PTS Flowbench Forum

Old Grey wrote:edit . . . I was also thinking about sound deading by lining my porting room with this foam

Grey: Check the frequency response of that foam. It is great for hi freq but poor dampening on very low frequencies. Sound waves are pressure waves. Mass is your most effective tool in dampening low freq. Tony has hit on the right idea with his high mass description. I plan to use both the foam pictured for hig freq and high mass for everything below 4KHZ.

Yes, its the low frequency single cylinder exhaust pulsing that is the biggest problem (25Hz @3,000rpm).
That is very objectionable, and that type of pulsing sound carries a very long way, especially at night.
Get rid of that, and everything else is just general mechanical clatter which is spread out much higher in frequency and not that difficult to attenuate.
This is a picture off the internet of the type of thing I am talking about.
Mine is about five feet tall, and the removable inspection cover makes it easy to stuff completely full with springy sound absorbing material. The secret is to have large internal volume, thick rigid walls, and a very small exit area. All you can hear is a faint air rushing sound at the outlet, its magic !

Tony wrote: (Severe edit . . .) I would much prefer a multi cylinder water cooled engine. The reason being very efficient sound insulation and high volumes of engine cooling air are not really compatible. A water cooled engine can be fully enclosed with the radiator located outside of the sound proof box.

I plan to use a cooling tower with the standard thermostat controlling incoming fresh water. Cool, city water is not a problem here. The motor will be totally enclosed (even the intake will be piped into my shop). No radiator or engine fan is needed.

Tony wrote: The biggest problem will be exhaust muffling with minimal exhaust back pressure. . . . It is huge ugly and heavy, but is almost totally silent and offers minimal exhaust back pressure.

Exactly! You have hit on the key to dampening low frequency. It is mass. Around here we have concrete building blocks. A tunnel with sufficient volume (above or below the ground) can be sized to have zero back pressure. All exhaust tuning takes place before the end of the collector (length, tubing size, etc.) It’s all over when you dump into the environment. My tunnel is the environment. It will have some of the foam pictured as well as doglegs to reflect back the sound waves. Eventually, they will get tired of fighting (or bouncing off the walls) and just die. My tunnel can be 30-60ft long if needed. Volume is the key to preventing backpressure. Mass is king. The best example is trying to get to sleep in a cheap motel. Do you want sound resonating wood paneling or a cement wall drowning out the moaning next door? Another idea is to use leaded sheet rock. It is used in dentist’s offices to attenuate x-rays. Its heavy stuff but very effective. I have some around my compressor.

Tony wrote: I suggest you buy a set of Forum flow bench plans to adapt to your needs.

Yep, will do.

I haven't looked into sound deadening yet, I was just going for the aniconic chamber.


When you grind with a carbide it has a high pitch ringing noise, that's what I was mainly thinking of.

Wikipedia wrote:Effectiveness over frequency
Pyramidal RAM
Waves of higher frequencies have shorter wavelengths and are higher in energy, while waves of lower frequencies have longer wavelengths and are lower in energy, according to the relationship \lambda= v/f where lambda represents wavelength, v is phase velocity of wave, and f is frequency. To shield for a specific wavelength, the cone must be of appropriate size to absorb that wavelength. The performance quality of an RF anechoic chamber is determined by its lowest test frequency of operation, at which measured reflections from the internal surfaces will be the most significant compared to higher frequencies. Pyramidal RAM is at its most absorptive when the incident wave is at normal incidence to the internal chamber surface and the pyramid height is approximately equal to \lambda/4, where \lambda is the free space wavelength. Accordingly, increasing the pyramid height of the RAM for the same (square) base size improves the effectiveness of the chamber at low frequencies but results in increased cost and a reduced unobstructed working volume that is available inside a chamber of defined size.

Oh, I forgot about the enclosed motor idea. if you are worried about hot exhaust I plan to use water cooling like boat headers. If it is a popular engine you can buy water-jacketed manifolds. If you think this sound deadening is getting extreme you don't live in a residential neighborhood.

saltfever wrote:Oh, I forgot about the enclosed motor idea. if you are worried about hot exhaust I plan to use water cooling like boat headers. If it is a popular engine you can buy water-jacketed manifolds. If you think this sound deadening is getting extreme you don't live in a residential neighborhood.

Jetskis have water mufflers.

Now you've got me thinking, I have a 40hp 550cc jetski

Grey, you got me thinking too . . . thanks.

Hi freg is usually not a problem. It is highly directional and it doesn't take much reactive force to kill it. Low frequency is very powerful and is non-directional. Think of elephants communicating from miles away at 2 or 3 Hz (below the human hearing threshold). To do an effective job your foam has to be used in conjunction with other high mass geometry.

All you need is a factory cast iron manifold and some thick walled commercial steam/water pipe.
The pipe itself will have enough strength and damping not to radiate any noise.
Might as well use "Pipemax" to size the diameter and length, and feed it into a humongous expansion chamber which will look like free open space as far as exhaust tuning goes.
Should make a very efficient as well as a quiet exhaust.

This was all a bit of a crazy experiment, but it has all worked far better than I had ever dared to hope for.

saltfever wrote:Grey, you got me thinking too . . . thanks.

Hi freg is usually not a problem. It is highly directional and it doesn't take much reactive force to kill it. Low frequency is very powerful and is non-directional. Think of elephants communicating from miles away at 2 or 3 Hz (below the human hearing threshold). To do an effective job your foam has to be used in conjunction with other high mass geometry.

I know something about low frequency sound transmission. Because I have tinnitus, when a "duff-duff" car pulls up next to my car, the low frequency sound goes through his closed window, through my closed window, and makes it feel like someone is poking a screwdriver into my ears.

Also, at kart tracks they have banned all air-cooled motors after 5:00pm, because the sound comes through the barrel and the fins ring, but because water-cooled motors have water insulating the bore they are quieter and fine to run any time.

Grey: with high frequency noise from die grinding I can see how you would be interested in the anechoic chamber and its pyramid-like material. I plan to use something like that in strategic areas of my muffler tunnel. But it is not a complete answer. It is my understanding the chamber is designed primarily to prevent feedback from sound waves bouncing back onto a subject of interest. It is very efficient on high frequency because the entire wave fits in and enters the valley between the peaks and continually loses energy as it travels toward the bottom, resulting in little energy left to be reflected back. To work well the sound wave has to be small enough to enter the valley. Very large waves get scattered.

Here are some sizes of waves that would be typical of a motor’s exhaust. Even with various cylinder count and RPM this would be a representative range that needs to be dampened. You can make anechoic foam larger but you quickly reach a point where it isn’t practical to rely solely on anechoic geometry in an exhaust muffler.
Frequency vs. wave length
4500HZ = 3”
1500HZ = 9”
50HZ = 270”

Lest you think I am a physicist or engineer, I cheated like most and looked for an on-line calculator. Wave length from frequency was calculated here. http://www.1728.org/freqwavf.htm

About the best way to kill exhaust acoustics would be to copy ANR (Active Noise Reduction) headsets. They take a wavelength and reverse it 180 degrees and send it back canceling out the frequency. Not particle at our level but it has been tried without electronics using parabolic reflectors and other devices inside a muffler to reflect back as much acoustic energy as possible without inhibiting viscous exhaust flow. I don’t have a design yet but I feel it will include some lead baffles, stuffing like you described and a long tunnel. Thanks for the Wikipedia reference. Apologies for the inch units.