 Working and studying on room correction I realised that I did not cover our auditory system. This is quite handy to know before you start with placing speakers and doing room correction. So how do we hear direction, localisation, tonal balance and spaciousness? Our auditory system, our hearing, is an extremely sophisticated system, even more sophisticated than we thought, say, 10 years ago for things concerning the brain are complex to evaluate. For the cognitive process involved with hearing is still for the most part a mystery. Compare our hearing with speech recognition as nowadays done by our smartphone, computer and systems like the Amazon Echo and Google Alexa. There's a microphone that picks up variations in air pressure caused by our voice. An analog signal is then converted into digital and sent to a computer in the cloud for interpretation. This interpretation is extremely complex and must require powerful computer systems, otherwise it would have been done inside your smartphone or computer. And this involves only interpreting content of spoken word and does not involve things like localisation and other spatial clues. Still, it can all be done by a system small enough to fit inside your head, our auditory system. It can be split up into four parts, the external ear, the middle ear, the inner ear and the brain. Comparable with the microphone, the mic amp, the analog to digital converter and the processor. The external ear starts at a pinna or ear cup that reflects the perceived sound into the ear canal. This canal amplifies the mid frequencies. At the end of the canal the sound hits the eardrum. As said, this can be seen as the microphone or rather the front part of the microphone, the membrane. On the other side of the eardrum we find three small bones, hammer, anvil and stirrup. They work as an impedance converter, more or less like a gearbox of your car. And the last bone, the stirrup, touches an aperture called the oval window of the cochlea. The cavity of the middle ear is filled with air through a pressure equalising tube called the Eustachian tube. It limits the lowest frequency we can hear since air pressure changes, that is sound, slower than about 20 times per second, that's 20 Hz, will reach the front and the back of the eardrum at the same time. The front via the auditory canal and the rear through the Eustachian tube. It is there to prevent the eardrum to react on atmospheric pressure changes. If the Eustachian tube is partly or totally blocked, for instance due to a cold, you feel the change in atmospheric pressure when driving up a mountain or when you are in a plane that is changing altitude. So the middle ear is comparable to the vacuum tube of veteran sister amplifier in the condenser mics plus the pressure equalising hole omni-directional mics have for the same reason we have the Eustachian tube. The oval window is found at the beginning of a rolled up tube that is called the cochlea. It is approximately 35 mm long. The tube is divided into three parts by two membranes and one of the membranes contain the organ of corti which holds the nerve terminals in the form of small hairs. The tubes are filled with the fluid and when the oval window is excited by the stirrup pressure waves will travel through the fluid in the cochlea and so stimulating the nerve terminals. Each frequency stimulates nerve terminals at another spot in the cochlea. This makes them fire pulses to the brain. Although things are changing due to advanced scanners that can register brain activities in real time, most of what we know of our auditory system is still based on physical tests. Suppose that a balloon explodes in a room. The resulting bang consists of nothing more than changing air pressure a bit but not completely like waves that arise as a stone is thrown into the water. Hence we talk about sound waves. The first pressure change caused by the bursting of the balloon reaches our ears through a direct part between the balloon and our ears. We call this the direct sound. Then the sound is reflected by the walls and reaches our ears a little later. These are called early reflections. The sound waves that are not directly stopped by our body or other obstacles in the room keep bouncing from wall to wall like balls on a pool table until all energy is gone. We call these reflections reverberation. All this information is used by our brain to determine the nature of the sound source, the place where it is located, the space in which it occurs and the sound timbre. In the bathroom there is a direct short loud reverberation with a bright character due to the short part the reflected sound travels, the straight walls that offer no dispersion and the reflective nature of materials used in bathrooms to cover the walls. In a church there is a full long reverberation with a larger delay between the direct and reflected sound. Also the shape of the church is more irregular, causing deflection and thus reducing early reflections. Let's take a look at how that works. Our hearing triggers on signals with a steep rise. Percussive sounds like the slamming of a door, the bursting of a balloon or the sound of percussion, drums and so on. The pinner reflects the signal into the auditory canal using a direction dependent coloration. So sound coming from one direction is colored differently than the sound coming from another direction due to interference patterns. So the coloration is a directional clue. A second one is caused by the difference in arrival time between both ears. The third is the difference in loudness between both ears. Since the head is in between the sound source and the far ear, that ear will hear a lower level signal. The spectral balance will be different too since high frequencies are stopped by the head as where the low frequencies, due to their longer wavelength, don't. And if the brain still has problems determining the location of the source, we turn our head a little, without noticing. The occurring changes are then used by our brain for extra information. The direct sound provides information about the direction from which the sound comes. Early reflections provide information about the location in space of the sound source and the distance between us, the sound source and the walls. While they can also blur the sound depending on the arrival time, length and density. The reverb that follows gives fullness to the sound. Due to the properties of our hearing, we want the early reflections to reach us after at least 5 milliseconds after the direct sound, otherwise they might merge with the direct sound and then cause coloration and errors in localization. 10 milliseconds or more is even better but not always realistic in a living room. Remember, sound travels 3.4 meters in 10 seconds. So if you have a speaker 1.7 meter of the wall behind the speaker, the reflected sound will be delayed by 10 milliseconds. By far not realistic in many living rooms on this side of the pond. Sounds that arrive 30 to 100 milliseconds after the direct sound will be seen as reverb. It will normally have a positive influence on the sound. Play music in an anechoic room and you will find it uninspiring, cold and non-immersive. Therefore you should be careful using absorbing materials in your room and rather use diffracting materials if possible. It might be clear that room acoustics play a key role in the sound quality of your stereo. If you, like me, have your stereo in the living room, in my case my setup one, you are limited to what you can do. Often the couch is against the wall, as with me. And that's not the best place since low frequencies hitting the wall will add up with the low frequencies reflected back from the wall. There is no real solution for that unless you want to sacrifice the sound in the rest of the room. The same goes for room modes that occur only on some spots in the room, although bass straps in corners might offer a solution, provided the aesthetics commission and the financial commission, often merged into one and the same significant other, agree. My solution is to place a chair at about two-thirds of the room away from the speakers when I want to seriously listen. I then use the parametric equalizers in room to cut problem frequencies when I'm on the couch and that's acceptable. The coming time I will investigate to what extent room correction offers agreeable solutions. But since acoustic problems are position bound, room correction will only work perfectly in a small area in the room. Over the coming period I will review several room correction solutions. And as always the equipment will cost a few hundred to max 2000 euros and no more. So if you are interested, subscribe to this channel or follow me on twitter, facebook or google plus. If you like this video, please consider supporting the channel through Patreon or Paypal. Any financial support is much appreciated, the links are in the comments. Help me to help even more people to enjoy music at home by telling your friends on the web about this channel. I am Hans Beekhuyzen, thank you for watching and see you in the next show or on theHBproject.com. And whatever you do, enjoy the music.