Article: The Essentials of Sound Quality: IMHO

[Hi-FiDelity]

TL;DR



In all seriousness though, this is an awesome resource.

[erinh]

updated posts 14-16.

[Lanson]

This is some great info!

On beaming: I have forever been using a different formula, and I'm wondering which one is accurate.

13740 / (driver size).

This is the one that MANY home audio folks use, including folks setting up D'Appolito arrays. I can link sites that use this exact math time and time again. Are you SURE your math is correct?


This is critical to my build because I designed and installed the size speakers I did in the off-axis positions I did, because of the math.

As comparison:

3/4" tweeter begins to beam just at the edge of the 20kHz envelope, right at 18.3Khz.
2" driver begins to beam at 6870 Hz
8" driver begins to beam at 1717 Hz


1145 feet per second, multiply by 12 for inches.




So which is right?

[erinh]

I'm using half wavelength. The reason why is based on the data I've seen. I understand different sources use different fractions (namely, whole, 1/2, or 1/4 wavelength). I think using 1/2 is a good middle grounds and represents what the data typically shows.

For example, look at the graph I provided. It's a 4" midrange with approximately 3.0" effective diameter.

Full wavelength:
Take the speed of sound (13600 in/s is 60*F, 50% humidity if I remember my calculations correctly) and divide it by the effective driver diameter. In this case, you'd have a beaming point at 4khz. Now look at the data; at 4khz your 0 degrees axis vs 60 deg axis measurements are about 8dB apart. That's quite a stretch if your goal is trying to achieve as close to Omni as you can at the crossover point (understanding that different slopes/types have different results).


Half-Wavelength:
Now use my formula: 13600/Effective Diameter/2. You get ~2.2khz. Now look at the data. That's practically right on the point where on/off axis diverge. I'm more concerned with omnidirectionality; if the driver is nearly 10dB down at 60 degrees at one full wavelength then it doesn't really do me any good trying to match it to a tweeter that's playing omni at it's HPF. Additionally, you're closer to the breakup point of the driver. If the driver breakup is pretty considerable (say, >15dB modal peak) then it's going to be hard to attenuate that via the crossover if the crossover point is practically right there. Furthermore, that breakup really screws up your power response because the driver's behavior off-axis (at least with poorer drivers) is quite different than it's on-axis behavior). If you're an octave lower (by using the 1/2 wavelength), then you're in that much more "clear". Especially if you're using steep slopes.

[erinh]

You're gonna have to link me to these other people's comments. Measuring from both seats is a waste of time in this environment unless your goal is to attain a balance (and believe me, there WILL be compromises made at your primary listening position) between the seats.

Also, NEVER, EVER... EVER EVER EVER use only a single point measurement of the system as your tuning guide. This is EXACTLY the stuff I talk about when I say "you're doing it wrong". SPATIAL AVERAGE. The guy who's measuring your car with only one position and not doing a head area spatial measurement is doing it wrong. Plain and simple. You can see why in my examples. The only time a single space measurement is worthy of doing is when you're measuring the response below the Schroeder Frequency; all of which was covered in my earlier posts and examples provided as to why.

[erinh]

That's averaging multiple measurements. Over a head area. One of those threads is actually mine. And that particular one would have the mic placed in 6 spots over about a 4x6" rectangle. And then those results would be averaged together.

I thought you were talking about putting the mic in one spot and using it to obtain only a single measurement. That would be bad.

[pocket5s]

And this is what we did , from the drivers seat ONLY ....

If you have TrueRTA (like most of us do) then it's a total breeze.

Here's how you get a spatial average:
Measure Center. Save results by going to the 'view' menu and saving or simply hit 'Alt+1'.
Move the mic about 45 degrees to the right or left of center. Mesure. Save results in #2 (Alt+2).
Move the mic to the opposite side of center (left or right). Measure. Alt+3 to save.
If you can, move the mic up about 2-3". Measure in the center. Save as #4. Alt+4.
With the mic in the same Y position (height position), move the mic left/right of center. Measure. Save. Alt+5.
Move the mic to the other side of center. Measure. Save. Alt+6.
Finally, average all the results together by doing this: Go to the 'Utilities' menu. Then 'Average'. 'Select Input': this will be #1-#6. Now 'Select Memory to use for Result': this will be #7 or whichever number you want to use. Just don't override one of your measurements.
That's it. Now, look at the difference between each singular measurement and the 'averaged' measurement. Notice the difference?

You have now measured and completed a spatial average of your system. Use this averaged measurement to tune for best results.

While this is certainly doable and Erin even shows examples I believe, some (including several who have taught me) just move the mic as you measure. Sweep it side to side in an arc in other words. Set your averaging on your trueRta to 40-50 or so. Much, much faster! You don't have to sweep fast either, just keep it moving.

As some know, Mark E built a stand that is motorized to do just this. It sweeps the mic in a roughly 270* arc. That way he can sit outside the car with the computer and run the processor and just let the mic sweep while he does it. When I do it I don't particularly like smacking my face with it, just I sweep about 180*, or side glass to side glass! that way you are accounting for the reflections you will get off the glass. An alternative would be to sit in the passenger seat, holding the mic in the area where the listeners head would be sweeping. Or, as I once did while Todd was tuning on my car before the gtg, I sat in the back seat and swept it.

Regardless, that way you aren't concerned with whether or not you had the mic in the same 6 points in space as you did the previous time when you go to do it again.

Just a thought

[JayinMI]

What happened to the Test CD? I was going to start messing around with some of this this weekend, and was planning on picking up some blank CDs on the way home. Is it somewhere else now?

Jay

[casey]

just re-read this for the third time. Going to try a few things when I get the new amps in before I let someone fine tune. You mentioned phone apps for level matching L/R. I have the JL app with the DB meter. Is that accurate enough to match left and right? just thinking of wedging it between the head rest and seat back or taping to head rest and monitoring

[JayinMI]

Setting gains with a DMM:

• Here is a quick guide to properly setting your system gains using nothing more than a DMM.
• Reset all your cd player DSP features, balance, fader, etc to ‘default’.
• Disconnect all speakers to keep from damaging them (and your hearing).
• We will assume your cd player puts out clean, undistorted signal at near maximum. Play your tone (-10dB suggested) and increase the headunit volume to a click or two below max.
• Knowing the amps’ rated power at a given load (ohm) obtained from manufacturer’s spec (assuming they are a reputable brand), use Ohm’s law to determine voltage needed.
• For example, with a 4 ohm load and 100wRMS rating, the output voltage should be 20VRMS.
• Set the DMM to “VAC”, which is AC Voltage.
• Connect the leads of the DMM to the amp’s appropriate speaker output and increase the amp gain until you achieve this voltage
• Repeat for each channel.

If I'm mixing 4 ohm and 8 ohm speakers, and I *don't* know my amp's rated power at 8 ohms (who does? lol) do I assume half power, or do I still try to match to total power?

In my case, my tweeters are 4ohm, but my mids are 8. and my amp is rated at 50w/ch@4ohm. They are right next to each other. At 4ohms, 50w is 14.14V, and at 8ohms 50w is 20V. So, would I set tweeter channels to 14.14v, and the mid channels to 20v? or would I assume 25w/ch @ 8ohm which would still end up being 14.14v?

Thanks.