***** this is a work in progress and will be updated as I go *****
The goal of this thread is to document my all-out attempt to build a transmission line subwoofer, one of the most difficult yet rewarding enclosures. It's worth mentioning
a thread started by aznboi3644 and the replies really intrigued me. I hope those interested in a TL box can learn from my experience. There is very limited knowledge on how to properly build one, this is no simple sealed box we're talking about! Nevertheless, here we go.
Design goals:
- frequency response from 20hz to 60hz at +-3db
- response must blend seamlessly into component speakers up front, including cabin gain
- high efficiency woofer and enclosure to minimize amp power
- class D amplifier for high efficiency, low heat, small size, and small power cables
- installation must be clean, and enclosure must be production quality
Steps taken:
- measure cabin gain of my vehicle and graph it
- design hypothetical subwoofer to compliment measured cabin gain
- select high efficiency woofer with low Fs to help achieve goal of 20hz and low power requirements
- select amplifier to compliment the woofer in power rating and crossover frequencies
- research to find useful information about building the transmission line (won't be easy!)
- design transmission line with cabin gain, woofer properties, and physical limitations in mind
- build enclosure striving for production quality
- install amp and enclosure, again striving for top quality
- last, measure frequency response, and celebrate!
---
Step the first: measure cabin gain of my vehicle
A subwoofer sitting on the shelf in a store will sound different when installed in your car, and it will sound completely different if sitting out in the open or in an anechoic chamber. Ever notice how subwoofers in an electronics store are usually contained in a small room you have to walk into in order to listen to them (hint hint)? The interior shape and size of your car, the materials used to build the car, and any stuff you have inside it will affect the frequency response of the subwoofer. Each car is unique in this respect, and so is every dining room for home theater installations (my other hobby). This effect is called cabin gain.
To measure how my car will affect the response of my transmission line, I needed to run a few tests. First, I placed my home theater sub (a
Bowers & Wilkins ASW600) on a chair in my backyard, careful to make sure it was not near any large walls or objects that might alter the response. Then I played test tones and measured the response of the sub out in the open, which is as close as I can get to an anechoic chamber which is what all enclosure simulation and calculating software assumes. Then I repeated the test placing my ASW600 sub in my car facing rearward like I plan the transmission line to.
I used a Belar spectrum analyzer connected to the best microphone I could find for this test. It is a Sennhieser 421 which has a frequency response rated down to 30hz, the lowest I can get my hands on. This is important to remember, any values below 30hz must be accepted with a grain of salt, they are not as accurate as the higher frequencies. There is a microphone preamplifier between the Sennhieser and the Belar, which should be transparent to these tests. The mic was placed one foot in front of the woofer in my backyard, and was sitting on a small mic stand in my front seat when the sub was in my car. After many attempts both inside the car and out, waiting for the wind to die down and averaging the data over many minutes, I came up with the following two graphs.
And here's the result I was looking for. This is simply the difference of the two lines in the previous graph, and it shows how my vehicle affects the frequency response of a sub.
So, what does this all mean? Looking at the first graph first, the sub placed in my backyard has a fairly gentle roll off from about 55hz and lower. When placed inside my car the response peaks sharply at about 55hz and rolls off in both directions. The data I want from this test is in the second graph. Other than a peak at 28hz, there is a rise of 8db from 20hz to 60hz. The slope from 60hz and up can be disregarded because I will have a low-pass filter on the amplifier anyhow. Good to know! Now I can keep this in mind going forward, to make sure my transmission line compliments the cabin gain of my car.
Step the second: design hypothetical subwoofer to compliment measured cabin gain