sound deadening, professional project

Start with a microphone. This will pick up the noise floor in the car. Calibrate it to an input on an operational amplifier, invert the output, feed it into an amplifier, drive the speakers always with said amplifier. This is how the Bose Active Noise Cancellation headphones work; just outside your ear canal, on the outside of the headphone cup, is a microphone. It's pickup is inverted, and mixed into the incoming source/music signal, cancelling the outside noise as it comes through the headphone cup. The inversion is done to drive the speaker itself with a 180° out of phase version of whatever the microphone picks up, cancelling or nearly cancelling it. It's called "active" because there is a powered signal used to cancel the environmental noise. That's really about all there is. As you can see, it IS the most complex, but since it uses the electronics already in the car, the weight gain is absolutely minimal. Additionally, properly implemented, it can be exceptionally effective. Four channels will be most effective, one pickup mic at each corner of the car, with a dedicated MUX for each corner to drive the noise floor at that point to a minimum.It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

As I understand it, there are two thoughts your dealing with. There is the resonant freq and the actual noise produced by the vehicle. The spray on and peel and stick stuff dampen some of the vehicle noise, but are intended to change the panels resonant frequency, usually the added weight lowers the frequency. Then there insulation's that absorb sound and change it into heat. Think fiberglass insulation or even styrofoam, vs the butyl stuff. You also have the vehicle's interior surfaces that act as an absorbing surface. Plastics, cloths, seat foams. Then there are also dampners like felt used around the plastic clips that make the pieces fit tighter, but I would also suspect because they fit tighter that it also changes the resonant freq of pieces. I've also head that the stippling (think about a golf ball)under the body/frame creates dead air zones that keep moving air from rubbing against the body and inducing the resonant freq. There is more than one way to skin a cat. It's an interesting project and I'd love to see the results, I still need more to quiet my tires down.

I don't know, the Sound Canceling Tech that they've been sticking in the Acura's and such are good but they make me feel like Im in an airplane. Because it cancels noise via white noise right? You know you can place your thumb over the canceling sensors and it will change the noise being emitted. If MPG wasn't an issue, cement would be the best choice lol. Cheap as hell and very effective. Im interested to see what the end result of this project will be. Could be pretty eye opening if something cool shows up as an alternative, to the butyl based dampeners that is.Studying for MECP basic certification and looking for job at an install bay.

abob89 wrote: Because it cancels noise via white noise right?

Not "white noise" at all. Not "pink noise", either. It's equal and opposite noise produced by the speakers within the car, to cancel the ambient noise.

abob89 wrote: but they make me feel like Im in an airplane.

Probably get this because (even though it's an Acura) the devices chosen have high ambient noise levels themselves. I have noticed this on nearly EVERY cheaper set or implementation of active noise cancellation. I personally believe that it is poor S/N ratio of the low-level devices chosen. Some better than ohers... While I am NOT BY ANY MEANS a Bose fan, I believe they have done a pretty good job on their ANC headphones.

haemphyst wrote: Start with a microphone. This will pick up the noise floor in the car. Calibrate it to an input on an operational amplifier, invert the output, feed it into an amplifier, drive the speakers always with said amplifier. This is how the Bose Active Noise Cancellation headphones work; just outside your ear canal, on the outside of the headphone cup, is a microphone. It's pickup is inverted, and mixed into the incoming source/music signal, cancelling the outside noise as it comes through the headphone cup. The inversion is done to drive the speaker itself with a 180° out of phase version of whatever the microphone picks up, cancelling or nearly cancelling it. It's called "active" because there is a powered signal used to cancel the environmental noise. That's really about all there is. As you can see, it IS the most complex, but since it uses the electronics already in the car, the weight gain is absolutely minimal. Additionally, properly implemented, it can be exceptionally effective.

And as I said, it can be VERY effective, even though it will be the most expensive, and complex to implement correctly.It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

haemphyst wrote: Start with a microphone. This will pick up the noise floor in the car. Calibrate it to an input on an operational amplifier, invert the output, feed it into an amplifier, drive the speakers always with said amplifier. This is how the Bose Active Noise Cancellation headphones work; just outside your ear canal, on the outside of the headphone cup, is a microphone. It's pickup is inverted, and mixed into the incoming source/music signal, cancelling the outside noise as it comes through the headphone cup. The inversion is done to drive the speaker itself with a 180° out of phase version of whatever the microphone picks up, cancelling or nearly cancelling it. It's called "active" because there is a powered signal used to cancel the environmental noise. That's really about all there is. As you can see, it IS the most complex, but since it uses the electronics already in the car, the weight gain is absolutely minimal. Additionally, properly implemented, it can be exceptionally effective. Four channels will be most effective, one pickup mic at each corner of the car, with a dedicated MUX for each corner to drive the noise floor at that point to a minimum.

To begin with, I found this post very helpful and it has a lot of great information on the whole. As far as using all of it, I will need some further clarification. To begin with, what sort of microphone would you recommend implementing? Keep in mind budget and size if possible. Any model numbers and other specifications would be greatly appreciated. As far as the "operational" amplifier, what exactly are you defining as such? And then to the subsequent amplifier, I am confused to what you are referencing with this. As far as inverting the output, that part is a little easier but some clarification would be great, especially outputting WHAT output to WHAT input, and how does this relate to the rest of the speaker system? I know amplifiers sometimes have a "phase" adjust, does this have anything to do with it? I know most of these types of amplifiers are aftermarket amplifiers, but for stock amplifiers in cars I don't think this option is available, but I could be wrong. As far as using a multiplexer in each corner, can you lead me to how I would implement it and in what fashion? Any models you are familiar with that offer great quality with a budget price? I know I am asking a lot, but I guarantee you this project will be vastly improved with everybody's help. I really appreciate all the time all of you are putting into this, as this technology could easily put our project over the top. Thank you, Anthony EE-Michigan Tech

filling the voids with expanding foam helps a lot...great stuff works well

michigan_tech wrote: To begin with, I found this post very helpful and it has a lot of great information on the whole. As far as using all of it, I will need some further clarification.

Thank you, and my apologies, all rolled up into one! My previous post might have been a bit vague. I will try to elaborate... I have the ideas in MY head, but that certainly doesn't put them into YOUR head!

michigan_tech wrote: To begin with, what sort of microphone would you recommend implementing? Keep in mind budget and size if possible. Any model numbers and other specifications would be greatly appreciated.

The microphone you want will ideally have a VERY flat frequency response. The flatter the better, but you certainly would want to be within a 1dB window, full range. 1dB would be the minimum design parameter, as 1dB is generally accepted the minimum difference the ear can hear. Additionally, whatever your response variance at the mic, this will be amplified at each step along the chain... Flatter is better. I know that the microphone I use in all my testing is the Behringer ECM8000, A very flat response mic, it would be ideal, were it not for the shotgun shape. As a proof of concept, however, it would do quite well for you. At only 80 dollars retail, it's a fantastic step for you! Here's the home page: http://www.behringer.com/ECM8000/index.cfm?lang=ENG It is an electret microphone, so a power supply will be required. ±15v is optimal. Balanced also, so electrical noise becomes a "not-issue". Again, for "proof of concept"... Perfect.

michigan_tech wrote: As far as the "operational" amplifier, what exactly are you defining as such?

Sometimes called a buffer, an op-amp uses VERY high impedance inputs, so their input frequency response is very flat. Here are the op-amps I used in my home crossovers: http://www.futurlec.com/Linear/OPA2134PA.shtml Additionally, they offer the option of inverting or non-inverting outputs, with the addition or removal of one ground point. Easy. They are used in buffer stages, as the first gain stage from the mic to the speaker signal chain. :::::::UPDATE/EDIT::::::: While searching for this answer, I came across a new IC from National Semiconductor. This guy would be small enough to attach RIGHT TO the speaker, direct speaker drive, single ended voltage power supply, vanishingly low THD... Worth exploring as an avenue: http://www.national.com/pf/LM/LME49600.html

michigan_tech wrote: And then to the subsequent amplifier, I am confused to what you are referencing with this. As far as inverting the output, that part is a little easier but some clarification would be great, especially outputting WHAT output to WHAT input, and how does this relate to the rest of the speaker system?

Once the low-level is inverted, the amplifier driving the speaker will stay non-inverted. The design and phase of the entire system will have to keep in mind that the ultimate end result will be that the speaker outputs a signal that is 180° out of phase from but of equal amplitude to the cabin noise.

michigan_tech wrote: I know amplifiers sometimes have a "phase" adjust, does this have anything to do with it? I know most of these types of amplifiers are aftermarket amplifiers, but for stock amplifiers in cars I don't think this option is available, but I could be wrong.

See my previous answer. Generally speaking the "phase" adjust is usually 0° or 180°, but this is normally only found on subwoofer amplifiers for better integration with the door or bass (as opposed to sub-bass) speakers. MUCH higher end amplifiers might also have a phase adjust, allowing a complete sweep from 0-180°. If more than that is necessary, one would simply invert the subwoofer's wiring, then re-adjust.

michigan_tech wrote: As far as using a multiplexer in each corner, can you lead me to how I would implement it and in what fashion? Any models you are familiar with that offer great quality with a budget price? I know I am asking a lot, but I guarantee you this project will be vastly improved with everybody's help.

haemphyst wrote: While searching for this answer, I came across a new IC from National Semiconductor. This guy would be small enough to attach RIGHT TO the speaker, direct speaker drive, single ended voltage power supply, vanishingly low THD... Worth exploring as an avenue: http://www.national.com/pf/LM/LME49600.html

michigan_tech wrote: I really appreciate all the time all of you are putting into this, as this technology could easily put our project over the top. Thank you, Anthony EE-Michigan Tech

It's what we do! I also am hoping that something comes of this for you! If you get an "A", be sure to mention our names! <grin>It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."