Standing waves in SQ and SPL installations

[Aaron Clinton 2,372]

I wanted to ask this question because I understand about them through extensive physics classes, but I wanted to ask on a clearer run down on how to deal with them in the car audio world

say for example a normal SQL for the everyday street beater

and then the SQ competitor

and then wackos like me who are building massive boxes to stupid loud numbers

(I understand the flow and movement of the air as controlled by the box designer, in my box case dave edwards is the genious behind it, but I wanted to start this for all members)

[dave_Edwards 1]

Three types of standing waves affect all previously manufactured loudspeaker enclosures. The first and most powerful is the axial standing wave---The axial wave involves only two enclosure walls that are parallel to one another. Next in order of strength is the tangential standing wave which involves four enclosure walls that are parallel to one another. This type of standing wave has only half the energy of an axial wave (3db down). The last and least powerful standing wave is the oblique which involves six enclosure walls and it has only one fourth the energy of the axial wave (6db down).

A sound wave has a positive portion starting at 0 degrees to 180 degrees and a negative portion starting at 180 degrees. If the positive portion of the wave (i.e. compression) meets the negative portion (i.e. rarefaction), then the waves will combine and will be out of phase relative to each other. Likewise, if the positive portion of the waves meets another positive portion of the wave then the waves are in phase (i.e. the waves reinforce each other) the sound will be inordinately loud (i.e. the high pressure zone). Likewise, at locations in the enclosure where the sound waves are out of phase (one being negative and the other being positive) the sound waves cancel each other out and no sound will be heard (a low or no pressure zone).

In any enclosure design standing waves produce a big problem but some simple ways of getting rid of them is to fiberglass the enclosed area of the enclosure on all walls--or to add wedge angles into the corners.

Just remember standing waves are created when two waves moving in opposite directions interfere. When a reflected wave reinforces a reflection of the original waveform the sound waves reinforce themselves, increasing in altitude.

hope this helps,

Dave

[Tirefryr 1,742]

I don't see how you can possibly have 4 walls parallel to each other.... Can you explain? Only thing I can come up with would be a multi-chambered enclosure with multiple drivers and dividers for each.

[Mark 0]

How will reflections occur when it requires 1/4 wavelength development in order to do so? 80hz = 3.52' . Standing waves are more of an issue elsewhere on the frequency spectrum.

Or has the 1/4wavelength theory been thrown out the window as of late? I'd heard rumblings of such things, but never any documented evidence to support it.

[Aaron Clinton 2,372]

Mark are you reffereing to bass reproduction at a distance? I am confused, because I have read about the 8' thing, where bass has not reached its full ????? until then

[Mark 0]

Mark are you reffereing to bass reproduction at a distance? I am confused, because I have read about the 8' thing, where bass has not reached its full ????? until then

To my knowledge, in order for the reflection of a wave to occur, it must first develop to 1/4 wavelength. At 80hz, 1/4 of a wavelength is 3.52 feet. In a subwoofer application, standing waves are essentially impossible due to the sheer distance required for the wave form to develop. Midbass and midrange drivers are a different story, however.

[Aaron Clinton 2,372]

do you mean impossible in a car audio application or in any application?

[Mark 0]

do you mean impossible in a car audio application or in any application?

Car

[Aaron Clinton 2,372]

unless we get a limo

[Mark 0]

unless we get a limo

Unless your enclsoure is 5+feet deep

[helotaxi 46]

Standing waves in the enclosure itself are a non-issue until you get to unreasonably large enclosures that won't fit in the normal car. Standing waves in the car itself are a different story. Depending on the size of the car and the frequency, there will be several nodes in the car where there is nearly total cancellation of certain frequencies and other spot where other frequencies are boosted by constructive interference to the point that they just scream. Most cars have a frequency that shows as a huge dip in the frequency response usually in the midbass region caused by cancellation. This is caused more by unequal path lengths between multiple drivers playing the same frequencies than by standing waves. I have yet to see a car with parallel surfaces conducive to establishing a true standing wave.

If you could establish a standing wave between the sub wall and the windshield (they would have to be parallel and 1/4 wavelength of the frequency that you want to tune to apart) you should be able to see a decent boost in that one freq and do pretty well on the meter.

Cars are an acoustic nightmare. Unequal pathlengths and the mixture of acoustically reflective and absorbative surfaces make getting a flat frequency response quite challenging. Standing waves are really the least of your concern.

[Aaron Clinton 2,372]

what about waves that are caught in corners, other than making corner cuts is there other build options?

I mean I have fiberglassed inside of boxes for better air flow

[GnomeAudio 0]

i think our friend helotaxi is catching on to something that the big dogs in the SPL world have known for years. its not about wavelengths or all that fancy-talk...its about knowing where the "hotspots" are and aiding their development near where the mics are to be placed. okay...i sorta contridicted myself, but follow me anyway.

the acoustics of our friend the automobile, in short, suck. you have to know your car inside and out and spend hours upon hours repositioning ports and moving seats in hope of finding that magic combination. that, ladies and gents is SPL. cars that compete in meca usually won't do as well in dB drag because the mic placement is different and vice versa. i'm sure there are exceptions to this rule, so feel free to correct me.

the box design isn't as huge a role as you might think. you're looking for efficiency and as little friction as possible. moving air is the name of the game. the correct port area to sd ratio is also very important. bigger ports move more air, but there is such a thing as over doing it. bracing is your friend, and you can even use sand or concrete to create a more acoustically perfect, non-resonant enclosure.

some exotic bandpass and transmission line boxes do get very loud. they also take a lot of time, practice and know-how to get right. i am in no way refuting mr edwards, but i don't think that most people need to be so well-versed in wave theory and mathematics to get loud. all you need is a mic, a few free evenings, a friend and a case of beer

ps. beer is optional. however, its a great motivating factor and most of my friends don't work free.

[helotaxi 46]

The design of the enclosure is important only in how well it works with the sub and the car to create the largest pressure wave possible at the mic location. Other than that it doesn't matter .