Resonant Frequency Results....

[profundus-sanus 10+ year member]

what's the significance of finding the resonance frequency of the enclosure outside the vehicle? with the results measured from inside/outside the cabin, what am i looking for?

subtract one graph from the other. then you'll more or less have a response graph from your vehicle.

the out of car is to attempt simulate an infinite envirpment to compare and see how the car effected the response.

 

[dleccord 5,000+ posts]

subtract one graph from the other. then you'll more or less have a response graph from your vehicle.
the out of car is to attempt simulate an infinite envirpment to compare and see how the car effected the response.

the part i dont get is how will the graph of the outside test affect the graph from inside the vehicle? how does the 2nd graph benefit the 1st graph?

so lets say the peak of my first graph is at 70hz, and the 2nd graph peaking at 50hz. am i still shooting for 70hz or somewhere in between?

 

[CJL 10+ year member]

I beleive it will tell you each freq loudness in a open envierment(sp?)...Therefore showing you which freq you car boosted the most.

 

[profundus-sanus 10+ year member]

the part i dont get is how will the graph of the outside test affect the graph from inside the vehicle? how does the 2nd graph benefit the 1st graph?
so lets say the peak of my first graph is at 70hz, and the 2nd graph peaking at 50hz. am i still shooting for 70hz or somewhere in between?

it tells you what the response of the subwoofer is in an open (quasi infinite enviroment). More or less what the natural response is of the woofer in the enclosure. Then when you throw it in the car and compare it shows you how much efficiency you gained at each frequency.

If you found a box that played PERFECTLY flat in your car and took it out and made a frequency response graph in the open enviroment it would probably roll off very soon.

Its not the best way to go about it but probably the cheapest.

 

[Immacomputer 10+ year member]

Just like mentioned above, measuring inside and outside the car with the same enclosure over the same frequencies at the same volume level will give you the actual transfer function for that system (it will vary some with different enclosures but you did a good job keeping it about the same size that you're going to make the final enclosure).

The transfer function gives you a graph that shows the relative gain per frequency. It basically shows you how much boost your cabin gives over different frequencies. If I was at home, I would be able to show you a graph I did for one of my old cars and you could see what it looks like. Instead, I ghetto mspainted what it looked like.

In my car, I had a set of frequencies that were boosted almost equally. I would take that and design an enclosure that would have a peak around those frequencies.

EDIT: Here is a link that explains it in more detail but his graphs don't show the in car/out of car plots. They just show the magnitude of gain.

http://www.aespeakers.com/phpbb2/viewtopic.php?t=518

 

[15nissen 10+ year member]

well did the woofer reach its full excursion?

What does that matter? since he peaks upper 50/60's it will not reach full throw. If he peaked low 30's then he would.

 

[GemaRastem 10+ year member]

technically, transfer function is the wrong word to use, but basically you are looking at the response in an anechoic environment, and then your car, if there is a peak in the anechoic environment, you know that's just the combo of the box and woofer, but if there is a peak in the car that isn't in the anechoic plot, then that is a resonant frequency of your car.

 

[Immacomputer 10+ year member]

technically, transfer function is the wrong word to use, but basically you are looking at the response in an anechoic environment, and then your car, if there is a peak in the anechoic environment, you know that's just the combo of the box and woofer, but if there is a peak in the car that isn't in the anechoic plot, then that is a resonant frequency of your car.

No, transfer function is 100% the correct word to use. The transfer function is Output/Input. It's used all the time in circuits and systems and your enclosure/sub/vehicle is the system we're discussing. The input "signal" is the sub in the enclosure. You're getting your signal's frequency response by putting it in an anechoic environment. You're then putting it through your system (the vehicle) and then you're graphing the output of the system with the known input signal. The input output relationship is the transfer function and in this case, it shows gain. It's just like hooking up a circuit and measuring your input voltage and your output voltage. Subtracting them gives you the transfer function of the circuit. The transfer function will show you the gain of the cabin over the frequency spectrum you measured.

For instance, your peak in car response might be 100dB at 60hz and in the anechoic chamber, you did 90dBat 60hz. Well lets say that in the vehicle, you did 98dB at 55hz. Then in the anechoic chamber, you did 78dB at 55hz. In this case, your cabin is boosting the 55hz signal 20dB while it's boosting the 60hz signal only 10dB. Now which makes more sense, to build an enclosure that will peak around 60hz or 55hz?

 

[GemaRastem 10+ year member]

A transfer function is H=Y/X where Y is the output and X is the input. Not H=Y1-Y2. The transfer function of a system is the zero input response, meaning the response without any initial conditions. You can use the transfer function to map the response of a system without initial conditions.

Taking the same input and getting two outputs and then subtracting them is not a transfer function.

 

[jmanpc 5,000+ posts]

just build your box, tune it to 50 or so if you want it to be a fart box, and run sweeps and see what you peak at.

done.

 

[thch 10+ year member]

please note that log - log(x) = log(y/x). this means that subtraction of two values expressed in dB is actually a ratio of the actual values. likewise addition of dB expressed values is multiplication of the actual values. (values non-negative). because dB expressed values are formed only from non-negative actual values, this is not a concern.

also, concider that the environment may have a significant impact on the response of the box. this can be shown by measuring impedance at a few frequencies outside of the car, then inside the car. changes to impedance (both magnitude and phase) mean a change in loading conditions.

 

[dleccord 5,000+ posts]

okay so how would you guys find the resonant frequency of your vehicle?

(step by step process would be nice)

 

[tommyk90 5,000+ posts]

okay so how would you guys find the resonant frequency of your vehicle?
(step by step process would be nice)

The way I did mine was like this:

1 cube box in my truck.

Played 20 to 70 hz, in 1 hz increments

Recorded each score

Placed the box outside, with the meter about 3 feet away.

Played 20 to 70 hz, in 1 hz increments.

Subtracted the ambient score from the in-car score. The frequency with the smallest difference is your resonant frequency.

In my case, it was around 44 hz. Now the exact note depends on my vehicle temperature, meter placement, and what setup I'm using. Funny thing about my truck is that it peaks in pretty much the same range no matter what setup i'm using.

Really, it just comes down to building and testing. You can do all this testing to find your resonant frequency, but there's far too many other variables for that to be considered the only method of testing you need to do.

Also, it is not uncommon for a vehicle to have two peak frequencies.

 

[snoopdan 5,000+ posts]

I love it when everyone gets over technical and still it means absolutely nothing. Reminds me of when I used to work with physicists...they'd dream up shit that would work wonderfully on paper but when it came to getting us engineers to actually build it, they couldnt understand what "in the real world" meant.

 

[Lakota 5,000+ posts]

I love it when everyone gets over technical and still it means absolutely nothing. Reminds me of when I used to work with physicists...they'd dream up shit that would work wonderfully on paper but when it came to getting us engineers to actually build it, they couldnt understand what "in the real world" meant.

tis' true