npdang
03-14-2005, 08:43 PM
This is a short, simple tutorial on how to interpret the results of your typical subwoofer box modelling software such as Winisd.
Here's our good old trusty Ascendant Audio Atlas 12" modelled up using the manufacturer supplied t/s parameters and Winisd. Notice the shape of the rolloff, and an F3 point of about 52hz. Not bad.
http://diymobileaudio.com/pics/winisd.jpg
Now take a look at the actual, measured frequency response (semi-anechoic) at 1 watt, 20 watts, and 75 watts of power.
http://diymobileaudio.com/pics/atlasbox.jpg
First off, you can see that while Winisd was fairly accurate, there was still some differences between the actual response and the calculated response. Note that the actual -3db point is around 56hz vs. predicted Winisd 52hz, and the actual -6db point is ~48hz vs. Winisd's prediction of 40hz.
This is due to variance in t/s parameters between the actual sub, and the manufacturer's specifications. We can also see that the frequency response changes with different power outputs. At 400 watts of power I'm sure the response looks quite different, but unfortunately my amp would only do 75 watts before clipping :)
Also, it's pretty evident that Winisd does not take into account the upper end response of the driver.
Now let's look at the same sub in the trunk of a Honda Accord (Gold line).
http://diymobileaudio.com/pics/example10.jpg
Now that looks nothing at all like what we modelled :) So you can see that the car's interior itself and placement has the greatest effect on the frequency response.
So to sum up, what do box modelling programs like Winisd tell you?
They can tell you:
1. The low end frequency response of the box+sub "ONLY", based on the manufacturer's supplied t/s parameters. The actual t/s parameters can vary by quite a bit, and will change the low end frequency response. For a ported enclosure, I wouldn't even think of using the manufacturer supplied t/s parameters, but rather I would measure the actual t/s specs myself. Reliable manufacturers are generally around 10-15% deviation from what I've noticed, while I've seen some t/s parameters that were off by at least 50%.
2. Frequency response changes with power output. The more linear your subwoofer's motor, the less shift you will see as power increases.
3. Frequency response changes DRAMATICALLY with the listening room/car, and placement.
In conclusion, Winisd can be a powerful tool for comparing different driver's low end response in a variety of enclosures against each other, but the actual frequency response in-car is going to depend on quite a few more factors that Winisd(and similar programs) cannot account for.
One last thing that I forgot to mention. If you subtract the in car response from the sub+box response, you can find the transfer function of your vehicle. Applying this transfer function to Winisd's calculated response can give you a much better estimate of how a sub wil sound in your car.
Here's our good old trusty Ascendant Audio Atlas 12" modelled up using the manufacturer supplied t/s parameters and Winisd. Notice the shape of the rolloff, and an F3 point of about 52hz. Not bad.
http://diymobileaudio.com/pics/winisd.jpg
Now take a look at the actual, measured frequency response (semi-anechoic) at 1 watt, 20 watts, and 75 watts of power.
http://diymobileaudio.com/pics/atlasbox.jpg
First off, you can see that while Winisd was fairly accurate, there was still some differences between the actual response and the calculated response. Note that the actual -3db point is around 56hz vs. predicted Winisd 52hz, and the actual -6db point is ~48hz vs. Winisd's prediction of 40hz.
This is due to variance in t/s parameters between the actual sub, and the manufacturer's specifications. We can also see that the frequency response changes with different power outputs. At 400 watts of power I'm sure the response looks quite different, but unfortunately my amp would only do 75 watts before clipping :)
Also, it's pretty evident that Winisd does not take into account the upper end response of the driver.
Now let's look at the same sub in the trunk of a Honda Accord (Gold line).
http://diymobileaudio.com/pics/example10.jpg
Now that looks nothing at all like what we modelled :) So you can see that the car's interior itself and placement has the greatest effect on the frequency response.
So to sum up, what do box modelling programs like Winisd tell you?
They can tell you:
1. The low end frequency response of the box+sub "ONLY", based on the manufacturer's supplied t/s parameters. The actual t/s parameters can vary by quite a bit, and will change the low end frequency response. For a ported enclosure, I wouldn't even think of using the manufacturer supplied t/s parameters, but rather I would measure the actual t/s specs myself. Reliable manufacturers are generally around 10-15% deviation from what I've noticed, while I've seen some t/s parameters that were off by at least 50%.
2. Frequency response changes with power output. The more linear your subwoofer's motor, the less shift you will see as power increases.
3. Frequency response changes DRAMATICALLY with the listening room/car, and placement.
In conclusion, Winisd can be a powerful tool for comparing different driver's low end response in a variety of enclosures against each other, but the actual frequency response in-car is going to depend on quite a few more factors that Winisd(and similar programs) cannot account for.
One last thing that I forgot to mention. If you subtract the in car response from the sub+box response, you can find the transfer function of your vehicle. Applying this transfer function to Winisd's calculated response can give you a much better estimate of how a sub wil sound in your car.