphone is placed at some distance from the speaker. The goal is to get a broader, natural mix of the sound source, along with ambient sound, including reverberation from the room or hall.
In room miking: This is used together with a close microphone, sometimes during the speaker Q&A.
To make the most out of a microphone, a speaker should consider these techniques:
Source: Boundless. "Volume." Boundless Communications Boundless, 14 Mar. 2017. Retrieved 31 May. 2017 from https://www.boundless.com/communications/textbooks/boundless-communications-textbook/delivering-the-speech-12/effective-vocal-delivery-64/volume-253-4174/