How do for Using a Real-Time Analyzer

You can try to set the frequency response (the level or volume of sounds the system reproduces within the audible frequency range) by ear, but it’s difficult and you have to know what to listen for. A much better way to do it is with a real-time analyzer, known as an RTA in car audio lingo (see Figure 20-2). 

Figure 20-2:

An Audio- Control SA-3055 real-time analyzer.

Courtesy of Audio Control

An RTA measures sound pressure with a microphone and displays graphically how much sound pressure there is at certain frequencies in decibels (dB). The Audio Control RTA shown in Figure 20-2 displays the amount of sound pressure level in frequencies from 25 to 20,000 Hz and in increments of up to + or - 16 dB.

In order to measure a system’s frequency response, you need to play a signal that has equal SPL at all frequencies. This signal is known as pink noise and has a static-like sound. Test CDs such as IASCA’s Official Sound Quality Reference CD (see Figure 20-3) contain a pink noise track. (You can order the CD from IASCA at www.iasca.com.)

To set frequency response with an RTA, follow these steps:

1. Play pink noise at about three-quarters of the maximum volume on your head unit, with all of the tone controls set to 0 and all signal processing turned off.
2. Using a microphone stand, place the RTA’s wired microphone close to where the driver’s head would be, or wedge the mic in the headrest. If your equalizers are mounted in the trunk, it’s best to move the RTA to the outside of the vehicle. Not only is this more convenient, but pink noise can damage your hearing at loud levels, and at the very least is annoying. If your EQ is in the passenger compartment, you’ll want to turn the volume down while playing pink noise.
3. With the pink noise playing, look at the RTA’s display to see the frequency response of the system. What you hope to see is a Rat response, with equal energy at all frequencies. But what you’ll most likely see is a series of peaks and dips, with more energy at some frequencies (peaks) and less at others (dips) (see Figure 20-4). 
4. Using the controls of the equalizer, start with all of the controls set to 0 dB (which is the center, neutral position) and adjust the frequency response of the system by cutting the amount of energy in the range where you see peaks and boosting it where you see dips. Keep in mind that it’s generally better to cut frequencies than boost them, and that as you make adjustments to one set of frequencies, adjacent frequencies are also affected.
5. After making adjustments, take another reading with the RTA to see how your EQ adjustments affected the frequency response, and then continue tuning until you get as close to a flat frequency response as possible.

As with setting gains, tuning with an RTA takes experience and patience, and you get better with practice. If someone with experience — and an RTA — can help you, even better. Many car audio shops have an RTA on hand for tuning customers’ systems.

A flat frequency response is an ideal you want to try to achieve, but it doesn’t mean that your system won’t sound good if it can’t achieve a flat response. In fact, quite the opposite. A flat frequency response sounds ... well, flat. If you like the bass cranked up and see a huge peak in the region below 125 Hz, more power to you. (Literally, “more power to you”: You’re gonna need it for all that bass!) Measuring for frequency response with an RTA is simply a way for you to see if your system is deficient in any particular area.

If your system is deficient and no amount of tweaking can compensate for huge peaks and dips, it could be caused by a flaw in the system design, such as improper speaker locations or crossover points.