Sound, Decibels, Doppler Effect

Transcription

1 Phys Lectures 3, 33 Sound, Decibels, Doppler Eect Key points: ntensity o Sound: Decibels Doppler Eect Re: -,,7. Page

2 Characteristics o Sound Sound can trael through any kind o matter, but not through a acuum. The speed o sound is dierent in dierent materials; in general, it is slowest in gases, aster in liquids, and astest in solids. The speed depends somewhat on temperature, especially or gases.

3 Characteristics o Sound Loudness: related to intensity o the sound wae Pitch: related to requency Audible range: about Hz to, Hz; upper limit decreases with age Ultrasound: aboe, Hz; nrasound: below Hz

4 ntensity o Sound: Decibels The intensity o a wae is the energy transported per unit time across a unit area. The human ear can detect sounds with an intensity as low as - W/m and as high as W/m. Perceied loudness, howeer, is not proportional to the intensity.

5 Sound Leel: Decibels The loudness o a sound is much more closely related to the logarithm o the intensity. Sound leel is measured in decibels (db) and is deined as: is taken to be the threshold o hearing:

6 Example: Sound intensity on the street. At a busy street corner, the sound leel is 75 db. What is the intensity o sound there? 75 log log W / m

7 Example: Loudspeaker response. A high-quality loudspeaker is adertised to reproduce, at ull olume, requencies rom 3 Hz to 8, Hz with uniorm sound leel ± 3 db. That is, oer this requency range, the sound leel output does not ary by more than 3 db or a gien input leel. By what actor does the intensity change or the maximum change o 3 db in output sound leel? Gien 3dB, log log log ind : 3 log db means times in intensity (or energy, power etc.)

8 Conceptual Example: Trumpet players. A trumpeter plays at a sound leel o 75 db. Three equally loud trumpet players join in. What is the new sound leel? log 4 log 4 4 log 4 log log 4 log dB

9 Example: Airplane roar. The sound leel measured 3 m rom a jet plane is 4 db. What is the sound leel at 3 m? (gnore relections rom the ground.) At 3 m: Sound intensity: Sound leel : log 4dB Sound At 3 m: r log intensity: Sound leel : r r 3,. r r 3. log log 4 db log. log

10 Example: How tiny the displacement is. Calculate the displacement o air molecules or a sound haing a requency o Hz at the threshold o hearing. [Solution] P S Sl A / t A A A A m

11 ntensity o Sound: Decibels The ear s sensitiity aries with requency. These cures translate the intensity into sound leel at dierent requencies.

12 Doppler Eect The Doppler eect occurs when a source o sound is moing with respect to an obserer. A source moing toward an obserer appears to hae a higher requency and shorter waelength; a source moing away rom an obserer appears to hae a lower requency and longer waelength.

13 elocity o sound we can igure out what the change in the waelength is, we also know the change in the requency. When the source is approaching, the obsered waelength is shorter: source T Then the obsered requency is: source T source source T sourcet

14 Doppler Eect The change in the requency is gien by: Source approaching: Higher req. > Similarly, i the source is moing away rom the obserer: Source receding: Lower req. <

15 Doppler Eect the obserer is moing with respect to the source, things are a bit dierent. The waelength remains the same, but the wae speed is dierent or the obserer. obs when the obserer is approaching obs obs obs

16 For an obserer moing toward a stationary source: Obsered sound elocity: Obsered requency: obs obs obs obs Obserer approaching: Higher req. > And i the obserer is moing away: obs obs obs obs Obserer receding: Lower req. <

17 Example: A moing siren. The siren o a police car at rest emits at a predominant requency o 6 Hz. What requency will you hear i you are at rest and the police car moes at 5. m/s (a) toward you, and (b) away rom you? (a) Moing source, approaching: Higher req. source Hz (speed o sound in air : 343m / s) (b) Moing source, receding: Lower req. source Hz

18 Example -5: Two Doppler shits. A 5-Hz sound wae is emitted by a stationary source. This sound wae relects rom an object moing 3.5m/s toward the source. What is the requency o the wae relected by the moing object as detected by a detector at rest near the source? (Photo Radar) Frequency receied by the moing object: obs Hz 343 Then, the object becomes a moing source when the sound is relected back to the detector. The requency receied by the detector is: source Hz This is known as a double Doppler eect. The moing object irst acts as a moing obserer and then a moing source.

19 Doppler Eect All our equations or the Doppler eect can be combined into one; you just hae to keep track o the signs! The signs: Approaching: higher req. Receding: lower req.

20 Phys Course Ealuation July 7 (Today) :noon August 8. On Webct % Bonus mark