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resonance and sealed enclosures

I was wondering: suppose you have a smallish sealed subwoofer with a resonance frequency of 40 Hz (obviously not up the quality of the things usually talked about here). This is right at the frequency of the low E on a bass. My concern is that the reponse at 40 Hz will be exagerrated. The resonance peak on a sealed sub isn't important when it's 20 Hz or below. Will the 40 Hz resonance really matter?

I am thinking about using this sub for computer speaker BTW.
daniel_newhouse5e69
sean
6,231 posts
 
El: I don't have a lot of personal experience with this type of design as compared to others that i've discussed, which for the record, is called an Aperiodic or "Vario-Vent" by some. From what i have heard, experienced and read about these designs, i like it better than most commercially available ported and passive radiator designs. Like anything else though, one would have to carefully select the proper box / driver combo to obtain optimum results.

As a side note, there was a commercial speaker designer that was posting on AA under the pseudonym of "Mr Bigglesworth". Whoever he was, he wanted to remain anonymous but at the same time, tried to share his experiences and educate the masses. When it came down to it, he stated that if one were do all of the measuring in the world and their goal was to achieve the highest levels of linearity possible, a sealed and stuffed low Q box would be what they would end up with. I asked him what his thoughts were on Aperiodic's and Transmission Lines and he referred me back to the sealed box along with an explanation why he had those thoughts. Given that i was already on the same page that he was, i didn't need much convincing. I was just looking to compare notes with someone inside the industry that didn't have a reputation to uphold or specific product to market. Sean
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danielnewhouse6adc
2 posts
 
Sean, what do you think of Miller and Kreisel's push/pull design?
sean
6,231 posts
 
Push / Pull lowers distortion, increases power handling, faster transient response, lower impedance peak at resonance, improved power transfer characteristics, etc... If properly implimented, it can be quite beneficial. I'm using a dipolar push-pull arrangement for my own personal subs. The dipolar pattern reduces standing waves, which further improves the linearity of in-room response. Problem is, you've got to use twice as many drivers, resulting in a lower impedance, much higher cost to build and the need for a much sturdier amp. Sean
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danielnewhouse6adc
2 posts
 
...I thought push/pull by nature required 2 subs. Are you using 4 subs?
drew_eckhardt
394 posts
 
A sealed speaker's response at resonance (Fb) depends on how the system is damped (Qtc).

A sealed speaker acts as a second order high-pass filter. Ultimately it will roll off at 12dB/octave with Fb determining where the roll-off starts and Qtc the shape of the transition. With Qtc <= 1/sqrt(2) (about .707) there is no peak.

A Qtc of 1/sqrt(2) is "maximally flat." It provides the slowest approach to the ultimate second-order roll-off without having a peak. The speaker will be 3dB down at Fb. Perceptually you get a bit of bloom.

A Qtc of .5 is "critically damped." The speaker will be 6dB down at Fb and have an F3 point half an octave higher (60Hz for Fb=40Hz). Such a speaker does not ring. Perceptually the bass is very tight.

Frequency response for various Qs can be seen here:
http://www.diyaudio.com/forums/attachment.php?s=&postid=88030

Note that small deviations in output are _much_ more noticeable in the bass area because of the equal loudness curve spacing.

Fb and Qtc also affect phase shift as a function of frequency (group delay). While you may not need a lower Fb for music, a 20Hz Fb gets you half the group delay at 40Hz as a 40Hz Fb. Low group delay is perceived as fast bass.
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