Personally, I'm thinking that's the kind of fence you actually need to stop noise from a neighbor's yard.

I'm certainly no expert on accoustics, but this web page should be read by anyone interested in reducing noise in their home:

ARCHIVED - Sound Transmission Through Building Components - IRC - NRC-CNRC

It's a study done by the Canadian National Research Council into how noise travels through walls and windows, and the most effective ways to stop that noise.

One of the points that study makes the loudest is that to stop noise you have to close any "flanking paths" where noise can go around the barrier you put in place to stop it. And, in the case of a fence outdoors, the higher you can make that fence, the more difficult it is for noise to go around it. So, a 6 foot or even an 8 foot fence isn't gonna do squat because noise will just go over it. And, unfortunately, your city probably has a by-law limiting the height of the fence you can build around your property. I'm sure 30 feet wouldn't be allowed.

Ignorance is fertile ground for misconceptions and misinformation to grow, and since MOST aspects of DIY work aren't taught anywhere, the resulting lack of knowledge has resulted in the whole area of DIY being full of people's misconceptions and misunderstanding of the principles. Sticking insulation inside stud cavities is a perfect example of that kind of misinformation. Insulation inside a stud wall will do almost nothing to reduce the noise level.

The best two ways to stop noise a

1. putting a two part barrier up to stop the noise, and physically separating the two parts so that the movement of one won't cause any movement of the other. That's because sound waves DO NOT go through barriers; they bounce off them and go around them.

When a sound wave hits a wall, it causes the wall to move slightly, and the movement of the wall causes another sound wave to be produced on the opposite side of the wall. It's that reproduced sound wave you hear, not the original. By having that wall made in two parts so that the movement of one side of the wall will not result in the movement of the other side, you prevent that second side of the wall from producing a sound wave.

Here, you need to have the fence high enough to prevent the noise from just going over the fence, but also you need to build the fence in two parts so that one side of the fence can move without causing the opposite side of the fence to move. Thus, a sound wave hitting one side of the fence won't result in a sound wave being reproduced on the other side of the fence.

2. What stops noise is MASS. This follows as a direct result of the manner in which sound propogates through walls, floors and ceilings. Remember, the sound wave doesn't pass through the walls, floors or ceilings, it causes them to move, and the movement of the walls, floors and ceilings causes another sound wave to be produced on the opposite side of the wall, floor or ceiling.

Imagine you have a noisy machine on one side of a wall, floor or ceiling and a listener on the other side. The MASS LAW of accoustics says that for every doubling of either:

A. the mass of the wall, floor or ceiling per unit area, or

B. the frequency of the sound hitting the wall, floor or ceiling,

the sound pressure level on the opposite side of that wall, floor or ceiling will be reduced by 6 decibels, or to approximately 1/4 of it's initial level. (But, our hearing isn't linear, and we hear quiet noises much better than we hear loud ones, and so the APPARANT reduction in noise level will be much less.)

The reason why the mass of the noise barrier and the frequency of the noise are important is ENTIRELY because sound doesn't travel through noise barriers, it's the movement of the barrier that recreates the sound waves hitting it.

Newton's law is Force = Mass X Acceleration. Sound is nothing more than air pressure variations, so the more rapidly the air changes pressure, the louder the sound wave will seem to your ears. There's only a certain amount of force with which the sound wave hits the wall. So, by increasing the mass of the wall, you reduce the acceleration with which the wall moves as a result, and that means that the rate of change of air pressure of the sound wave reproduced on the other side of the wall will be lower. Slower change in air pressure means a quieter reproduced sound wave.

Also, the heavier the noise barrier, the more inertia it has, and that inertia prevents it from changing it's direction of motion quickly. So, the higher the frequency of the sound waves hitting a wall, floor or ceiling (or any kind of noise barrier) the more the inertia of the noise barrier will prevent that noise barrier from reproducing those sound waves on the other side. It's precisely this filtering out of the higher frequency sound waves that result in your hearing a "BOOM - BOOM - BOOM" in your apartment when someone else in the apartment block is having a party at 3:00 in the morning. The walls, floors and ceilings are heavy and have too much inertia to respond to the high frequencies in the music, and so all the midrange and high frequencies get filtered out, and all you hear is the base line. It's not until you get closer to the apartment and can hear the music coming from under and reproduced by the much lighter door that you hear those higher frequencies and can identify what song is being played.

So, knowing how to stop noise requires that you know how it propogates. And, it's precisely that physics that tells you that stuffing the stud cavities of a wall with light weight insulation isn't going to do much. While that insulation might dampen the movement of the drywall between the studs, the drywall on both sides of the wall is connected by the wooden studs so any movement of one side of the wall is going to be reproduced near perfectly on the other side of the wall with or without that insulation. So, stuffing a fence with a light weight movement damping material won't do any more good than stuffing stud cavities in a wall with insulation. Pouring sand in between two fences would do much more to stop the noise from on yard getting into another, but only if the fence is high enough to prevent the noise from just going over the fence.

Anyhow, now you know WHY the noise barriers between highways and homes are 30 feet high and made of 8 inch thick solid concrete. You need the height to prevent the noise from just going around the noise barrier by going over it, and you need the mass to minimize the sound waves reproduced by the noise barrier.

But, that kind of fence is what's needed to be effective in stopping noise from one yard from annoying the neighbors in the next yard. The shorter and lighter weight you make the fence the less effective at stopping noise it will be.