So I found something odd under a car - ideas?

[anonymous-user]

RVVUNM said:

I love PH, I had no idea these existed.

This, you learn something new everyday!

[mwstewart]

On a similar 'note' BMW always designed their shells to have a specific resonant frequency. The 3 series for example was - I think - 37Hz or thereabouts, the aim being to minimise hum/transmission of road noise within the commonly used road speed range in their main markets (likely optimised for Germany!).

BMW do put a lot into their cars.

[Adz The Rat]

williamp said:

geniunly interetsing response, many thanks!

I agree, very interesting and new to me!

[Fastdruid]

robinessex said:

If you want to experience a similar event, try gradually opening a window at motorway speeds, you’ll eventually feel/hear a deep drumming sound, quite uncomfortable. Basically the car has become a Helmholtz resonator, and you are sitting in it!

I can't open *just* a single rear window on my car at >30mph for that reason. A very painful throbbing noise that vanishes if a second window is opened.

[robinessex]

mwstewart said:

On a similar 'note' BMW always designed their shells to have a specific resonant frequency. The 3 series for example was - I think - 37Hz or thereabouts, the aim being to minimise hum/transmission of road noise within the commonly used road speed range in their main markets (likely optimised for Germany!).

BMW do put a lot into their cars.

All manufacturers do it. It's called Point Mobility Analysis. Performed with a mathematical analysis system called Finite Element Analysis. FEA for short.

The physical meaning of point mobility is that it tells you how much velocity you can create at that point for a given force, at each frequency. Velocity is used for several reasons, not least of which is that sound pressure generated by a radiating panel is a function of velocity.

Edited by robinessex on Friday 23 September 11:03

[FiF]

Learn2MergeInTurn said:

RVVUNM said:

I love PH, I had no idea these existed.

This, you learn something new everyday!

Agreed, though it has had the unfortunate side effect of destroying a cynical misconception of mine, that it's only Ford who would come up with bodges like this.

[robinessex]

FiF said:

Learn2MergeInTurn said:

RVVUNM said:

I love PH, I had no idea these existed.

This, you learn something new everyday!

Agreed, though it has had the unfortunate side effect of destroying a cynical misconception of mine, that it's only Ford who would come up with bodges like this.

No not a bodge. Even a crankshaft had a damper, it's usually the pulley at the front. Inspect it carefully, and you will see it has a rubber section in it. Almost any industrial machinery that moves usually has a mass damper somewhere. At home, washing machines and spin driers have them, that is why they are so heavy. Try using a new washing machine WITHOUT removing the transit bolts!!!

https://www.youtube.com/watch?v=tX1sVBEp6lQ

And this gets real bad !!!!

https://www.youtube.com/watch?v=DOhQWTBGEf8

[mwstewart]

robinessex said:

All manufacturers do it. It's called Point Mobility Analysis. Performed with a mathematical analysis system called Finite Element Analysis. FEA for short.

The physical meaning of point mobility is that it tells you how much velocity you can create at that point for a given force, at each frequency. Velocity is used for several reasons, not least of which is that sound pressure generated by a radiating panel is a function of velocity.

Edited by robinessex on Friday 23 September 11:03

FEA of course has long been in use at a component level i.e. chassis sections, but with the E46 I am pretty sure that BMW were the first to tune the entire structure to a particular frequency in order to actively improve refinement. I used to work in Ford Computer Aided Engineering and it definitely was not done there - many years after the E46.

[robinessex]

mwstewart said:

robinessex said:

All manufacturers do it. It's called Point Mobility Analysis. Performed with a mathematical analysis system called Finite Element Analysis. FEA for short.

The physical meaning of point mobility is that it tells you how much velocity you can create at that point for a given force, at each frequency. Velocity is used for several reasons, not least of which is that sound pressure generated by a radiating panel is a function of velocity.

Edited by robinessex on Friday 23 September 11:03

FEA of course has long been in use at a component level i.e. chassis sections, but with the E46 I am pretty sure that BMW were the first to tune the entire structure to a particular frequency in order to actively improve refinement. I used to work in Ford Computer Aided Engineering and it definitely was not done there - many years after the E46.

I worked in body engineering for a while at Dunton. I remember when I was at Canewdon Consultants, a couple of the guys there did some work for FoMoCo on this subject. May have been the early days.

[Steve_W]

Fastdruid said:

robinessex said:

If you want to experience a similar event, try gradually opening a window at motorway speeds, you’ll eventually feel/hear a deep drumming sound, quite uncomfortable. Basically the car has become a Helmholtz resonator, and you are sitting in it!

I can't open *just* a single rear window on my car at >30mph for that reason. A very painful throbbing noise that vanishes if a second window is opened.

Both my F31s (current 3 series Touring) were bloody awful for sounding like a Chinook inside if you opened one or both rear windows at speed.

[hondafanatic]

robinessex said:

hondafanatic said:

charltjr said:

They damp out vibrations at a certain range of frequencies that would otherwise cause noise/buzzing/ringing in other components or cause a sensation of vibration in the cabin.

Do all cars have them?

No. Selective, some car in the manufacturers range yes, no. It tends to be the smaller cars that have them, more likely to suffer resonance’s due to lighter construction. That's why large, heavy, luxury cars sound naturally quiet. No flimsy stuff to vibrate.

Brill...thanks!

Now I remember why I joined PH...threads like this.

[405dogvan]

Aha - I knew PH would come through!!!

Awesome stuff, fascinating to know - thanks!

p.s. ironically, my other job was working on an Insignia, once I was done I was clearing-up tools and I could hear a weird noise - I thought it was the radio but the closer I got to the bonnet of the Insignia, the louder it got.

As far as I could tell, the engine idling was making the 2 layers of the bonnet skin 'sing' - pretty annoying (bonnet had been painted, my guess is that sound deadening hadn't been refitting properly?)

Diesels eh...

Edited by 405dogvan on Saturday 24th September 13:03

[kev b]

I remember seeing two huge lumps of metal attached, flexibly, to the rear axles of certain Vauxhalls in the 1970s, might have been the Victor but seem to think it was on larger engined Viva/Magnums.

[CanAm]

kev b said:

I remember seeing two huge lumps of metal attached, flexibly, to the rear axles of certain Vauxhalls in the 1970s, might have been the Victor but seem to think it was on larger engined Viva/Magnums.

I think it was the Chevette.