# What is sound class 9th answer?

Answer: Sound is a mechanical wave and needs a material medium to propagate. It cannot travel in vacuum and can be shown by the following experiment. Take an electric bell and an airtight glass bell jar.

## What is sound in physics in 9th class?

Sound is a mechanical energy which produces sensation of hearing. Sound is produced due to vibration of different objects. • Sound wave propagates as compressions & rarefactions in the medium. Sound waves are longitudinal waves.

## How do you solve a sound wave in physics?

Assuming the speed of sound in air is 345 m/s, determine the wavelength of the sound corresponding to the note of middle C. Let λ = wavelength. Use v = f • λ where v = 345 m/s and f = 256 Hz. Rearrange the equation to the form of λ = v / f.

## What is 9 9th class sound?

Sound is defined as vibrations that travel through air or another medium as an audible mechanical wave. It is produced from a vibrating body. The vibrating body causes the medium (water, air etc.) around it to vibrate these producing sounds.

## Why do we fall ill?

Infectious diseases are caused by pathogens such as bacteria, viruses, fungi and parasites entering the body through the air we breathe, the food and drink we consume or through openings in the skin, such as cuts. As an example, through cough and sneezing a person can spread the cold virus to another.

## Why do we fall ill Class 9?

Answer: This is because the immune system of our body is normally fighting off microbes. Our body have cells that are specialised in killing infecting microbes. Whenever any microbes or foreign body enters our system, these cells become active and kill the microbes that could cause any damage to the body.

## What is the SI unit of sound?

Since this energy can be measured, the most common SI unit of sound that is used is Decibel which is abbreviated as dB. This unit is basically used to express the ratio of some value of a physical property to another, mainly on a logarithmic scale. This is called as Decibel Scale or DB Scale.

## What is Echo Class 9?

An echo is a sound caused by the reflection of sound waves from a surface back to the listener. It is the reflection of sound, arriving at the listener some time after the direct sound.

## What are the 5 characteristics of sound?

There are five main characteristics of sound waves: wavelength, amplitude, frequency, time period, and velocity. The wavelength of a sound wave indicates the distance that wave travels before it repeats itself.

## What are the 4 types of sound waves?

So what type of wave is sound? Sound waves fall into three categories: longitudinal waves, mechanical waves, and pressure waves.

## What does Hz mean in sound?

High frequencies produce more oscillations. The units of frequency are called hertz (Hz). Humans with normal hearing can hear sounds between 20 Hz and 20,000 Hz. Frequencies above 20,000 Hz are known as ultrasound.

## What is reverberation Class 9?

Reverberation is the phenomenon of overlapping of sound caused by multiple reflections. It causes the overlapping of several reflected waves.

## How do we fall ill notes?

Immediate cause: The organisms that enter our body and causes disease is called immediate cause. For example, virus, bacteria, protozoa etc. surroundings, contaminated food, improper nourishment, poverty, poor standard of living etc. Diseases may be due to infectious and non-infectious causes.

## What does disease look like?

Note: Disease can be viewed by examining symptoms of disease. The common symptoms of diseases include headache, fever, vomiting etc. Like jaundice can be examined by pale skin. Diarrhoea can be examined by loose motions.

## What is health class 9?

Health is a state of complete physical mental and social well being. Its not only the absence of disease. ​ A person is said to be healthy when he/she is free of any type of disease (infectious/deficiency), when he/she is mentally happy and healthy and when his/her social relationships are healthy in a society.

## What is difference between health and disease?

Hint: Health is defined as an individual’s mental, physical, and psychological well-being. The disease is defined as the abnormal dysfunctioning of the mental-physical states, which results in illness in a specific individual and does not take into account society or community.

## What are vectors Class 9?

Vectors are those infectious agents which carry infections from a sick person to a healthy person. Example: Female Anopheles mosquito work as a vector causing Malaria disease. It has 2 life cycle one in human where they reach to sporophyte stage and then the gut of mosquito where they reproduce and grow.

## What are symptoms Class 9?

A symptom is any subjective evidence of disease, while a sign is any objective evidence of disease. Therefore, a symptom is a phenomenon that is experienced by the individual affected by the disease, while a sign is a phenomenon that can be detected by someone other than the individual affected by the disease.

## How many books are in 9th class?

NCERT Books for Class 9 Hindi NCERT prescribes 4 books for the class which are – Kshitij, Sparsh, Kritika and Sanchayan.

## What is dB noise level?

It is measured in decibels (dB). The decibel scale is logarithmic, which means that loudness is not directly proportional to sound intensity. Instead, the intensity of a sound grows very fast. This means that a sound at 20 dB is 10 times more intense than a sound at 10 dB.

## How is dB value calculated?

For all decibel calculations, use the common logarithm. = Exp × log (value). So, in the case of the voltage and current ratios, the exponent of 2 is brought outside the logarithm calculation as 10 × 2 log (ratio) = 20 log (ratio).

## What is i0 intensity?

Making use of the sound intensity associated with zero decibels, I0 = 10-12 watts/m2, the sound pressure level (SPL) P0 associated with the threshold of hearing is. This can be used to develop the expression for sound pressure level related to the threshold of hearing above.

## What is the formula of echo?

Hence, to hear an echo of a sound the interval between reaching the main sound and echo must be $\dfrac110$ seconds. We can use the formula, distance $d = V \times t$.

## What is amplitude class 9?

Amplitude is the maximum displacement or distance moved by a point on a vibrating body or wave measured from its equilibrium position.

## What is sonar short?

Sonar uses sound waves to ‘see’ in the water. Sonar, short for Sound Navigation and Ranging, is helpful for exploring and mapping the ocean because sound waves travel farther in the water than do radar and light waves.