Resonance & room modes: why Touring bass sucks & the deck rattles

[josby]

Here's an 1/24th octave RTA measurement of the stock Touring system from 20 to 200 Hz from the driver's headrest. I think that big spike and big dip are why the Touring bass sounds pretty decent on some songs but crappy on others. They're about +10 dB and -10 dB compared to the average level of the rest.



Here's a measurement of just the sub, taken directly above the sub. The spike's still there, but the dip is gone. So the dip isn't coming from the speaker itself. It happens at the driver's headrest because of a room mode. It's caused by the interior dimensions of the car and the location of the sub and seat within it. Not much to be done about that.

Also notice the difference in the bass below 40 Hz - the extra low bass at the headrest is due to cabin gain.



Here are two headrest measurements taken with the trunk open. The reason for opening the trunk is that for the purple one, I ran back before the sweep started and pushed hard on the metal on either side of the sub to see if bracing the rear deck would have an effect. Sure enough, it made the spike at 50 Hz drop by 3 dB.

So, the flimsy metal deck the sub is mounted in must have a natural resonance around 50 Hz, so when the speaker tries to play that frequency, you get a big increase in output.

That explains why, using these bass tones, the rattling seems to be the worst around 45 to 50 Hz - it's not just the speaker making the plastic cover vibrate, it's the speaker making the metal deck vibrate against the plastic.

Moral of the story: get an aftermarket sub I'm getting more bass AND less rattle with a cheap 12" sub in a box in the trunk.

 

[hondo]

Thanks for this analysis. Early on I added dynamat to a lot of the deck's metal frame, I assume that has helped reduce the rattle along with the sorbothane and foam. I'm sure an aftermarket sub will be much better, but my wife won't give up the trunk space.

 

[DontDrinknDerive]

Nice Bode analysis! I have an aftermarket 10 in the trunk but I still get rattles from the rear deck. I'm thinking of sound dampening the thing...but I haven't gotten around to it.

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