The difference in sound produced from the top of a snorkel versus underwater is primarily due to the change in the medium through which sound waves travel.
When you are on the surface and the top of the snorkel is above the water, the primary medium for sound propagation is the air. Sound waves generated from your mouth or any other source travel through the air and reach the listener's ears. In this case, the sound travels as air pressure waves, and our ears perceive it as typical airborne sound.
However, when you submerge underwater and the snorkel's top is submerged as well, the medium for sound propagation changes. Water is denser than air, and sound waves travel more effectively through water due to its higher molecular density. When you make a sound underwater, the sound waves now propagate through water instead of air.
The change in medium affects the way sound waves interact with the surrounding environment. In water, sound waves travel faster, and they also attenuate (weaken) at a different rate compared to air. This leads to changes in the perceived sound characteristics.
Underwater sounds tend to be louder and more muffled compared to sounds in the air. This is because water is denser and transfers more energy from the vibrating source to the surrounding medium, resulting in stronger sound transmission. However, the higher density of water also causes more rapid attenuation, meaning that sound waves lose their energy and intensity over shorter distances underwater.
Additionally, the different density and composition of water can alter the way certain frequencies are transmitted, resulting in a change in the overall sound quality. High-frequency sounds, in particular, tend to be absorbed more quickly in water compared to low-frequency sounds.
Overall, the change in medium from air to water when submerging with a snorkel leads to differences in sound transmission, resulting in a distinct sound experience underwater compared to on the surface.