Originally Posted by Archaea
IMO --- nearfield sealed subs need to be immediately next to the seats --- firing into the bottom, back, etc. Probably ideally within the proximity of the driver's diameter.
IE --- if an 18" sub - the cone should be within 18" of your back. Luke Kamp said something to me like that one time, and I generally agree with his preference. I started nearfield with three 10" Infinity drivers in sealed boxes, and while it was fantastic if they were immediately next to the back of the chair, I got basically nothing by the time I put them on the floor. HUGE tactile feedback as compared to nothing. As carp mentioned earlier - when he leans back over his angled driver as his chair reclines - it produces much more feel. Same here, though my boxes aren't angled. Just reclining the chair back is a BIG difference.
There's a lot of good discussion with regards to the near field and its benefits. I wanted to take a moment and define it based on what I've researched. Caveat: I'm an enthusiast not a scientist.
I originally talked about the nearfield almost 4 years ago in this post
I will try and summarize it below and use the ISO 12001 definition of near field:
First defining Sound Intensity:
Sound Intensity (SIL) = pressure (SPL) * particle velocity (PVL)
Sound intensity has a direction as particle velocity is a vector
quantity. This means it has a magnitude and direction. Pressure is a scalar
quantity and has a magnitude, but no direction.
Sound Energy is a form of energy associated with the vibration of matter.
Sound Intensity is sound energy per unit time per unit area.
Sound Intensity is how we 'feel' sound on our bodies.
While bass waves are said to be omnidirectional (meaning we cannot locate the sound with our ears), the intensity of those waves are directional. This means we can feel where the waves are coming from in the right conditions. (e.g. Feel a NF sub with the driver pointed directly at your LP and then turn it 180 degrees away and see if you can feel the difference; the answer is yes.).
Now, on to Sound Fields...
Sound travels through various Sound Fields.
What does this all mean in practice...
- Far Field - Pressure and Particle Velocity are in phase. In this sound field, because they are in phase, when SPL peaks, so does PVL. Therefore, to understand Intensity, you just need to measure one of the quantities (SPL). When we measure SPL in the far field, we can expect that the Intensity will be the same regardless of the distance (e.g. 115db 25ft away will feel the same as 115db 40ft away).
- Near Field - Pressure and Particle Velocity are NOT in phase (as defined by ISO 12001). This means that when PVL is at its peak, SPL is not. In fact, in the Near Field, PVL is greater than SPL in the Near Field (see page 2-15 in this paper). This is commonly referred to as "the near field effect". This means to get an understanding of Intensity (or how we feel sound) in the near field, we need to know SPL and PVL.
- Near Field Region - ISO 12001 (see 184.108.40.206 in prior link) defines that a frequency is in the near field if it is within a wavelength. An 80hz wave is 13.7Ft. A 20hz wave is 54.8Ft. Given those lengths and the placement of our subs from the LP, most bass waves in the typical home theater room will be considered in the near field; meaning to understand Intensity or how we feel sound, SPL measurements are just not enough. We also need to understand PVL.
- The Very Near Field It is the sound field that is very close to the source (see page 2-16 in prior link) where it behaves more like an incompressible fluid. This means there is very little SPL, and mostly PVL. The paper describes that this region occurs when the distance r from the moving object (speaker cone) is much smaller than both the wavelength *and* the object dimension L (eq. 2.36), which in our case would correspond to the driver diameter. Being that these ULF frequencies are much greater in length than the typical subwoofer diameter, the very near field would be the driver diameter (15in, 18in, 21in, depending on sub).
The majority of sub frequencies in our HT's exist in the near field. This means that just understanding SPL is not enough to understand Intensity, or how we 'feel' sound.
Sound intensity is dependent on distance. The closer to the source, the more intensity you will have. It is also directional.
In the Very Near Field
's description), the physics starts to enter fluid dynamics and starts to behave differently than the near field. It is said that this has high levels of PVL in this field relative to SPL.
To optimize (have the highest levels) Intensity:
- Put the sub as close as possible. You'll get additional Intensity if the proximity of the LP is less than the driver diameter of the sub.
- Use ported, horn, bandpass, etc.; anything with a vent. Hornresp has modeled particle velocity, and the models show increased levels of PVL compared to sealed in the near field. However, what it does not model is the behavior of the Very Near Field.
Measuring Sound Intensity:
1st - Microflown Acoustic Camera
2nd - Sound Intensity Mapping of a Speaker
3rd - Low Frequency Mapping of Sound Intensity of Speaker/Subwoofer