Physics

• Sound radiated by a source in a free field propagates away from the source and is never reflected back.

• Sound spreads three-dimensionally from the source such that the intensity falls off (6 dB decrease as distance from source doubles).

liquid

Sound travels faster in

liquids than in gases because molecules are more tightly packed.

• The indoors acoustic environment introduces boundaries which reflect sound.

• If the boundaries completely reflect all incident sound without any absorption then the resulting sound field is termed diffuse or reverberant.

Source moving with vsource = vsound ( Mach 1 - breaking the sound barrier )

Frequency is directly proportional to Pitch

Sound

solid

Sound is the vibration of molecules, and since there is absolutely nothing in a vacuum, there are no molecules to vibrate

doppler effect

is the shift in frequency and wavelength of waves which results from a source moving with respect to the medium, a receiver moving with respect to the medium, or even a moving medium.

• 500 people talking continuously for one year to produce enough energy to heat a cup of tea.

• Typical male and female speakers generate 34 µW and18 µW, respectively, at a distance of 3.28 ft.

• Common sound sources are not excessively powerful, the sound energy in the enclosure travels about the enclosure and slowly decays as it is absorbed by the boundaries and the medium.

Several ocean-dwelling animals rely upon sound waves to communicate with

other animals and to locate food and obstacles.

• If the boundaries absorb some of the incident sound and reflect the rest

• Energy flows in more than one direction.

• In a diffuse sound field the time average of the mean square sound pressure is the same everywhere through the enclosure.

• The flow of energy is equally probable in all directions.

Everything in between can be heard by a human with normal hearing.

"cutoffs" are very different for various individuals and changes with age, surroundings, etc.

Molecules at higher temperatures have more energy, thus they can vibrate faster. Since the molecules vibrate faster, sound waves can travel more quickly.

Think about the volume

• There’s no single volume setting appropriate for everyone.

• You may experience a different sound level with different earbuds or headphones and with different EQ settings.

• Doppler Radar uses the doppler effect for electromagetic waves to predict the weather.

• Radar Speed Gun is a Doppler radar unit that may be hand-held, vehicle-mounted or static. It measures the speed of the objects at which it is pointed by detecting a change in frequency of the returned radar signal caused by the Doppler effect.

Keep Track of Time

• You should also pay attention to how long you listen to audio at high volume.

• Remember: you can adapt to higher volume settings over time, not realizing that the higher volume may be harmful to your hearing.

• measurement of the ability of a material to absorb a sound

• measurement of the ability of a partition to transmit a sound

Source moving with vsource < vsound ( Mach 0.7 )

• A sound field is said to be a free field if it is uniform, free from boundaries, and undisturbed by other sources of sound.

• Anechoic chambers and well-above

the-ground outdoors are free fields.

Unfortunately, the iPod doesn’t tell you how the limit you set equates to actual sound levels—and it really can’t, considering that a given headphone-output level produces different sound levels depending on the headphones you’re using.

A decibel is a scientific unit that measures the intensity of sounds. The softest sound that a human can hear is the zero point. When the sound is twice as loud, the decibel level goes up by six. Humans speak normally at 60 decibels

• The absorption coefficient of the walls must be very low (α<0.2)

• The room must have geometrical irregularities to avoid nodes of pressure of the resonance modes.

• The hearing threshold (level at which humans begin to perceive sound) is 0 dB.

• When a sound reaches upwards of 120 dB, it is above the threshold of pain (point at which most people begin feeling pain).

• "loudness" is a psycho-physical response, it can be said that an increase 10dB is a doubling in "loudness" even though the dB scale is base 10 logarithmetric.

Temperature also affects the speed of sound.

At freezing (0º Celcius),sound travels through air at 331 meters per second (about 740 mph).

At 20ºC, room temperature, sound travels at

343 meters per second (767 mph).

If a speaker's diaphragm is vibrating back and forth at a frequency of 900 Hz, then 900 condensations are generated every second, each followed by a rarefaction, forming a sound wave whose frequency is 900 Hz.

RESearch:

Most research about noise-induced hearing loss has focused on prolonged exposure to loud sounds in industrial workplaces.

• If you experience ringing in your ears or hear muffled speech, stop listening and have your hearing checked.

• The principle of this room is to simulate a free field.

• In anechoic rooms, the walls are very absorbent in order to eliminate these reflections.

• The materials used on the walls are rockwool or glasswool, which are materials that absorb sound in relatively high frequencies.

• Much of the energy is truly diffuse, though there are regions of the sound field that have a definable direction of propagation from the noise source.

• Semireverberant fields are the most widely encountered in the majority of architectural acoustic environments.

• plays an important role in architectural acoustics, the design of recording studios and listening rooms, and automobile interiors (seat material is responsible for almost 50% of sound absorption inside an automobile).

The difference

between

sound & noise

Source moving with vsource > vsound (Mach 1.4 - supersonic)

• When a sound source is enclosed the radiated sound energy is retained within the enclosure.

• If the boundaries are perfectly reflective then the sound energy inside the enclosure could theoretically grow until a pressure is reached that would be explosive.

• For example the sound power produced by human speech is very small.

Gas

The speed of sound depends upon the properties of the medium it is passing through.

Stationary Sound Source

lambda symbol of wavelength

• The theory explaining reverberation time, and the acoustical behavior of large and small rooms, was developed by Sabine.

• This theory requires a knowledge of the absorbing properties of the materials which cover the surfaces of the room under analysis.

• The walls of a reverberation room consist of concrete and are covered with reflecting paint.

• the sound reflects a lot of time on the walls before dying down.

• It gives the same impression of a sound in a cathedral.