Sizing corner reflectors for frequencies in the audible range

[Aubergine Agonistes]

TL;DR Summary

Asking how large corner reflectors need to be to reflect road noise (800 Hz -- 1300 Hz)

I'm contemplating extending the concept of corner reflectors to wavelengths in the audible spectrum, specifically road noise. I read somewhere that road noise is predominantly between 800 Hz and 1300 Hz. The corresponding wavelengths (at 20°C) are 16.9 inches and 10.4 inches.

I read elsewhere that the planes of a corner reflector had to be "several times the wavelength" of the frequency of interest, but that was in relation to RF signals. That size fudge factor is acceptable when the wavelengths are minuscule, but the size becomes much more important when dealing with longer, audible wavelengths.

My question: To reflect all road noise do I need corner reflectors that are, at minimum, 17 in. x 17 in. x 17 in.?

I will have multiple reflectors (5-sided cubes) side-by-side like pigeonholes, either single or multiple tier, which I will place on top of a brick wall.

 

[berkeman]

Welcome to the PF.

which I will place on top of a brick wall.

Is there a reason that you don't just use sound absorbing material instead? Reflecting road noise back at cars (most with their windows rolled up) wouldn't seem to server any useful purpose.

Also, do you have permission to be building something on that wall? If it's a sound wall next to a freeway, most likely you would need to get permission (and permits and inspections) to build on top of it.

 

[Aubergine Agonistes]

Yes, there is. I'm experimenting. One reason some people experiment is to learn something new.

At the moment, I'm experimenting with sound reflection, hence the question about sizing corner reflectors. However, I'm also experimenting with sound cancellation (destructive interference), which is mostly completed, and sound absorption (mass-loaded vinyl, etc.), which efforts are less advanced than either cancellation or reflection. This question is about sound reflection.

Reflecting sound in itself is useful, otherwise speaker designers and auditorium designers would be unemployed. It's a city street, not a freeway. My concern is for sound pollution in my yard, not for drivers, or even the neighbors across the street.

Permits and inspections? This is a little far afield, isn't it?

None of these issues are related to the size of corner reflectors.

When I ask for help in building my zero-point-energy doomsday device, will I be bombarded with questions about zoning, or ethics, or bicycle path easements, or squirrel abatement?

 

[anorlunda]

Here's a clue as to the size you need.
https://en.wikipedia.org/wiki/Corner_reflector

Corner reflectors can also occur accidentally. Tower blocks with balconies are often accidental corner reflectors for sound and return a distinctive echo to an observer making a sharp noise, such as a hand clap, nearby.

A simple web search for sound reflectors reveals many sites, most of them commercial, but which have essays on reflecting versus absorbing sound.

 

[berkeman]

My concern is for sound pollution in my yard, not for drivers, or even the neighbors across the street.

Yoiks. I feel bad for your neighbors. You are going to increase the vehicle noise power level that they hear because of your sound reflectors? (Quiz Question -- by a factor of 2 or √2 ?)

Permits and inspections? This is a little far afield, isn't it?

Well, you can see how your OP suggested that you were going to build this thing on top of a freeway sound wall, right? That would definitely require permits and inspections and approvals. Just think of a poorly built structure blowing over in the wind onto a busy freeway. Not good...

None of these issues are related to the size of corner reflectors.

You seemed to have a pretty good handle on the numbers in your post. So I was more interested/concerned about the overall concept, not the numbers.

When I ask for help in building my zero-point-energy doomsday device, will I be bombarded with questions about zoning, or ethics, or bicycle path easements, or squirrel abatement?

We do not allow discussions about squirrels here at the PF -- they are way too distracting.

 

[marcusl]

You’ll likely get better performance with 17” more brick on top instead of corner reflectors.

 

[Baluncore]

The same rules of thumb apply to acoustic waves as to RF, only the materials are different.

Your reflector will have an aperture, d, it's dimension perpendicular to the signal arrival. For signals with a wavelength of λ it will have a beamwidth of BW = 57° * d / λ. That means, for small reflectors, the reflected beam will spread over a large spatial angle. Also, high frequencies will be reflected more accurately.

If the reflector is solid and does not move, then 100% of the incident pressure wave will be reflected. But energy passing close to the edges will be partly refracted around into the acoustic shadow of the reflector in your yard. That is called “knife edge diffraction”.

Consider a fence leaning backwards at 45°, that would reflect noise upwards. You should get interesting echos from atmospheric stratification, such as an inversion layer.

 

[DrClaude]

Thread closed pending moderation.

 

[anorlunda]

Thread closed permanently.