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diff --git a/www/devmaster/lesson7.html b/www/devmaster/lesson7.html index ddb54a3..4a716da 100644 --- a/www/devmaster/lesson7.html +++ b/www/devmaster/lesson7.html @@ -33,138 +33,144 @@ OpenAL Tutorials from DevMaster.net. Reprinted with Permission.<br> Maurais</a><br> Adapted For Java By: <a href="mailto:[email protected]">Athomas Goldberg</a></p> <h1>A Look at Real-World Physics</h1> -<p>I know this will be boring review for anyone with a course in high school -physics, but lets humour ourselves. The Doppler effect can be a very tricky -concept for some people, but it is a logical process, and kind of interesting -when you get right down to it. To begin understanding the Doppler effect we -first must start to understand what a "sound" really is. Basically a sound is -your minds interpretation of a compression wave that is traveling through the -air. Whenever the air becomes disturbed it starts a wave which compresses the -air particles around it. This wave travels outward from it's point of origin. -Consider the following diagram.</p> -<p><img src="sound_waves.jpg" width="150" height="132" hspace="5" vspace="0" border="0" align="left">In this diagram -(on the left) the big red "S" stands for the sources position, and the big -red "L" stands for (you guessed it), the Listener's position. Both source and -Listener are not moving. The source is emitting compression waves outward, which -are represented in this diagram by the blue circles. The Listener is -experiencing the sound exactly as it is being made in this diagram. The Doppler -effect is not actually present in this example since there is no motion; the -Doppler effect only describes the warping of sound due to motion.</p> -<p>What you should try to do is picture this diagram animated. When the source -emits a wave (the circles) it will look as though it is growing away from it's -point of origin, which is the sources position. A good example of a similar -effect is the ripples in a pond. When you throw a pebble into a calm body of -water it will emit waves which constantly move away from the point of impact. -Believe it or not this occurs from the exact same physical properties. But what -does this have to do with the Doppler effect? Check out the next diagram (on the -right).</p> +<p align="justify">I know this will be boring review for anyone with a course + in high school physics, but lets humour ourselves. The Doppler effect can be + a very tricky concept for some people, but it is a logical process, and kind + of interesting when you get right down to it. To begin understanding the Doppler + effect we first must start to understand what a "sound" really is. + Basically a sound is your minds interpretation of a compression wave that is + traveling through the air. Whenever the air becomes disturbed it starts a wave + which compresses the air particles around it. This wave travels outward from + it's point of origin. Consider the following diagram.</p> +<p align="justify"><img src="sound_waves.jpg" width="150" height="132" hspace="5" vspace="0" border="0" align="left">In + this diagram (on the left) the big red "S" stands for the sources + position, and the big red "L" stands for (you guessed it), the Listener's + position. Both source and Listener are not moving. The source is emitting compression + waves outward, which are represented in this diagram by the blue circles. The + Listener is experiencing the sound exactly as it is being made in this diagram. + The Doppler effect is not actually present in this example since there is no + motion; the Doppler effect only describes the warping of sound due to motion.</p> +<p align="justify">What you should try to do is picture this diagram animated. + When the source emits a wave (the circles) it will look as though it is growing + away from it's point of origin, which is the sources position. A good example + of a similar effect is the ripples in a pond. When you throw a pebble into a + calm body of water it will emit waves which constantly move away from the point + of impact. Believe it or not this occurs from the exact same physical properties. + But what does this have to do with the Doppler effect? Check out the next diagram + (on the right).</p> -<p> -<img src="doppler_effect.jpg" width="150" height="132" hspace="5" border="0" align="right">Wow, what's going on here? The source is now in motion, indicated by the -little red arrow. In fact the source is now moving towards the Listener with an -implied velocity. Notice particularly that the waves (circles) are being -displaced inside each other. The displacement follows the approximate path of -the source which emits them. This is the key to the Doppler effect. Essentially -what has happened is that the source has emitted a wave at different points in -it's path of travel. The waves it emits do not move with it, but continue on -their own path of travel from the point they were emitted.</p> -<p>So how does this effect the perceived sound by the Listener? Well, notice too -in the last diagram that the waves (circles) that are between the source and the -Listener are kind of compressed together. This will cause the sound waves to run -together, which in turn causes the perceived sound seem like it's faster. What -we are talking about here is frequency. The distances between the waves effects -the frequency of the sound. When the source that emits the sound is in motion, -it causes a change in frequency. You may notice too that distance between the -waves varies at different points in space. For example, on the opposite side of -the moving source (anywhere along the previous path of travel) the distances are -actually wider, so the frequency will be lower (the distance and frequency have -an inverse relationship). What this implies is that the frequency perceived by -the Listener is relative to where the Listener is standing. </p> -<p>The motion of the Listener can also affect the frequency. This one is a -little harder to picture though. If the source is still, and the Listener is -moving toward the source, then the perceived frequency by the Listener will be -warped in the same exact manner that we described for the moving source. </p> +<p align="justify"> <img src="doppler_effect.jpg" width="150" height="132" hspace="5" border="0" align="right">Wow, + what's going on here? The source is now in motion, indicated by the little red + arrow. In fact the source is now moving towards the Listener with an implied + velocity. Notice particularly that the waves (circles) are being displaced inside + each other. The displacement follows the approximate path of the source which + emits them. This is the key to the Doppler effect. Essentially what has happened + is that the source has emitted a wave at different points in it's path of travel. + The waves it emits do not move with it, but continue on their own path of travel + from the point they were emitted.</p> +<p align="justify">So how does this effect the perceived sound by the Listener? + Well, notice too in the last diagram that the waves (circles) that are between + the source and the Listener are kind of compressed together. This will cause + the sound waves to run together, which in turn causes the perceived sound seem + like it's faster. What we are talking about here is frequency. The distances + between the waves effects the frequency of the sound. When the source that emits + the sound is in motion, it causes a change in frequency. You may notice too + that distance between the waves varies at different points in space. For example, + on the opposite side of the moving source (anywhere along the previous path + of travel) the distances are actually wider, so the frequency will be lower + (the distance and frequency have an inverse relationship). What this implies + is that the frequency perceived by the Listener is relative to where the Listener + is standing. </p> +<p align="justify">The motion of the Listener can also affect the frequency. This + one is a little harder to picture though. If the source is still, and the Listener + is moving toward the source, then the perceived frequency by the Listener will + be warped in the same exact manner that we described for the moving source. +</p> <p>If you still have trouble picturing this, consider the following two diagrams:</p> <p align="center"><img border="0" src="sin_wave.jpg" width="255" height="135"> <img border="0" src="compress_sin_wave.jpg" width="255" height="135"></p> -<p>These two diagrams will represent the sound in the form of a sine wave. Look -at the first one. Think of the peaks as the instance of the wave. The very top -point of the wave will be the same as the instance of the blue circle in the -previous set of diagrams. The valleys will be like the spaces in between the -blue circles. The second diagram represents a compressed wave. When you compare -the two you will notice an obvious difference. The second diagram simply has -more wave occurrences in the same amount of space. Other ways of saying this are -that they occur more often, with a greater regularity, or with a greater -frequency. </p> -<p>For anyone who is interested in some added information: The velocity of the -waves is the speed of sound. If the velocity of the source is greater than that -of the wave, then the source is breaking the sound barrier.</p> +<p align="justify">These two diagrams will represent the sound in the form of + a sine wave. Look at the first one. Think of the peaks as the instance of the + wave. The very top point of the wave will be the same as the instance of the + blue circle in the previous set of diagrams. The valleys will be like the spaces + in between the blue circles. The second diagram represents a compressed wave. + When you compare the two you will notice an obvious difference. The second diagram + simply has more wave occurrences in the same amount of space. Other ways of + saying this are that they occur more often, with a greater regularity, or with + a greater frequency. </p> +<p align="justify">For anyone who is interested in some added information: The + velocity of the waves is the speed of sound. If the velocity of the source is + greater than that of the wave, then the source is breaking the sound barrier.</p> <h1>The Physics of OpenAL</h1> -<p>Ok, either you have understood my ramblings on the Doppler effect from above, -or you have skipped it because you already have full knowledge of the Doppler -effect and just want to know how it effects the OpenAL rendering pipeline. I -think the best start to his section will be to quote the OpenAL spec directly:</p> +<p align="justify">Ok, either you have understood my ramblings on the Doppler + effect from above, or you have skipped it because you already have full knowledge + of the Doppler effect and just want to know how it effects the OpenAL rendering + pipeline. I think the best start to his section will be to quote the OpenAL + spec directly:</p> <blockquote> - <p><i>"The Doppler Effect depends on the velocities of Source and Listener - relative to the medium, and the propagation speed of sound in that medium." - - chapter 3, subsection 7"</i></p> + <p align="justify"><i>"The Doppler Effect depends on the velocities of + Source and Listener relative to the medium, and the propagation speed of sound + in that medium." - chapter 3, subsection 7"</i></p> </blockquote> -<p>We can take this to mean that there are 3 factors which are going to affect -the final frequency of the sound heard by the Listener. These factors are the -velocity of the source, the velocity of the Listener, and a predefined speed of -sound. </p> -<p>When we refer to a "medium", what we mean is the kind of material that both -the source and Listener are "in". For example, sounds that are heard from -underwater are much different than sounds that are heard in the open air. Air -and water are examples of different mediums. The reason that sound is so -different between these mediums has to do with the particle density. As we said -before, sound is nothing but the motion of particles in the air. In a medium -with a much greater particle density the sound will be much different because -the particles are in closer contact. When they are in closer contact it allows -for the wave to travel much better. As an example of the opposite, think of -outer space. In outer space there is an extremely low particle density. In fact -there is only a few very light particles (mostly hydrogen) scattered about. This -is why no sound can be heard from space. </p> +<p align="justify">We can take this to mean that there are 3 factors which are + going to affect the final frequency of the sound heard by the Listener. These + factors are the velocity of the source, the velocity of the Listener, and a + predefined speed of sound. </p> +<p align="justify">When we refer to a "medium", what we mean is the + kind of material that both the source and Listener are "in". For example, + sounds that are heard from underwater are much different than sounds that are + heard in the open air. Air and water are examples of different mediums. The + reason that sound is so different between these mediums has to do with the particle + density. As we said before, sound is nothing but the motion of particles in + the air. In a medium with a much greater particle density the sound will be + much different because the particles are in closer contact. When they are in + closer contact it allows for the wave to travel much better. As an example of + the opposite, think of outer space. In outer space there is an extremely low + particle density. In fact there is only a few very light particles (mostly hydrogen) + scattered about. This is why no sound can be heard from space. </p> -<p>Ok, lets get back on topic. OpenAL calculates the Doppler effect internally -for us, so we need only define a few parameters that will effect the -calculation. We would do this in case we don't want a realistic rendering. -Rather if want to exaggerate or deemphasize the effect. The calculation goes -like this.</p> +<p align="justify">Ok, lets get back on topic. OpenAL calculates the Doppler effect + internally for us, so we need only define a few parameters that will effect + the calculation. We would do this in case we don't want a realistic rendering. + Rather if want to exaggerate or deemphasize the effect. The calculation goes + like this.</p> <p><span class="codeNormal"> shift = DOPPLER_FACTOR * freq * (DOPPLER_VELOCITY - l.velocity) / (DOPPLER_VELOCITY + s.velocity)</span></p> -<p>Constants are written in all caps to differentiate. The "l" and "s" variables -are the Listener and source respectively. "freq" is the initial unaltered -frequency of the emitting wave, and "shift" is the altered frequency of the -wave. The term "shift" is the proper way to address the altered frequency and -will be used from now on. This final shifted frequency will be sampled by OpenAL -for all audio streaming that is affected. </p> +<p align="justify">Constants are written in all caps to differentiate. The "l" + and "s" variables are the Listener and source respectively. "freq" + is the initial unaltered frequency of the emitting wave, and "shift" + is the altered frequency of the wave. The term "shift" is the proper + way to address the altered frequency and will be used from now on. This final + shifted frequency will be sampled by OpenAL for all audio streaming that is + affected. </p> -<p>We already know that we can define the velocity of both source and Listener -by using the 'AL_VELOCITY' field to 'alListenerfv' and 'alSourcefv'. The 'freq' -parameter comes straight from the buffer properties when it was loaded from -file. To set the constant values the following functions are provided for us.</p> +<p align="justify">We already know that we can define the velocity of both source + and Listener by using the 'AL_VELOCITY' field to 'alListenerfv' and 'alSourcefv'. + The 'freq' parameter comes straight from the buffer properties when it was loaded + from file. To set the constant values the following functions are provided for + us.</p> <pre class=code><font color="#0000FF">public void </font>alDopplerFactor(<font color="#0000FF">float</font> factor); <font color="#0000FF">public void </font>alDopplerVelocity(<font color="#0000FF">float</font> velocity); </pre> -<p>For 'alDopplerFactor' any non-negative value will suffice. Passing a negative -value will raise an error of 'AL_INVALID_VALUE', and the whole command will be -ignored. Passing zero is a perfectly valid argument. Doing this will disable the -Doppler effect and may in fact help overall performance (but won't be as -realistic). The effect of the Doppler factor will directly change the magnitude -of the equation. A value of 1.0 will not change the effect at all. Passing -anything between 0.0 and 1.0 will minimize the Doppler effect, and anything -greater than 1.0 will maximize the effect. </p> -<p>For 'alDopplerVelocity' any non-negative non-zero value will suffice. Passing -either a negative or a zero will raise an error of 'AL_INVALID_VALUE', and the -whole command will be ignored. The Doppler velocity is essentially the speed of -sound. Setting this will be like setting how fast sound can move through the -medium. OpenAL has no sense of medium, but setting the velocity will give the -effect of a medium. OpenAL also has no sense of units (kilometer, miles, -parsecs), so keep that in mind when you set this value so it is consistent with -all other notions of units that you have defined.</p></p> +<p align="justify">For 'alDopplerFactor' any non-negative value will suffice. + Passing a negative value will raise an error of 'AL_INVALID_VALUE', and the + whole command will be ignored. Passing zero is a perfectly valid argument. Doing + this will disable the Doppler effect and may in fact help overall performance + (but won't be as realistic). The effect of the Doppler factor will directly + change the magnitude of the equation. A value of 1.0 will not change the effect + at all. Passing anything between 0.0 and 1.0 will minimize the Doppler effect, + and anything greater than 1.0 will maximize the effect. </p> +<p align="justify">For 'alDopplerVelocity' any non-negative non-zero value will + suffice. Passing either a negative or a zero will raise an error of 'AL_INVALID_VALUE', + and the whole command will be ignored. The Doppler velocity is essentially the + speed of sound. Setting this will be like setting how fast sound can move through + the medium. OpenAL has no sense of medium, but setting the velocity will give + the effect of a medium. OpenAL also has no sense of units (kilometer, miles, + parsecs), so keep that in mind when you set this value so it is consistent with + all other notions of units that you have defined.</p> +</p> <table border="0" cellspacing="1" style="border-collapse: collapse" width="100%" id="AutoNumber2" bgcolor="#666699"> <tr> <td width="40%"> <p dir="ltr"><font color="#FFFFFF" size="2">� 2003 DevMaster.net. |