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. |
...I thought push/pull by nature required 2 subs. Are you using 4 subs? |
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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Sean, what do you think of Miller and Kreisel's push/pull design? |
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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Sean...We know how you feel about vented speakers, sealed speakers, and even TL, but what is your take on the resistive vent, such as used with great success in the old Dynaco speakers? I tried this once with a box that was a trifle too small for the driver, but didn't see much effect, good or bad. |
This has to do with port velocity and turbulence within and / or near the port entrance and exits. Using a port that has radiused flares tends to reduce these problems quite a bit. This allows the port to maintain more consistent tuning / proper operation over a wider spl range. If such a port is not used, the output of the port will be far peakier in a specific spl range and lower the performance standard if used outside of a narrow range of average spl's. Using a port that is flared on one side rather than at the inlet and the outlet is better than a straight port, but it will still run into the same problems, albeit to a lesser extent. Sean
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I have read a revuew about a similar setup from M-Audio, slightly more expensive, that features an 8" ported sub. The complaint is that the sub sounds right at middle volumen levels but is too weak at high volume levels and too strong at low volume. Is this a classic case of the type of distortion that is found when using ported subs? |
A port is typically tuned at the point of resonance that the woofer achieves within that given box. By "counter-acting" this resonance, two smaller resonances are created. It is this elevated bass plateau due to these resonances that typically results in greater extension. At the same time, more resonance equates to less control and definition, which is why most vented designs sound "sloppy" compared to a well designed sealed box. The sealed box loses when it comes to output levels though, so you have to choose which trade-offs you want to live with and go from there. As i've mentioned before, sealed vs vented is equivalent to quality vs quantity.
I don't think that a 10" sub would be out of place filling in the bottom for 5.25" mid-woofers. If you cross it over properly, you can play around with the placement and probably get it to work pretty well. As far as using it on a desk-top, that may be a bit much depending on the size of the cabinet and the size of the desk : ) Sean
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Isn't a properly designed port supposed to counteract the resonance?
The speaker in question is the Samson Resolv 120a, part of a cheap line of "studio monitors" though I would never spend money on any recording studio that used these things. They're really marketed at wannabe musicians.
Would a 10" sealed sub be an appropriate fit for main speakers with a 5.25" mid speaker or would the sub be too big? Desktop space is a concern here. |
Poor old Dr Bose had a really neat idea long ago. Instead of trying to lower the resonant frequency as much as possible, push it UP, to 200 Hz or higher. The advantage is that rolloff below resonance, although very great, is smooth and predictable, and can be compensated by electronic means. (This is quite different from the practice of a modest (3-6 dB) boost at 30 Hz or so commonly used with sealed subwoofers). Downside is that a lot of amplifier power is needed.
This idea always appealed to me, as I am a "contrarian". If everyone thinks that one approach is best, I try the opposite. This served me well in a long engineering career. |
It looks like we're all in agreement.
Most sealed designs come in between 0.7 and 1.0 when it comes to Q, with 0.77 problably being most common, due to the way people perceive the sound. |
For non-critical listening...such as your application...i wouldnt be too concerned...and Sean is dead on...it is all based on the "Q" tuning for sealed designs...such as .70, etc...I would also agree that for truly quality deep bass response...if that is goal...a sealed design on paper would have the edge,,,I have also heard ported designs that surprised me as well...all in the construction I suppose...good luck..one last note... |
It will depend on the "Q" or tuning of the driver in that specific cabinet. Most sealed designs typically have a lower Q, meaning that they have a much smaller peak ( if any ) at resonance. This is why they sound far more natural and offer better control than a vented design, which typically has a tremendous peak ( both in output and impedance ) at the point of resonance.
While this type of speaker would be phenomenal for a computer type system, you can always vary the type and quantity of stuffing material to alter the bass response to your tastes as needed. More stuffing will reduce the peak at output, provide more extension and give the bass a tighter, drier sound. Reducing the stuffing will give you greater output at a higher frequency, making the bass sound more "lively", but also with less control. Finding the right spot for what you desire in your specific installation shouldn't be that difficult. Sean
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