posted
Does any one belive in the 1/4 wave lenth theory anymore? 1/4 wave, is where the mic on the windshield is reciving a tone exactly 90 degrees out of phase. So if your using 75hz it would be at 90 degerees at 45". Has anyone tried this? Does this work?
If so, how about in nebraska where we are a bit above see level. How does the speed of sound change by elevation to compensate for this?
posted
I don’t believe it. The volume of the car is very important though. It helps load the subs. The most important thing you can do in a car is prevent cancellation by not allowing multiple paths to the dash. Example, don’t let the wave travel 50" to the dash, and 25" behind the subs then bounce off whatever and go 25" back to the sub, then 50" to the dash. Whatever freq has half a wavelength equal to 50" will be canceled out. The freqs around that will be partially cancelled as well until you get far enough away from that freq.
------------------ 2-DD 9512's 2-2000X's (for now) 8.0ft^3 box in a civic trunk One big ass port ?????dB's
It's not the size of your woofer, It's the size of the waves your woofer makes!
Posts: 1259 | From: Fullerton. CA ,USA | Registered: Oct 1999
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posted
I'm not following you here. Mulitpul paths?
Like two subs wounted on different planes? Or that are being reflected? I am using a the drivers in a wall, so the most direct path would be straight forward, to the mic. There is nothing for the wave to hit but the windshield.
And quarter wavelenth, makes common sense, But it won't work? I am looking for answers, for I think I might be confused.
Are you talking about, like waves from the back of the box reflecting off the rear trunk plate, and then comming forward being exactly 180 degrees out of phase?
Thanks,
quote:Originally posted by mr.dank: I don’t believe it. The volume of the car is very important though. It helps load the subs. The most important thing you can do in a car is prevent cancellation by not allowing multiple paths to the dash. Example, don’t let the wave travel 50" to the dash, and 25" behind the subs then bounce off whatever and go 25" back to the sub, then 50" to the dash. Whatever freq has half a wavelength equal to 50" will be canceled out. The freqs around that will be partially cancelled as well until you get far enough away from that freq.
posted
Thank god!!! I went pale by thinking of the possibility of this happening in a vehicle with a sub wall....a full wavelength for 75 hertz is 190 inches according to your calculations. Keep in mind that the distance between your speaker and windshield would never be that far in most vehicles. The second best thing would be to hope for half-wavelength, or 90 inches. Again we usually do not have that so we settle for the third best thing which is 1/4 wave length, or 45 inches in your case. Any of these three would essentially work, but it is best to use the smallest since we take advange of the least ammount of loss by applying the shortest usable cabin space. you see, any other frequencies can be used, but none of them would hit the mic at its full potential of 1/4, 1/2, or full wavelength. You are also taking advantage of second and third harmonics of this 75 hertz waveform, which also have the same wavelenth (mmmmm then again...that is more of an educated guess/ recollection LOL).
quote:Originally posted by skipper: you see, any other frequencies can be used, but none of them would hit the mic at its full potential of 1/4, 1/2, or full wavelength. Skipper
????????????????? 1/4, 1/2, 3/4, or any fraction of the phase hits the mic! Phase moves (travles in air) and every part of the wavelength hits everywhere you hear sound, no matter what.
I thnk that not everyone understands what the phase means.
A speaker causes the air pressure in your car to change 60 times a second at 60Hz. so in one second there are 60 oscilloations. For one oscillation, the air pressure starts at normal 14psi. at 1/4 of the way through the oscillation its compressed to the maximum say 14.1psi, them at 1/2 way through the oscillation its back down to 14psi again. At 3/4 of the way through the oscillation its at its minimum air pressure (maximum decompression) say 13.9psi. Then at the end of the oscillation its back to 14psi again. as far as wavelength goes its like this, frequencies. sound travles at what 850ft/s (or is it 1150ft/s?) anyway, in one second at 60 hz there will be 60 wavelengths in the 850ft, If you could stop time, and walk that distance away from the speaker You would pass a max, then normal pressure, then min, then mormal pressure, then max then mormal pressure, then min, and on and on untill you walked through 60 max,120 normals, and 60mins. ONLY IF YOU COULD STOP TIME!
Its a little trickier then that because air movement travles at the speed of sound, while phase travles at the phase velocity which is where the term group delay comes from. So the pressure can actually lag behind the flow movement. Here is a link that kinda explains this more. http://www.phys.unsw.edu.au/music/z.html
Hope this helps
------------------ 2-DD 9512's 2-2000X's (for now) 8.0ft^3 box in a civic trunk One big ass port ?????dB's
It's not the size of your woofer, It's the size of the waves your woofer makes!
[This message has been edited by mr.dank (edited 03-27-2001).]
Posts: 1259 | From: Fullerton. CA ,USA | Registered: Oct 1999
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posted
Its not absolute but most cars Ive seen have notes at least CLOSE to a multiple of the 1/4 wave. But all cars are different and it will only get you in the ballpark. Lots of testing is what will get you home.
While a sound wave propogates an undulating form, the intensity of this form will be relatively constant through a complete cycle. By the reasoning proposed by the 1/4 wave folks, you should get NO SPL if you play a tone that winds up with the 1/2 or full wavestriking the mic. Obviously, this isn't so. Personally, I believe that the whole 1/4 wave thing came about from someone 'discovering' that a human cannot hear a sound if there is not the space for a 1/4 wave to develop (excluding near field stuff) and incorrectly transferring that theory somehow to SPL...
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quote:Originally posted by Dukk: I agree with Mr. Dank.
While a sound wave propogates an undulating form, the intensity of this form will be relatively constant through a complete cycle. By the reasoning proposed by the 1/4 wave folks, you should get NO SPL if you play a tone that winds up with the 1/2 or full wavestriking the mic. Obviously, this isn't so. Personally, I believe that the whole 1/4 wave thing came about from someone 'discovering' that a human cannot hear a sound if there is not the space for a 1/4 wave to develop (excluding near field stuff) and incorrectly transferring that theory somehow to SPL...
Well actually mine came from those darn physics experiments..
posted
[QUOTE]Originally posted by Dukk: [B]I agree with Mr. Dank. By the reasoning proposed by the 1/4 wave folks, you should get NO SPL if you play a tone that winds up with the 1/2 or full wavestriking the mic. Obviously, this isn't so.
Now that makes sense. I am no longer confused, that was a simple. Thanks guy/als. In concept it sounds right, but with the offen ovelooked common sense thrown in there you get the answer....very good indeed.
posted
I stand very corrected about the 1/2 or full wave being useful. If I can have my head back after being bitten off, I will research comments before I use a 6 year old memory from some comversation LOL. I do agree with Mr. Dank. But I am more captivated by The Buzz and his simple explanation. I have been told that this is the ball park and to do lots of testing for fine tuning. RTA, audio generator, frequency counter, different enclosures, reference speakers, blah, blah...
posted
Yeah, like how I can measure the distance for the 1/4 wave and come out with 55 Hz, yet 45 Hz is the tested peak, and 59 Hz is the absolute worst possible. This testing was all done with a sealed box, and then dividing the metered number by the out of car response. I still think something is screwy.
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posted
Then why when I play my half wave note (65Hz) in my car is it really loud and the whole car vibrates?
Colin
------------------ Warning SPL in my car exceeds government health guidelines, civilians are advised to proceed with caution. Failiure to comply may result in serious injury or death.
posted
Buzz, Are you maybe thinking of the 1/4 wave resonance experiments in physics? That’s different. Those are done usually in a perfect cylinder sealed at one end.
Db Don, I know you know more than you say, So show us some love and elaborate on the comment you made
------------------ 2-DD 9512's 2-2000X's (for now) 8.0ft^3 box in a civic trunk One big ass port ?????dB's
It's not the size of your woofer, It's the size of the waves your woofer makes!
Posts: 1259 | From: Fullerton. CA ,USA | Registered: Oct 1999
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posted
Lets take a string and say it represents the cars acoustical interior, and take the end of the string in one hand (representing the driver), the other end would represent (ideally) the microphone.
Now, which frequency causes, due to the length of the string, the biggest sympathetic motion on the measurement end of the string? Hint : I doubt its the one with the length of the string being the 1/4 wavelength!
ShadowStar
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Posts: 2578 | From: Somewhere In the Northeast | Registered: May 1999
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posted
The 1/4 wave in my car is the loudest, or at least close to it. No matter where my box was tuned last year, from 35-60 hertz, 40-43 hertz was always the loudest. At 70 -80 hertz there was a 12+ db drop in sub output.
Is the 1/4 based on different cars? I have a wagon and it is around 9-10 feet from the subs to the mic.
posted
OK, just to get the brain going....You can measure a pressure wave at any point in its cycle for a fraction of a second and not get a reading, now if you left it measure long enough to get the 3/4 wave you would have enough for a good reading. I am not saying that any math will give you an exact answer, it will give you a ball park place to start. You have to take into acount the velocity of air and pressure wave(non complete wave of sound) completion to maximize spl. For example: An extreme vehicle will benifit from a small cab and a larger enclosure,why? I found that allowing a pressure wave to semi form it increases velocity as it alternates from the box to enclosure(hense the large port on drivers side). There are two systems to spl cab designs(sorry for the non use of convention terms) they are Absolute pressure systems which are sealed cab designs that use brute power to compress the air inside as fast and as much as possible. Then the one that maximizes efficiency by introducing velocity/pressure exchange of air through a cavity that have a specific resonance/resistance that couples the output of both front and rear of the subwoofer. Now heres the sweet part, I am almost finished the reasearch but I found that the pressure can be followed in a specific path through the port and peaking out in a patern that resembles a waveform. It is so hard to measure this in an absolute pressure system cause the velocity is not there. The diagram can be seen with the results in the link below. The pattern will reverse if the port changed to the other side. Its hard to beleive but the mic does not lie. http://www3.sk.sympatico.ca/hebig1/port_placement.jpg
posted
after you click the link, delete the junk at the end on your adress bar.
Interesting picture. Its good to see your doing your research in a constructive manner. Im still tripping on the area near the port being the loudest. I would have bet money the corners would be loudest before I saw the picture. If this is in a car, would the top corner not be a 90 degree angle then, and be less? Your note says 45 deg but shows a 90 degree angle.
Have you tried other speaker port placement combos? I wish I had a mic. THANKS Don
------------------ 2-DD 9512's 2-2000X's (for now) 8.0ft^3 box in a civic trunk One big ass port ?????dB's
It's not the size of your woofer, It's the size of the waves your woofer makes!
Posts: 1259 | From: Fullerton. CA ,USA | Registered: Oct 1999
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posted
Db Don, I thought about how the port came out the loudest. Have you considered what effect the velocity of air moving across the mic has on its reading? At work we design ventilators. When looking at pressure changes you must look at the pressure when or where you will get no air movement across the sensor. If there is air moving across it, the sensor will see a pressure drop for the same reason an airplane can fly. Its much more complicated than that for your application though. The pressure change is caused by moving air, but because of the properties of air (impedance) they are not in phase. I think the more I think about this stuff the more questions I have and the more confused I get. I need to find a book with all the answers in it.
------------------ 2-DD 9512's 2-2000X's (for now) 8.0ft^3 box in a civic trunk One big ass port ?????dB's
It's not the size of your woofer, It's the size of the waves your woofer makes!
Posts: 1259 | From: Fullerton. CA ,USA | Registered: Oct 1999
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