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Analysing the Spectrum

Audiosource RTA1 Spectrum Analyser

Dave Simpson takes a break from counting his savings to tell you what he's spending it all on.


How spectrum analysis and adjustment of room EQ can help you achieve a better sound in your studio... plus a review of Audiosource RTA1 Analyser & Sound Pressure Meter.


How many times have you spent days on a particular recording; carefully monitoring levels and EQing every step? Finally you mix down and after playing it back to ensure the master is alright you hurry off to a friends house clutching your cassette to impress him with your skills. Proudly you switch on his tape machine only to find your tape sounds completely different on his system. Carefully regulated bass has become a booming nightmare, the hi-hats which you thought were way back in the mix cut through you like a knife; the vocals have-become inaudible, whilst loads of top end hiss has suddenly manifested itself.

We've all experienced this, from the guy with a Portastudio to the full B16-based home studio but the problem is not confined to home studios. Many so-called professional studios suffer from the same syndrome, and it adds insult to injury when you have to pay for a tape which sounds nothing like you originally intended.

It's amazing how many excuses are made in order to explain this imbalance. Some people, having recorded in a professional studio assume that big monitors and loud power amps necessarily produce accurate sound, and blame their home systems as incapable of accurately reproducing this. Others assume that their perception of the mix was faulty, perhaps due to the unfamiliar surroundings. Neither reason can explain the same thing happening in a home studio though. Why do records or prerecorded tapes usually sound so much better than demos, whether they're recorded at home or in a professional studio?

Monitoring



Basically the answer does lie with inaccurate monitoring but it is worth making the point that even the most expensive studio monitors can be rendered useless as points of reference in certain circumstances.

When you listen to sound coming from a pair of speakers, the sounds you hear are coloured by the environment. Sound waves are less disciplined than you might imagine, and instead of all of them obligingly making a beeline from the speakers to your ears, most of them hurtle off at various angles until they encounter an object (usually the sides, ceiling or floor of the room) where their behaviour is modified by the nature of the material they encounter. Absorbent materials such as carpets or curtains will tend to suck them in, and the higher the frequency, the more the sound will be absorbed. Hard surfaces, such as bare walls reflect them, whereupon they bounce off and continue on their way. Eventually some might find their way to your ears where they combine with those waves which travelled in a direct line from the speaker.

As you might appreciate, it is therefore erroneous to assume that the sound leaving the speaker is identical to that heard by your ears. Few people suspect though, how drastically a room can affect a monitoring system. Ironically, it is often so-called acoustic treatments which do the most damage. Covering a room in sound absorbent materials for instance leads to a perceived high frequency loss. When you mix down you cannot hear the high frequencies as well, and so you boost the EQ until it sounds good. Unfortunately in doing this you are compensating for the acoustic deficiencies of the room, and as soon as the tape is played in another environment the same acoustic factors won't apply as another room will carry its own distortions, unlikely to correspond to the original studio environment. Imagine this problem repeated over the entire range of the audio spectrum, and you begin to appreciate the problems involved. So what can you do about it?

Well, the easiest way to avoid the problem is to monitor every step of the recording process on headphones, thus avoiding any distortions created by poor speakers or an acoustically deficient environment. However, most headphones are themselves deficient in certain frequencies and even good sets - when used for long periods. Near field monitoring is another way of reducing environmental effects, but most small monitors are incapable of accurately reproducing the whole audio spectrum, particularly at the bass. In addition they require accurate positioning in order to produce a good stereo image. Both methods are better than nothing though. Assuming though that you don't want ears permanently squashed flat like a dog thats just been kicked in the features, and assuming also you like listening to a tape at more than three watts, what else can you do?

Acoustic Treatment



Well, you can alter the room acoustically using various materials with known acoustic properties, which is what the big boys do. If a particular frequency, say 2kHz, is particularly resonant, then materials which absorb those particular frequencies are introduced into the room's construction. As previously mentioned, this treatment needs to take place over the entire audio spectrum, so this method is far from cheap! It is however the best form of treatment because the positioning of the listener within the room becomes far less critical.

The second thing you can do is to install a stereo graphic equaliser over the monitoring system to cut or boost frequencies as needed. Obviously, the more bands the better: ten will do at a pinch, but if you are really serious about the idea, a dual 31-band graphic is the only answer.

Even in a home studio then, a small ten-band graphic EQ will make all the difference when it's set to flatten the room. Having done this, you'll be amazed at how much more flexible the EQ on your mixing desk will become. Instead of compensating for the room, the treble control will become a creative tool and you'll hear what's actually going on to tape.

Suddenly all your tapes sound better and the recording companies are lining up to throw contracts at you, and all because you installed a teensy weensy graphic into your system to cut and boost the correct frequencies. (I'm sure there's more to it than that! - Ed.) Hold on, though: how do you know which are the correct frequencies?

Audiosource RTA1 Spectrum Analyser.



A spectrum analyser is basically a device that literally analyses the audio spectrum. A qualified broad band noise source is connected to the system and played back through the speakers. This is usually pink, as white noise tends to burn out tweeters. A microphone with a flat response (usually omnidirectional) is placed in the position to be occupied by the engineer/producer and connected to the analyser. The device 'listens' to the pink noise being played through the speakers and displays each frequency band relative to a flat position - ie. what it knows the pink noise should sound like in a perfect acoustic environment. The number of bands monitored and the method of display vary from machine to machine, but they usually correspond to the bands on graphic equalisers; 10, 15 and 31, (or octave, ⅔ octave and ⅓ octave).

From this reading you can alter the acoustics of a room or set a graphic equaliser to compensate for them. If the analyser indicates that 125Hz needs to be cut by 7.5dB, you can do this on the graphic. In fact I'm oversimplifying things to some extent, because cutting or boosting a particular frequency band will affect the bands either side and acoustic discrepancies rarely fall into convenient bands. You have to play around for a little while to find the best slider position.

The Audiosource RTA1 operates over ten bands and consists of a black box containing an analyser, with a built in electret condenser mic. The readout is shown by ten rows of LEDs, each representing a frequency band; -31.5, 63, 125, 250 and 500Hz, and 1, 2, 4, 8 and 16kHz. Each row consists of nine LEDs showing up to 10dB of boost or cut. All the LEDs are red except the centre row ones which are green and indicate a flat response.

Controls



Underneath the LED display screen are three indented rotary controls. The one marked Power Decay refers to the display itself and has three positions - off, slow and fast. Off switches off the display so that no readout is visible. Fast and Slow refer to the speed of the display (or if you prefer, the decay). Factors such as momentary noise pulses, charges in mic positioning or even the level of a particular frequency falling between any two of the LEDs on the display can cause the LEDs to fluctuate rapidly, with a given frequency band showing levels varying by as much as 5dB. Switching the display to Slow will cause the display to stabilise around the average level, making it easier to read. In practice though, I found that given a good graphic equaliser it was possible to obtain a stable display even on the fast setting.

The middle knob is marked Input Display and gives a choice of two inputs; mic and line, and for each of these two choices of display are available. Switching the unit to either of the line inputs will display a signal fed through the line input jacks (phono) on the front of the analyser. Switching to mic inputs will display a signal monitored by either the built-in mic or a mic connected to the external microphone jack.

For both of these input selections (mic and line) the Level display will put the RTA1 in the Sound Pressure Level (SPL) mode. The LEDs will display as a horizontal row indicating SPL in dBs. For the home recordist this facility tends to be of limited value, and is more useful for monitoring amp and speaker performance relative to specification. Setting the mic or line inputs to RTA (Real Time Analyser) is where the device becomes invaluable, with the LEDs indicating as a curve and each column of LEDs indicating the relative SPL in dBs of each of the ten octaves of the audio range.

The third knob is marked Level (dB), and has six settings, each representing the SPL in dB at the centre 0dB reference point on the LED display. In other words, with the level knob set at its furthest left detent (60) the 0dB row of green LEDs represents 60dB, the next row of red LEDs representing 62.5dB and so on. The total range of the RTA1 is from 50dB to 70dB with the range varying with the settings of the level control.

In practice this is only critical when using the device to measure across the board sound pressure levels. When used as a real time analyser, the level control is set so that the reading is fairly central on the display, with further fine tuning possible by altering the output volume on the system you are analysing.

In order to analyse any system a noise source is required, composed of equal amplitude of all frequencies in the audible audio range (since all frequencies are equal in amplitude the pink noise is 'flat' in frequency response at the time it is generated). Included in the RTA1 package is a battery operated pink noise generator with an adjustable output. Although the analyser can work off batteries, a mains adapter is also provided, with the whole system packed inside a neat imitation leather carrying case. To make analysis easier and more accurate, a tripod fixture is provided on the analyser itself.

To use the analyser, first connect the pink noise generator to your system and adjust playback level to a moderately high volume. Turn the input/display knob to the MIC/RTA setting and adjust the level knob so the LEDs are clearly displayed across the screen. Basically, any LEDs below the green band have to be boosted in frequency, and any above have to be cut (the green line representing a 'flat' response).

Remedial Action



Having ascertained the response of the system being analysed, the amount of remedial action possible depends on resources. A stereo ten-band graphic equaliser will take care of many problems, but if you haven't got one there are still other things you can do, such as altering the speaker placement. For instance boomy bass is sometimes caused by nearby walls. Changing the room acoustically (reducing the amount of area covered by carpet and so on) also has an effect. Even the controls on a hi-fi amp will have some value, and the mere fact of your being aware of frequency anomalies can allow some corrective action when mixing down.

Once the room is equalised, this need not be the end of the units usefulness. As well as regular re-equalising (every new item of gear or change of equipment positioning will alter the response of the room slightly), the unit can be patched in to the system permanently using a couple of 'Y' cords. Thus every signal monitored will display its frequency bands on the analyser. This is both informative and useful. I monitored the frequency response of the bass drum on an MXR drum computer and shaped a live bass drum using a 31-band graphic to show a similar curve, at which point it also sounded good. Knowing the frequency response you are after means that using a graphic equaliser is much less a matter of chance than it used to be.

Retailing at less than £120.00, this is by far the cheapest analyser on the market, and to my mind it should be seriously considered as an addition to any home recordists system. The cost of remedial treatment could prove high (though I have seen ads for spectrum analysis plus installation of a graphic for under £100.00). In the long run, however, I feel it would be worth it. All in all, this unit is attractively presented at a good price, with a degree of usefulness impossible to overemphasise. If you think you can't afford to have your room spectrum analysed, take it from me - you can't afford not to!



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Home & Studio Recording - Copyright: Music Maker Publications (UK), Future Publishing.

 

Home & Studio Recording - Aug 1985

Donated & scanned by: Mike Gorman

Topic:

Acoustics


Gear in this article:

Metering > Audiosource > RTA1 Spectrum Analyser

Review by David Simpson

Previous article in this issue:

> Making the Most of...

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> Take the Money


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