# Sizing corner reflectors for frequencies in the audible range

• Aubergine Agonistes
In summary: It reflects a certain amount of sound, but it also absorbs sound. A sound fence also emits a certain amount of sound, which is why you hear both the sound of the fence and the sound of the ambient noise together. 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.In summary, the author is considering using corner reflectors to reflect road noise at audible wavelengths. The dimensions of the reflector will be based on the wavelength of the noise being reflected. The reflector will be placed on a brick wall to increase the sound level heard by drivers.
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.

Welcome to the PF.

Aubergine Agonistes said:
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.

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?

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.

Aubergine Agonistes
Aubergine Agonistes said:
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 ?)
Aubergine Agonistes said:
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...
Aubergine Agonistes said:
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.
Aubergine Agonistes said:
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.

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

Aubergine Agonistes and berkeman
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.

Aubergine Agonistes

Aubergine Agonistes

## 1. How do you determine the appropriate size for a corner reflector in the audible range?

The size of a corner reflector for frequencies in the audible range is determined by the wavelength of the sound. This can be calculated by dividing the speed of sound (343 meters per second) by the frequency of the sound in hertz. The resulting number is the wavelength in meters. The size of the corner reflector should be at least half the wavelength in order to effectively reflect the sound waves.

## 2. Can I use the same size corner reflector for all frequencies in the audible range?

No, the size of the corner reflector should vary depending on the frequency of the sound. This is because the wavelength of sound changes as the frequency changes, so a different size reflector is needed to effectively reflect the sound waves at each frequency.

## 3. Are there any factors that can affect the effectiveness of a corner reflector in the audible range?

Yes, there are several factors that can affect the effectiveness of a corner reflector in the audible range. These include the material of the reflector, the shape and angle of the corners, and the distance between the reflector and the sound source.

## 4. How can I test the effectiveness of a corner reflector for a specific frequency in the audible range?

To test the effectiveness of a corner reflector for a specific frequency, you can use a sound level meter to measure the sound intensity at different distances from the reflector. The intensity should increase as you get closer to the reflector, indicating that it is effectively reflecting the sound waves.

## 5. Can corner reflectors be used for frequencies outside of the audible range?

Yes, corner reflectors can be used for frequencies outside of the audible range. However, the size and shape of the reflector may need to be adjusted to effectively reflect the sound waves at those frequencies. It is important to consider the specific wavelength and properties of the sound when designing a corner reflector for non-audible frequencies.