How to achieve awesome mid bass
I have recently received many questions about mid bass. Instead of making several replies I have opted to make this post. Questions and additions welcome.
This is one of the most complex and misunderstood topics in car audio for the following reasons that I will explain.
You must be able to grasp the concept of sound in the incremental phase domain as well as sound in the frequency domain. I will explain.
2. The cars dimensions
The car itself conflicts with good mid bass response do to various interior dimensions of a typical automobile coinciding with the physical size of sound waves in this frequency range.
3. The misconceptions
The misconception that poor mid bass is due to low amplitude in this range. More won’t help. I will explain.
4. Futile attempts
Many of the attempts people make to increase mid bass response only make it worse including gaining up, expanding crossover pass band, adding more speakers, and placing the mid bass drivers in an enclosure.
The term mid bass can be interpreted many ways. In car audio however this band is pretty much set as the band between 50hz and 250hz. This is due to many reasons that are beyond the scope of this article. What’s important to understand is that in the 50hz-250hz range humans are the very least sensitive to variations in frequency response but we are the most sensitive in this range to changes in phase. As frequency rises above 250hz the opposite becomes true. Humans become less aware of phase response and much more aware of frequency response.
Phase is the angle measured at a given point on a sound wave.
Here you can see that from where Homer sits the sound wave arrives at his ear at one angle but below you can see that when he changes is position relative to the speaker the angle of the sign wave changes as well.
he can’t hear any difference in the sound. But when you add a second speaker things change.
Now his brain is picking up one sound coming from two different places. His brain knows its coming from two different places partially because the phase angle is not the same. When this happens your brain will red flag this sound information as “invalid” and discard it. In nature this helps us localize where a sound is coming from. By discarding conflicting information our brain can better hone in on non conflicting information. This is one major reason why boosting the volume of your mid bass drivers will not make your mid bass sound better or much louder. The more you crank up the volume the more your brain will refuse the information and the worse it will sound.
The second reason is that because the sound waves from both your speakers are colliding with each other and due to the phase angles not lining up with each other they will add and subtract from each other, not necessarily amounting up to more volume. Again more speaker and power will not alleviate this effect. This also causes severe deviations of amplitude at higher frequencies causing what is known as the comb filter effect. When comb filtering happens your brains ability to sample that sounds qualities is greatly diminished because much of the information is either physically being destroyed due to phase cancellation or being discarded by your brain as it is perceived as invalid. This will actually make your highs sound metallic and harsh not to mention trashing any chance that you have at good imaging or authoritive mid bass.
The cars dimensions.
All modern vehicles have been purposely designed to emit all of their noises in the mid bass range. From the engine to the rumble of the tires to the sound of the AC system, every device in your car has been designed to keep their noises with in the 50hz-250hz range. From Mercedes to Honda, auto manufacturers have done their homework and know that people have an inability to perceive sounds based on frequency in this range. By limiting noises to this range the car will seem much quieter and any noises made are much more tolerable. Because this frequency range is already so cluttered your mid bass drivers need to be in proper acoustic phase and all precautions taken to ensure that they are performing correctly in order to overcome this noise floor. When this is done the end result is that your brain will accept this information (music) and reject all of the cars noises.
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.
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.