Flight Service
Flight Service MegaDork
4/21/16 11:47 a.m.

Ok, Need to find a vibration in your car? Here is how to help find it.

Now disclaimer. I did not write the next post. It was from Locost USA forums.

All intellectual property and advice are from the original author 1055 (his sig says he is called Tucker and is from Outside Hartford, CT.)


Before you get started.. know that none of this is possible without a very VERY simple tool and understanding one simple rule. The rule is: everything that rattles in your interior or noises that you hear outside the car are all caused by vibration and sound waves whether or not we can physically hear them. infact, everything that moves emits a Hertz rating. the tool we will be working with is called a Sirometer and is not something you can go to your local autozone and find. fortunately. Small engine supply has them for sale at about $21. As far as I know, there are only two places that sell sirometer's and only one company that makes them. The company is Treysit and you can get them direct for 14 euros (about $17 + shipping). (Flight Service edit: This is also called a Vibra Tach Amazon)

What is a Sirometer? a sirometer is a unit that converts vibrations into cycles per second, more commonly known as Hertz (from here on out known as Hz).

It does this using Frequency and resonance. This is a very basic lesson in sound; the slower the frequency or cycles per second (Hz) the lower (pitch wise) the noise will be. subsequently, the faster the frequency the higher pitched the sound.

For those that do not know what resonance is; Resonance by definition is “The increase in amplitude of oscillation of an electric or mechanical system exposed to a periodic force whose frequency is equal or very close to the natural undamped frequency of the system.” (thefreedictionary.com) in laymans terms it is the point where any one object will begin to deflect, vibrate, oscillate or just generally move. An objects natural resonance is dependent on pretty much every single characteristic of that object (build material, length, width, height, density) any fraction of difference in any of these characteristics will change the point of resonance.

What people don't know about the random weight or bulges in a cars frame or body. Say your an engineer and you spend millions of dollar and countless hours designing a car. then you go to road test it and at 45 mph the entire thing hums like a mother****er. well.. instead of re-designing the entire car to change its structural resonance.. lets just throw some random weights on that bitch! seriously though. if you ever have your car up in the air and notice random structural points on the frame or body and your like wtfbbq why would they put that there.. that's the sole reason. remember what I said about changing resonance and how it is completely unique to each individual object etc? these small weights positioned at certain points are designed to completely eliminate or move the point of structural resonance to a pitch we can not hear or not notice via butt diagnosis. People take the weights off of there differentials all the time.. notice a weird noise now? if you don't, your sure lucky.

Why is this **** important? obviously, you are trying to chase sounds and vibrations through your car. how do you hone in on them? sounds and vibrations travel through whatever is adjoining what is creating them... so.. we trace them down using amplitude. as you can see on the picture of the sirometer, there is a small wire extruding from it. the small, bumpy part on top actually spins and controls how much wire is extended from the sirometer.

Using the Sirometer. The way you operate this doohickamig is by simply pressing it to whatever is vibrating (enough to maintain a solid connection without actually dampening the vibration). starting with the wire fully extended or spooled in the unit and extending or retracting it until the yellow loop starts to oscillate or deflect. when it moves backs and forth the most, stop changing its length. you have found the Hz of the noise/vibration you are after. from here, believe it or not it sadly is mostly math to determine what is causing the vibration.

The Important Stuff

This section is all about the different equations you use to determine the different “orders” of frequency based on the different things that can cause these vibrations or oscillations.

First order of business: tires and wheels. the single biggest cause of noise and vibrations in cars. This includes more then just tires and wheels though. it also includes hubs, rotors, axles and differentials.

To determine the Hz of tires we follow this equation:

Vibration occurs at X mph divided by 5. this leaves us with increments of 5. (if x = 30 mph, that leaves us with 6 increments of 5. essentially X/5= # of increments of 5. you then take this number and multiply it by the Tire revolutions per second at 5mph. this is where it can get complicated. believe it or not, this is actually a perfectly normal number to request of any tire on the market. More common and easy to find is the tires rolling circumference in inches. If you have the tires rolling circumference, you simply have to divide it into 88 (88/rolling circumference = revolutions per second at 5mph). for this example we will be using a 235/75/15 tire. it just happens to have a perfect RPS/5 of 1. Once you have obtained the RPS/5, you need to multiply it times the increments of 5 you have already found. this will give you the Hz First order.

Order is essentially the number of degrees off the 1st order Hz that a frequency lies. this is an incredibly easy concept to follow. take first order, multiply by 2. that gives you second order. to find third order, multiply first order by 3, to find fourth order (as far as you'll need to go with regards to tires and wheels [there are only four of them on your vehicle......])

To sum everything up in an example. Say a vehicle experiences a vibration of 36Hz @ 60mph with a 235/75/15 tire.

60mph/5 = 12 increments of 5 (60/5=12)
12 increments of 5 x 1 revolutions per second at 5mph = 12hz. (12x1=12)

this gives you a first order frequency of 12Hz. well.. this isn't close to our 36Hz vibration.. so lets check out the other orders.. second is easy, 12x2= 24Hz, still not close. 12x3 (3rd order) however equals 36Hz. This means that that the tires and wheels are a very very strong suspect in the vibration we are seeking. From here, I would personally check the wheels for balance and the tires for un-even wear. 3rd order indicates that there are 3 imposing factors contributing to the vibration. It could be two bulges in a single tire and a wheel that is out of round.. or two wheels that are out of balance and a tire that is cupped or feathered. anyways, you get the point.

Tires and wheels are the single biggest/most common cause of vibration. it is a good idea to always start here. if it is not in the tires/wheels them selves.. its a good idea to look into rotors, hubs, wheel bearings etc. these are easy to narrow down by using amplitude. Amplitude is the measure of the strength of the wave length.

this picture shows differences in amplitude. notice the frequency always stays the same, but the distance between the peak and trough of the wave grow larger. the larger the distance, the stronger the amplitude, the closer you are to the source of the vibration. The way to determine this is how far the wire on the sirometer is deflecting. if its moving very fast, but only an 1/8th of an inch back and forth.. your not very close to the source. if its vibrating to speed, but moving 3/4th of an inch.. chances are your very close.

The rest of the driveline. This covers Prop shafts (drive shafts), differentials, U joints (not commonly used on our vehicles) and CV joints.

This section is actually an extension of tires and wheels (surprise!) so you cant really jump to right here without doing some of the above calculations first. For this.. you will actually need your axle (gear) ratio. it is a simple equation and there are only two orders we really need to look at. Take the First order Hz calculation from the tire/wheel section and multiply it by the Axle ratio (1st order wheel/tire Hz x Axle ratio). For example, we will use the same thing from above, but with an axle ratio of 3.5:1. 12x3.50 = 42Hz. this is first order propshaft frequency. To find second order, we multiply this number by 2. (42x2=84Hz). Why bother with either of these? if you find a value of 42Hz rattling your dash.. chances are its something in a drive-shaft or differential. could be your drive shaft is out of balance or has excessive run out. could be that it is out of phase after recently doing work on your car, could be that it got dented after your last dirt excursion. who knows, thats up for you to determine after you confrim this is the frequency you are picking up.

2nd order drive shaft is a little bit trickier to diagnose. it involves the joints and their necessary angles. if you've messed with your suspension geometry at all (lowering, raising, camber kits, wider wheels etc, etc.) [yeah thats 95% of you on this board ] proper operating angle for most driveshaft joints is within 3 degrees positive or negative of 0. the operating range is -3 to 3. now.. that sounds like a decent amount of play right? wrong. each angle on either end of the shaft or half shaft (axles) needs to be within 1 degree of each other. on that note, it is also bad to have less then one half degree difference between each end (example: each angle is measured at 0 degrees, this is no good. it is necessary for one end to be at least positive or negative .5) degrees and the other to be at least 0.5 degrees above it or, in this specific case, 1 degree below it (-0.5 degrees) (since it is never a good idea to have a joint operating at 0 degrees.)

CV (constant velocity) joints are a bit more forgiving when it comes to this as they are not loaded with needle bearings like U joints that need to be rotated through the sleeve. They [CV joints] can take a bit more abuse and can stand being rotated at 0 degrees, but they where designed to be operated at an angle so keep that in mind. anyways.. why is this important? if they are operating at too extreme or too little of an angle, they will cause a vibration. how do we fix this? shims (something as small as 1/8th of an inch will change angles) in diff mounts, trans mounts etc.

Engine Frequencies. To determine engine frequency, we actually take the RPM's and just convert them to revolutions per second (Hz, duh.) RPM's is just a much easier, more common way of talking about engine speed. you could in-fact walk into your local dealership and say “I want my idle tuned to 12Hz”. they'd probably look at you like you had 17 heads, but you're technically just asking them to set your idle speed to 720 rpm. Again, only two orders with engine frequency. First is simply RPM divided by 60 (rpm/60=first order Hz). Most first order vibrations from engines are normally caused by motor mounts or something touching the engine in a way it was not supposed to. both can be determined with a visual inspection of each. 2nd order vibrations from engines generally point to internal items in an engine, which obviously require a greater amount of diagnosis then just a visual inspection. they can be found by multiplying first order Hz by 2. your on your own after you've narrowed it down!

Examples: 1500/60= 25Hz first order
25x2 = 50Hz second order.

Engine Accessory Frequency. I view this as the most complicated series of formula's.. only because there are the most So surprise surprise this section deals with all your engine accessories and the noises they make. For this section you will need to know the diameter of each of the belt driven pulleys that are driven by the crank pulley. For this example, I will use an 8” diameter crank pulley, 3.75” water pump pulley, 3” A/C pulley, 4.5” power steering pulley, 2.5” idler pulley, 3.25” alternator pulley. Divide the crank pulley by the different accessory pulleys to get the pulley size ratio.

this leaves us with a pulley ratio of 2.13 :water pump, 2.6: A/C, 1.7: P/S, 3.2 Idler, 2.46: alternator. once you have the pulley ratios, multiply them by engine RPM and this gives you each individual accessory pulley rpm. to find Hz, divide the accessory pulley RPM by 60.
so.. example time, ill use the A/C pulley and 1200rpms for example.
8 / 3 = 2.6. 1200 x 2.6 = 3,120. 3,120 / 60 = 52Hz.

This is the Hz created by the A/C compressor @ 1200 rpm for this particular vehicle. Again, doesn't tell you how to fix it, but that it is what is causing your other objects to vibrate. For reference; the Hz of the other pulley's in the above situation ended up being: W/P: 42.6Hz, P/S: 34Hz, Idler: 64Hz and Alt:49Hz.

The last force to reckon with: Engine Firing Frequency. More commonly known as Exhaust frequency. This is the simplest one to figure out.. although it has a lot to do with exhaust pulses. Take the number of cylinders in your engine and divide it by 2. I'll use 4 for this example. This equals 2 cylinders fired per crankshaft revolution. take your engine speed (1200 rpm ) and multiply by cylinders fired per crankshaft revolution (2). this leaves us with 2400 cylinders fired per minute. Take your cylinders fired per minute and divide by 60. this gives us 40Hz.


4 cylinders / 2 = 2 (cylinders fired per revolution). 1200 rpm x 2 (cylinders fired per revolution) = 2400 cylinders fired per minute. 2400 cylinders fired per minute / 60 = 40hz.

Once you have your firing frequency, you need to trace it through the exhaust using the amplitude. Ill throw this warning in there because I dont want to be responsible for someone obtaining a 1200 degree cat burn. exhausts get ****ing HOT! when running. be careful.

Fixing an exhaust rattle could be as simple as isolating the mount that is causing the racket and repairing it.. or changing the resonance of the exhaust components.

So, lets diagnose vibration!


RossD UltimaDork
4/21/16 12:03 p.m.

Give that man a medal.

Flight Service
Flight Service MegaDork
4/22/16 6:55 p.m.

it was impressive

Flight Service
Flight Service MegaDork
4/25/16 7:27 a.m.

updated link

mmurray New Reader
5/3/16 3:01 p.m.

Here from the newsletter link... this is fantastic. Time to find out if it is the A/C compressor after all.

RossD UltimaDork
5/3/16 4:26 p.m.

I'm surprised this hasn't garnered more attention.

russ_mill New Reader
12/4/17 6:11 a.m.

Wonderful. Commenting so I can bookmark this!

02Pilot UltraDork
12/7/20 4:01 p.m.

Missed this the first time around and just came here through the link in dusterbd13's pinion angle thread. This looks like it could be very useful in tracking down the noise/vibration I'm getting in my 128i, but I'm not entirely clear how you use this thing. If it's a rotating assembly, you can't really put the tool on it, so what do you do? Put it on something nearby and connected? I'm sure there's some trial and error involved in using it, but it would be nice to have a better idea of where to start.

No Time
No Time SuperDork
12/7/20 4:17 p.m.

In reply to 02Pilot :

Exactly, you put it on something nearby and connected. The surface you place the sirometer on doesn't need to be the one causing the vibration. 

If you can feel the vibration then it will be measurable by placing the sirometer on the surface in the vehicle where the vibration is noticeable.  

The small engine use of the sirometer is to measure engine RPM. By placing it on the engine case you can measure the speed of the rotating assembly. 

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