How many oscillations each second?

The idea

One complete oscillation is the change of density at a fixed point from maximum to minimum and back; frequency ν is the number of such oscillations per unit time, measured in hertz (Hz = s⁻¹).

Watching one spot

Imagine a tiny density meter fixed at one spot in the air. A sound passes. The meter shows the density going up and down, again and again.

If the sound has a higher frequency, what do you think the meter shows?

• More ups and downs in each second
• Fewer ups and downs in each second
• The same number of ups and downs, but taller

Count the oscillations

The graph shows the density at one fixed point over 2 seconds. Change the frequency and count the complete oscillations in the shaded first second. The numbers are only examples.

Oscillations per second

One complete oscillation is the density at a fixed point going from maximum to minimum and back to maximum. The number of oscillations in one second is the frequency. One hertz means one oscillation per second.

One complete oscillation is the change of density at a fixed point from maximum to minimum and back. The frequency ν is the number of such oscillations per unit time. Its SI unit is the hertz (Hz), which is s⁻¹.

Notes

Frequency ν is the number of density oscillations per unit time at a fixed point. Its SI unit is the hertz (Hz = s⁻¹).

Check yourself

There are 12 density oscillations in 3 seconds at a given position (an example). What is the frequency, in Hz?

Answer: 4 Hz

Frequency = 12 oscillations ÷ 3 s = 4 Hz.

What is one hertz?

The density at a fixed point goes from maximum to minimum and back to maximum. How many complete oscillations is this?

A sound has a frequency of 50 Hz. What does this mean?

• One oscillation per second — correct. Yes!
• One oscillation per hour. A hertz is one oscillation per second, also written s⁻¹.
• One metre of wavelength. A metre is the unit of wavelength. One hertz is one oscillation per second.
• Two. The change from maximum to minimum and then again to maximum makes one complete oscillation.
• One — correct. Yes!
• Half. The change from maximum to minimum and then again to maximum makes one complete oscillation, not half.
• The sound lasts 50 seconds. Frequency is the number of oscillations per unit time. It does not tell how long the sound lasts.
• The wavelength is 50 m. Frequency is the number of oscillations per second. Wavelength is a distance and is measured in metres.
• There are 50 density oscillations at a fixed point in each second — correct. Yes!