What can I learn about vehicle cabin acoustics? I know next to nothing. What causes cabin gain is it a harmonic thing? Why does mutual coupling work in Physics?
If you have any explainations on acoustics and waves when using ported/sealed subwoofers let me know as I am interested in the knowledge. The physics aspect.
If you have any explainations on acoustics and waves when using ported/sealed subwoofers let me know as I am interested in the knowledge. The physics aspect.
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Didn't you already have a thread sorta like this?
Cabin Gain is based on the fact that the sound waves can't escape in 360deg the way they want to. Boundary gain. Place the sub next to a wall or in this case a body panel, and the wave is reflected back by the wall instead of moving through it. This makes the SPL higher on the non-wall side than it would have been without the wall.
As the frequency gets lower the wavelengths get longer, so more and more "boundaries" are effectively right next to the enclosure, so gain increases. The lower the frequency the more constrained it is, until it is eventuality contained on all sides, and this is called pressure vessel gain, where there are no modes anymore.
Think of if like a flashlight, and its reflector. The boundaries focus all the "waste" pressure waves in the direction you need them.
Mutual coupling is when the sources creating the waves are close enough together that the waves work together with one another from the "relative origin" point out. The waves are in almost exact phase, so they build on one another and increase the waves strength/amplitude. I said "relative origin" as the drivers do not need to be right next to each other to couple, as it is again frequency based, and you can have coupling of low frequencies even with a good distance between two subs.
Cabin Gain is based on the fact that the sound waves can't escape in 360deg the way they want to. Boundary gain. Place the sub next to a wall or in this case a body panel, and the wave is reflected back by the wall instead of moving through it. This makes the SPL higher on the non-wall side than it would have been without the wall.
As the frequency gets lower the wavelengths get longer, so more and more "boundaries" are effectively right next to the enclosure, so gain increases. The lower the frequency the more constrained it is, until it is eventuality contained on all sides, and this is called pressure vessel gain, where there are no modes anymore.
Think of if like a flashlight, and its reflector. The boundaries focus all the "waste" pressure waves in the direction you need them.
Mutual coupling is when the sources creating the waves are close enough together that the waves work together with one another from the "relative origin" point out. The waves are in almost exact phase, so they build on one another and increase the waves strength/amplitude. I said "relative origin" as the drivers do not need to be right next to each other to couple, as it is again frequency based, and you can have coupling of low frequencies even with a good distance between two subs.
That thread was all cluttered with my babbling, this is more specific and easier i think.
So, two sources of exact same phase radiations will build a stronger one? It just always confused me the extra 3 db, so it is a phenomena that happens with waves together? Could you say that it is like water waves how they come close to eachother and then push away stornger?
See, i know that when you place a subwoofer to the rear wall/vehicle it is supposed to make the waves in phase because low freqs are 360 right? I figgured this was cabin gain?
I am not too smart with modes, standing waves etc even though iv'e been reading about it. I figured standing waves was the cause of cabin gain but im not sure of it all.
Any easy explainations via links etc to waves and the rest like resonation someone could give me would be great..
So, two sources of exact same phase radiations will build a stronger one? It just always confused me the extra 3 db, so it is a phenomena that happens with waves together? Could you say that it is like water waves how they come close to eachother and then push away stornger?
See, i know that when you place a subwoofer to the rear wall/vehicle it is supposed to make the waves in phase because low freqs are 360 right? I figgured this was cabin gain?
I am not too smart with modes, standing waves etc even though iv'e been reading about it. I figured standing waves was the cause of cabin gain but im not sure of it all.
Any easy explainations via links etc to waves and the rest like resonation someone could give me would be great..
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I go to them i just wanted the theory on it because alot of those sites don't like theory. Even here i see that people don't really seem to want to give me some theory explainations. It is not that i need help doing basic installs, it is that i want to know all about acoustics because it is a hobby to learn about what is going on and do the best i can when i do my setups. I just want to know all the possibilities and know what is pysically going on, it interests me to know.
Cabin gain is definitley your friend in car. In general further towards the rear of the vehicle is best. This will avoid some of the out of phase cancellation that occurs with reflexions. Cabin gain can also lower your perceived F3 allowing for a very compact sub ( if you're not going for raw SPL ). It's very rare that I'll design an enclosure tuned lower than 40hz for the car.
Standing waves only have an effect above the lowest frequency that will form a standing wave in the space. The longest dimension of a typical car cabin is about 2.5 metres / 8 feet. The lowest frequency that will form a standing wave is about 70 Hz. Below that, the SPL in the cabin is entirely due to pressurisation - the difference in cabin volume betweeen "cone in" and cone out" position of the subwoofer cone, minus any air leaks.
Strictly speaking, the "cabin gain" due to pressurisation is there at higher frequencies too. It just doesn't show up at higher frequencies because it is proportional to cone movement, and the cone movement decreases with increasing frequency. It also gets swamped by the standing waves and reflections at higher frequencies.
For a "normal" speaker, the cone movement has to increase by 4 times with each halving of the frequency in order to maintain a flat response in open air or a large (compared to the speaker) room. This 4 times air displacement results in a 6 dB per octave increase in the "pressurisation SPL" when the speaker is in a sealed room. In order to maintain flat response in a car (a small sealed room), the frequency response of the speaker has to drop at the same rate: 6 dB per octave. This results in the cone displacement "leveling out" to a constant amount and thus resulting in a constant SPL.
You can see this for yourself with any speaker modelling software that shows you the cone displacement. Modelling a closed box that performs well in a car should show the excursion increasing as the frequency drops, then levelling off.
The design process to "get you in the ballpark" goes something like this:
- Take a sub, just about any sub. Preferably sealed.
- Measure its bass frequency response (below about 100 Hz, or below the lowest "standing wave" frequency of your car) in open air.
- Repeat the measurements with it placed in the location you want to use in the car. If the frequency response is "lumpy", try moving it about to find a better position.
- From the two responses, find the frequency where the response is up by about 6 dB in the car. This is your car's "6 dB cabin gain" frequency.
- In your design software, pick a driver and a closed box that is approximately the size you want to use. Set the power input to the speaker's max or the intended amp's max, whichever is lower.
- You now need to balance 2 factors.
1: The frequency response of the speaker will need to be about 6 db down at the "6dB cabin gain" frequency. Adjust the box volume to achieve this. If you can't get a smooth curve with no peak and with -6dB at the desired frequency, your driver is not suitable. Pick another one.
2: The driver should not exceed xmax at any frequencies. If it does, accept a lower amp power or use a driver with higher xmax.
- Examine the SPL figure. If it's more than you need, you can use a smaller driver (and likely a smaller box and /or amp.) The converse applies if the SPL is too low.
- Build. Measure again - open air vs in car. Repeat as required.
Note that this is just a "first approximation". You may want a non-flat bass response because the ear's sensitivity drops off at lower frequencies and SPLs. You may find that if your mids can do 100 dB SPL, your sub may need to hit 120 dB SPL at, say, 25 Hz to give a perceived "flat" response. It will depend on the levels you listen at and the type of music you listen to.
At least one speaker design program includes the ability to specify a "cabin gain" frequency and plot the resultant theoretical in-car response.
It's this Excel spreadsheet:
http://www.diysubwoofers.org/prt/ported.zip
The name implies that it is for ported enclosures, but you can specify sealed enclosures as well (set "fb" to 0).
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I think that there is a pretty good reason that even the specialty forums don't like to discuss cabin acoustics. Because it's pretty pointless to analyze it in cars. Even with finite element analysis, multidimensional modeling and high powered software it is impossible to nail down theoretically and model the inside of a car. Every car is different, and when you add engine noise, ignition noise, road noise, the noise of other cars, whether the windows are open or closed, whether the vehicle has cloth or leather seats, how many passengers are in the car....
you can see how it can get out of hand pretty fast.
The best audio shops "tune in" your car with a mixture of educated guesswork, experience, knowledge of product, a decent spectrum analyzer, skid-loads of dynamat and a good quality graphic equalizer in the system.
It is one thing to calculate the proper dimensions for different types of subwoofer enclosures whereby equations exist for sealed, ported, bandpass and isobaric enclosures -the woofer driver manufacturers will all disclose this information so that you can achieve the best "pound" for you money ("bang" for your buck?)
I've tackled my own problematic cars in the following manner:
- get the best source components you can afford (or these days the best high level to line level transformers you can get since it's so difficult to remove factory head units)
- install high quality electronic crossovers and a good graphic equalizer - preferably one with a parametric eq for the low frequencies or a one third octave eq.
- buy the best drivers you can find like MB Quart, Focal, Image Dynamics
- carefully wire everything to avoid ground loops
- use the biggest most powerful amplifiers in class AB with MosFET or bipolar outputs.
Don Hill's advice is excellent, but it is extremely difficult to make box volume adjustments unless you're not worried about cargo space. More practical is to use a small sealed enclosure and correct the acoustic problems with equalization. This is where power is your friend. And a good quality single 10" subwoofer in a 0.75 cuft sealed enclosure can put out some pretty awesome bass with the right amplification.
In a challenging acoustic like the inside of a car high power is your closest ally and you best tool for overcoming problems.
The graphic equalizer is an absolute must to tame standing waves, complimentary wave addition and to bring up the suckouts in the spectrum. Borrow a spectrum analyzer or go to a car audio shop for this.
It is futile to worry about phase shift, and small amounts of harmonic distortion in a car environment. It's a lot easier to control frequency response, especially with high quality speakers.
you can see how it can get out of hand pretty fast.
The best audio shops "tune in" your car with a mixture of educated guesswork, experience, knowledge of product, a decent spectrum analyzer, skid-loads of dynamat and a good quality graphic equalizer in the system.
It is one thing to calculate the proper dimensions for different types of subwoofer enclosures whereby equations exist for sealed, ported, bandpass and isobaric enclosures -the woofer driver manufacturers will all disclose this information so that you can achieve the best "pound" for you money ("bang" for your buck?)
I've tackled my own problematic cars in the following manner:
- get the best source components you can afford (or these days the best high level to line level transformers you can get since it's so difficult to remove factory head units)
- install high quality electronic crossovers and a good graphic equalizer - preferably one with a parametric eq for the low frequencies or a one third octave eq.
- buy the best drivers you can find like MB Quart, Focal, Image Dynamics
- carefully wire everything to avoid ground loops
- use the biggest most powerful amplifiers in class AB with MosFET or bipolar outputs.
Don Hill's advice is excellent, but it is extremely difficult to make box volume adjustments unless you're not worried about cargo space. More practical is to use a small sealed enclosure and correct the acoustic problems with equalization. This is where power is your friend. And a good quality single 10" subwoofer in a 0.75 cuft sealed enclosure can put out some pretty awesome bass with the right amplification.
In a challenging acoustic like the inside of a car high power is your closest ally and you best tool for overcoming problems.
The graphic equalizer is an absolute must to tame standing waves, complimentary wave addition and to bring up the suckouts in the spectrum. Borrow a spectrum analyzer or go to a car audio shop for this.
It is futile to worry about phase shift, and small amounts of harmonic distortion in a car environment. It's a lot easier to control frequency response, especially with high quality speakers.
What do you consider a theory explanation?
Cabin Gain is a mobile audio term that combines Boundary/Spacial Loading, and Pressure Vessel Loading. All you have to do is look these things up. There are no simple wave table or spring demos that I know of for these pressure wave phenomenon. They are in Toole's book, IIRC.
No. A standing wave is a standing wave. They act the same way in a car as they do anywhere else. The smaller the area the higher the Standing wave cutoff point. Below that Pressure Vessel mechanics are the driving force.
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Thanks for your explainations. So standing waves form at higher frequencies in a room? I guess i had it backwards, i thought it was the opposite.
Is that book you are talkign about "sound reproduction" by floyd toole? I will give it a read.
One of my faviorite things in audio is not just hearing it, but knowing how it works! That's why i like all of this information and appreciate it. Ofcoarse i could just go by manufacturers reccomendations etc, but i want to know how it works too.
EDIT: I just found this which explains some. So can you tell me about the whole space thing? Half space, 4pi space etc, how that works?
Is that book you are talkign about "sound reproduction" by floyd toole? I will give it a read.
One of my faviorite things in audio is not just hearing it, but knowing how it works! That's why i like all of this information and appreciate it. Ofcoarse i could just go by manufacturers reccomendations etc, but i want to know how it works too.
EDIT: I just found this which explains some. So can you tell me about the whole space thing? Half space, 4pi space etc, how that works?
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No, standing waves are based on the dimensions of the room.
In a car the dimensions are small, and thus the wavelengths that are effected are based on these dimensions. As the distances get smaller the frequencies that are effected are pushed higher up.
A room will have larger dimensions, and thus the frequencies where Standing Waves will form are lower, as wavelength get longer as the frequency decreases.
Let's say you have a room that is 20Ft wide (let's stay in 2d for simplicities sake.) You go (Speed of Sound/20ft)/2=28.25Hz this is the First Harmonic mode for the room along this axis, and the frequency of the first standing wave.
Now let's say you have a car with a long internal length of 10Ft. (Speed of Sound/10ft)/2=56.5Hz, and this is the First Harmonic mode for the car.
As you can now see, as the space decreases the standing waves are shifted up in frequency. In reality you get get standing waves from every wall up, down, side to side, and even at diagonals. Standing Waves are bad, unless you are trying for an SPL record. In which case you tune everything to hit a standing wave for the extra SPL, but that is another topic.
This is a decent page that will do the calculations for you in 3d. Room Mode / Standing Wave Calculator
Yes, that is the book. There is a lot of good info in there.
In a car the dimensions are small, and thus the wavelengths that are effected are based on these dimensions. As the distances get smaller the frequencies that are effected are pushed higher up.
A room will have larger dimensions, and thus the frequencies where Standing Waves will form are lower, as wavelength get longer as the frequency decreases.
Let's say you have a room that is 20Ft wide (let's stay in 2d for simplicities sake.) You go (Speed of Sound/20ft)/2=28.25Hz this is the First Harmonic mode for the room along this axis, and the frequency of the first standing wave.
Now let's say you have a car with a long internal length of 10Ft. (Speed of Sound/10ft)/2=56.5Hz, and this is the First Harmonic mode for the car.
As you can now see, as the space decreases the standing waves are shifted up in frequency. In reality you get get standing waves from every wall up, down, side to side, and even at diagonals. Standing Waves are bad, unless you are trying for an SPL record. In which case you tune everything to hit a standing wave for the extra SPL, but that is another topic.
This is a decent page that will do the calculations for you in 3d. Room Mode / Standing Wave Calculator
Yes, that is the book. There is a lot of good info in there.
Okay i get that, soho54.
I hear the term loading, boundary loading etc, what does the "loading" term depict?
Also when you said for spl to tune so that it is a standing wave, does this have to do with the quarter wave theory because i have been reading about spl competing and ported boxes.
I know how a ported box works, how it aligns the rear wave with the front and how it changes with frequency. A question i have is, when it is at tunning, say 35hz and the port is virtually emitting all output while the subwoofer dosn't move far, would you say that the rear wave is 3/4 the subwoofers front wave(quarter wave)?
I hear the term loading, boundary loading etc, what does the "loading" term depict?
Also when you said for spl to tune so that it is a standing wave, does this have to do with the quarter wave theory because i have been reading about spl competing and ported boxes.
I know how a ported box works, how it aligns the rear wave with the front and how it changes with frequency. A question i have is, when it is at tunning, say 35hz and the port is virtually emitting all output while the subwoofer dosn't move far, would you say that the rear wave is 3/4 the subwoofers front wave(quarter wave)?
This is boundary/spacial loading.
Let's stick to bass frequencies for now.
If you take any sub, and play say a 30Hz wave, the sound doesn't just shoot out forward from the front of the driver. It oozes around the sides, and back trying to bleed the pressure off as fast as possible. The end result is a large sphere of sound being emanated from the sub. You can test this by pulling a sealed sub out into the middle of your room, or better yet outside. Turn it on and walk around it, and you can see that the sound doesn't change.
Whenever you add a wall of sufficient size anywhere around the sub it will prevent the sound from continuing to move in that direction. In the experiment above if you did it outside you would have been in half space (2PI) as the ground was acting like a wall preventing the sound from moving downward, and reflecting the sound back up. This means all the SPL that would have been going down is now going everywhere else, and is making it a little louder.
If you took the same setup, and moved it next to a building you now have the ground wall energy reflecting back, as well as the energy the building in now reflecting. This would be quarter space(1PI.)
The more you focus the sound, the more gain you will get at a set distance. A car is a bit over eighth space, and can approach infinite horn loading like gains in some cases.
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I think my last post covers this.
It is similar. The main difference is that a standing wave starts at half wave length, and then hits every harmonic on up, odd and even.
Not exactly sure what you are asking here. Are you referring to the power/SPL output split maybe?
I see. Alright i remember reading an older thread here that was talking about mutual coupling and a guy believed it is caused by 2 subwoofers each radiating in half space, so does that mean that the radiation of each driver causes the other to radiate in half space since the waves are the same?
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No. Note sure where that guy was going. It is just easier to spread the pressure load across two motors than it is with one.
That is the simple way of understanding it. If you were doing something by yourself it is always easier with someone else right? You can both put out 100% effort, and do things quicker, or you can both slack off to 50% and still pull your original solo 100% pace.
The only key here is that you have to have the speaker within a quarter wavelength of each other, so that the phase between the two is close enough for their outputs to gel properly. It like when working with someone else, and their timing is off. You have to be in-sync to pull it off correctly, or you are just fighting each other.
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