Anybody ever heard of tuning the frequency of your box to the resonance of the car?

Jstas

I was talking to guys who like to do the SPL stuff last night. They build these gigantic boxes with massive slots to pressurize the car. But they tune the boxes to the resonant frequency of the car. They get the resonant frequency by placing a 10 inch sub in the car and running test tones and measuring with a microphone when the highest response level is and then tune to that frequency.

I'm honestly not sure how that will achieve what they want which is higher SPL and they weren't explaining it well so my understanding of it is weak at best. I've always built enclosures to the resonant frequency of the subwoofer in the enclosure, not the resonant frequency of the car.

Needless to say, they want to try it my way and see what makes more noise.

The way I see it, you want that sub working at its peak efficiency in the enclosure. If it is operating at it's peak efficiency, it is using the power you are providing it the best that it can and it is moving air through that vented box as best as it can. In order to do that, you want to build the enclosure and tune it to a frequency that is smack in the middle of the subs response peak. That would be Fs. So if the sub's Fs is 40 Hz, you build a box that will be tuned to a frequency somewhere between say 37 Hz and 43 Hz. That will give you maximum performance from the sub because you efficiency will be high.

Now a concept I don't think they totally grasped was port flow vs. port velocity and what inertia of the cylinder of air in the port has to do with it. They have gigantic slots which move air by sheer volume alone. They are not understanding how you can get similar air flow with a few smaller ports. The big fat slot has low velocity but large volume. The small, round tube has low volume but velocity that can be as much as 10 times as fast. But when you are going for big SPL, you need to make sure that you aren't moving that air in the tube so fast that it cavitates insides the tube and reduces port efficiency and overall velocity.

Is any of this making sense?

Anyway, if some can explain to me that car resonance thing, I'd appreciate it.

RuSsMaN

Wow, nevermind.......

Jstas

Dude, you just said what I have been doing since I built my first box at 16. What I'm saying is that they find the resonance of the car. Then they tune to the box to the resonance of the car. It doesn't make sense to me. Mainly because if the car's resonance is 67 Hz and the sub's resonance is 38 Hz with a pretty progressive drop off above say 50 Hz, by the time you reach 67 Hz, you could be pretty close to a -6 dB drop. If you tuned to that frequency and had a 38 Hz resonance that was at say a +1 peak, you are going to have to push a serious amount of power through that woofer to get the same output at 67 Hz that you could at 38 Hz.

The woofer is a driver in a cabinet. When you buy a sub for home use, does the dude from HSU or SVS or even Polk Audio call you and ask you what your room or house resonance is?

I'm asking what the benefit to tuning to the car's resonance is? As long as I have been doing this, two things have been the utmost importance. Stop vibrations in the car and make sure you have enough mass and/or rigidity in your enclosure to tame resonances of the materials that make up the enclosure. To get the car to stop vibrating, you do stuff like add mass in the form of Dynamat, sound-deadening foam and even some guys fill floorpans with concrete. Adding mass changes the overall resonance of the car. That tames the vibrations. You don't want the car to vibrate because it decreases efficiency. You end expending mechanical energy by flexing the car at it's resonance. You don't want that. You want all of your energy going in to pressurizing the cabin. If you aim for the car's resonance, you are going to shake it.

If you aim for the driver's resonance and tune your enclosure to take advantage of the backwave off of that driver, you can use the pulses of air to boost your output out of the port. When you "tune" the enclosure, you are basically adjusting the resonance of the enclosure through airspace management to match the resonance of the woofer. That way, the woofer is using as much power as it can to move as much air to pressurize the cabin. The more you pressurize the cabin, the more SPL you build and the louder you are. The more efficient the sub is, the more pressure you will get out of it at a given power level. The reason higher pressure builds SPL is because of the pressure wave that the pressurized air constantly wanting to expand gives you. That wave compresses the lower pressure air in front of it. That builds inertia in the air mass and can "load" the cabin. Just like corner loading in your house.

What I am essentially saying is that tuning to the resonance of the car goes against everything I've been told, know and practice and even the math behind it says it's not right.


I don't care what anyone says. This is physics. Numbers with explain everything you see and hear in this hobby. They are not a deciding factor in how the equipment will perform but all those differences in sound come down to something that can be explained by numbers. Physics governs this hobby. You can't change physics. There is so much more to this than just a bunch of guys in a garage doing trial and error and getting a lucky guess. Thermal dynamics, fluid dynamics, the law of conservation of energy, Newton's laws...there is no shortage of ways to describe what is happening with all of this. I don't see how how tuning to the resonance of the listening/metering environment can be beneficial when it totally ignores not only the resonance and efficiency of the driver but also the enclosure the driver is mounted in.

Maybe I'm missing something, I don't know. I'm just looking for someone to explain it to me.

ShinAce

If you know anything about building enclosures, you realize that a home subwoofer is tuned to a 'flat alignment'. You tune the box to modify the woofer's response into the flattest result.

In a car, this doesn't apply. In a car, you tune the box to whatever frequency seems reasonable. By going higher, you'll get a peak at the port resonance(look, free SPL). If you can get that peak to match the peak of the cabin, more free SPL. Your friends are right(which means you're off base), but failed to explain that adding a wall of woofers in the car will change the resonant frequency of the cabin. Even opening a window will modify the car's peak.

The theory is sound, but still requires a lot of tweaking to find the sweet spot.

BradimusMaximus

By tuning it to the cars resonant frequency hey want to use the cars natural resonance like free power and efficiency. There is already a response hump there so is more efficient to amplify that hump if you just want it to get loud. The fact that this hump is also usually a higher resonance has its advantages as well. A woofer needs much less mechanical excursion to produce a 67hz tone compared to a 40hz tone. This means they can use more power without the woofer mechanically failing. They will then put as much power possible to their woofers with a 67hz test tone. If they keep the sub within its mechanical limits the main danger they face is cooking voice coils with to much power...but they are probably fine with that.
It is true however that every thing to change in the car, adding a sub box, opening windows, sealing holes, all change the resonant frequency.

Jstas

If you tune the enclosure to the port resonance, you get "free SPL". That is the whole idea behind a vented enclosure. However, if you have a port resonance of 35 Hz and your car's resonance is 47 Hz, if you tune the enclosure to the 47 Hz, you are cutting your port resonance response because you are no where near your peak. So if you want to have maximum SPL and your enclosure peaks at 35 Hz, why should you sacrifice performance of the enclosure and tune to the resonance of the car?



If you add mass to the car and stop vibrations, you change the resonance of the car. It makes no logical sense to forgo the enclosure efficiency in order to obtain the added pressure at the car's resonance while sacrificing enclosure efficiency. If I take a sub and enclosure that performs at it's peak efficiency at 35 Hz and tune it to 47 Hz to match the car's resonance, I would need to pump much more power into the sub in the 47 Hz enclosure to match the output of the sub in the 35 Hz enclosure.

So while I need less excursion at a 47 Hz tone, I need twice as much power to make up the -dB drop being so far up the response curve.

So if I can change the resonance of the car, why tune to the resonance of the car when my constricting factor is the resonance of the sub? Tune the enclosure of the sub to the sub and then change the resonance of the car to match the sub enclosure. You may need more excursion but if you built your box correctly, the air cushion behind it will be a bigger boon to performance than the lower excursion levels. If the excursion concept is is even true. Excursion affects SPL, not necessarily frequency response. You are still going to need to move the cone to get the SPL you need at whatever frequency you play at. I don't doubt that lower tones will move the sub but I don't think it has as big of an effect as it is being made out to be. You also have to remember that lower frequencies have a longer wavelength so the sub will seem to move more to the naked eye because you can see the movement better. Just because it seems like it is moving farther at a lower frequency doesn't mean that it is.

Besides, if my sub is operating at it's performance hump, I will be getting the most pressure for my given power because the sub will be operating at it's most efficient. It's just like a drag race car. The fastest cars don't pick up the front wheels because they aren't wasting the energy picking the front of the car up, they are using it to move it down the track. The chassis is setup to put the power down as best as possible and that makes the car more efficient.

No matter what frequency you tune for, you have to put power through it. If your sub is not strong enough to handle the extra power needed to go louder at the higher frequency, you are going to destroy the whole setup. So why cripple yourself but moving your tuned frequency farther away from your sub's peak efficiency for power handling and performance?

Jstas

ShinAce wrote: »

If you know anything about building enclosures, you realize that a home subwoofer is tuned to a 'flat alignment'. You tune the box to modify the woofer's response into the flattest result.

In a car, this doesn't apply. In a car, you tune the box to whatever frequency seems reasonable. By going higher, you'll get a peak at the port resonance(look, free SPL). If you can get that peak to match the peak of the cabin, more free SPL. Your friends are right(which means you're off base), but failed to explain that adding a wall of woofers in the car will change the resonant frequency of the cabin. Even opening a window will modify the car's peak.

The theory is sound, but still requires a lot of tweaking to find the sweet spot.

I think you make unfair assumptions here.

Number one, home subs are tuned to a flat response because they are meant for sound reproduction. How many people do you know drag their house to a sound-off competition to see how loud their HT sub will boom? I know of nobody.

Car subs are also tuned to a flat response if you are looking at sound reproduction. They are no different. Flat responses give you the widest usable frequency range to help your sub do the job of reinforcing the lower end of the spectrum. The hostile environment and low-frequency white noise from road noise, engines and other stuff is what necessitates the higher power levels usually experienced in a car.

When you are looking to compete for sound pressure levels, you tune for the highest response and that becomes your test tone that you run at. Tuning for SPL is the exact opposite of tuning for music reproduction and reproduction accuracy.

Not every car is tuned for maximum loudness. My truck's sub has a response from about 80 Hz where the crossover cuts off the high end down to about 22Hz where the sub's response trails off and quickly approaches a -6 dB drop. My enclosure is sealed and not tuned for loud at all but accuracy.



Lastly, since everyone so far seems to think so, I do know how a car's acoustic response can change given things like walls and open windows and such. I thought that was evident from my previous posts. Makes me wonder if anyone actually read my posts or just read one or two sentences and gave me a knee-jerk response.

RuSsMaN

I didn't give you a knee jerk response, I was trying to participate in your thread, and I thought I relayed some info on the topic as it relates to home audio, and tech Polk has been using for years.

F all that, you are obviously off your meds today, and just looking to babble or fight.

exalted512

The resonant frequency of a sub does not mean that that is the frequency is capable of playing the loudest at.

Take the Adire Brahma 15"...just picking this because its already in my WINisd program and I'm familiar with it. FS=26Hz.

Tuning a 15ft^3 box with 3 drivers at 26 Hz, there is a 2.75dB peak at 34.45 Hz. Same box, same driver, in a 70 Hz tuned box, there is a 14.8dB increase at 72.7 Hz.

Tuning at the resonant frequency will give you a much, much flatter frequency response than tuning higher, which is why for SQ you tune your box low, to get as flat a frequency as possible.

Cabin gain is a very powerful thing, coupled with the fact that you increase output with a higher tuning frequency, its a win win situation.
-Cody

exalted512

Also, you need to tune your box a little less than the resonant frequency...according to my post above, if your resonant frequency is 72.7 Hz, you need to be tuning at 70Hz to get the most out of it, not 72.7, but I think I read somewhere in this thread you understood that, I was just re-reading your first post and saw you mentioned they were tuning at that frequency.
-Cody

mhardy6647

With enough power, you can blow the car up.

Did you ever see what happened to the Tacoma Narrows suspension bridge, "Galloping Gertie", in Washington state when storm winds hit the bridge's resonant frequency?

http://www.youtube.com/watch?v=P0Fi1VcbpAI

BradimusMaximus

"I don't care what anyone says. This is physics. Numbers with explain everything you see and hear in this hobby." Jstas

So...just out of curiosity...how about those numbers?

BradimusMaximus

"Now a concept I don't think they totally grasped was port flow vs. port velocity and what inertia of the cylinder of air in the port has to do with it. They have gigantic slots which move air by sheer volume alone. They are not understanding how you can get similar air flow with a few smaller ports. The big fat slot has low velocity but large volume. The small, round tube has low volume but velocity that can be as much as 10 times as fast. But when you are going for big SPL, you need to make sure that you aren't moving that air in the tube so fast that it cavitates insides the tube and reduces port efficiency and overall velocity." Jstas

Am I mistaken or did you just answer your own question?
Why do they use large slot ports that can move a large volume of air? Isn't this SPL? Shouldn't large volumes of air be moving? Didn't you just say that many small ports could result in reduced port efficiency? Well I think I have something to support your theory.

"The port area must be commensurate with cone area. If the port velocity gets too high, the port no longer functions as a port, you end up with a leaky sealed box, double bad.

Lets say we have a 12 inch speaker in a box, roughly 100 sqin of surface area. Many programs and manufacturer sites will suggest a 4" diameter port for a 12" vented box. A 4" diameter port has about 12 sqin of area. This is about an 8-1 ratio of cone area to port area. If the 12 inch cone moves 0.25", the port must move 2.0". It can handle this, but when the cone is moving 1.0", the port must move 8 inches! Now you've got a leaky sealed box. "
from Digital Designs
As for the higher tunning frequency. You I think are thinking you should tune the box for maximum efficiency over a wider range than is necessary for spl. We do not need to play 35hz for spl so we are not going to build a box to suite that purpose. We are not building a box to sound good. What we are trying to do is build a one note wonder that takes advantage of the fact the car likes to resonate at 67 hertz.
You should read this article, I think it will help.
http://www.caraudiomag.com/technical/0203cae_audio_speakers_basics/index.html

Jstas

exalted512 wrote: »

The resonant frequency of a sub does not mean that that is the frequency is capable of playing the loudest at.

Tuning a 15ft^3 box with 3 drivers at 26 Hz, there is a 2.75dB peak at 34.45 Hz. Same box, same driver, in a 70 Hz tuned box, there is a 14.8dB increase at 72.7 Hz.

Cabin gain is a very powerful thing, coupled with the fact that you increase output with a higher tuning frequency, its a win win situation.
-Cody

OK, sorry I may have mis-spoke or implied that the resonant frequency was where you played loudest. The resonant frequency is where most subs seem to be most efficient.

Now I get what you are saying about the 26 Hz vs 70 Hz and how drivers can have a very peaky response at different levels. I can see how that will affect drivers in an enclosure. I also know how cabin gain can affect SPL. Believe me, when a single 12 inch woofer with 300 watts on it in a sealed box can hit around 122-125 dB, that's cabin gain right there. Cause the sub/amp combo is only really good for around 115 dB honestly. But what I'm not seeing is how the car's resonant frequency comes in to play. I know that the resonance inside the cabin can change and how the cabin gets pressurized affects it too.

It comes down to what is preached to every n00b. Sound deadening. If you are going to do it, do it right. I mean, look at your truck. All of that extra mass changes the vehicle's resonant frequency.

Now if the guys I was talking about in the first post go and test the car's resonance with a single 10 inch woofer with the woofer placed near the center of the cabin, how is that going to give them an accurate resonance of the cabin? Especially if the box they are going to throw in there is a 6 cubic foot beast with two 12's in it? Also, if the cabin is tested before any sound deadening is done, how is the resonance they test accurate? If this is how everybody tests for resonance of the car, how is it accurate? How can they tune to the resonance if the resonance changes just by virtue of the amount of air space the box takes up? Do you see what I'm asking?

I understand cabin gain, I understand peaky response. But when two guys have similar setups where one has a slightly smaller box in a hatch compared to the 6 cubic foot beat in a sedan. One averages 155 dB and the other averages 144 dB and they tell me that they can't understand why one dude hits higher than the other and they ask me. My immediate response was the the 6 cubic foot box was too big for the sedan, the enclosure was fighting itself in the cabin and they have it tuned for the wrong frequency. They asked me for my opinion and then told me I was wrong. Then they ask how I came up with my idea and that started a conversation several hours long.

I'm trying to figure out where they are coming from and I was baffled by some of their responses and wanted to see what other people thought. I'm not trying to argue, just trying to make sense.

Jstas

RuSsMaN wrote: »

I didn't give you a knee jerk response, I was trying to participate in your thread, and I thought I relayed some info on the topic as it relates to home audio, and tech Polk has been using for years.

F all that, you are obviously off your meds today, and just looking to babble or fight.

Honestly, I forgot you responded and I wasn't trying to argue with you. I was trying to tell you I agree with what you posted and elaborate more on what they were saying. I've seen the Polk ARC stuff and it makes perfect sense to me. Given that, when you're told everything you know and have been successful with is wrong, it makes one want to find out. Sorry you took it differently. Didn't mean it. You had good info in your posts and I think you're an idiot for deleting it.


BradimusMaximus, I would go into the math but it honestly requires signs and symbols that aren't available on this forum. But on top of that I would just be subject to more hostility from you when I'd try to explain what I was looking at and failed miserably. I am only trying to related was is in my head where things make sense because I can visualize them. Explaining a picture in words to people on an internet forum is difficult at best.

eloplayspolo

just wondering... as far as a small port pushing air at a higher velocity but pushing a lower volume.... and a big port pushing more volume at a lower velocity... wouldnt there be a balance point? and why wouldnt that be the best point?

exalted512

Jstas wrote: »

Now I get what you are saying about the 26 Hz vs 70 Hz and how drivers can have a very peaky response at different levels. I can see how that will affect drivers in an enclosure. I also know how cabin gain can affect SPL. Believe me, when a single 12 inch woofer with 300 watts on it in a sealed box can hit around 122-125 dB, that's cabin gain right there. Cause the sub/amp combo is only really good for around 115 dB honestly. But what I'm not seeing is how the car's resonant frequency comes in to play. I know that the resonance inside the cabin can change and how the cabin gets pressurized affects it too.

It comes down to what is preached to every n00b. Sound deadening. If you are going to do it, do it right. I mean, look at your truck. All of that extra mass changes the vehicle's resonant frequency.

Now if the guys I was talking about in the first post go and test the car's resonance with a single 10 inch woofer with the woofer placed near the center of the cabin, how is that going to give them an accurate resonance of the cabin? Especially if the box they are going to throw in there is a 6 cubic foot beast with two 12's in it? Also, if the cabin is tested before any sound deadening is done, how is the resonance they test accurate? If this is how everybody tests for resonance of the car, how is it accurate? How can they tune to the resonance if the resonance changes just by virtue of the amount of air space the box takes up? Do you see what I'm asking?

I understand cabin gain, I understand peaky response. But when two guys have similar setups where one has a slightly smaller box in a hatch compared to the 6 cubic foot beat in a sedan. One averages 155 dB and the other averages 144 dB and they tell me that they can't understand why one dude hits higher than the other and they ask me. My immediate response was the the 6 cubic foot box was too big for the sedan, the enclosure was fighting itself in the cabin and they have it tuned for the wrong frequency. They asked me for my opinion and then told me I was wrong. Then they ask how I came up with my idea and that started a conversation several hours long.

I'm trying to figure out where they are coming from and I was baffled by some of their responses and wanted to see what other people thought. I'm not trying to argue, just trying to make sense.

You're 100% right. That is by no means a good representation of their car's peak frequency at all. Even the test of putting a 10" in a small box isnt all that accurate.

Also, a hatch is a VERY VERY efficient car. Hence why a lot of lower power classes have nothing but CRXs competing.
-Cody

Deadof_knight

Well if you guys havent noticed all the big spl boxes (for car audio )are built like the pro concert speakers now which are waaaaay loud and built for spl. Sound quality wise they sound loose an boomy but their LOUD! Thats the point of the vented (long slot) Quality wise forget it. I havent heard one yet that sounded good IMHO. I mean they sound allright and can get loud enough to mess with your breathing but .....Quality just isnt there.

Dude since the eighties CRX's have been awesome in audio contests