Room gain at low frequencies

[michael hurd]

Here is a test of a sealed box car audio subwoofer, with measurement in and out of the car. If you notice, the sealed box rolls off starting at around 57 hz, without the reinforcement or "gain" of the vehicle interior.

http://www.carsound.com/reviews/subs/jl15w3.html

http://www.carsound.com/reviews/subs/jl15w3chts.html

[michael hurd]

After loading into the interior of a vehicle, the resultant frequency response is basically flat to 10 hz. The woofer is relatively inefficent, but suffers very little from power compression, compared to others in it's class.

3 db power compression starts at about 300w input, and at 1kw, there is 5 db of compression.

[michael hurd]

I would apply the same thoughts to adding a subwoofer to a small room. Ie: do you really need a subwoofer that has flat anechoic response to 16 hz, resulting in a lift or gain at the lowest frequencies?

Would you not want to utilize a subwoofer that might roll off a little early in this case, resulting in a flat frequency response? Just a thought, without testing equipment, it would be hard to find out the transfer function of the room, where gain starts in the low frequencies.

[Olorin]

Michael, maybe you can use this.

room060d.zip

[DrWho]

Mike's not talking about modes, but rather the pressure loading that occurs at frequencies below the lowest room resonance. You can calculate this by this equation:

frequency of lowest resonance = speed of sound / ( 2 x longest length in the room)

using METERS and a rectangular square room, we have

f = 344/(2 x SQUAREROOT( H^2 + W^2 + L^2))

In a 8x15x20 foot room this comes to 43Hz (don't forget to convert to meters). Using this number, we can apply a linkwitz transform:

(f0 = 43, Q0 = .707, fp = 15, Qp= .707)

inside our modelling software and we get about 12dB of room gain at 20Hz (which is consistent with real measurements).

So should this be calculated for when designing subwoofers? Yes and no. If we look at the Fletcher-Munson curves for loudness, this 12dB boost will actually help flatten the response out if we're listening at lower volumes (say around 70dB, which is just a bit louder than a normal conversation).

It's been my limited experience that relying on the room gain for a flat response doesn't sound as powerful as a speaker that has the same anechoic response. Part of this probably has to do with the nature of the real source material below 40Hz...for music it's basically nothing and for HT it's explosions. Who wouldn't want a low kick drum or a low bass note or an explosion to be slightly exagerated? Heck, in most recordings there is very little control and processing performed on the frequencies below 40Hz because there's rarely ever a need to worry about it. When you've got a system that can faithfully playback these low frequencies, then it's pretty much up to the operater to dial in what sounds best. I personally would rather have a driver that would overdo the lower frequencies because you can always bring in an EQ and reduce the peak without other side effects (in fact, it will lower distortion). Trying to use EQ to bring up a lacking response reduces power handling and max SPL, and increases distortion.

All that said, if you were strictly limited by the size of your subwoofer enclosure and were confident with the room gain in your room, then I would say let it come into play. I don't think it'd ever hurt to check what the response might look like with and without room gain. I know I check it all the time.

Now to relate this back to car audio, the only difference is the size of the listening space. In car audio, the pressure function adds over 20dB at 20Hz and effects frequencies as high as 80Hz. I guess what I'm trying to say is that the pressure gain has a larger effect the smaller the listening space is. In a car this becomes critical (unless you wanna be drowned with bass).

[jnorv]

That F-M curve you show is the one I am used to. However, in another search I came across this one:

http://www.allchurchsound.com/ACS/edart/fmelc.html

I have not develed into their analysis but it is a differnetn curve. Somewhere in my data bank I think I have the original data from F-M. M

Jim N

[DrWho]

What's the difference between the two? It looks to me like the two graphs are on different scales, but the data is the same...

[sfogg]

"It's been my limited experience that relying on the room gain for a flat response doesn't sound as powerful as a speaker that has the same anechoic response. "

If you can find a comparible speaker that actually has the same anechoic response that would somewhat be expected since it would have the low end boosted by room gain too.

Where room gain can be most interesting (IMO) is with sealed subwoofers to extend their low end response. Sealed woofers roll off second order, room gain idealy is a second order gain. If done well the two can extend response quite a bit further down as evidenced in the original post in the thread. This doesn't really work as well for ported boxes or horns since they roll off faster then the room can boost.

In my room with my subs on some material I tend to think my pair of sealed subs (which by design are -3dB around 35hz) feel more powerful then my ported sub which is -3dB around 14hz outdoors. I haven't spent a lot of time measuring that though but my guess is my sealed subs reach deeper thanks to room gain.

" Now to relate this back to car audio, the only difference is the size of the listening space. In car audio, the pressure function adds over 20dB at 20Hz and effects frequencies as high as 80Hz. I guess what I'm trying to say is that the pressure gain has a larger effect the smaller the listening space is. "

Not the only difference, the other difference is a car will typically be better sealed too if the windows are up, sunroof closed...etc...etc. 'Leaks' in a room reduce the room gains effectiveness.

It would be interesting to do a comparison of gain in a vehicle with the windows open vs. closed and see how the response differs. I doubt Iget any room gain in my convertible for example....

Shawn

[DrWho]

Whenever I roll them down, response below 60Hz just goes out the window (wow ok, I didn't intend that lame pun, lol). That's prob cuz my subs start dropping around 60Hz though. Theoretically I should have a -3dB point at 20Hz with the windows rolled up, but I can tell this is nowhere near the case. I think once you hit near -12dB point of the speaker that the harmonic is such that you don't really get to hear the lower frequencies...at least this is where the sub in my car seems to stop making sound, even though cabin gain says otherwise. I've swapped in a few other speakers with different -12dB points and noticed an insane drop-off at the same spot.