The SI unit of time is ______.

Reverberation occurs only in ________ .

A loud unpleasant sound is called a/an ______.

• An ultrasound has frequency more than 20kHz.
• And an infrasound has frequency less than 20Hz.

A decrease in amplitude makes a sound wave _______.

Sounds that are pleasant to hear are called ultrasounds.

• Sounds which have frequencies more than 20kHz are called ultrasounds.
• Human ears are cannot perceive ultrasound.

Communication in the submarines takes place using infra waves.

• SONAR stands for sound navigation and ranging.

The device that transmits ultrasounds in SONAR is called a/an _______.

• The receiver receives the sounds reflected from underwater objects.

Echo is the_________ of sound.

• Refraction is the bending of waves when they enter one medium from another medium.

Sound cannot travel through _________.

The multiple reflection of sound by a closed surface is called _______

• Amplified sound is any sound which is made louder.
• Echo is the reflection of sound after a delay.

If a sound is amplified, it will be comparatively ______.

Echo is the reflection of a sound.

If the amplitude of a sound increases, then it is said to be ______ .

• An echo is a reflection of sound that arrives to the listener with a delay after the original sound.
• Reverberated sound is when you hear the same sound continuing to echo.

Sound cannot travel in space because sound cannot travel through _______.

154Hz sound frequency comes under ________ .

• Sounds with frequencies less than 20Hz are called infrasound.
• Sounds with frequencies more than 20kHz are called ultrasounds.

Infrasound have more ____________.

ECG uses _________ for the medication.