Measuring "Right"
Now you're obviously asking how you measure the "right" way, or "listen right". Well, truth be told, it's not very easy to relay over the internet. It really takes time and practice. Luckily there are a few sources already that I think do a good job of illustrating the methods of measuring and listening. Here's some links:
Analyzing the Results:
One thing that really needs to be done is spatial averaging. The two measurement tutorials above discuss this but I want to reiterate it because it's just that important. If you'd like additional information, please read the attached PDF in this post . The document is by Dr. Earl Geddes and is a study he did for Ford Automotive. It's not too technically heavy which should keep the "I don't like science" excuses to a minimum.
Note: The following is plucked straight from my build log.
The cliffs version is simply this: your car (and home) are wrought with reflection inducing panels/walls. When measuring response in the environment, you have two options:
Doing the first in the car?... good luck. How about... don't bother. At least not until you've gotten really good at measuring and understanding what you're measuring. Let's just say for all intents and purposes you won't be doing the first... like... ever (thank you, Taylor Swift, for making that phrase weird now). Really, it's just pretty much trivial unless you have a very specific goal and understanding of how to achieve it this way.
So, we do the second option. The issue, then, is the fact every measurement you take is a measurement of EVERYTHING occurring at the mic. This is good and it's bad. It's good in the way that there's not a whole lot you can do to the speaker itself so it kind of keeps you from worrying about it. - Although, this is why I really encourage people to study independent tests or do their own to understand the issue(s) with the speakers they've chosen before they use them in the car. - It's bad because, thanks to the nature of the reflective environment, you can't really trust a single point measurement (a measurement taken with the mic in one location). If you move the mic as little as one-half inch you'll get a different result. Most notably in the higher frequencies. This means RTA'ing your car for any desired curve by using one mic measurement is a TOTAL WASTE OF TIME. It's ideal to take multiple measurements in the "head area" and average them together. TrueRTA, OmniMic, and REW allow you to do this pretty easily. Then you have what is known as a good spatial average. It's not an exact method but it's the most realistic and approximates a very realistic response in the seated position.
For this spatial average there are a couple methods: one is a 'live average', discussed in my tutorial, the second is using various single measurements and averaging them together.
For this purpose, I did (6) individual measurements.
Now I've got 6 measurements. What next? Simple: average them all together to get one measurement.
Here's an example....
All six measurements taken by the method described above (no smoothing applied):
Same as above, but with 1/3 Octave smoothing:
All of the above averaged in to one response:
Let's talk about the above... at least my personal take on the above. I'm sure others may key on to some other aspects I might otherwise ignore or just overlook.
Notice how the response varies more the higher you get in frequency? This is exactly why I said using a single point measurement to tune to a curve is a very bad idea.
I'm going to ignore the shape of the curve, however, for this post... what I really want to focus on is midbass/subbass response, so let's look below 300hz. Anyone notice the one glaringly different thing about the response below this frequency versus the response above it? No matter where the measurement was taken, the response is pretty much the same. This is the critical frequency area (schroeder frequency (Fs)). Linkwitz gives the most simple definition I can think of here:
The frequency fs is also called the Schroeder frequency and denotes approximately the boundary between reverberant room behavior above and discrete room modes below.
Which makes sense, right? Look again at the graphs I provided. Reverberation is occurring above about 300hz as evidenced by the diverging responses from the 6 head area measurements. Below this, the response is pretty constant in this area so it is modally (sp?) dominated. What does this mean to us? You can ignore spatial averaging (multiple measurements) when focusing on low(er) frequency response! This saves you time! Of course, every car is different so I suggest you always do a spatial average to determine where this Fs occurs in your car, but you can expect it to occur around the 200-400hz area, depending on car size. The larger the 'room' the lower the Schroeder frequency. This means once you do a spatial average you'll know where this frequency is. From then on, when you only care about working on the low frequency response, you can ignore spatial averaging and just put the mic at the seated position and measure, tune, measure, tune, rinse, wash, repeat until you're satisfied. I will caveat this by saying that tuning low frequency response with graphic EQ's isn't easy because modal peaks and dips are often too narrow and too specific of a frequency to effectively be targeted by graphic EQs. This is where my subsequent posts will sort of pick up.
Cliffs:
Now you're obviously asking how you measure the "right" way, or "listen right". Well, truth be told, it's not very easy to relay over the internet. It really takes time and practice. Luckily there are a few sources already that I think do a good job of illustrating the methods of measuring and listening. Here's some links:
- Dr. Sean Olive's "How To Listen" Software
- http://www.diymobileaudio.com/forum/how-articles-provided-our-members/163234-first-timers-guide-measuring-your-system.html
- My RTA Walk-Through Thread
Analyzing the Results:
One thing that really needs to be done is spatial averaging. The two measurement tutorials above discuss this but I want to reiterate it because it's just that important. If you'd like additional information, please read the attached PDF in this post . The document is by Dr. Earl Geddes and is a study he did for Ford Automotive. It's not too technically heavy which should keep the "I don't like science" excuses to a minimum.
Note: The following is plucked straight from my build log.
The cliffs version is simply this: your car (and home) are wrought with reflection inducing panels/walls. When measuring response in the environment, you have two options:
- "Gate" the response so you obtain only the response of the speaker you're trying to measure and you essentially ignore everything else.
- Measure everything: speaker response and reflections.
Doing the first in the car?... good luck. How about... don't bother. At least not until you've gotten really good at measuring and understanding what you're measuring. Let's just say for all intents and purposes you won't be doing the first... like... ever (thank you, Taylor Swift, for making that phrase weird now). Really, it's just pretty much trivial unless you have a very specific goal and understanding of how to achieve it this way.
So, we do the second option. The issue, then, is the fact every measurement you take is a measurement of EVERYTHING occurring at the mic. This is good and it's bad. It's good in the way that there's not a whole lot you can do to the speaker itself so it kind of keeps you from worrying about it. - Although, this is why I really encourage people to study independent tests or do their own to understand the issue(s) with the speakers they've chosen before they use them in the car. - It's bad because, thanks to the nature of the reflective environment, you can't really trust a single point measurement (a measurement taken with the mic in one location). If you move the mic as little as one-half inch you'll get a different result. Most notably in the higher frequencies. This means RTA'ing your car for any desired curve by using one mic measurement is a TOTAL WASTE OF TIME. It's ideal to take multiple measurements in the "head area" and average them together. TrueRTA, OmniMic, and REW allow you to do this pretty easily. Then you have what is known as a good spatial average. It's not an exact method but it's the most realistic and approximates a very realistic response in the seated position.
For this spatial average there are a couple methods: one is a 'live average', discussed in my tutorial, the second is using various single measurements and averaging them together.
For this purpose, I did (6) individual measurements.
Now I've got 6 measurements. What next? Simple: average them all together to get one measurement.
Here's an example....
All six measurements taken by the method described above (no smoothing applied):
Same as above, but with 1/3 Octave smoothing:
All of the above averaged in to one response:
Let's talk about the above... at least my personal take on the above. I'm sure others may key on to some other aspects I might otherwise ignore or just overlook.
Notice how the response varies more the higher you get in frequency? This is exactly why I said using a single point measurement to tune to a curve is a very bad idea.
I'm going to ignore the shape of the curve, however, for this post... what I really want to focus on is midbass/subbass response, so let's look below 300hz. Anyone notice the one glaringly different thing about the response below this frequency versus the response above it? No matter where the measurement was taken, the response is pretty much the same. This is the critical frequency area (schroeder frequency (Fs)). Linkwitz gives the most simple definition I can think of here:
The frequency fs is also called the Schroeder frequency and denotes approximately the boundary between reverberant room behavior above and discrete room modes below.
Which makes sense, right? Look again at the graphs I provided. Reverberation is occurring above about 300hz as evidenced by the diverging responses from the 6 head area measurements. Below this, the response is pretty constant in this area so it is modally (sp?) dominated. What does this mean to us? You can ignore spatial averaging (multiple measurements) when focusing on low(er) frequency response! This saves you time! Of course, every car is different so I suggest you always do a spatial average to determine where this Fs occurs in your car, but you can expect it to occur around the 200-400hz area, depending on car size. The larger the 'room' the lower the Schroeder frequency. This means once you do a spatial average you'll know where this frequency is. From then on, when you only care about working on the low frequency response, you can ignore spatial averaging and just put the mic at the seated position and measure, tune, measure, tune, rinse, wash, repeat until you're satisfied. I will caveat this by saying that tuning low frequency response with graphic EQ's isn't easy because modal peaks and dips are often too narrow and too specific of a frequency to effectively be targeted by graphic EQs. This is where my subsequent posts will sort of pick up.
Cliffs:
- Tuning based on one mic measurement is a waste of time. This has a caveat...
- Take a few measurements in the head area, where you sit. Look at them all overlaid. Where do they really start to diverge? This is your car's Schroeder frequency.
- Above the Schroeder frequency you must take multiple measurements and average them if you want to tune via RTA.
- Below the Schroeder frequency, one mic measurement will suffice since the response doesn't change enough to matter.
- Graphic EQs aren't the best tool for fixing response issues low in frequency. Parametric EQs are MUCH better. But, if all you have is a graphic use it to the best of your potential.
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