[taregg (OP)]

What is better bricks for a soundproof wall, outdoor sounds.... hollow bricks or solid bricks .....and why.

[Colin2B]

What is better bricks for a soundproof wall, outdoor sounds.... hollow bricks or solid bricks .....and why.

If you have a choice go for greater mass. You need to minimise vibration which is passed through the wall and far side acts as a speaker.

[Bored chemist]

A lot of sound is reflected by the first surface of the wall. A hollow wall means two surfaces to reflect the sound so it might do a better job of soundproofing.
It probably depends on a lot of things like the wavelength of the sound.

[CliffordK]

I think porous materials such as foam would make the best sound insulation.  And a solid foam block may be better than a hollow one.  Does the wall need to be structural?  Durable? 

You might also benefit from foam sandwiched between two brick layers or foam filled hollow concrete blocks.

Also, are you interested in sound reflection or dampening inside the structure?

[Colin2B]

A lot of sound is reflected by the first surface of the wall. A hollow wall means two surfaces to reflect the sound so it might do a better job of soundproofing.
It probably depends on a lot of things like the wavelength of the sound.

I think porous materials such as foam would make the best sound insulation.  And a solid foam block may be better than a hollow one.  Does the wall need to be structural?  Durable? 

You might also benefit from foam sandwiched between two brick layers or foam filled hollow concrete blocks.

Also, are you interested in sound reflection or dampening inside the structure?

Yes additional reflective surfaces do help, but the measurement of transmission loss through a wall includes both reflective and adsorption losses. Transmission loss is mainly dependant on density rather than material type. The density is usually expressed as surface density or kg/mē, you typically get 5dB transmission loss for every doubling of density eg thickness. Discontinuous structures such as bricks and mortar are slightly better than homogeneous eg concrete.
Yes, Transmission Loss varies with frequency with higher frequencies suffering greater attenuation. Rather than produce a loss for each frequency, a single number Sound Transmission Class STC is produced by comparing standard curves with the experimental plot. For general use this gives a good indication of sound insulation, but the curves only go down to 125Hz and if your interfering noise is below this you will need a greater thickness of insulation than indicated.
Yes, a cavity does theoretically increase transmission loss for a given density (this is not constant thickness however). In practice it is almost impossible to fully isolate the 2 walls and sound will pass over any bridging material - this is known as flanking. Bridges will include wall ties, water pipes, conduits, door and window frames and even roof joists resting across the 2 walls. Porous material in the cavity makes a only a minor improvement to the loss, but can prevent any resonances setting up in the cavity.
Transmission loss of porous materials eg rock wool Is only proportional to thickness. Porous material can be of benefit inside a room as pointed out by CliffordK. Concrete and brick are highly reflective making a room feel 'live', a layer of absorbing material over the walls will tame the reverberation time. It works better if there is a small air gap between absorber and wall. Another way of using porous material is in doors, which are rarely made of concrete. In this situation you will need to stagger the studs in the door to prevent bridging. Remember that sound will flank through any air gaps between door and frame so think about baffles. If you have problems building a high density wall you will have to use plasterboard panels with porous material in between, but this will be thicker than a high density wall. It can however be used for constructing a room within a room for extra soundproofing eg sound studio.
Give some thought to room modes. If low frequency interference gets through it can set up standing waves. Knowing the problem frequencies will help with room design and absorbers on wall will cut down resonances. If you find a problem frequency remember not to design a room with reinforcing modes eg don't make length 2x width!
All the above also applies to ceiling and floors.
What are you trying to keep out? (and don't say sound [])

[Bored chemist]

If this
"Yes additional reflective surfaces do help, but the measurement of transmission loss through a wall includes both reflective and adsorption losses. Transmission loss is mainly dependant on density rather than material type."
is true, how come sound carries so well under water?

Hanging a curtain on the wall may also help.

[Colin2B]

If this
"Yes additional reflective surfaces do help, but the measurement of transmission loss through a wall includes both reflective and adsorption losses. Transmission loss is mainly dependant on density rather than material type."
is true, how come sound carries so well under water?

Hanging a curtain on the wall may also help.

Easy one 1st. Yes curtain definitely would help absorption and also contain excessive reverb time, they are often used for this purpose. Sabine in his original experiments used cushions on seats in the lecture hall as absorbers. If you want to do the opposite you can lay plastic sheeting over auditorium seats or under a piano to increase reflections.

The water question is a good one, but we are not comparing like with like. When sound passes through a wall it starts in air and hits the concrete (say) where we get a lot of reflection. As the wave passes through the concrete friction converts wave energy to heat and some absorption takes place. At the far side the diaphragm effect converts the wave back to sound in air.
When we hear sounds underwater we generally hear those generated and received underwater. The equivalent property causing friction in water is viscosity, the particles slide over each other more easily and actual attenuation is quite low. Conversely reflection at the air water interface is very high (this is due to the huge difference in acoustic impedance between air and water) so if we set up the equivalent of the wall we have air-water-air and very little sound would get through. You can get one half of this by being underwater and listening to sounds originating above the surface and you will hear them very muted.
OK, at some time this evening you are going to get really bored and I can feel the next question forming "so why do submariners hear sounds inside the sub so clearly?". Again the source sound originates in the water (prop noises etc), but we are not dealing with an air water interface for the sub it is a water metal interface and the metal makes a really good diaphragm.
Enjoy a nice boring evening []