If cost is not a factor, I'd go for the foam as it is the most perfect air block, thus blocking sound. Open cell foam is often said to work better than closed cell as it can also "absorb" sound. However, I haven't found any scientific proof of the claims.

Our current home is clad in XPS, plus 6ins. of fiberglass and seems quieter than previous homes. But that's purely subjective.

None of the insulation products is very good for sound insulation. Air block or not, it's usually not the solution. For exterior sound, you're usually trying to address resonance and low frequency sound issues. if its high-frequency, the walls aren't the limiting factor but doors/windows. So whatever you do in the walls won't do much there either.

for low frequency, its quite difficult - you want/need more mass (additional layer of dry wall helps), de-coupling helps, staggered studs help, RSIC channels help. None of which are usually done in a normal house build, so its probably not something you can/will address.

I'd not look at sound insulation as a determining factor for insulation types. If you are going to be willing to pay for spray foam - look at the advantages in terms of a sealed house from the hvac point of view.

Anthem is correct. It is virtually impossible to measure the difference in sound attenuation between different kinds insulation in the field because they don't do much to start with, perform in a similar manner, and sound easily bypasses the walls through cracks and windows.

If exterior noise is a serious problem consider a second layer of interior drywall and a double casement window system. I've seen systems in Italy that could completely block the noise from a busy city street. Of course, they probably cost a great deal.

Here are the sound transmission classes for various insulation materials. Interestingly, cellulose tops foam by a large margin.

But, as pointed out, the choice of insulation by itself is not sufficient to create a quiet wall.

We had foam put into our exterior walls on the house we're building and we have excellent sound proofing. The proof is the train that passes just next door to us. It does make a difference. We have the Green Bean product.

I love anecdotal evidence that isn't backed up by anything. . . . To make a claim that something works, you really need a baseline or at a minimum a before/after. . .

Worthy - in terms of cellulose being 'better' at STC, I'll take a wild guess - density. cellulose is a bit more 'dense' than spray foam, so it should rate better at those frequencies affected. That being said, dense packed cullulose might actually act as a conductor of low frequency noise since it acts as a bond between the inner and outer surfaces.

However, we should back up and look at STC ratings in general. STC may be a bit more applicable for interior noise attenuation but I would rate its applicability for exterior noise attenuation very very low. STC does not measure any sound below 125hz. That pretty much means any sounds created by planes, trains, automobiles, etc are all not even factors in the STC rating system. . . STC is more applicable for normal ambient sound ratings like human speech, etc.

I am in Phoenix also and I am using the spray in for a few walls and the underside of the roof and using a blown in blanket fiberglass system for the basement walls that are block and furred on the inside. For sound issues between certain rooms I am using Ultra Touch from bondedlogic.com. This is insulation made from recycled blue jeans, is supposed to be better for sound and is also a green product. In addition, it is made in Chandler, so less resources to get it to you. I am just a homeowner, no expert, but that might be something to look into that would help with both sound and normal insulating properties.

Here is a link that might be useful: blue jean insulation

The difference in the sound isolation properties of different insulations would be worth comparing if walls were thick enough for insulation to actually do much. Since most walls are not very thick it would be wiser to concentrate on adding drywall or resilient channels and improving the performance of the windows if you are concerned about sound transfer.

As someone here said, "sound easily bypasses the walls through cracks and windows".

The spray foam is harder to install wrong, preventing gaps or voids through which the sound will travel. That is why - regardless of tests of materials - spray foam walls are very quiet. Same for ICF walls (which also have the advantage of foam on outside absorbing sound energy and what gets through the wall doesn't radiate well from the foam.

After quality sealing with the foam insulation and quality well sealed windows with at least double glazing, de-coupling is the key. You want to minimize direct transmission paths and isolate materials that transmit sound well.

Outside:

Whatever your outer wall is, the sheathing get modulated with vibration energy from sound energy. To stop sound at this "origin", you can spray the sheathing with one or two inches of closed cell spray insulation, covering everything except intentional openings and the furring; this has an astounding improvement on the thermal efficiency of the wall as well, as the heat absorbing outer surface is insulated it will perform far better than the resulting R value would suggest. The wall siding goes on the furring (sized to allow for the spray foam), which goes in place before the spray form goes on. Also, cut a long strip of sill plate gasket (foam strip, 1/4" thick) and staple on the sheathing where the furring goes. Depending on the siding going on the furring, another strip of sill plate gasket on the furring first. These are two thermal/acoustic isolators between the siding and the sheathing. A bead of acoustic seal on the outside of the studs and plates before putting the sheathing on also seals the space.

Inside:

Staggered studs mean that the studs are not direct energy transmission paths (thermal or acoustic) from the sheathing to the drywall. The studs will still receive energy from the plates, but this greatly reduces the area/cross section for transmission. Now the drywall, which will be your sound radiating "speaker" surface, needs to be decoupled from the studs. There are rubber o-ring style washers that can be used to do this; difficult to use. Where I am, we vapor seal on the inside, so I'll include that.
A bead of acoustic seal on the inside surface of the studs and plates (to seal the resulting "panel" between two studs, acoustic seal the seam between the stud and plate), then install the vapor barrier. Another bead of acoustic seal, then a sill plate gasket on the studs and plates. Another bead of acoustic seal, then you install the drywall. The sound has trouble transmitting through the energy absorbing mediums.

For more isolation, do the acoustic seal and sill plate gasket as above, then install furring on the inside of the wall to mount the drywall. The furring is isolated from the plates were the studs aren't. Of course, acoustic seal and sill plate gasket to the furring before mounting drywall. This creates a decoupled wall.

You want to up it from there, you can find a rubberized spray and coat the back side of your drywall before installing it. But, unless you've got amazing sound proofed, windows, you'll never notice the difference with this.

The thermal energy performance of this wall is amazing. Why the little extra acoustic seal to make sealed panels? No gaps for sound in air to penetrate, and, no paths for flame. The panels are now non-vented panels. The only way for fire transmission is for a complete burn through.

Energy in the structure will vibrate ceilings too. Don't for get to install acoustic seal and sill plate gasket for the ceiling drywall.

There are more complex and thicker wall systems with completely de-coupled inner walls with an air gap between them, but you lose inside space and the cost goes up again.

I'd highly recommend the following combination:

1. outside furring (with sill plate gasket between furring and sheathing) with closed cell spray foam;

2. closed cell spray foam insulation; if you need to save some money, then one to two inches of closed cell spray form on inside of sheathing - balance to fill cavity can be open cell foam (cheaper) or the insulation of your choice;

3. staggered stud walls, six inch wide plates (six inch "interior" for insulation);

4. sill plate gasket with acoustic seal on studs and plates.

With two inches of foam on the outside, the nominal R value is higher than R34, but the performance is closer to R50. You could use a four inch wall/plate and get R40 performance, but now you're using non-standard sized materials and introduced structural questions.

You'll love the thermal comfort and savings along with the peaceful interior.

Modeling suggests a home like this, along with quality vinyl double-glazed two-way low-E inert gas windows, will use 1/3 the heating/cooling energy of a R2000 home.