If the source vibrate continuously, a continuous sound is produced. In most cases in ultrasound, the source vibrates briefly, producing a pulse of sound, which travels through the tissue. After echoes are picked up, another pulse is sent, and so on.
The material in this section gives a brief introduction to some of the terms used in building acoustics and the test methods used to characterize systems and materials. A very basic understanding of some fundamentals of acoustics and terms used in building acoustics, is all that is necessary to understand the material in this and the following chapters. To emphasize the simplicity of the approach, equations are kept to a minimum. Sound is generated by creating a disturbance of the air, which sets up a series of pressure waves fluctuating above and below the air's normal atmospheric pressure, much as a stone that falls in water generates expanding ripples on the surface. Unlike the water waves, however, these pressure waves propagate in all directions from the source of the sound. Our ears sense these pressure fluctuations, convert them to electrical impulses, and send them to our brain, where they are interpreted as sound. There are many sources of sound in buildings: voices, human activities, external noises such as traffic, entertainment devices and machinery. They all generate small rapid variations in pressure about the static atmospheric pressure; these propagate through the air as sound waves. As well as travelling in air, sound can travel as vibrational waves in solids or liquids. The terms airborne and structure-borne sound are used depending on which medium the sound is travelling in at the time. For example, the noise from a radio set may begin as airborne sound, enter the structure of the building and travel for some distance as structure-borne sound, and then be radiated again as airborne sound in another place. (Figure 1) The importance of structure-borne sound will become more apparent when flanking sound transmission is discussed, in Acoustics in Practice.
Air pressure is usually measured in units of Pascals (Pa). Atmospheric pressure is about 100 kPa. Sound pressure is a measure of the fluctuation of the air pressure above and below normal atmospheric pressure as the sound waves propagate past a listener. Generally, the larger the fluctuations, the louder the sound.
The pressure variations in an individual sound wave are much less than the static atmospheric pressure, but the range of sound pressures encountered in acoustics is very large. The threshold of hearing is assumed to correspond to pressure fluctuations of 20 microPascals; some individuals will have more acute hearing than this, some less. The threshold of pain in the ear corresponds to pressure fluctuations of about 200 Pa. This second value is ten million times the first. These unwieldy numbers are converted to more convenient ones using a logarithmic scale, the decibel scale. Sound pressure levels are expressed as a number followed by the symbol dB. Sound level metersconvert electrical signals from a microphone to sound pressure levels in dB. Table 1 gives some representative sound pressure levels encountered in a range of situations.
Decibels are more easily related to the response of the human ear, which also responds logarithmically to sound.The response of our ears, that is, our perception of loudness, does not increase linearly with a linear increase insound pressure. For example, a 10 dB increase in sound pressure level would be perceived as a doubling of the loudness. In practical situations, level changes of about 3 dB are just noticeable.
It is very important to remember that decibels and similar acoustical quantities have properties different from more conventional units. Sound pressure levels, for example, cannot be added together as can kilograms. The combination of two noises with average levels of 60 dB does not give a sound pressure level of 120 dB, but 63 dB.
The addition of a noise with a level of 70 dB to a room with a level of 80 dB will result in no measureable difference in the overall level. This does not mean, however, that a large number of secondary sources can be introduced into an environment without increasing the overall level. If ten 'negligible' 70 dB noise sources are combined with one 80 dB noise source, the resulting level will be 83 dB. Fortunately in building acoustics, there is seldom a need to combine noise levels in this way or to do many complicated calculations with decibels or other logarithmic units. These examples merely emphasize the peculiarities of the decibel scale.
On October 15, 1997 the British built " Thrust SSC " vehicle became the first land based vehicle to break the sound barrier. To be official it had to break the sound barrier twice within one hour. It did this, with an average top speed on the two runs of Mach 1.020. The runs took place in early in the day so that the temperature of the air (and the speed of sound) would be lower. As an interesting side note, this record was set one day after the 50th anniversary of the first supersonic flight made by Chuck Yeager on October 14, 1947 in the " Bell X-1 ."
Example 1 : Determine the speed of sound when it is –5°C.
v = 331.5m/s + 0.6(-5) v = 331.5m/s + -3m/s v = 328.5 m/s
You can observe an example of how the speed of sound affects when you hear it compared to the occurrence of the event that caused the sound.
During a thunderstorm, watch for a lightning strike. You see it first because light travels at a very high speed (3.00e8m/s), which is so fast it travels to your eye from the lightening almost instantly.
Now listen for the thunder. The sound is traveling at a sluggish (compared to light!) 340m/s behind the flash of light.
For every 3 seconds that you count between the flash and the sound there is a distance of about one kilometre between you and the lightning.
Sound can also travel through solids and liquids, not just gases.
This is why you can still hear stuff, even if it’s distorted, when you are under water at a swimming pool.
The speed of sound in liquids is quicker than in gases, and the speed of sound in solids is even quicker. <ul><li>o This is because the atoms are closer together, so they transfer the sound more efficiently. </li></ul>
You might have even seen people in movies listening for an approaching train by putting an ear on the train tracks and listening for it. Don’t try this! It’s very dangerous! <ul><li>o The sound of the train travels faster and more efficiently through the solid train tracks. </li></ul>
If the sound you are measuring is at a constant temperature then the velocity will be constant… about 340 m/s.
What sort of frequencies of sound will you typically be talking about?
Most often we will be looking at sound waves that humans can actually hear, which are frequencies from 20 – 20 000 Hz.
Check out the specifications for headphones printed on the back of the package. They’ll probably list their range from 20 – 20 000Hz, since that’s what the average person can hear. <ul><li>o 20 Hz would be very deep, low, rumbling sounds. </li></ul><ul><li>o 20 000 Hz would be a very high pitched, squealing sort of noise. ( N.B. In music “pitch” means the same as frequency. </li></ul>
Infrasonic 0 - 20 Very low frequencies of sound that the human ear can’t detect, but you may feel the rumbling of the waves through your body.
Sonic (AKA Audio) 20 - 20 000 Normal range for human ears, although not everyone (especially the elderly) will hear to the extremes of this range.
Ultrasonic 20 000 + Beyond normal hearing for humans, although some animals (like dogs) hear part ways into this range. Also used in medicine (e.g. ultrasounds for pregnant women).
Example 2 : My wife and I are listening to my favourite Bugles song, “Video Killed the Radio Star” from the 1980’s. At one point the singer hits a note that my wife thinks has a wavelength of 0.014m. I tell her this is impossible… explain why.
We will assume that the speed of sound is 340 m/s. That means that we will get…
v = f λ f = v / λ f= (340m/s) / (0.014m) f = 24 286 Hz = 2.4e4 Hz
This frequency is beyond the range of normal human hearing. We wouldn’t be able to hear it, and it is unlikely that our stereo system could produce a sound with a frequency that high.
If the speed of sound changed due to a change in the temperature of the air, it would make notes sound "off key."
This is why an orchestra “warms up” before a performance.
If a flute was tuned to the right frequency when the metal is cold, the frequency will change as the person plays for the first few minutes and the instrument heats up from their breath.
Every instrument gets played for a few minutes to make sure that it is at a constant temperature for the whole performance and is then tuned.
Example 3 : I am playing the flute (yes, I actually can, I’m just not very good!), and tuned it straight out of the case. The temperature of the flute was 17°C and I tuned it to 15 000 Hz. I start playing the flute, and by the time I’m a few minutes into the song I notice that the notes all seem wrong. If the flute has warmed up to my body temperature (37°C) , determine what my original tuned note has changed to.
First we need to calculate the speed of sound at 17°C…
v = 331.5m/s + 0.6(17) v = 331.5m/s + 10.2m/s v = 341.7 m/s
Next we figure out the wavelength of the note I tuned. This wavelength will remain constant, even if the flute warms up.
v = f λ λ = v/f λ = (341.7m/s) / (15 000Hz) λ= 0.02278 m
Third, what’s the speed of sound at 37°C (body temperature)…
v = 331.5m/s + 0.6(37) v = 331.5m/s + 22.2m/s v = 353.7 m/s
Which means, finally, I can calculate what the frequency of the note has changed to. This is based on the constant wavelength and the new speed of sound.
v = f λ f = v / λ f = = (353.7m/s) / (0.02278m) f = 15 527Hz
The frequency has jumped up by more than 500Hz! This will be a very noticeable difference, even for someone that doesn’t know anything about music… the notes will just sound wrong.
The loudness of a sound depends on the wave’s amplitude .
This is why a stereo system has an “amplifier”, a device that increases the amplitude of sound waves.
The louder a sound, the bigger the amplitude .
This is also a way of measuring the amount of energy the wave has.
The system used to measure the loudness of sounds is the decibel system , given the unit dB .
The decibel system is based on logarithms, which means for every step up by one, the sound is actually ten times louder. For example, a 15dB sound is ten times louder than a 14dB sound.
The decibel is actually a fraction of a bel, the original unit for measuring sound (1 db = 0.1 b). The "bel" was originally named after Alexander Graham Bell, the inventor of the telephone. Because the bel was too high a value for day to day situations, the decibel became a standard.
0 - 30 Very Quiet This is the threshold of human hearing, up to the sound of a quiet whisper.
31 - 50 Quiet This is an average quiet house, with maybe the sound of a fridge running or someone moving around.
51 - 70 Normal Regular daily sounds like people talking.
71 - 90 Loud This is the point where a sound becomes annoying or distracting. Vacuums or a noisy car on a busy street are at these levels.
91 - 110 Very Loud Most people will try to avoid being in areas this loud. Prolonged exposure can cause permanent ear damage. Temporary effects, like "stereo hiss", may happen.
111 + Painful!!! Even limited exposure to levels this high will cause permanent hearing loss.
You want to know the scary part? Most concerts you go to will have sound levels between 100 – 130 dB… easily into the permanent damage range.
Lot’s of old rock stars have permanent hearing loss.
Many modern day musicians wear ear protection of some sort while in concert.
One of the loudest man-made sounds is created by the space shuttle lifting off. It will generate sounds at an incredible 215 dB!!! The sound is so loud that it would actually cause damage to the launch tower, and as a reflected echo, to the shuttle itself. To absorb the energy, huge amounts of water are pumped to the base of the launch pad seconds before takeoff. The water absorbs the sound, as well as a lot of heat. When you see video of a shuttle launch, most of the white stuff you see billowing from the launch pad right at takeoff is not smoke... it's steam!