Why tune a sub so low?? (1 Viewer)

[Brian D B]

In the past I have designed and built several mobile audio systems including dozens and dozens of speaker enclosures and consider myself to be fairly knowledgeable on the topic. Depending on the driver, I usually got really good results from tuning to 30-35hz.

However, since I have been perusing though the conversations on this site the past couple of weeks I have notice a lot of people tuning their sub enclosures to frequencies below 20 hz. Now, if recall correctly the frequency to which an enclosure is tuned is the frequency that is produced most efficiently with this efficiency trailing off at about 6-12 db per octave. If this is indeed the case, why would you tune your sub to be most efficient at producing a frequency that you can't even hear?

What am I missing? Does it have something to do with the different acoustical environment? Have the laws of physics changed?

 

[Chris*Liberti]

Actually there is a slight boost at the tuning frequency followed by a drop off below the pitch. The speaker will still produce sound above the tuning frequency. For example the SVS 16-46 which is tuned at 16Hz. It has flat response from 16Hz to about 100 or so Hz.

 

[Andrew Walbert]

I think the big reason for tuning very low is because it ensures that you'll get good reproduction throughout the frequency range. This is more important to HT than music, since most music rarely has notes that reach into the subsonic area, but more and more movies have scenes (ex. LOTR:EE) that have extremely low bass.

 

[Neil Joseph]

... and tuning low takes the load off the driver and allows the enclosure/ports to reproduce the low frequencies that mroe and more soundtracks come with.

 

[Mark_E_Smith]

You also dont have the same room rise in a home as in a car.

 

[Brian Bunge]

Brian,

Also, for a ported enclosure the rolloff below Fb is 24dB/octave. Sealed enclosures rolloff at a rate of 12dB/octave.

 

[KyleGS]

I believe Mark Smith probably hit this one on the head. Coming from car audio I had the same question. The cabin gain in a car is incredible and allows you to tune to mid 30's and still get great output in the 20's. You have to tune lower in a home environment to get that same bottom end- sometimes you can never get that same low output that you get in a car. Example- Beyond Audio Inhuman 15" hitting 150db @ 30hz. (hatchback) That's gonna take ALOT more than a single 15" in a home environment.

 

[Mark_E_Smith]

This is why car subs usually don't work out for HT for the most part as their FS is to high.

 

[Brian D B]

Thanks for all of these great responses. I'm glad I asked before I started to build!

 

[Shawn Solar]

Plus group delay is usaully highest around tuning. By lowering the tuning to 18-20hz the spike in GD is out of the passband of the sub for the most part.

 

[Dan Wesnor]

OK, I'll throw the tomatoes.

I build sealed subs, and my last one has an F3 of 30Hz. It produces plenty of output at 20Hz despite the fact that my room is much larger than average, and thus produces less room gain.

So why would you want it to be flat to 20Hz?

Maybe those ultra-low-tuned subs are the reason why many of you guys need equalization to flatten the sub out, and I don't.

 

[Dan Wesnor]

BTW, here is that sub (the blue line).

In room, it's flat to 20Hz, +/- 3dB without any equalization.

 

[Greg Yeatts]

Yo uh, Loc D,

What is the sealed sub you are describing? Is it your Shiva or SB10? What is the Fb of the sealed box? I am currently working on a sealed box design with the Adire Tumult. I am using a linkwitz transform and a big amp and a tiny sealed box. Should have an anechoic F3 of 21 hertz. Should be much better in room.

 

[Dan Wesnor]

It's the Shiva with the LT.

 

[Scott Simonian]

Oh you have a LT in there. Well that explains a lot!
Im pretty happy with the responce Im getting with my tempest. I have it in a pretty large room that opens to the rest of the house too. I will be making a second.

 

[Greg Yeatts]

I am beginning to think that with sound mixers throwing ever more copious amount of extremely LFE into movies that we will have to go sealed to protect our woofers. Both Monster Inc and AOTC had my current woofer flapping. It's a serious woofer too. Its a Dayton DVC 12" in a 88 liter PR box with an Adire PR15 weighted to 600 grams. This gives me about a 21 hertz tuned. It cried for mercy on the two aforementioned flicks.

A sealed woofer offers better cone control at frequencies below Fb. This is something we all now have to consider when buying or designing subs.

Dan Wesnor:

Your LT Shiva sub looks like a great idea. Bet it souund good too. As I said above, I will soon be using an LT sub too. I already have the LT. Just waiting on the driver.

 

[Owen Bartley]

Greg, won't the rumble filters on a lot of sub amps take care of ported subs?

 

[Dan Wesnor]

Rumble filters limit power, but it takes very little power below tuning frequency to send a ported sub past it's excursion limits.

 

[Peter Jessee]

I build sealed subs, and my last one has an F3 of 30Hz. It produces plenty of output at 20Hz despite the fact that my room is much larger than average, and thus produces less room gain.
So why would you want it to be flat to 20Hz?

You can go to this link and see measurements I made with a 12" DVC in a 1.0 cu ft box in a 2-car garage. Theoretical F3 is around 42 hz, but it's flat in-room (er, in-garage) to about 25 hz, and maybe -4 dB at 20 hz. We're talking about 4500 cu ft here.
http://home.att.net/~pjessee.or/wsb/...tos.html-.html
Moral of the story - you don't want the sub to be anechoically flat to 20 hz, or your in-room response will have a huge peak in the low bass. One advantage of DIY subs is that you can design the sub to give a flat INSTALLED bass response IN YOUR ROOM.
Makes you wonder what the in-room curve for a SVS 16-39 is like, doesn't it? I'm sure it's impressive, but maybe not so accurate.
Peter

 

[Sebastian]

Moral of the story - you don't want the sub to be anechoically flat to 20 hz, or your in-room response will have a huge peak in the low bass.

That may be good thing since our ears are not as sensitive to low frequencies.
I don't hear too many SVS owners complaining about huge peaks just in low bass output.

This thread just may turn into a, "is a flat frequency response really needed?" thread.