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How to Achieve Awesome Mid Bass.

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Member - Posts: 1
Member spacespace
Joined: June 04, 2007
Location: United States
Posted: June 30, 2008 at 10:16 AM / IP Logged  
Lots of good information Speakermakers. Thanks. You mention in the post using an "enclosure with a large resistive hole." for midbass and midrange speakers. Midrange enclosures is a topic I've been researching and can't really find even a general rule of thunb for. Or I guess I should say, the proper way to mount them for best SQ and accuracy. Subs are easy enough using software, speaker parameters, and listening experience but with mids, what is "accurate" for their reproduction? I've used sealed back mids and regular open back. They've been door mounted, kick panel, door pod, and dash mounted and all sound pretty good while covering the required frequency spectrum. But I've also noticed that changing the enclosure size, stuffing, even the enclosure shape all have an effect on the sound to an extent. How do I know what the mid is producing from the enclosure is truely accurate musical information, and not adversely affected BY the enclosure?
Member - Posts: 10
Member spacespace
Joined: March 04, 2008
Location: Oregon, United States
Posted: July 14, 2008 at 9:05 PM / IP Logged  

This is a very interesting thread, and I hope I'm getting the right ideas, as I dial in my newly-installed car stereo.  Please help me with the following questions.

speakermakers wrote:
In the case of midbasses pay attention to the phase at the lower limit of the driver (crossover point).

If my crossover point is 85Hz, does this mean I'd want to pay particular attention to the 85Hz wavelength, and where my listening position is located within the phase of the 85Hz wavelength?

speakermakers wrote:
Though phase angle changes with frequency it is not a concern. As long as phase is consistent with other drivers emitting that same frequency band at that frequency. Shifts in phase due to changing wave length (frequency) occur naturally and are undetectable by the human ear so long as they are not drastic shifts (like those caused by poor enclosure and crossover design). Use a wave length calculator to determine the wave length at the crossover frequency of a driver. Knowing the physical size of this wave will help you understand how much phase will be affected by any distance changes you might make.

Do I understand this right?  If a given frequency has a wavelength of "x," and I move the driver 1/2x (one-half x), the wave's phase would be 180 degrees different from where it was originally.

speakermakers wrote:
Regardless of the degree of phase shift be prepared to compensate for acoustic loss any time a phase shift is encountered. This is why there is typically a ripple in the frequency response at a crossover point. Even with a good crossover design.

In this case, do you compensate by boosting the affected frequency?

speakermakers wrote:
Keep all of your phase sensitive drivers, such as mid basses and mid ranges (group delay afflicted might be a better term) crossed over (hi passed)at 12db or better and at twice the resonance (Fs). All speakers are least accurate at or near resonance (Fs).

Again, do I have this right?  My speaker's Fs is 42.5Hz, so I'd set the high-pass at 85 Hz and at least 12dB (would 24dB or higher be even better?).

Member - Posts: 1
Member spacespace
Joined: July 25, 2008
Location: Vanuatu
Posted: July 25, 2008 at 12:33 AM / IP Logged  

speakermakers wrote:
We already know that boosting the volume, power, speaker, and equalization is not the answer here. So you might be wondering, what’s the answer. Who do I have to kill to get good mid bass.
The best way to understand this is to first under stand what we are really after. Humans perceive strong mid bass best when there is a sudden change in volume that is coherent with the rest of the sound systems volume level. And when this sound has a fast attack and decay rate with little phase distortion. Not a bunch of power in a very limited frequency range that causes phase distortion all the way up to 10khz, which is too often the case.

Your very eloquent post, is this spoken based on experience or theory based expectations?  Are you discussing dedicated midbass or the usual 2 way front set up?  I am very curious why you say more is not better?  Maybe ENOUGH is the right answer?  In real world, road noise seems to be the king, given the typical urge to run a pretty strong sub bass section, achieving a balanced sound may very likely warrant very strong midbass, capable of producing levels sufficient to make the sub bass and midbass transition very smooth (I think this may warrant a lower subwoofer lo pass filter point?)  From what I know (and I know very little), humans perceive strong midbass when there is a strong midbass, not through the sudden changes but through sheer output within frequency band that is needed).  Not sure why too much power is bad?  Considering music dynamics and energy needed to support output in lower freqs, having more is considerably safer than having just enough (what do you do trying to deal with 6dB peak in midbass region?)


The solution
Choose speakers with a low Fs (resonant frequency). Closer to 50hz than 100hz. The fact is that as a speaker attempts to reproduce sounds that are to close to its Fs phase distortion and consequently comb filtering becomes a problem. You need to try to keep your speakers operating above this range. Fortunately the cars transfer function will help with this. Often times allowing a hi pass of 80hz while still allowing strong reproduction down to 50hz. There are many tricks here that can be accomplished by using asymmetrical crossovers and parametric equalization but all of that is outside the scope of this article. Maybe some other time.
Choose speakers with a low Qts. Lower than .5 closer to .35. Keep in mind that as your Qts value gets lower you must choose a lower Fs as well. For example a speaker with an Fs of 85 and a Qts of .52 might reproduce 50hz just fine but a speaker with a Qts of .35 might need an Fs of 60hz to do the same. There is more to this subject but I am limited on space. Just keep that in mind.
If your speaker ends up in a small enclosure (less than 1 cubic foot) there is a very good chance that you will end up with a sharp resonant peak that will cause many of the problems that we have already discussed as well as increase the lowest usable frequency that can be reproduced. This may cause a gap in the lower mid bass frequency where the fundamental of drum sounds occur. With out the fundamental wave off a drum all that is left is unconvincing upper harmonics.
Doors work real well as mid bass enclosures. Just make sure to seal off all opportunities for the rear wave to ever meet the front wave.
In kick panel speakers vent the rear of the enclosure with a large resistive hole. This forms an aperiodic enclosure. This is also a useful technique to utilize on mid range enclosures.
Always attempt to align the phase of your sub with the phase of your mid bass speakers, and your mid ranges and your tweeters. As dumb as it sounds this is rarely ever done. You know the destructive nature of phase cancellation from just the one speaker on the other side of your car. What do you think that a 10” speaker can do? The fact is that a lot of musical information bleeds past the crossover points between the sub and the mid bass speakers and the transfer function of the car amplifies this.
Last but not least the easiest, fastest, least expensive, and most effective way to improve mid bass response is to simply add .4-.9 milliseconds of time delay to the left speaker. Doing this will largely correct both phase distortion and comb filtering while simultaneously increasing volume by eliminating cancellation. The best way to get time delay is to choose either a head unit or amplifier that has it on board. Be aware though that head units that have factory preset positions are of little or no use. You will need incremental adjustability.
Questions welcome.   

This solution, what size of drivers are you discussing?  There may be some drivers out there capable of functioning perfectly in a tiny sealed enclosure (if you pick the right driver) that do not have the issues you mention, in fact, I am pretty sure there are a few ... Seems to me this solution may be just another band aid to avoid the only realistic solution which would involve some larger (make the greater sensitivity your friend when efficiency is at stake) decoupled from the typical door cavity, unless you prefer to post another article with advise how to deal with interior limitations due to flexing and rattling door skins?  And of course, there is resonance to deal with but parts of your advice, discussing filter points and eq should take care of this rather easily?

About that left side delay, this sound like another trouble waiting to happen, considering the relative distances between other drivers and then the subwoofer itself... What can 10 inch driver do while being used as a dedicated midbass?  You tell me... For one thing, it should allow you to produce enough low end extension and output to allow the subwoofer to function merely as a filler without supplying too many directional cues that can cause way more headache than issues you mentioned... Or have you tried those 10 inch midbasses already? 

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