# What is Beat frequency: Definition and 69 Discussions

In acoustics, a beat is an interference pattern between two sounds of slightly different frequencies, perceived as a periodic variation in volume whose rate is the difference of the two frequencies.
With tuning instruments that can produce sustained tones, beats can be readily recognized. Tuning two tones to a unison will present a peculiar effect: when the two tones are close in pitch but not identical, the difference in frequency generates the beating. The volume varies like in a tremolo as the sounds alternately interfere constructively and destructively. As the two tones gradually approach unison, the beating slows down and may become so slow as to be imperceptible. As the two tones get further apart, their beat frequency starts to approach the range of human pitch perception, the beating starts to sound like a note, and a combination tone is produced. This combination tone can also be referred to as a missing fundamental, as the beat frequency of any two tones is equivalent to the frequency of their implied fundamental frequency.

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1. ### Beat frequency heard from two tuning forks

Is my solution correct? I used v=lambda*f, i.e. f=v/lambda to get the frequency for each wave. Then I calculated the average of the frequencies to get 258Hz and found the beat frequency by doing f1-f2 to get 4Hz. I then said that this means the observer will hear a tone of frequency 258Hz which...
2. ### I Time period of Beat (acoustics)

$$y=2 A \cos 2 \pi\left(\frac{\nu_{1}-\nu_{2}}{2}\right) t \sin 2 \pi\left(\frac{\nu_{1}+\nu_{2}}{2}\right) t$$ Can you explain me the significance of the above equation in the context of waves and oscillations? It's something to do with 'beats,'.
3. ### Doppler effect and beat frequency

Summary:: Two speakers A and B are at rest, and a listener L stays on the line that connects the two speakers (see picture). The speakers have almost the same frequency. Assume that the speed of sound in air is 340 m/s. When the listener is at rest, he/she hears beats with frequency 6 Hz. The...
4. ### Beat frequency and Doppler shift

I found the beat frequency to be 4 Hz and the carrier frequency to be 260 Hz, but I'm not sure how to apply them to the solution/integrate the doppler and beat frequency equations?
5. ### Travelling towards a Canyon Wall: Calculating Beat Frequency

Homework Statement 1. On a long trip that you take to forget for a while about physics, you find yourself driving directly towards a canyon wall. You’ve previously noticed that your car horn has a frequency of 440 Hz. You glance at the speedometer and notice you’re moving at 30 km/h...
6. ### Understanding Beat Frequencies in Sound Waves

Homework Statement Homework Equations ##f_beat=f_1-f_2## The Attempt at a Solution Why are the two different answers? Is it because the first question is asking for how often it fluctuates, and the other is actually asking for the frequency of the sound? Why is the resultant tone the...
7. ### How do you find speed given the beat frequency?

Homework Statement Using a radar gun, you emit radar waves at a frequency of 2.6 GHz that bounce off of a moving tennis ball and recombine with the original waves. This produces a beat frequency of 730 Hz. How fast was the tennis ball moving? Homework Equations fbeat = fa - fb v=λf f=1/TThe...
8. ### Guitar Strings & Tuning Forks: Investigating Beats

Homework Statement A guitar string with 0.60 m and 0.012 of mass vibrates with frequencies that are multiples of 109 Hz. Approaching to the string a tuning fork of 440 Hz we verify beats between the sound signals of the string and the tuning fork. Calculate the time interval between consecutive...
9. ### Flute player initial frequency?

Homework Statement A flute player hears four beats per second when she compares her note to a 587 Hz tuning fork (the note D). She can match the frequency of the tuning fork by pulling out the "tuning joint" to lengthen her flute slightly. What was her initial frequency? Homework Equations Not...
10. ### Problem Related to Beat Frequency of a hollow tube

Homework Statement Q. A hollow metallic tube closed at one end produces resonance with a tuning fork of frequency n at temperature T. The temperature of the tube is increased by dT. If the coefficient of thermal expansion of the tube metal is alpha, the beat frequency will be - (there are 4...
11. ### A question about wave motion and beat frequency

Homework Statement 3 tuning forks of frequencies 200, 203, 207 Hz are sounded together.find out the beat frequency. Homework Equations Beat frequency= n1-n2 (n=frequency). The Attempt at a Solution I know that beat frequency is the difference in the frequencies of two superposing notes. But...
12. ### Beat Frequency of 2 Waves in Free Space

Homework Statement Two waves of different wavelengths λ1= 1.5μm, λ2 = 1.63μm, are combined in free space Et = 1.5cos(k1x - ω1t) + 2cos(k2x -ω2t) Find the beat frequency Homework Equations (ω1- ω2) = 2ωm=beat frequency E02 = 4E012cos2(kmx - ωmt) The Attempt at a Solution I am quite stuck on...
13. ### Solving Beat Frequency with Tuning Forks: f2 = f1 ± fb

Homework Statement Tuning Fork A has frequency of 320 Hz. When it is sounded with Tuning Fork B, 14 beats are detected in 7.0 seconds. Elastic bands are placed on the tines of Tuning Fork B and the beat frequency drops down to 1.0 Hz. What was the original frequency of Tuning Fork B? Explain...
14. ### Marching Band doppler effect with Beat Frequency.

Homework Statement Emma and Don are playing in a band. While emma is marching toward Don, Don tunes his instrument to match the note that emma is playing. When emma stops, don immediately notices a beat frequency of 5 Hz. If the speed of the sound is 330m/s what is the relationship between...
15. ### What is the beat frequency for a cyclist ringing a bell towards a wall?

Homework Statement A cyclist with a bell ringing with a frequency of 659.7 Hz drives towards a wall with a speed of 8.15 ms-1. Just before colliding with the wall the cyclist hears beats, due to the bell itself and the reflection of the sound from the wall. What is the frequency of beats...
16. ### Beat frequency and length question. (grade 11)

Homework Statement Two pipes, Identical in length and closed at one end are producing notes. You are very, very annoyed because the notes are creating an audible beat frequency of 20Hz. If one pipe is producing a note at 10Hz and the temperature is 15C (which correlates to a speed of sound of...
17. ### Doppler effect and Beat frequency

Homework Statement A runner is running with speed V along the straight line connecting two identical speakers. Both speakers are playing a tone of the same frequency f. a)What is the beat frequency that the runner hears? b)If the standing wave forms between two speakers, how frequently will the...
18. ### Beat frequency and velocity of observer

Homework Statement Two sound sources 15m apart producing identical 229 Hz sounds. As you move from one to the other, you hear beat frequency of 2.5. How fast are you moving?[/B] Homework Equations The question asks for the velocity of the observer Vo. The trick here is to use the standing wave...
19. ### What Was the Initial Frequency of the Trumpet Player?

Homework Statement A trumpet player hears 3 beats per second when she plays a note and simultaneously sounds a 440 Hz tuning fork. After pulling her tuning valve out to slightly increase the length of her trumpet, she hears 5 beats per second against the tuning fork. What was her initial...
20. ### What is the tension of the string needed for a 440 Hz fundamental frequency?

Homework Statement [/B] A violin string which is 30 cm long is tuned using a 440 Hz reference tone. A) What is the wavelength fundamental mode of the string? B) When the string has a tension of 34 N a violinist hears 5.7 beats per second. What are the frequencies at which the string might be...
21. ### What is the relationship between beat period and cosine cycles in a graph?

Homework Statement please refer to the notes , since we know that a cycle of cosine is as in the photo 1 , why the beat period has 8 complete cycle of cosine? Homework Equations The Attempt at a Solution
22. ### Harmonics, Notes, Beat frequency

Homework Statement When an instrument plays a note, the resulting sound is a combination of all the possible harmonics for that instrument in its momentary configuration. For instance, a musician changes notes on a violin by pressing the strings against the neck of the instrument, thus...
23. ### Beat Frequency of tuning forks

Homework Statement Two tuning forks, first fork has freqency of f = 273Hz. The beat frequency is 3.00Hz. What is the frequency of the second fork? Homework Equations f_{beat} = |f_1 - f_2| The Attempt at a Solution This was a question on my Physics I exam today, and my prof is...
24. ### What is the Beat Frequency in Doppler Ultrasound Measurements?

Doppler ultrasound is used to measure the speed of blood flow. If the speed of the red blood cells is v, the speed of sound in blood is u, the ultrasound source emits waves of frequency f, and we assume that the blood cells are moving directly toward the ultrasound source, then the frequency fr...
25. ### Envelope, carrier and beat frequency equations?

Hello, What are the equations used to calculate the carrier, envelope and beat frequencies? Thanks
26. ### Beat frequency: which equation to use to find the mass of string 2

Homework Statement Two strings of equal length, L, are each clamped at both ends and are under the same tension T. String 1 has a mass of 45.00 g and vibrates at a frequency of 512 Hz. String 2 is slightly heavier. When the two strings are plucked at the same time, beats with a frequency...
27. ### How to get beat frequency from graph

Homework Statement how do you get the beat frequency from the beat envelope of two intersecting waves as in the attachment Homework Equations fb=f1-f2 The Attempt at a Solution i thought maybe to get the frequency of the actual wave but don't think that's right
28. ### Beat frequency- universal mathematical proof?

It is easily derived from trigonometric identities that cos(2*pi*f1*t)+cos(2*pi*f2*t)=2cos(2*pi*(f2+f1)*t)*cos(2*pi*(abs(f2-f1)*t)) which proves that superposition of two cosine wave will generate a beat frequency, but what about about a universal proof that applies to any kind of...
29. ### Beat frequency for hanging bar between two wires with added offset weight

Homework Statement A uniform 165 N bar is supported horizontally by two identical wires A and B. A small 185 N cube of lead is placed 3/4 of the way from A to B. The wires are each o.75 m long and have a mass of 5.50 g. If both of them are simultaneously plucked at the center, what is the...
30. ### Laser beams and beat frequency.

If I combine 2 laser beams of different wavelengths, would I get a beat frequency of the difference between the 2 lasers? Thanks, Bill
31. ### Calculating Speed of Approaching Police Car Using Beat Frequency

Homework Statement Two police cars have identical sirens that produce a frequency of 660 Hz. A stationary listener is standing between two cars. One car is parked and the other is approaching the listener and both have their sirens on. The listener notices 8.7 beats per second. Find the speed...
32. ### Help Needed: Solving Homework Assignment on Beat Frequency

So I had a homework assignment which I couldn't complete in time. One of the questions was what the title is about. I also asked for help earlier in the homework section. See my post: https://www.physicsforums.com/showthread.php?t=487676. Right now I just want to learn how this works for the...
33. ### Beat frequency, find intensity at distance from sources?

Homework Statement Three sources are emitting sound at 0.8W, one at 200 Hz, second one at 201 Hz and third one at 202 Hz. The sound travels in a shape of a half sphere. Find the intensity at 3m distance from the source (the sources are said to be very close to each other so I guess they can be...
34. ### Possible Frequencies of Tuning Fork for Beat Frequency of 4.85 Hz

Homework Statement A 0.0145 kg, 2.18 m long wire is fixed at both ends and vibrates in its simplest mode under a tension of 201 N. When a tuning fork is placed near the wire, a beat frequency of 4.85 Hz is heard. What are the possible frequencies of the tuning fork? (enter the smaller...
35. ### Calculating Fractional Increase in Tension for Beat Frequency Problem

Homework Statement Two identical piano wires have a fundamental frequency of 600 Hz when kept under the same tension.What fractional increase in the tension of wire will lead to the occurrence of 6.0 beat/sec when both wires oscillate simultaneously? Homework Equations fbeat=f2-f1...
36. ### Understanding Beat Frequency: Equations and Solutions

Homework Statement See attachment. Homework Equations The Attempt at a Solution f1 - f2 = 5 What's there for me to interpret in the graph?
37. ### Beat frequency when there are phase constants

Homework Statement If we have two vibrations with angular frequencies ω1 and ω2 with ω1≈ω2. Then we will have beats with beat frequency ω1-ω2. But suppose we have two different phase constants, for example x1 = Acos(ω1t + ϕ1) and x2 = Acos(ω2t + ϕ2). What happens to the beat frequency...
38. ### Understanding Beat Frequency in Sinusoidal Waves: Exploring the Envelope Concept

So far every source I have read define "beat" frequency as the difference between two frequencies or f2-f1. The common example is in the audio world where we have two tones very close in value, say 100 MHz and 101 MHz. In this case the beat frequency would simply be 1MHz. These same sources...
39. ### What is the beat frequency of two strings with different tensions?

Homework Statement Two strings with otherwise identical properties differ in tension by 0.25 N. If both strings are struck at the same time, what is the resulting beat frequency? Homework Equations frequency is proportional to the root of tension? The Attempt at a Solution I...
40. ### What Causes Beat Frequency in Reflected Laser Light?

Homework Statement A laser emits a monochromatic beam of wavelength \lambda , which is reflected normally from a plane mirror, receding at speed v . What is the beat frequency between the incident and reflected light? I know that the f_{beat} = |f_2 - f_1|. When the wave hits the...
41. ### Can beat frequencies occur when the amplitude of the waves are different?

When two waves of same amplitude but differing frequencies are added together, the frequency of the amplitude modulation of the resulting wave is the difference b/w the frequencies of the two parent waves. How about beat frequencies from the interference of more than 2 waves, say 3, 4, or 10...
42. ### What Beat Frequency Results from Unequal Tensions in Piano Strings?

Homework Statement In certain ranges of a piano keyboard, more than one string is tuned to the same note to provide greater intensity. For example, the note at 162 Hz has two strings at this pitch. If one string slips from its normal tension of 645 N to 516 N, what beat frequency will be...
43. ### What Is the Interval ΔT Between the Periods of Two Synchronous Strobe Lights?

Homework Statement The strobe lights on top of two tall radio towers both have a period of about 1.00 s. When turned on simultaneously, a keen observer notes that during an 813 s interval, the lights gradually fall out of synchronization and then come back into flashing synchronously. What...
44. ### Beat frequency of strobe lights

Homework Statement The strobe lights on top of two tall radio towers both have a period of about 1.00 s. When turned on simultaneously, a keen observer notes that during an 813 s interval, the lights gradually fall out of synchronization and then come back into flashing synchronously. What is...
45. ### Doppler Effect & Beat Frequency

Homework Statement Horseshoe bats use the Doppler Effect to determine their location. A horseshoe bat flies toward a wall at a speed of 15 m/s while emitting a sound of frequency 35 kHz. What is the beat frequency between the emission frequency and the echo? Assume that the air temperature is...
46. ### How Does Temperature Affect Beat Frequency in Organ Pipes?

Homework Statement Chapter 12, problem 34. An auditorium has organ pipes at the front and at the rear of the hall. Two identical pipes, one at the front and one at the back, have fundamental frequencies of 264.00 Hz at 20.0°C. During a performance, the organ pipes at the back of the hall...
47. ### Beat Frequency: Find Frequency with Wavelength of 3.141m & 3.135m

Homework Statement What would be the beat frequency if radio waves of wavelenght 3.141m and 3.135 are mixed? The Attempt at a Solution I need frequency of wave1 and wave2, because Fb=|f1-f2| But i don't have nothing but wavelength... Any hints?
48. ### Unraveling the Mystery of Beat Frequencies in Musical Instruments

Two-out-of-tune flutes play the same note. One produces a tone that has a frequency of 260 Hz, while the other produces 266 Hz. When a tuning fork is sounded together with the 260 Hz tone, a beat frequency of 1 Hz is produced. When the same tuning fork is sounded together with the 266 Hz tone, a...
49. ### What beat frequency will the observer detect?

Homework Statement Two trains emit whistles of the same frequency, 282 Hz. If one train is at rest and the other is traveling at 45 km/h away from an observer at rest, what beat frequency will the observer detect? Homework Equations The Attempt at a Solution I used listener...
50. ### Beat Frequency of xylophone

Homework Statement A xylophone has a set of vibrating bars. A bar with a frequency of 343 Hz is struck. A very short time later, a second bar is also sounded, and one hears the volume of the xylophone increase and decrease 5 times each second (called beats). What are the possible frequencies...