posted
This is a good method with cheap cost and high precision result. First go to a Radio-Shack store and get a Sound level meter(analog is best). After it, you will need a sealed subwoofer(with only one woofer and if you dont have it take a ported sub and seal the port). You will also nead a stef-ladder and a CD test that contain frequency playable between 40 to 70hz. That could appear strange but all these thing are usefull to find what you want. The first thing to do is to wait at the begining of the night, when outdoor noise are low. Go on a large ground like the middle of the ground back of your house and put your stef-ladder at this place. Take your sealed subwoofer and attache it on the top of the stef-ladder with the woofer perpendicular to the ground. After. put the Sound level meter at one meter of the woofer and mesure and note all the sound level for each frequency between 40 and 70 hz using a cd player connected to an amplifier. That will give you the frequency responce of your sub. The outdoor space and the stef-ladder is use to simulate the no reverberation effect of an anecoic room(like in a recording studio) Keep a level of sound between 90 and 100dB when mesuring thr woofer, to never be influenced by ambient noise like a car passing in your street. After it, take the same measurement but in your car with the speaker placed in te same position that you want to place the sub built for your car.(with the spl meter ajusted in a range that permit you to got a reading) (same condition and WITHOUT THE STEEP-LADDER!!) After you finished this work,You should have spl result for the sub in car and outdoor. Calculate the difference between the spl result for the sub in car and the sub outdoor.(Microsoft EXCEL could help to do this job) That will give you the gain in dB for each frequency that you measured before. Checkout the highest number and his corresponding frequency and that will give you the most resonant freq in your car.
This work take you a little 3 hour but the answer is exact!! We allready got a gain of +27.5dB at 51hz in a VW golf!!
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Posts: 645 | From: Quebec city | Registered: Mar 2000
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posted
Couldn't you do the same thing in the car to find the loudest frequency, just keep the input amplitude constant and watch the Db meter. The loudest freq. should be close to the resonant freq. of box and vehicle. I may be wrong, it just sounds so much simpler.
------------------ Without Inno there can be no Innovation!
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Posts: 490 | From: Fort Frances, Ontario, Canada | Registered: May 1999
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posted
I use this method BEFORE to build the future subwoofer, because my team think that before to build a speaker system it is better to determinate what will be the freq it will be most efficient. Exemple: after finding the most resonant freq in the car, We built a subwoofer as efficient as possible on THIS FREQUENCY. Often, we simulate the desing of the subwoofer on a software like BassboxPro, and we tune our system to have a curve with the highest peak at the resonant freq of the car. THIS IS ONLY USE FOR SPL COMPETITION NOT FOR SOUND Q..!!
NOW, you can use it (the secondpart of the measurement) to find the most resonant freq of your TOTAL system already installed.You dont need to find the curve response of your speaker only.
I dont know if John Henry is using this method, but i personally think that this is a way to win!
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Posts: 645 | From: Quebec city | Registered: Mar 2000
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posted
ok now john henry isnt doing this to his van next the part about a sealed enclosure and outside makes no sense. if u want the resonant frequincy of a woofer look at the paramiter sheet that came with the woofer if your not using a sealed enclosure in the install then it will change when u build the box that u are going to put in the car then u take the spl meter from radio shack and put the volume verylow and go threw the tracks of the disk to find your highest frequincy this doent help to find the volume which u are the loudest at just the frequincy because the mics that are from radio shack only go up to 120 db so the volume isnt very high just enough to move the mic to 2 db when the mic is set at 120 db if u have the anolog meter. as far as finding the loudest frequincy of the woofer itself well that all depends on the box , power , car , so to find what box and tuning to use most use the termpak program i know this helps out alot when i do spl cars peace all
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Posts: 118 | From: spl city | Registered: May 2000
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posted
I think I see what you mean. If you know what the anecholic (no room effect) response curve of a specific driver is in a specific box, then you can measure the response curve of that box in the vehicle. The difference between the two curves will be your vehicles response curve regardless of what sub/enslosure you use. Then you can select a woofer/enclosure that is tuned for the peak in your vehicle's freqency response, or resonant freq. Right?
I think a better way to win is make your vehicle so that it doesn't resonate, therefore you pressurize the cabin and don't have to worry about the vehicle's resonant freq, no? I've been thinking lately about a completely cement cab!
------------------ Without Inno there can be no Innovation!
inno73@hotmail.com
Look for Innovative Electronics, coming soon...............
Posts: 490 | From: Fort Frances, Ontario, Canada | Registered: May 1999
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posted
When you pay attention to the dBdragRacing rules, you will see that it is not possible to pressurise the cabin, the only way possible to increase the sound level pressure is with some woofers...I saw that you perfectly understod the method i use, and this method permit me to win all the 4 last competition... I did 149,7 with only 250 watts RMS total power regardless the speaker impedance at the freq i used. These score have been hit in a oem VW Golf. I want to know why you dont think that the resonant freq of the cabin is'nt the best way to get the highest score?.. Because I think taht you also have a good knowledge in acoustic and i am not sure that i perfetly understod when you talk about pressurising the cabin...
Doctorbass
------------------ Show me the bass !
Posts: 645 | From: Quebec city | Registered: Mar 2000
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posted
The method discussed here has actually been detailed in several car audio magazines and is proven to be effective, even for SQ (in this case you can design the sub enclosue to account for the peak).
However, the enclosure can be placed on/above any flat surface to do the measurements (such as on pedestal in the driveway.
------------------ Scott CMD AUDIO Hello, My name is scott, and I am addicted to Atomic Thunder. Currently: 2 Shocker 15's, ported wall, Ford Tempo (haha)...148.8 in first compeition Owner of CMD Audio Authorized Dealer of THE SHOCKER!
Posts: 626 | From: Rochester, NY | Registered: Jan 2000
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posted
This is what i did to find my freq. I used the radio shack meter turned up the volume a little bit. Placed the meter pointing towards the window in about the same area as a mic would be. Then played through frequencies till i found the highest one. For me 63,64,65 all did about the same. Then i moved the box around until i get highest w/ that. Only problem is my connections came off the woofer again. So i am not done yet w/ testing but will be soon.
------------------ One Explorer One Memphis 500d One 12" Shocker One hundred??db's
posted
i tried the radio shack meter method it only goes up to 125db and at 125db my res freq is around 58-60hz but at 154 it is alot lower because of flexing, so this didnt work for me
posted
Anyone know about how much can be gained from this? Right now I'm using my box tuning frequency of 80 Hz. By measuring, my guess for a 1/4 wave is 50 Hz. Anyone have a guess as to how many dbs can be gained with a properly tuned box?
------------------ Overcoming by out-thinking
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posted
This is the best way to do this. It is what I am going to do for my system. The only problem that I can see(with the Rat Shack meter) is what lowered90 said about the frequency changing due to flexing.. The radio shack meter is still a great start to fine tuning your system though.
posted
My findings show that this only works for low to mid level spl systems. (140+- dB)
Once you get beyond that, you need more output during the tests to come close to the proper frequency. The big problem is we all know how hard it is to get 1 woofer in a sealed box to do 155dB. Most (not all) vehicles have drastically different peaks between 140, 150 and 160 dB.
Again, this is just what I've found trying the methods mentioned above....but with AC & termlab meters (both 170+ capable)
------------------ Jeff Official member of the 1k post club!
posted
first i would like to say this .. radio shack meters are not that bad .
radio shack meters are weighted so the 126 db you see there will be at least 145 db on audio controll or termpro ... dont beleive me ya say measure them at the same time you will get such a difference that you cant even compare ..
however since they are weighted the curves will not be the same as a termpro or a audio controll meter thus it is still best to tune with the meter you are using to compeate with
just my $.02
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posted
I'll try to post the compensation curve for the analog Radio-Shack meter this week. I had compared it with HIGH PRECISION B&K METER in perfect measurement condition and got the real response curve of it between 30 to 80hz with 1 Hz resolution and accuracy of +/-0.5dB. The curve that i calculated on microsoft Excel is regular and is around -7dB at 30Hz to -1,5dB at 80hz. EX: That mean if i read 112dB at 80hz on the B&K precision, i read 110,5 +/-0,5dB on the Radio Shack meter.
MDSPL
I DONT BELIEVE THAT RS METER MEASURE 145dB WHEN YOU READ 126dB ON IT because the electret mic in it is rated 128dB and hit his Xmech at this pressure. I tested it with many high precision B&K equipement and I am sure of the results they have. Wayne use B&K to calibrate his meter and WE KNOW THAT IS THE BEST REFERENCE! I know that many model and manufacture have different result for the same measurement but I know that in the SAME CONDITION IN SAME TIME, are result should have a little diference, but NEVER 19dB of diff !!
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Posts: 645 | From: Quebec city | Registered: Mar 2000
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