3rd order+Noise

[par1]

I have heard that 3rd order is Sporty sound source? Can anyone explain me the meaning & physical significance of order in terms of NVH, that would be a great help

 

[Rob45]

"Order" relates to, in your case, multiples of engine rotational frequency.
For example, 3rd order indicates the frequency that is 3 x the engine rotational speed; if an engine is turning at say 1200 rpm, or 20 rps, 3rd order would be a frequency of 60 Hz (3 x 20 = 60).

Third order is engine firing frequency for 6-cylinder (4-stroke) engines, so I'm not sure it relates to "sporty sound" for anything other than 6-cylinders. You'd be very unlikely to encounter a significant 3rd-order component of the exhaust noise in for example a V-8 engine.

 

[SomptingGuy]

The area of subjective noise is HUGE. Most sporty noises you hear from a car are either intake or exhaust or a combination of the two. Assuming this is what you mean...

Sporty sounds tend to have an order balance that comprises the fundamental, plus a good contribution from the first few harmonics. For a 6-cylinder engine, this would be 3rd, 6th, 9th, 12th, etc. Half orders (a term used to describe orders that aren't harmonics of the fundamental, but are harmonics of the 1/2 order) tend to give a rougher sound.

Many many dollars, euros & yen are spent in refining the sound quality, usually through modifications to the order balance.

Sometimes roughness needs to be added to a vehicle's sound to match the rough, tough image required. Sometimes smoothness is required. Sometimes downright aggressive power is required. These can usually be obtained from the same base engine, by modifications to the exhaust and intake system geometries.

 

[par1]

If Engine is running at 1200 rpm than i have to worry about only 20rps (20 Hz)-

1. Like when engine will run 6000 rpm than I will worry about 100 hz so than why people study upto 1000 Hz or may be more? (due to other components?)

So the Plot for Noise vs Frequency should be from 0 to 100 Hz?

2.If i see any pick at specific rpm & i would like to reduce that peak Noise(dB) how could i achieve it at specific rpm?

(add some more stiffness or mass?)

Thank you for quick reply.

 

[SomptingGuy]

1) If your main order is 3rd order (i.e. if you have a 6-cylinder engine), this'll have a frequency of 60Hz at 1200 rev/min and 300 Hz at 6000 rev/min. As I mentioned earlier, the noise will also contain many harmonics, which is why people need to measure up to and beyond 1kHz. E.g. 12th order at 6000 rev/min is 1200Hz. Note also that your ears are at their most sensitive at 1kHz, so noise around that frequency is prety important. For intake/exhaust noise studies, it's quite common to measure up to 1200Hz.


2) If it's a particular frequency/speed combination that's the problem (order cuts through resonance), the normal approach would be to tune something to absorb it. In the exhaust/intake world we add resonators (Helmholz & 1/4 wave).

 

[par1]

I understand that you are trying to help me out but ur sentences are little bit confusing me.

1) If your main order is 3rd order (i.e. if you have a 6-cylinder engine), this'll have a frequency of 60Hz at 1200 rev/min and 300 Hz at 6000 rev/min.

(a) What do you mean by if main order is 3rd order

My understanding is
If engine is running at 1200 rpm than (1200/60) so it would be 20 Hz why 60 hz?


So 2nd order 40 Hz (2400 rpm)

3rd order 60 Hz (3600 rpm)

4th order 80 Hz (4800 rpm)

5th order 100 Hz (6000 rpm)

2) Human is sensitive to near by 1kHz but engine is running max at 6000 rpm so only frequency is 100 hz than how come I plot upto 1000/1200 Hz?

May be the question won't be appropriate but will appreciate if you answer me

 

[SomptingGuy]

Without wishing to sound dissmissive, you really should talk to the NVH people where you work. You are asking some extremely basic questions. There is no way you will be able to learn the fundamentals of automotive NVH by asking questions here. Specifics yes, but fundamentals no.

 

[Drej]

par1 All the information you need is here, kindly given by SomptingGuy.

3rd Order
1200 rpm = 1200/60 x 3 = 60 hz
6000 rpm = 6000/60 x 3 = 300 hz

12th Order @ 6000 rpm
6000 rpm = 6000/60 x 12 = 1200 hz

etc.


------------
See faq569-1083 for details on how to make best use of Eng-Tips.com

 

[DaveBidner]

Hi par1, order is how NVH folks track and relate vibration/noise to rotating machinery thorough different rotational speeds. For a 6 cylinder 4 stroke @600 rpm, (600/60)= 10Hz, exhaust firing frequency tracted at same conditions lines up with 30Hz (600/60)*3 or 3rd order (30Hz). A lifter tick would line up with 2nd order 20Hz. At 6000rpm its 100Hz, 300Hz and 200Hz respectfully. On 8 cylinder some noises line up differently @600 rpm, (600/60)= 10Hz still first order. but exhaust firing frequency lines up with 30Hz (600/60)*4 or 4th order (40Hz). A lifter tick still lines up with 2nd order 20Hz. At 6000rpm its 100Hz, 400Hz and 200Hz respectfully. As rpm is swept the order related excitation will stimulate time based systems responce like resonators in an exhaust and add or subtract to the exhaust note. Fun stuff.
Dave B

 

[GregLocock]

While I agree with SomptingGuy's most recent post, there is a specific SAE paper by Mazda that explores this and similar ideas.

Personally, I think that it was an oversimplified experiment and (as I remember) it ignored phase. Really the best way of doing this stuff is with a good binaural setup, such as that made by HEAD or B&K.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[SomptingGuy]

Greg,

Do you recall the SAE paper number?

- Steve