What does your KF25 bellow hose look like?

Photo of your set up?

What sort of velocity are you doing when it start vibrating?

Any type of support or fixing?

Any visible movement or just "noise"?

Any bending or iss it dead straight?

Any axial force to get it to fit?

You can sometimes get or need inner liners to smooth the airflow.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Helmholtz resonance - see link

https://m.media-amazon.com/images/I/51fBDrtr61L._A...

"a high frequency noise"

identifying the frequency content of a noise can be very helpful. Crucial, actually.

A cheap microphone and free PC based spectrum analyzer softwsre can provide a lot of needed insight.
There probably some smart phone apps to do the same thing.

My hunch, while standing uncomfortably in the shadow of vastly incomplete background info, is that the frequency of the sound is "blade pass".
ASSuming a typical single outlet centrifugal blower like this -
https://www.aircontrolindustries.com/wp-content/up...

The number of vanes/blades times the rpm is the frequency of the pressure pulses created as each blade passes the tight spot (tongue, or cutwater in a cent pump) in the blower scroll/housing. If that is endeed the frequency, there are ways to "soften" the pressure pulses.
In addition if the length of inlet or discharge ducting is a numerical match for the wavelength of the frequency, or an unfortunate factor of the wavelength, the sound (noise!) can be amplified a lot.

https://www.toppr.com/ask/en-us/question/a-musical...

Here is a spreadsheet that I wrote a while back for Helmholtz resonance -

• https://files.engineering.com/getfile.aspx?folder=a7607f52-8483-4e28-b2cf-d

Thank you for everybody's responses so far. I've hopefully addressed them in some way below:

Images of my setup and KF25 Hose: https://imgur.com/a/XEP8JFF

Velocity I don't have that information. Could look into it.

No the tube does not move its only a loud audible noise.

Supported/fixed at two points seen in CAD model image.

Link for blower fan data sheet: PN 9BMB24P2K01
https://www.mouser.ca/datasheet/2/471/San_Ace_97BM...

It is bending in the model as shown in the picture but have tested it while relatively straight and open/not fixed at intake end as seen 3rd image.

I'm not sure if it is what you were looking for @Tmoose but I've also included a picture in the link of the sound I recorded on an app with my phone.

Best,

I think it might be your fan.

They seem to be PWM fans but you're trying to vary speed with voltage.

How exactly are you controlling speed?

Those fans are not designed to be used below 21V. I suspect its seeing up a pulsing flow which is resonating in your tube.

Look at how the fan is supposed to work or use a fixed speed fan.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

@LittleInch Yes, currently I am using the voltage to regulate the fan speed. Implementing a PWM control is not possible at this stage. In terms of the fan operation, would a PWM not average out the voltage and be comparable to using a lower speed through voltage control? If a PWM and voltage control both result in lets say 50% of rated operating RPM, would they then not both be resonating if one or the other would do so?

Best,

Ok, so your phone is showing some noise up around 1000 Hz, and a defined tone at 120 Hz. You think the noise you hear is 120 Hz (low hum or buzz), or the higher frequency stuff (wailing noise at around 1000 Hz)?

Dvd's calculator puts the first longitudinal acoustic mode for your hose (I guessed about 24" long by 1" diameter) at about 120 Hz. Shortening or lengthening the hose might knock that tone down a bit, or you could add a 1/4-wave tube or tuned helmholtz cavity at one end to absorb some of it.

But your ear is probably more annoyed by the higher freq. stuff, which is likely blade passing noise from the blower (or in your case sucker). Using a double-walled hose or stiffer material might keep the high freq. noise from propagating from the hose so much, i.e. using only short lengths of flex to make the connections easy, and some type of rigid plumbing for the majority of the length.

NVH issues like this are pretty common and straightforward to resolve without any real analysis. You can affect the bellows, flow, or both. I would start off by damping the bellows with a hand, strap, blanket, etc and listening for effect. If you need more then try damping the flow with a larger diameter bellows, resonator, or baffling.

If you find it is the tube a loose fitting cover over it will create damping as it vibrates, and the mass loading will reduce noise transmission directly.

Cheers

Greg Locock


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