# Ports aligned

#### Arenith

I have back-ported speakers with one ported sub right in the middle, would it be beneficial to align the ports of the mains and the sub on the same axis, in an even row? For an even wave launch (from the ports) Could this also give some gain?

#### tmuikku

Hi, the main output of the port is at very low frequency, around the tuning frequency. If it is 40Hz for example, wavelength at 40Hz is about 8.5m long. Any two sound sources interfere costructively below frequency, whose wavelength is quarter of the distance between the sources. So, any sound sources, that are within 2m of each other, would sum constructively at 40Hz. 100Hz is about 3.4m long, so roughly 85cm, and so on.

They would interfere destructively when path length from each source to observer is half wavelength, and so on as the condition would repeat every n+1/2wl, constructive and destructive interference when path length difference is multiple of wavelength, and destructive when there is half wavelength difference, a comb filter.

Fear not, average room dimensions are roughly about 100Hz in size, any box store speakers are about 300-1000Hz in size, so location of 40Hz port on the speaker doesn't matter at all, but location of speaker(s) in room matters a lot. Speaking Hz here, to highlight how sound wavelength is more useful metric to figure out/imagine/think/calculate such things, how sound sources interact with surroundings, and how distance and location of observer relates

So, to answer your question, assuming output of the ports is around 40Hz, it's fine to have them within about 2m from each other and it doesn't matter as they would act like a single point source. But, considering how 40Hz interacts with a room it would matter a lot, how both sound sources relate to room and your listening position.

Main speakers you want and need to position to get a stereo image you want, but low frequency sources like subs position (and processing) could be adjusted to tune the room modes at listening position. The room dominates low frequency sound in the room, due to long wavelengths. Best position for a sub could be opposite side of room, or multiple subs all around the room. If you have measurement setup you could use REW or some other program to find this out. You could also try to position things with ear, but it's quite difficult.

Anyway, when sources are within 1/4 wavelength from each other, they would seem like a point source below frequencies of the wavelength as their phase is less than 90deg apart to any direction and would make only constructive interference to any direction which seems like there was one source and not two.

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#### AllenB

Paid Member
room modes
Yes I agree the room participation at that frequency is such a concern, and adjusting or even mixing the source locations may be helpful when done for the right reason. There's nothing relevant to imaging happening at 40Hz.

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#### tmuikku

Yeah. Taking idea of "to get more gain with positioning", the speakers are already close enough to "combine maximally", but the location of low frequency sources and observer in room matters a lot and there is likely destructive interference that makes one feel like "i need mo base".

There could be +10db peak at 40Hz and -15db dip at 60Hz, another peak at 80Hz, crazy response in general depending on your room dimensions, construction and location of sources and the observer. Change listening position and it could be reverse, dip at 40Hz, peak at 60Hz and so on. To give perspective 10dB is ten times the power, so if you have 10W going from amplifier to the speakers the room makes it appear as if you had 1W amplification on a dip and 100W on a peak.

Practical example: You'd might feel/hear that kick is missing and you boost bass, but now the sound is overwhelming due to some other frequency having a peak and you'll want to turn it back down and it never feels quite right. There just isn't a good balance and it's due to room modes, constructive and destructive interference on long wavelengths, wavelengths size of the room and longer. If you happen to sit at location where there is dip at kick frequency no amount of boost fixes that.

To combat this you must control the room sized sound somehow, and for that you need sound source that is size of the room as well. As woofers come only up to size of 24" you must just use multiple (any size) all around the room Or, use what you've got but change positioning until it's best possible. If you are locked to (practical) positioning you can still do something with DSP using delay and filters.

If you have a single sub, you could put that behind your listening position or something. Try various positions and figure out if dips (and peaks) get less severe at listening position by measuring with a mic. After finding best compromise positioning, one can further tune for example putting notch filters to the peaks or use some more elaborate DSP scheme, or try various things and develop experience what sounds good, what is practical and so on.

Key takeaway, even if you now have nice response at your listening position the response is likely still a mess everywhere else, so don't get confused, what you hear depends on where your ears are at.

To give example it is not a simple thing to get bass fine in a room: I've got my sub at a practical position left of my listening spot, and the whole system bass response is somewhat optimized for one listening spot with DSP. I might turn the sub off sometimes if I listen somewhere else than at the spot to get better bass. Sole purpose of the sub is to even out the response which it can possibly help achieve, at one spot. Also, need to adjust it again as I've changed xo of my mains which changed their group delay and messed up the response of the complete system with sub at listening spot.

Buying a sub doesn't take one's system any closer of good or loud bass. The whole system, all low frequency sources and the room, must be aligned as a system. Have fun!

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