Reducing noise of electric vehicle inverter?

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MementoMori
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Reducing noise of electric vehicle inverter?

Post by MementoMori »

I would like to reduce the noise from the inverter in my electric vehicle. I have excellent hearing plus tinnitus, and find the 10-12 kHz sustained sound to be fatiguing over time.

Has anyone sound proofed their EV inverter? Obvious concerns are adding heat and interfering with the airflow around the unit. The car is a MINI Cooper electric (same drivetrain as the i3S).
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Re: Reducing noise of electric vehicle inverter?

Post by arber333 »

MementoMori wrote: Mon Nov 14, 2022 7:21 pm I would like to reduce the noise from the inverter in my electric vehicle. I have excellent hearing plus tinnitus, and find the 10-12 kHz sustained sound to be fatiguing over time.

Has anyone sound proofed their EV inverter? Obvious concerns are adding heat and interfering with the airflow around the unit. The car is a MINI Cooper electric (same drivetrain as the i3S).
I know it sounds like scifi...
Have you considered makeing an active damping system? Try to measure inverter sound frequency in general.
Then reproduce the same sound by a large speaker and experiment with harmonics. There should be a response frequency where both sounds would cancel each out.
I have seen (heared) this done in a Saab 2000 turboprop aircraft. As long as you are in your seat propeller vibrations are dampened by active speakers in the cabin. If you lean out in the aisle you can hear much more prop sound then seated.
https://www.flightglobal.com/saab-sound ... 40.article
https://www.ultra-pcs.com/mission-criti ... n-control/
https://mdpi-res.com/d_attachment/appls ... 1654069794

Could be that you can solve this by sound proofing your boot.
MementoMori
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Re: Reducing noise of electric vehicle inverter?

Post by MementoMori »

arber333 wrote: Mon Nov 14, 2022 7:56 pm I know it sounds like scifi...
Have you considered makeing an active damping system? Try to measure inverter sound frequency in general.
Then reproduce the same sound by a large speaker and experiment with harmonics. There should be a response frequency where both sounds would cancel each out.
I have seen (heared) this done in a Saab 2000 turboprop aircraft. As long as you are in your seat propeller vibrations are dampened by active speakers in the cabin. If you lean out in the aisle you can hear much more prop sound then seated.
https://www.flightglobal.com/saab-sound ... 40.article
https://www.ultra-pcs.com/mission-criti ... n-control/
https://mdpi-res.com/d_attachment/appls ... 1654069794

Could be that you can solve this by sound proofing your boot.
That is a wonderful idea, thank you. I’m not sure the PWM noise is regular enough for a static phase-reversed recording to be effective, but who knows. Certainly a sound could be created that includes those high frequencies plus some lower harmonics as you said, for a masking effect. Some automakers like Mercedes are including sounds like that paired with the throttle, but unfortunately my vehicle’s “pedestrian” noise fades out around 25 mph.

The engine bay has some room for sound dampening, and also has numerous points of escape for sound (top of bonnet, hood scoop, bottom of bay, etc.
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Re: Reducing noise of electric vehicle inverter?

Post by MattsAwesomeStuff »

I think the motor itself is your speaker. Kinda hard to shut it up.

You could insulate it, yeah. Would hurt the cooling a bit (not everything acoustically insulating is thermally insulating, but, in general).Or, put a big magnet on it. Blacksmiths do that to shut up their anvils, though I suspect it's increasingly less useful at higher frequencies.
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Re: Reducing noise of electric vehicle inverter?

Post by arber333 »

MattsAwesomeStuff wrote: Tue Nov 15, 2022 8:15 am I think the motor itself is your speaker. Kinda hard to shut it up.

You could insulate it, yeah. Would hurt the cooling a bit (not everything acoustically insulating is thermally insulating, but, in general).Or, put a big magnet on it. Blacksmiths do that to shut up their anvils, though I suspect it's increasingly less useful at higher frequencies.
Well motor is kind of source and sink for noise. To effectively dampen the sound you have to transmitt the same frequency in the opposite phase shift.
Maybe it would be too demanding for DIY.

Instead of insulating motor directly i propose to use sound foam under the boot cover and one the sides of the boot. Also firewall needs to be insulated too. You will loose some noise on the underside, but road noise will sufficiently dampen this i think.
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Re: Reducing noise of electric vehicle inverter?

Post by mjc506 »

The ultimate solution would be changing the pwm frequency continuously, generating (white?) noise instead of a fixed pitch tone. But that would need massive changes to the Open Inverter code...

It could be worse of course, you could be stuck at 4 or 8kHz... :)
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Re: Reducing noise of electric vehicle inverter?

Post by Pete9008 »

Not sure a sound dampening field will work in this case. Its good at low frequencies where the wavelength is long (around 3m at 100Hz) but by the time you get to 10KHz the wavelength has dropped to 3cm. The size of the nulled area scales with this so at say 100Hz you might get a dampened region of say 0.5m, at 10kHz maybe 5mm :(

The good news is that higher frequencies are much easier to attenuate than low frequencies using acoustic techniques. The first thing I'd do is check that none of the standard vibrations mounts have been bridged and if so sort. If not then try to work out what the source is and what the transmission path is. If it is an air path then strategically placed padding or insulation should help, if conducted then dynamat or similar may help.

The other option is turn the radio up a little bit - that's what I do to stop me worrying about all the noises that cars normally make ;)

Edit - I like the white noise idea, wonder how much variation it would need to be effective (and how much of a nightmare it would be making the control loops cope!)
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Re: Reducing noise of electric vehicle inverter?

Post by MattsAwesomeStuff »

mjc506 wrote: Tue Nov 15, 2022 10:35 amThe ultimate solution would be changing the pwm frequency continuously, generating (white?) noise instead of a fixed pitch tone. But that would need massive changes to the Open Inverter code...
This is what modern lathes do to eliminate vibration.

Just a very subtle and constant wandering of the rotating frequency so that it can't ever resonate.

Might be a good place to start looking if need be.

I don't know if it would require massive changes, you end up with a waveform that's changing many times a second, shouldn't be too hard to just add one small random bump to whatever the calculated result should be. Apparently it can be really small and that's good enough.
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Re: Reducing noise of electric vehicle inverter?

Post by MementoMori »

Wow, your replies are so heartening. I didn't expect much of a response at all, and just wow.. Thank you! :D

Funnily enough playing music loud is really the only solution, so long as it contains enough high frequency content to mask the motor and inverter. To me, that's kind of a "don't shower, just spray on some more axe body spray" solution, but it truly is the best fix I have come across.

I did try some absorption materials at the top of the bonnet and they actually helped to reduce some of the more offensive frequencies. The firewall was also mentioned. I made numerous recordings with a cardioid lav mic, and indeed there is substantial noise coming through the front of the vehicle to the occupant compartment. Next steps, attempting sound dampening material under the bonnet, and textile/foam + MLV on the interior firewall.

For fun and testing I taped the lav mic to plastic cover on top of the motor. Here's what that looked like. Pretty lines! If only it had some lower harmonic content to balance it out..
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Screen Shot 2022-11-15 at 9.16.37 PM.png
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Re: Reducing noise of electric vehicle inverter?

Post by Pete9008 »

Pretty pictures!

I'm surprised the noise is that bad in the Mini and would have expected the BMW engineers to have done a better job of the noise suppression. Is you car representative of others, it is possible that there is actually a problem with it?

Or is it that the car is so much quieter without the IC engine that even relatively low level noises become intrusive?
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Re: Reducing noise of electric vehicle inverter?

Post by MementoMori »

The sound is actually exceptionally quiet, even compared to other EVs. My wife and other passengers are not bothered by it one bit. I think the engineers did a great job, it's just a case of a particular set of ears. :)
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Re: Reducing noise of electric vehicle inverter?

Post by Pete9008 »

I know exactly what you mean, ever since moving house a few years ago I've been conscious of a low frequency persistent sound (google 'The Hum') which is why I researched sound cancelling in the past. Never got round to building anything though.

No one else can even hear it but it used to bug me constantly. Must have got used to it though, I can still hear it if I concentrate but am otherwise largely oblivious to it now :)
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Re: Reducing noise of electric vehicle inverter?

Post by SuperV8 »

I presume the standard 'acoustic' covers including motor cover - and 3x transmission covers are present and correct?
image.png
image.png
As mentioned I would check all the rubber mounts are 'free' and not in hard contact with the body?

I would also try filling the void behind the motor with acoustic insulation to see if this helps - old bath towels have been shown (saw a video on video on YouTube so must be true :) ) to be virtually as effective as actual acoustic panels but for free!
You could also try covering the fire wall in dyna mat type damping panels.

How about a white noise generator - picking a frequency which doesn't offend your ears?

Noise cancelation is interesting - but in an automotive environment (a metal box) I would say (guess) is really rather difficult to implement effectively as you will have many sound sources reflecting around the cabin. Bose and JLR have noise cancelling systems but I think these both work on lower frequency noises which don't bounce/reflect as easily.
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Re: Reducing noise of electric vehicle inverter?

Post by P.S.Mangelsdorf »

Finally a topic that is actually in my area of expertise! So far, the comments are headed in the right direction.

As Pete9008 said, the good news is that higher frequencies are easier to attenuate than low frequencies, the bad news is that our ears can detect high frequencies at a lower amplitude.

As several people have said, whenever we're addressing machinery noise, the first step is to check the attenuation, but there is more to check than just the mounts.

Below are the steps I would take, in the order I would take them, as someone that does similar things (in buildings, not cars) for a living.

1: Check that the motor and inverter mounts include a resilient element (rubber, neoprene, spring) and that the mounting method does not short circuit this. i.e. a rubber mount does no good if the bolt runs right through it. If this was a variable frequency drive in a building's HVAC system, I'd use a mount like this https://mason-ind.com/raa-rda/ Note that the bolt in the center is mounted in the neoprene and doesn't actually physically contact the housing or structure.

2: Check that no connections to the motor or inverter are rigid. There should be significant slack in electrical cables and coolant lines. Shields should not rigidly bridge between the units and the rest of the car. Any of these provides a pathway for sound, especially high frequency sound.

3: Check that the firewall is well sealed. Anywhere air can go, sound will follow. High frequency sound will have a difficult time making it through the firewall if it is properly sealed. Seal any holes with a non-hardening compound (dries to not be sticky but is still flexible)

4: Improve the sound isolation of the firewall. To do this, you need to add mass. Many sound "deadening" products do this, but you should look for one with a surface weight of at least 1lbs/sqft, preferably 2lbs/sqft. This would need to cover the entire firewall. Other applications where people will recommend only covering a portion (prevent a stereo from causing rattles), the purpose is just to shift the natural frequency.

5: If all else fails, mask the sound by raising the noise floor in the car, i.e. turn up the radio.


Now, in regards to active cancellation: in theory, this is an ideal use case - steady sound, consistent frequency, controlled environment. In reality, it does require a more advanced system than a hobbyist would tend to be able to develop. There are cars that do it, and there are noise canceling headphones that do it well (Bose pioneered the tech), but I don't think it's feasible in this circumstance.
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Re: Reducing noise of electric vehicle inverter?

Post by Tremelune »

You can gain a lot here just buy doing standard sound deadening techniques: CLD + CCF + MLV

Any audio installation shop can help, it's just not that cheap or light.
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Re: Reducing noise of electric vehicle inverter?

Post by MementoMori »

Thanks again for the amazing replies. I have been working on the vehicle since with MLV, absorption, etc., but have not made any notable progress.

Another issue is high frequency tonal noise from the instrument displays and hvac/dash area even when the vehicle is parked. It’s essentially coil whine sounds. I have noticed this in virtually all EVs. I wonder how much of my personal annoyance is derived from those sounds versus the drivetrain. The annoyance manifests as pressure in my left ear and temporary muffling of hearing in addition to the fatigue. The effects last a day or two after a drive, so it’s impossible to ignore. Even pumping ambient noise and music doesn’t help.

I’ve driven many different EVs on road trips, and ICEV to compare. It’s clear that there is a problrm with me and electric vehicles, whether it be medical or purely psychosomatic. Really ready to give up on EVs for good after having promoted them for years.
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Re: Reducing noise of electric vehicle inverter?

Post by Pete9008 »

Must say I'm a little surprised that you find EV dashboards worse than ICE ones - or have I misunderstood?

These days pretty much everything has switchmode supplies in although the low power/LV supplies in the instrument cluster/dash are likely to be operating well above the audible range (100Khz - 2Mhz typically) and the hardware is likely to be pretty similar on EV and ICE models. All I can think is that it's actually inverter or DC:DC noise that it making its way though the bulkhead somewhere behind the dashboard. If it is present even when the drive is inactive then the DC:DC converter seems more likely, do you know where it lives in your car?

Do any other power supplies (e.g. energy efficient lighting) cause similar problems or is it just the car?
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Re: Reducing noise of electric vehicle inverter?

Post by johnspark »

I have seen measurements of electric fields emanating from charge cables. Pretty bad, err very bad. When I last parked at a EV charging station my FM radio played up that's how bad the electrical interference was. I suspect these charging cables are not shielded.

I have a trifield EM meter which I have found very useful, I will go and measure some of this one day.

[sort of on track]
PS: I put the meter on RF in front of the cheap microwave oven and found very high RF emission! This is not supposed to happen! Microwave oven is supposed to be a Faraday cage? When I made a measurement behind the oven the field died away. Also 1 metre away from the front door as well.

[way off track]
PPS: I heated a glass of water with the microwave oven, and a glass of water with an electric kettle. Then I let the water cool down. Then I put a flower in one glass and a flower in the other glass from the same plant. The flower in the electric kettle water lasted a lot longer.

Wondering if electrical noise is somehow changing to acoustic noise for you.

I actually have tinnitus. Never really noticed it until someone mentioned the phenomena. Now I always hear it.
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Re: Reducing noise of electric vehicle inverter?

Post by johu »

johnspark wrote: Tue Jan 24, 2023 10:34 am PPS: I heated a glass of water with the microwave oven, and a glass of water with an electric kettle. Then I let the water cool down. Then I put a flower in one glass and a flower in the other glass from the same plant. The flower in the electric kettle water lasted a lot longer.
More samples needed!

Got tinnitus, too. Doctor just said "oh another software guy"
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Re: Reducing noise of electric vehicle inverter?

Post by arber333 »

johu wrote: Tue Jan 24, 2023 1:20 pm More samples needed!

Got tinnitus, too. Doctor just said "oh another software guy"
Years and years spent in close proximity of heavy machinnery will do that :). For me it was turbine propeller aircraft. The thing is you dont notice it until you put on really good protective earphones that cancel out any external sound. World is full of noise.
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Re: Reducing noise of electric vehicle inverter?

Post by johnspark »

Very interesting Johu:
Got tinnitus, too. Doctor just said "oh another software guy"
[/quote]

Well I thought I would measure the electric field with my Trifield meter in front of a node Ethernet phone connected to the WiFi box for a friend. I thought I would measure just a couple of volts / m. To my shock I measured about 40 volts / metre! I found out the phone was connected to the WiFi box by a 10 metre long Cat 5. So the V/m was transferred to the phone by the CAT 5 cable...

Then looked at the WiFi box and realized it is supplied with voltage from a plug in power supply that has no earth pin!

What I did was get from eBay a USB breakout board and soldered an earth wire onto it. Phone and WiFi box V/m dropped to ~5V/m.

Also, I put a 10mm steel bar in front of my monitor and earthed it too. This reduced V/m of my monitor I am in front of a few hours a day.

All these remedies cost virtually $0.
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Re: Reducing noise of electric vehicle inverter?

Post by johnspark »

oh here is a picture:
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Re: Reducing noise of electric vehicle inverter?

Post by johnspark »

arber333 wrote: Tue Jan 24, 2023 3:06 pm Years and years spent in close proximity of heavy machinnery will do that :). For me it was turbine propeller aircraft. The thing is you dont notice it until you put on really good protective earphones that cancel out any external sound. World is full of noise.
Very interesting arber333, interesting to see what backgrounds people have.
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