Originally Posted by dominguez1
This is a very interesting observation...and one that has me scratching my head a bit as it doesn't seem intuitive to at least how I understand it (and I'm no expert, mind you).
Sound Intensity is sound energy per unit volume. It is what we 'feel' from a sound source. The equation for Sound Intensity is:
I = p * v
I (Sound Intensity Level) = pressure (SPL) * particle velocity (PVL)
All are measured logarithmically in db.
When in the nearfield, PVL is the dominant quantity; meaning that any change in Intensity (tactile feel) in the nearfield is related to fluctuations in PVL as opposed to SPL. This was demonstrated in my test here
. To summarize, I measured the tactile feeling of a sub 13ft away at a constant SPL, and measured a sub 3ft away at the same SPL, and the sub 3ft away quantitatively had far more tactile feeling. Two subs, different distances, exact same SPL, but felt entirely different. The reason? Give the equation above, it could only mean higher levels of PVL and thus more Sound Intensity (since SPL is kept constant).
I often refer to PVL as the cause of tactile feeling, when really it is Sound Intensity, but in the nearfield, PVL is the driver for the increased Intensity.
Back to your observations...I would have thought that tactile feeling would be directly related to the frequency response; a null at a certain frequency would reduce SPL and thus Sound Intensity, just as a peak would increase SPL and Sound Intensity as well.
You observed quite the opposite; essentially that the tactile feeling was NOT linked to the FR but more to whether or not the sound waves were in phase. Time alignment was the driver for great tactile feel, not FR (or spl levels).
I don't quite understand the science behind that...not saying its wrong, just don't understand. Particle Velocity and Pressure have can be in or out of phase with respect to each other, but wasn't sure if particle velocity can be out of phase by itself as combines with different waves (in your case another sub).
Perhaps this is what's happening:
Let's say that the nearfield sub produces 100db at 15hz by itself. Let's also assume that the farfield sub also produces 100db at 15hz by itself. But, when you turn both on, the summed output is only 90db (i've experienced this before); I would assume this means that they are not in phase. If they were perfectly in phase, you would get 106db.
Same two subs, but now they are in phase. So the nearfield sub produces 84db at 15hz, as does the farfield sub. Since they are perfectly in phase, the summed SPL level is 90db.
Here's the question: Which scenario will have more tactile feel?
There is more Sound Power being produced in Scenario A, even though it nets to the same level SPL as Scenario B. Does that mean that there's more tactile feel? My qualitative observations in my room say yes, but I don't understand the science behind it.
Going to have to call for backup...
or others, please weigh in?
Originally Posted by Okv
I would think a would have a larger velocity potential, out-of-phase sources creates velocity.
To be sure, you would need to test it.
Listen and experience, then measure, both velocity and pressure, with this information you can then try to see if there is a correlation between measurements and listening experience.
I have tested many different configurations, and generally the ones with flat (smooth) fr and phase, clean and nice step response and at the same time a decent velocity potential sounds best.
A double bass array is one example of a setup that gives a predictable behavior for both pressure and velocity.
So I decided to test my hypothesis...
I'm testing a single frequency (27hz) using a sine wave. 27hz is one of the resonant frequencies of my room.
I placed my android phone with the Vibration Meter app at my main LP. Looped the 27hz sine wave and recorded the results. I also changed the phase of my pair of FTW21s and left the phase of my FV15HPs alone.
The Vibration Meter recorded a mean of .6 on the phone.
Recorded Max SPL was 120.4 db
Phase was at ~125 degrees.
In this video, I only changed the phase (kept the main volume the same). As a result of changing the phase:
Vibration Meter recorded a mean of .5 on the phone.
Recorded Max SPL was 104.1 db
Phase was at 0 degrees
That's a 16db swing just by changing the phase!
You can see that there was a very slight difference in tactile feel as the mean dropped by .1. I think for all intensive purposes we can say they are pretty much equal in tactile feeling. So even after a 16db swing in SPL, the tactile feeling is pretty much the same.
So I decided to match the SPL of the first test when the subs were in phase. In the first test, the subs produced 120.4 db. I turned up the main volume by 16db and reran the test with the subs being out of phase.
Vibration Meter recorded a mean of 1.4
Recorded Max SPL was 120.8 db
Phase was at 0 degrees
To summarize, I had two different scenarios producing ~120db, but one had double/almost triple the tactile impact. Same SPL, same distances, far different tactile feedback.
So not only does distance have an impact on tactile feedback (nearfield vs farfield), but phase also plays a big part of tactile feedback; specifically, given a constant SPL, out of phase conditions will produce more PVL, causing more Sound Intensity and thus tactile feedback.
If you think about it, it is intuitive. Think about it from a Sound Power standpoint...in the first test the subs aren't working that hard, but in the third test, they are working a lot harder to produce the same SPL. As a result, even though SPL is the same, there is more Sound Power in the third scenario, thus producing more Sound Intensity and tactile feeling.
This is just another example of how SPL (and thus frequency response) only tells a part of the story...and for this region of frequencies (LF/ULF), one could argue that this is the more important quantity to focus on...yet, most often don't because they don't know about or have heard of PVL or Sound Intensity, or don't have the tools to properly dial it in.