The Doppler effect affects both the tone and the perceived velocity of sound waves. The effect describes the change in frequency (and therefore the pitch or tone) of a wave as observed by an observer when there is relative motion between the source of the wave and the observer. The velocity of sound itself, however, remains constant in a given medium.
When a sound source is moving towards an observer, the waves get compressed, resulting in a higher frequency and a higher pitch. This is commonly experienced when a siren approaches and the pitch of the sound appears to increase. Conversely, when a sound source is moving away from an observer, the waves get stretched, resulting in a lower frequency and a lower pitch. This is experienced when a siren moves away and the pitch of the sound appears to decrease.
It's important to note that the actual velocity of sound in a medium, such as air, remains constant under normal conditions. The Doppler effect only affects the perceived frequency and pitch of the sound waves as observed by a moving observer. The velocity of sound itself is determined by the properties of the medium through which it travels, such as air temperature, humidity, and pressure.
In summary, the Doppler effect affects both the tone (pitch) and the perceived velocity of sound waves. It is a phenomenon that occurs due to relative motion between the source of the sound and the observer.