Frequency, time period and audible range: Class 9 practice
Frequency is the number of oscillations per second (Hz) and time period T = 1/f. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Fractions
- Units
The idea behind frequency, time period and audible range
Frequency is the number of oscillations per second (Hz) and time period T = 1/f. Humans hear about 20 Hz to 20 kHz; lower is infrasound, higher is ultrasound.
Step-by-step method
- T = 1/f or f = 1/T.
- Compare f with 20 Hz and 20 000 Hz.
Worked example 1 (Basic)
A source vibrates at 15000 Hz. Find its time period.
- T = 1/15000 = 6.67e-5 s
- Answer: 6.67e-5 s
Worked example 2 (Standard)
A source vibrates at 5 Hz. Find its time period and say whether it is audible.
- T = 1/5 = 2.00e-1 s
- infrasound
- Answer: 2.00e-1 s, infrasound
Worked example 3 (Challenge)
A wave has time period 0.002 s. Find its frequency and say whether humans can hear it.
- f = 1/0.002 = 500 Hz
- Audible
- Answer: 500 Hz, audible
Common mistakes and misconceptions
- Mixing kHz and Hz
- Calling 15 Hz ultrasound
Practice questions
- (Basic) A source vibrates at 256 Hz. Find its time period.
- (Basic) A source vibrates at 25000 Hz. Find its time period.
- (Standard) A source vibrates at 12 Hz. Find its time period and say whether it is audible.
- (Standard) A source vibrates at 40000 Hz. Find its time period and say whether it is audible.
- (Challenge) A wave has time period 0.00004 s. Find its frequency and say whether humans can hear it.
- (Challenge) A wave has time period 0.01 s. Find its frequency and say whether humans can hear it.
Answer key with full working
- Q1: T = 1/256 = 3.91e-3 s → Answer: 3.91e-3 s
- Q2: T = 1/25000 = 4.00e-5 s → Answer: 4.00e-5 s
- Q3: T = 1/12 = 8.33e-2 s → infrasound → Answer: 8.33e-2 s, infrasound
- Q4: T = 1/40000 = 2.50e-5 s → ultrasound → Answer: 2.50e-5 s, ultrasound
- Q5: f = 1/0.00004 = 25000 Hz → Above 20 kHz → ultrasound → Answer: 25000 Hz, ultrasound
- Q6: f = 1/0.01 = 100 Hz → Audible → Answer: 100 Hz, audible
Syllabus note
This skill is listed under "Sound" for Class 9 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 frequency, time period and audible range in?
"Sound", Class 9 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.