an ambulance is approaching a stationary observer. for the observer, the apparent frequency of the siren compared to the actual frequency of the siren
The apparent frequency of the siren as heard by the stationary observer will be higher than the actual frequency of the siren. This effect is known as the Doppler effect and occurs when there is relative motion between the observer and the source of sound.
When the ambulance approaches the observer, the sound waves from the siren get compressed, causing an increase in perceived frequency. As a result, the apparent frequency of the siren will be higher than its actual frequency. This is often described as a higher pitch sound.
Conversely, when the ambulance is moving away from the observer, the sound waves get stretched, causing a decrease in perceived frequency. In this case, the apparent frequency of the siren will be lower than its actual frequency, leading to a lower pitch sound.
It is important to note that the actual frequency of the siren remains constant. It is only the perceived frequency that changes for the observer due to the relative motion between the observer and the source of sound.
The observed frequency of the siren compared to the actual frequency of the siren can be determined using the Doppler effect.
The Doppler effect is the change in frequency or wavelength of a wave as perceived by an observer moving relative to the source of the wave. In this case, the observer is stationary, and the ambulance with the siren is approaching.
When a source of sound (such as a siren) is moving towards an observer, the frequency of the sound waves received by the observer appears higher than the actual frequency of the source. This increase in frequency is known as a "blue shift." Conversely, if the source is moving away from the observer, the frequency appears lower, creating a "red shift."
To calculate the observed frequency (f_obs) when the ambulance is approaching a stationary observer, we can use the formula:
f_obs = f_source * (v_sound + v_observer) / (v_sound + v_source)
Where:
- f_obs is the observed frequency
- f_source is the actual frequency of the siren
- v_sound is the speed of sound in air (approximately 343 m/s)
- v_observer is the velocity of the observer (0 m/s for a stationary observer)
- v_source is the velocity of the source (the ambulance)
Keep in mind that the observed frequency will depend on the velocity of the ambulance. If the ambulance is stationary, the observed frequency will be the same as the actual frequency.