Beats: Class 11 practice
Two sounds of slightly different frequency produce beats; beat frequency = |f₁ − f₂|. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Frequency
The idea behind beats
Two sounds of slightly different frequency produce beats; beat frequency = |f₁ − f₂|. Loading a tuning fork with wax lowers its frequency; filing it raises it.
Step-by-step method
- Beat frequency = difference of frequencies.
- Find two possible frequencies f ± beats.
- Use loading/filing information to choose.
Worked example 1 (Basic)
Two forks of 299 Hz and 301 Hz sound together. How many beats per second?
- |301 − 299|
- Answer: 2
Worked example 2 (Standard)
A fork gives 7 beats/s with a 338 Hz fork. What could its frequency be?
- 338 ± 7
- Answer: 331 Hz or 345 Hz
Worked example 3 (Challenge)
Fork X gives 7 beats/s with a 313 Hz fork. When X is loaded with wax, the beats decrease. Find X's original frequency.
- X = 306 or 320
- Loading lowers X; beats fall only if X > 313
- Answer: 320 Hz
Common mistakes and misconceptions
- Adding the frequencies
- Assuming wax raises frequency
Practice questions
- (Basic) Two forks of 442 Hz and 444 Hz sound together. How many beats per second?
- (Basic) Two forks of 357 Hz and 365 Hz sound together. How many beats per second?
- (Standard) A fork gives 4 beats/s with a 363 Hz fork. What could its frequency be?
- (Standard) A fork gives 6 beats/s with a 329 Hz fork. What could its frequency be?
- (Challenge) Fork X gives 6 beats/s with a 473 Hz fork. When X is loaded with wax, the beats decrease. Find X's original frequency.
- (Challenge) Fork X gives 2 beats/s with a 458 Hz fork. When X is loaded with wax, the beats decrease. Find X's original frequency.
Answer key with full working
- Q1: |444 − 442| → Answer: 2
- Q2: |365 − 357| → Answer: 8
- Q3: 363 ± 4 → Answer: 359 Hz or 367 Hz
- Q4: 329 ± 6 → Answer: 323 Hz or 335 Hz
- Q5: X = 467 or 479 → Loading lowers X; beats fall only if X > 473 → Answer: 479 Hz
- Q6: X = 456 or 460 → Loading lowers X; beats fall only if X > 458 → Answer: 460 Hz
Syllabus note
This skill is listed under "Waves" for Class 11 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.
Related practice and notes
Frequently asked questions
Which chapter is beats in?
"Waves", Class 11 Physics (NCERT/CBSE). Other boards may place it differently.
How were the answers checked?
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.