Before we take the car environment into consideration, I'm going to give a tutorial upon the human ear. It is imperative that you understand how we percieve sound in order to achieve good results within a vehicle. This is going to be a VERY long post, for your warning.
Human perception of sound
The backbone of human hearing is due to stream formation. Primary sound events MUST be separated from secondary sounds. Primary sound would be the source signal, secondary being reflections, reverberations, etc. Primary sounds would be in streams-being in a common direction, a common timbre. A set of sound events from a single source becomes a sound stream.
Intelligibility of a sound is dependant upon the separation of these sound streams from secondary sources. This means we have to minimize reflected energy and noise. Fluctuations in the level within a given time period are percieved as a sense of distance and "air". These fluctuations are caused by reflections. Time periods are crucial here, though. If a reverberation occurs quickly (within ~50ms of the original signal), we percieve that as having space around the source. If longer than that, we perceive these reflections as space around the listener. This is referred to as envelopment. Reflections are how we percieve distance as well. If the reflected energy occurs within around 150ms of the original signal, we percieve that as distance.
Now, the first set of problems. Reflections up to ~50ms do not impair intelligibility. Reflections from 50-150ms add to the perception of distance, but they also create muddy sound
Quick summary:
Primary sound stream: The major portion of the sound. In music, this would be the band, a vocalist. Intelligibility is the indicator for this one.
Secondary sound stream: An example would be background noise. Sounds that occurs a long time after the end of a primary sound stream. Reverberation is the key to this one.
Spatial impression: This would be an early reflection that occurs very shortly after the primary sound. The perception is the presence of the music ("air", distance, acoustic space).
Last one relates to secondary reflections. Late reflections of the original source. We percieve this as distance and/or muddy sound (loss of intelligibility).
Imaging and Localization
Localization is related to our above points. For localization, we depend upon:
IID
Interaural Intensity Difference. The difference in the intensity of sound arriving at the two ears. Volume level, basically.
ITD
Interaural Time Difference. The difference in the time of arrival of a sound wave at the two ears. This is primarily dependant upon pathlengths, but in a car we also have to take reflections into account. More on that in the next tutorial
Pinnae
The outer ear. When sound comes in contact with the outer ear, its frequency characteristics are modified. These modifications vary depending on the position of the sound source and provide an important directional cue.
Those are the basic things to take into account for imaging. But wait, there's more:
Distance. How do we percieve distance? The brain perceives distance as a function of the relative amount of reverberant or reflected sound to the amount of direct or unreflected sound received by the ear. This quantity is expressed as the reverberant/direct (R/D) ratio. This is critical in acoustic engineering, especially when referring to architecture.
The Haas (precedence) effect. It's also called "the law of the first wave front". This one is VERY related to car audio. The Haas effect describes our perception of imaging when using two or more sources. Note that this is a BIG explanation of situations with differing pathlengths (such as in a vehicle). After hearing a signal, the ears will suppress any subsequent signals for about 25-35 milliseconds. If the difference is greater than 25-35ms, we hear two distinct sounds. Here we're describing the human psychoacoustic phenomena of correctly identifying the direction of a sound source heard in both ears but arriving at different times. Obviously we have two ears spaced apart, which are separated by a barrier. The direct sound from any source first enters the ear closest to the source, then the ear farthest away. The Haas Effect tells us that humans localize a sound source based upon the first arriving sound, if the subsequent arrivals are within 25-35 milliseconds. If the later arrivals are longer than this, then two distinct sounds are heard. This is even true if the later sound is louder than the first (even by 10db-twice as loud).
So what does that mean? It means that if your right speaker is delayed in comparison to the left (within the time window), the total sound will actually seem to come from the left speaker alone. But what's also important: sound arriving at both ears simultaneously is heard as coming from straight ahead, or behind, or within the head. To simply sum this up: You have two sources. Source A, source B. The sound created at source A, which is closer to the listener than source B, arrives first. To the listener, this creates the impression that A is the only source of the sound. If both arrive simultaneously, the sound seems to come from in front of you, behind you, or within your head.
Now, localization after all these terms. Requirements:
- The rise-time of the sound event must be more rapid than the rise-time
of the reverberation
- The IID (Interaural Intensity Difference) and the ITD (Interaural Time Difference) must be unaffected by reflections, and during this period we detect the direction of the sound source. Then the brain holdsthe detected direction during the reverberant part of the sound.
- The conversion between IID and ITD and the perceived direction
is simple in natural hearing, but complex (and unnatural) when
sound is panned between two loudspeakers.
Our perception at different frequencies
HRTF
Head Related Transfer Function. The HRTF represents the spectral or frequency component filtering that occurs to a sound as it travels from outside the head into the ear canal. HRTF captures all of the physical cues to source localization. This is EXTREMELY complicated and takes four variables into account: three space coordinates (azimuth, elevation & range) and frequency. To make matters even worse, they change from person to person. HRTF is dependant upon YOU. Your head shape, the space between your ears, your shoulders, amongst many different variables. Next point:
Panning of two speakers
The pressure at each ear is the sum of the direct sound pressure from one speaker and the diffracted sound pressure from the other, which interfere with one another and create a highly frequency dependent signal.
A two channel pan is completely different from the localization process in natural hearing.
Why?
Localization is dependant upon IID and ITD. Naturally these alone vary due to HRTF. With two channels, IID and ITD vary substantially because of the above described interference. This is physical, not perceptual. It's measurable and able to being calculated. This makes ITD and IID highly frequency dependent.
Expectation
Hey, this one's easy. We have a visual field or expected location in mind. We also go on a "past history" type of perception. Basically, this can be proven by playing a known source such as speech. We could then alter the bandwidth and/or frequency of the source and we will still percieve this as having the same localization (despite the drastic differences in IID and ITD). On top of that, our thought process tends to guess at localization over the entire stream, and not the localization of the individual streams. This is also in relation to our hearing mechanism being reluctant to change it's perception.
Width
I'll make it short and sweet. Syllabic sources (when one note is related to one syllable in the text) are percieved as being very sharp and narrow. Broadband sources (such as an instrumental section, chorus) are percieved as being much wider than they are due to inconsistencies in IID and ITD.
Applying this in the car
Nothing is perfect. The two properties you need to consider the most will be IID and ITD. I'll keep this simple:
Our perception of imaging is mostly dependant on ITD below 500hz or so, in fact volume level isn't very important below that point at all.
From 500hz to about 1500hz, IID becomes increasingly important. You want to have good performance in both areas.
Above 1500hz, we become less dependant upon ITD and more dependant upon IID for imaging.
At around 6000hz, our heads create an acoustic shadow. IID is the dominant form of percieved imaging-which means pathlengths aren't as important.
More to come in the next tutorial
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Choosing midbass drivers
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USACi Certified SQ Judge
MECA Certified SQ Judge
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CVD Audio Sales and Acquisitions Team
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Part 2-The Human Ear
The above relates mainly to our ability to localize sources in the horizontal plane , which would be a center image, stage width, etc. But how do we localize sources in the vertical plane, which gives us our perception of stage height?
HRTF
Defined above. It is our heads and ears that allow us to perceive stage height. Consider the anatomy of the human head, though. It is essentially a sphere with two holes in the sides. Which means that sounds of ALL directions, from ANY point source could absolutely NOT be localized vertically by our ear canals alone.
The Pinnae
Outer ear, again described in the last tutorial. Our perception of stage height depends solely upon the frequency modifications that the shape of our outer ears create. Look in the mirror. Obviously the shape of your ear would cause frequencies to be modified in different ways from sources of different heights.
This relates to two points above, HRTF and Expectation. Our brains remember how our ears modify given sounds, and due to this memory, it is how we perceive stage height. Meaning that if a sound comes from below you, our brain has predetermined what characteristics a sound from below would have. And the same applies to a sound emanating from above.
What frequencies are important for stage height
The dimensions of the outer ear can only have an effect upon frequencies above ~2kHz. Which means frequencies below that point can be physically low and still be perceived as high. That is assuming the 2khz and up region is high or is perceived as high
)
The most important point of this post: Our ears do not perceive physical height, they perceive the frequency modifications that our ears make upon sources of different heights.
What does that mean in your car? It means that you have to replicate the characteristics of a high stage.
Second point: Your drivers (even tweeters) do NOT have to be physically high to have stage height
One way to get stage height would be to make sure frequencies above 2khz are physically high. BUT, what about tweets down low, such as doors or kick panels? If you tune your tweeters to replicate the frequency characteristics your ears would have if they were mounted higher, you will perceive just as high a soundstage That's a BIG point. It means that if your tweeters are low, you can tune them to replicate a frequency response associated with a higher source. A lot of times you reap the benefits of not being directly beside a reflective surface as well.
Later on I'll try to give more specific info on where to tune to accomplish this, but I need to finish all the primers
Also, Glass, with this ever expanding thread, I think the FAQ title should perhaps be something other than choosing component systems. It's turning into a SQ tutorial in combination with choosing equipment. And hopefully in the end it'll help the guys that already have their system and want to improve upon it.
The above relates mainly to our ability to localize sources in the horizontal plane , which would be a center image, stage width, etc. But how do we localize sources in the vertical plane, which gives us our perception of stage height?
HRTF
Defined above. It is our heads and ears that allow us to perceive stage height. Consider the anatomy of the human head, though. It is essentially a sphere with two holes in the sides. Which means that sounds of ALL directions, from ANY point source could absolutely NOT be localized vertically by our ear canals alone.
The Pinnae
Outer ear, again described in the last tutorial. Our perception of stage height depends solely upon the frequency modifications that the shape of our outer ears create. Look in the mirror. Obviously the shape of your ear would cause frequencies to be modified in different ways from sources of different heights.
This relates to two points above, HRTF and Expectation. Our brains remember how our ears modify given sounds, and due to this memory, it is how we perceive stage height. Meaning that if a sound comes from below you, our brain has predetermined what characteristics a sound from below would have. And the same applies to a sound emanating from above.
What frequencies are important for stage height
The dimensions of the outer ear can only have an effect upon frequencies above ~2kHz. Which means frequencies below that point can be physically low and still be perceived as high. That is assuming the 2khz and up region is high or is perceived as high
)The most important point of this post: Our ears do not perceive physical height, they perceive the frequency modifications that our ears make upon sources of different heights.
What does that mean in your car? It means that you have to replicate the characteristics of a high stage.
Second point: Your drivers (even tweeters) do NOT have to be physically high to have stage height
One way to get stage height would be to make sure frequencies above 2khz are physically high. BUT, what about tweets down low, such as doors or kick panels? If you tune your tweeters to replicate the frequency characteristics your ears would have if they were mounted higher, you will perceive just as high a soundstage That's a BIG point. It means that if your tweeters are low, you can tune them to replicate a frequency response associated with a higher source. A lot of times you reap the benefits of not being directly beside a reflective surface as well.
Later on I'll try to give more specific info on where to tune to accomplish this, but I need to finish all the primers
Also, Glass, with this ever expanding thread, I think the FAQ title should perhaps be something other than choosing component systems. It's turning into a SQ tutorial in combination with choosing equipment. And hopefully in the end it'll help the guys that already have their system and want to improve upon it.
Frequency dependant. How do you have the drivers installed, especially the tweeters?car1990 wrote:Jonathan,
Since my x-over region is about 2K for woofer and tweeter, for imaging purpose, I persume the bigger impacts will come from adjusting time-alignment of woofer (ITD) and adjusting SP gain on tweeter (IID)?
T/A of the woofers should help your imaging (especially center image). Your tweet will benefit from bothj T/A and level adjustment given the lower x/o point.