17.3 Sound Intensity and Sound Level

The larger must be and the smaller must be because the product of multiplication by equals a constant.

When sound travels from one medium to another, the speed of sound can change.

The Frequency usually remains the same because it is like a driven oscillation and has the Frequency of the original source.
The wavelength must change if it stays the same.
The wavelength of a sound is determined by the speed of the sound.

The speed of sound is slower than the speed of light.

The speed difference is not noticeable because the first firework is very close.
The sound wave arrives at your ears before the light arrives at your eyes, because the firework is farther away.

You can't identify two musical instruments.

One plays high-pitched sounds and the other plays low-pitched sounds.

High-pitch instruments generate a smaller wavelength than low-pitch instruments.

It is hard to hear others unless they shout.

Sometimes you can hear a leaf fall in a forest.

You may hear blood flowing through your ears after you sleep.
You can't hear what the person next to you is saying when a passing driver has his stereo on.

It is common for musicians to have hearing losses that are so severe that they interfere with their ability to perform because of high noise exposure.

Sound intensity is a concept that is valid for all sounds, even if they are not audible.

The power per unit area is called intensity.

The wave transfers power at a rate.

The power is through an area.

The unit is called the SI.

The pressure variation is the difference between the maximum and minimum pressure in the sound wave.

The energy of air due to a sound wave is proportional to its amplitude squared.
The density of the material in which the sound wave travels, in units of, and the speed of sound in the medium, in units of m/s are related to this equation.
The pressure variation is a function of the amplitude of the oscillation.
The relationship is consistent with the fact that the sound wave is produced by some vibration and that the greater the pressure, the more air is compressed in the sound it creates.

The source that produces the more intense sound has larger-amplitude oscillations and greater pressure maxima and minima.

It can exert larger forces on the objects it encounters because of the higher-intensity sound.

Sound intensity levels are quoted in decibels more often than sound intensities.

In the popular media and scientific literature, decibels are the unit of choice.
The reasons for this choice of units are related to how we perceive sounds.
The logarithm of the intensity can be used to describe how our ears perceive sound.

The lowest or threshold intensity of sound a person with normal hearing can perceive is 1000 Hz.

Sound intensity is not the same as intensity.
It is a unitless quantity that tells you the level of sound relative to a fixed standard.

The units of decibels are used to show the ratio is greater than 10.

Alexander Graham Bell was the inventor of the telephone.

The threshold of hearing is zero decibels.

Table 17.2 shows decibels and intensities in watt per meter squared.

The intensities in Table 17.2 are quite small for most sounds.

Air molecules in a sound wave of this intensity vibrate over a distance of less than one molecule, and the gauge pressures involved are less than atm.

The sounds in Table 17.2 have a numerical range.

The sound intensity is determined by a number of factors, from the threshold to the sound that causes the most damage.
How your ears respond to sound intensity can be described as the logarithm of intensity.
Sound intensities in decibels fit your experience better than they do in watt per meter squared.
The decibel scale is easier to relate to because most people are used to dealing with numbers such as 0, 53, or 120.

Several government agencies and health-related professional associations recommend that 85 decibels not be exceeded for 8-hour daily exposures in the absence of hearing protection.

Table 17.2 shows that each factor of 10 in intensity corresponds to 10 dB.

A 90 decibel sound is 30 decibels louder than a 60 decibel sound, and three factors of 10 are as intense.
If one sound is more intense than the other, it is 70 decibels higher.

Determine the sound intensity level in decibels for a sound wave that travels in the air at a pressure of 0.656 Pa.

We can use the equation because we are given.

The density of air is at atmospheric pressure.

An 80 dB sound has an intensity five times greater.

The value is true for any intensities that are less than five.

If one sound is more intense than the other, it has a sound level that is 3 decibels higher.

The ratio of two intensities is 2 to 1, and you are asked to find the difference in their sound levels.

The properties of logarithms can be used to solve this problem.

We want to show the difference in sound levels.

The two sound intensity levels differ by about 3 decibels.

This result is true for any intensities that differ by a factor of two, because only the ratio is given.
A sound of 56.0 decibels is twice as intense as a sound of 53.0 decibels, a sound of 90.0 decibels is half as intense as a sound of 100 decibels, and so on.

In applications where sound travels in water, this scale is used.

It is beyond the scope of most introductory texts to treat this scale because it is not commonly used for sounds in air, but it is important to note that very different decibel levels may be encountered when sound pressure levels are quoted.
For example, ocean noise pollution produced by ships may be as great as 200 decibels expressed in the sound pressure level, where the more familiar sound intensity level we use here would be something under 140 decibels for the same sound.

There is a CD that has rock music.

Place the player on a light table and play the CD.
Place your hand on the table.
When the rock music plays, increase the volume and note the level.
The volume control should be increased until it doubles.