Ask an Acoustic Engineer (me)

[Justin Zazzi]

Hi Jazzi,

- How do you set time alignment? Do you fine tune the timing after initial TA? If yes, how would you fine tune timing? Ear?
- Is there any case you give up correct timing to exchange for other benefit (for example phase?)

Please be details.

Thank you very much.

If I only have a tape measure, I use that to start with.
If I only have an RTA, I will use a technique called "nulling" where I flip the polarity of one speaker and then adjust time alignment so there is maximum cancellation between the two speakers (largest null in the RTA).
If I have a dual-channel FFT system like SMAART or OpenSoundMeter, then I use the phase response directly.

After I get close using the tools, I will fine tune by ear using the technique below. If I have tools then this process is much faster and I can match phase over a larger range of frequencies and use other corrections like all-pass filters which I find difficult to do by ear. If I have no tools available, then I use the technique below but it takes a lot longer.

To fine tune, I will set the lowpass filter to somewhere around 200hz and play only bass. I will play a mono track with a male voice like the introduction tracks on the Chesky Ultimate Demonstration Disc. I set the time delay for all speakers to the middle of the range on the equipment, like if the maximum delay per channel is 10.0ms then I set delay to 5.0ms on all speakers. I choose one speaker as the time anchor, usually front left woofer. Then I'll close my eyes and listen to a pair of speakers and adjust time delay until the mono sound appears to come from the "center" of the speakers. I will also check polarity by flipping the polarity of one speaker and ensure that it sounds more focused or not.

There are always cases when I give up "perfect" time alignment. The two most common are when a customer wants the sound to be more enveloping or surround-sound instead of precision focused in front. The other case is when I have a rear speaker that is very close to the listening position (like a rear speaker in an off-road vehicle) and the sound appears to be coming from that near speaker only. I will adjust the time delay of that nearest speaker slightly so that the sound stage moves away from that speaker and appears to be more balanced with the other speakers in the vehicle. A good example is the rear speaker in the Polaris Slingshot, where the rear speaker is directly behind your head about 6 inches away.

[Kb86]

There are always cases when I give up "perfect" time alignment. The two most common are when a customer wants the sound to be more enveloping or surround-sound instead of precision focused in front. .

Hi, this is exactly what i am asking for.

Can you explain how adjust timing to be less perfect can make the sound more enveloping or surround like?

In my experiment, making midbass and midrange perfectly in time will lead to perfect sum at xover point (+6db) but crossover zone tend to be narrower.

Adjust timing to aim for wider crossover zone will make the sound blend better and more envelop?

Is that the case you talking here?

Would be great if you explain in technical perspective. This is very interesting.

Thank you.

[Justin Zazzi]

Hi, this is exactly what i am asking for.

Can you explain how adjust timing to be less perfect can make the sound more enveloping or surround like?

In my experiment, making midbass and midrange perfectly in time will lead to perfect sum at xover point (+6db) but crossover zone tend to be narrower.

Adjust timing to aim for wider crossover zone will make the sound blend better and more envelop?

Is that the case you talking here?

Would be great if you explain in technical perspective. This is very interesting.

Thank you.

I do not do this very tso it might be difficult to describe.

I always prefer to have good time/phase alignment in the lowest frequencies so that the system has more bass output. Everyone wants more bass output, and even if you don't want more bass output, then you can still have the benefit of less bass boosting. So when I say I sometimes give up on perfect time alignment, I mean perfect alignment in the not-subwoofer and not-midbass frequencies first.

I do not have a good scientific formula because it do not do this very often, but some things that we can try are changing the polrity and time delay of the tweeters, and if that doesn't get the effect then try adjusting polarity and timing of the midrange speakers. If you only have a woofer/tweeter 2-way, then try adjusting the time delay of the woofers very slightly to shift the phase of the midrange frequencies without disturbing the lower frequencies as much.

I would try to adjust the front speakers VS the rear speakers first, and if that doesn't work then try adjusting the left VS the right speakers. Sometimes polarity and timing of the rear speakers can make a huge difference in envelopment.

Sometimes it is also very simple, just make the rear speakers a little louder.

[Kb86]

Thank you

[Grinder]

I don't know if this has been covered already, so here goes...

I have read from one seemingly knowledgeable source that:

A: Port compression begins to reduce output at port velocities as low as 22m/s

...whereas another seemingly knowledgeable source says:

B: Port compression begins to reduce output at 60m/s.

What say you?

Here is a link to A: https://www.diymobileaudio.com/threa.../#post-4820809

Here is a link to B: https://www.diymobileaudio.com/threa.../#post-6241236

[Justin Zazzi]

I love Triticum Agricolam's method of taking more variables into account. I don't remember where their formulas come from but the theory of it seems sound. I wish we could also take the woofer's motor strength as a variable too since woofers with higher BL^2/Re ratios should be able to create more force per watt which means more air compression at port tuning and higher port velocities.

daloudin's post mentions they run the simulations at xmax. At port tuning when velocity is maximum, the cone motion is minimum. If the simulation is run at tuning frequency AND the cone is simulated moving all the way to xmax, wouldn't this result in dramatically higher port velocities than we would actually see? Or is the author saying they use xmax at other frequencies like below tuning frequency to look for air movement in the port. I am slightly unclear about the conditions of the simulation and what the data is showing in this particular example. I like how the author mentions there are other variables like proximity the listener, if the port is firing towards the listener or buried behind other stuff, etc.

Overall there are many variables that can determine how to design and it looks like both authors are trying to account for these instead of a one-size-fits-all method. But how do you choose which of these variables are important to you? If you're an SPL competitor then the sound of chuffing or other distortions might not matter as much as power compression and pure output right? If you are a pure SQ enthusiast then you have different tradeoffs you're willing to make. If you're making a commercial product and you know consumers like to shop with their eyeballs and big numbers in a review article are impressive, then there is motivation to make different design choices again.

The shape of the port matters too. Port area is super nonlinear in that a "two inch" port in the shape of a tube will behave much different from one in the shape of a rectangle. This is a boundary layer thing where air closest to the inside walls of the port is moving slowly but air in the center of the port (away from the walls) is moving fastest. The same thing is why water along the edge of a river is calm and not moving very fast, but you can find fast flowing water near the center of the river. Then the terminations on either end influence how the port "sounds" and if we get chuffing or other undesirable distortions. So many factors at play here, difficult to make super simple rules that apply to everyone.

By happy accident, I have some COMSOL training this week. Maybe I'll look at this port tuning and air velocity thing and see what I can find. It seems like there are two questions: how to minimize the port sounding "bad" and how to get the most output from it.

[Grinder]

I asked the question in hopes of gaining some insight as to whether or not it would be preferable in terms of output efficiency to increase port area beyond that which would be sufficient to prevent audible port noise, assuming very close proximity of port to listener (maximum potential for hearing any port noise).

Triticum's viewpoint seems to be that it would be preferable in terms of output efficiency to increase port area beyond that which would be sufficient to prevent audible port noise, particularly in the case of a non-flared port.

Daloudin's viewpoint seems to be that increased port area beyond that which would ordinarily prevent considerable port noise (certainly considerable port noise would be produced through a non-flared port at 60m/s, and perhaps even through an aeroport?) would yield no increase in output efficiency.

Although both of their points of view appear to be based on the premise that port compression = decreased output efficiency, they differ in terms of the rate of port velocity at which port compression causes a decrease in output efficiency

[2015TexasSuperDuty]

I’m trying to hook up my new double din aftermarket stereo to my 2015 Ford F250, I figured everything out but the steering wheel controls. I have what’s labeled on the bracket/harness, and the new stereo connected and steering wheel controls still do not work. The original stereo had two harnesses, could the steering wheel controls be located on the smaller 16 pin harness? I’ve scoured the Internet, and cannot find the answer to this, and finally found your thread here, hoping to gain some insight! Thanks for any help you guys can provide!

[2015TexasSuperDuty]

Also, is there a way to tap into the existing microphone above the driver seat for phone calls?

[Jdunk54nl]

That info should be available with a wiring diagram. You can get 48hr (or something like that) access for under $20. Grab all the electrical diagrams you need and you should be able to find your info.