What makes a sub "bottom"? (1 Viewer)

[Craig_B]

I was wondering what was the biggest factor in a driver reaching its limits and "bottoming out". Is it mainly the sheer volume you are pushing through it or is it related more to the frequency of the signal feeding the sub vs the frequency the sub is tuned to, or neither or both? I assume it is probably both, so I guess my question is: Which factor is more important?

For example, assuming everything else was equal (ie both subs calibrated to 75 with VE and output at reference levels), would the sub tuned to 16 hz be less likely to bottom out with the intro to Toy Story 2, TPM or the door knock on the Monsters trailer than a sub tuned to 25 hz? I know the 25hz tuned sub will pump out more spl's at 25hz and higher, but if your main goal was not to bottom on those really low, bass heavy scenes, are you better off going for a sub that is tuned lower? I am talking about passive subs with an external amplifier and no filters.

 

[Dustin B]

Cuz I don't want to study for my final on Friday, I'll write way more than I should for this.
To start go download and install this program:
http://www.adireaudio.com/Files/Adire.zip
Then after your run it do the following:
1) File -> New -> Bass Reflex
2) Driver -> Configuration
3) In the window that pops up in the "Dual Voice Coil" area select the parallel radio button and then close the window.
4) Meas. Setup
5) In the window that pops up change the "Stop Frequency" to 120. Then change the "Watts into 8ohms" to 125. Then close that window.
6) Box
7) Change "Volume" to 142.5L and "Min Dia" to 10.2. Leave this window open and place it on the right side of your screen.
8) Now the "Free air spl at 1m" window should already be on your screen. If not, open it using the steps in 9 as well.
9) Window -> Cone Excursion followed by Window -> Airspeed in port.
10) Arrange the 4 windows on your screen so you can see them all.
Now in the "Bassreflex Box" window change the "Res. Freq." to 16, 20 and 25 and watch what happens in the graph.
You can also reopen the "Meas. Setup" window and change the "Watts into 8ohms" to see how things change with more or less power. The driver is 4ohm, so the actual power applied is twice what you enter into that box.
The important graph for bottoming is the "Cone Excursion" one. This isn't the SVS driver and enclosure. It's an EBS Adire Audio Shiva, but the concepts are the same. Xmax on a Shiva is 31.8mm. This is the linear travel limit. It starts to sound bad once you pass that much excursion. However the physical travel limit (Xmech) were bottoming will occur is 52mm. Whether the difference between Xmax and Xmech is as high with the SVS driver as it is with the Shiva I don't know.
Whatching what happens in the three graphs will also let you see what the port does.

 

[Steve_Ma]

If you want that much bottom, somethin's gotta give....

Sorry...I couldn't resist. And oh yeah..What Dustin said.
--Steve

 

[Harold_C]

Simply put, a subwoofer bottoms when the voice coil moves further than it is designed to move.

Many things can contribute:

Driving the sub too hard (i.e. levels set too high) can cause it to bottom.

A poorly designed driver can contribute. A really ideal subwoofer driver needs to be designed from the ground up for long excursion. It's quite expensive to build drivers with long excursion. The average stereo speaker woofer is really not up to the task.

Improper tuning of a vented enclosure can cause a sub to bottom.

Equalization or bass boost is probably the biggest culprit. Boosting low frequencies in an effort to achieve some theoretically "ideal" frequency response will be very likely to overdrive the sub.

In vented enclosures, control of driver excursion is totally lost below system tuning. Therefore, it is a very good idea to use a 20 Hz subsonic filter if you are going to "beat on" a vented sub.