using signalscope in car audio installations?

[acousticalz]

I have been reading up a lot on car audio, and i hear a lot of professionals use RTA's to get a flat response in order to get good sound quality in their car...

I am interested in doing the same, but was wondering is it possible to do this with signalscope and using my powerbook's built in microphone to analyze sound?

If not, what is a good accurate microphone to use for this type of work? and do i need any extra equipment to plug that mic into my powerbook or does it directly hook up to my line-in?

Most people told me that in order to get good sound quality in your vehicle, is to get a pink noise stereo and play it on your car's headunit, and then using an RTA to view the graph to see where the dips are... and then adjust the eq/crossover/etc on your headunit to make up for those dips in the frequency graph in order to get as much of a flat response as possible. Is this correct?

Will this only work with signalscope or do i need the pro version? I'm kind of new to this RTA, stuff and would really like to learn how to use this properly in order to achieve the best possible sound i can get in my vehicle....

Would appreciate the advice on what i need to get to do this...

I been looking for a long time for an RTA type of application for mac os x, and ran across a few... not sure which one is the best/most accurate for what I want to do.

Thanks in advance!

[FaberAST]

Broadcasting pink noise through your car stereo system is certainly one way to start getting a "look" at what how your system sounds.  If you do that, you will want a microphone with a reasonably flat frequency response and an RTA that can display a spectrum in octave or third-octave bands, like SignalScope Pro.  Pink noise should have a flat spectrum on an octave band analyzer.

An alternative to pink noise is white noise, which will have a flat spectrum on a narrowband (FFT) analyzer, like the one in either SignalScope or SignalScope Pro.  FFT analysis lets you see the spectrum with higher resolution, particularly at higher frequencies.

In either case, I don't recommend using the built-in mic on your PowerBook.  If you want an inexpensive measurement mic, you might want to consider the Nady CM100 or the Behringer ECM8000, both of which are electret condenser mics.  You will also need an audio interface with at least one built-in mic preamp that offers phantom power for the microphone.  I would recommend the Echo AudioFire 4 (www.echoaudio.com) or the Edirol FA-66 (www.edirol.com).  Either will connect to your PowerBook via FireWire cable without requiring a separate power supply.

If you need to generate a white or pink noise signal, take a look at SignalSuite, as well.  If you can't connect the audio output of your PowerBook directly to your car stereo, you can use SignalSuite to generate an audio file containing the test signal, which can then be burned to a CD.

I hope this helps you get started.

Ben

[dannyboy]

I was going to start a new thread but then stumbled across this one and realized that it described my situation perfectly! I do have a couple a of additional questions I was hoping maybe I could get cleared up.

Like acousticatz I too am new to the RTA world and I`m using the signal scope pro, a test disc w/ pink noise from 25 to 20k, a 13" g4, (I plan on getting the behringer mic Ben listed above run thru my Mbox2) but right now I have been using the built in mic.

My questions are what should I have the weighting set @ I know for SPL measurements its usually reccomended to set it at C would it be the same for these types of measurments? Or would flat be the way to go? And I have been having a heck of a time tryng to get the calibration info for my powerbook (full scale voltage for the adc) I have been using the 5.00 setting according to the example in the help menu for a 2003 15" g4 powerbook I dont know if it would be different for a newer model 13".   

The issue I`m having is the high freq readings (6.3k and above) are considerably lower than the rest of the readings and I dont know if thats a problem w/ my internal mic, calibration, settings or all of the above ...lol. 

Thanks in advance

Danny     

[FaberAST]

Danny,

A and C frequency weightings are designed to mimic human perception of sound levels.  They are generally used for measuring overall SPL's, but can be useful for seeing how the frequency content of an acoustic signal is affected by human perception.  Generally speaking, A weighting is more appropriate for low sound levels and C weighting is more appropriate for high sound levels.  This is open to debate, but my recommendation would be to perform your equalization using a flat weighting, unless your personal preference tends toward emphasizing frequencies in a manner consistent with a particular frequency weighting.

To calibrate the voltage level measured by SignalScope Pro, it would be good to apply a single, steady tone, perhaps with a frequency between 200 and 2000 Hz, with a known rms voltage level to the input.  Then you can adjust the full-scale voltage in the Preferences window, until the Oscilloscope gives you an accurate rms level that agrees with the known input.  It might be good to use a horizontal scale of 20 or 50 ms per division.

The drop in your signal level at high frequencies is probably due to the frequency weighting filters.

Good luck!

Ben

[foko]

I'm gonna use SS pro for RTA and equalization of my car audio system.  Some questions though....

1.  It is recomended we use an external mic rather than the internal mic on the powerbook.  Is this simply due to the internal mics frequence response not being flat?  Or is there some other reason?

 When I was just doing some initial frequency scans, the obvious issue was the fan on the powerbook coming on and hence disturbing the RTA.

2.  I don't really understand the calibration issue. Will an external microphone need to be calibrated in some way?  If so, what is that procedure?  I read the above description, but I'm not completely clear.....

The way I understand it......Play a test disc with say a 1000 hz signal and measure the output with a seperate db meter and adjust volume  to say ......80 db.  Then using SS pro, calibrate the software to 80 db at 1000 hz. 

Is that right?

Thanks
Fabian

[FaberAST]

The problem isn't just the frequency response of the built-in microphone on a Powerbook--it's also that the presence of the Powerbook affects the sound field near the microphone, further degrading the measurement.  Using a dedicated measurement microphone for equalization is highly recommended.

The need for microphone calibration depends on what you are trying to accomplish.  If you are only trying to equalize the sound spectrum, and you know that your microphone's frequency response is flat enough for your needs, then no additional calibration is necessary.

If you want to measure accurate sound levels, using a properly calibrated sound level meter is recommended.  A relative calibration at 1kHz, as you described it, can be problematic even in an anechoic chamber, not to mention in your car!  Then again, it all depends on how precise your measurements need to be.

Ben

[foko]

That answers my questions, thanks.

One last thing, I've found some condenser mic's with USB connections which make them very easy to use on the powerbook.  They are available with a cardioid reception pattern as well as an omnidirectional pattern.  Does that matter for the RTA application?

 The omnidirectional mic is almost twice the price (actually its a selectable pattern (cardiod and omni) whereas the cardiod pattern mic is only that.

[FaberAST]

Generally, when making acoustical measurements, or when equalizing an audio system, using an omnidirectional microphone is the best approach.  Directional microphones exhibit different frequency responses, depending on which direction the sound is coming from when it arrives at the microphone.  Therefore, the orientation can significantly affect the measurement.  Omnidirectional mics are not so sensitive to orientation at a particular location within the sound field.

Ben

[arlimfilho]

I was thinking about buying a M-Audio Mobilepre instead of audiofire4 or edirol fa-66, because they are too expensive. Is mobilepre a good option? does it have a flat response for measurement purposes?

[FaberAST]

The Mobilepre USB from M-Audio is certainly one of the least expensive options for getting a signal from a condensor microphone (via phantom power) into your Mac.  You'll just want to be sure the specs are satisfactory for your particular application.

The Mobilepre samples analog input signals with 16-bit precision at sample rates up to 48 kHz, while the AF4 and FA-66 operate with 24-bit precision and sample rates of 96 kHz and 192 kHz, respectively.  The higher precision also allows for a higher dynamic range and signal to noise ratio (SNR) for those FireWire devices.

As far as the frequency response goes, M-Audio advertises that the Mobilepre's frequency response only varies by +/- 0.8 dB over the full audio band (20 Hz to 20 kHz).

If you think the Mobilepre suits your needs, then go for it--you'll certainly save some money.