2 to 1 Microphone Rule

Most audio engineers know about the 3 to 1 rule, when dealing with 2 microphones and 2 sound sources. The rule is that if the sound sources are 1 foot way from the microphones then the two microphones need to be 3 feet apart from each other. In this article we are going to look at the 2 to 1 rule, when dealing with microphones and boundaries.

2 to 1 rule

The 2 to 1 rule is that if the sound source is 1 foot way from the microphone then the microphone needs to be 2 feet way from any boundary surface that will reflect the sound wave back towards the microphone. These are things like walls, gobos, music stands, ect.

In order to understand the 2 to 1 rule we have to understand phase distortion and reflections. Lets first define reflection; this is when a wave hits a boundary like a wall and the wave will then bounce off the wall and travel outwards away from the wall.

Phase distortion is when a sound is duplicated and is not in sync with the original sound source. When it comes to phase distortion, reflections and the 2 to 1 rule, this is what is happens. The original sound hits the microphone then travels past the mic, then hits the wall and bounces back towards the microphone and causes phase distortion.

Why is this a problem? Next time you get a chance to find a quiet room, go next to the wall and talk into the wall and slowly step back words until you can't hear your voice change tones. This exercise will give you a hint as to what is happening to the tone of the sound, as it bounces off the wall. It colors the sound and can be unnatural and undesirable. This exercise can be used to determine how close a microphone can be to a boundry. This may look odd to the recording artist, however; it will make a significant difference in the recording.

Problem spaces that ignore the 2 to 1 rule

Sound booths are probably the worst offender of the 2 to 1 rule. These tiny rooms can be too small to get a nice sound recorded. I went into a recording studio once to help them set up their sound booth. The booth was probably a 5' x '5 x 8' room. There was foam on the walls and when I went in there to listen to my voice I was extremely annoyed with the reflections in the mid range. I told them that this would not be the ideal place to record vocals. I couldn't imagine putting a opera singer in there or  any recording artist for that matter. I would be embarrassed. A better option would be to record the vocals in a larger space.

The next thing that comes to mind are gobos (go between). A gobo is an acoustic baffle that usually has a slanted window. This makes it so the players can see each other, but the sound is blocked. It would be a good idea to listen to the microphones with the gobo in between and with out the gobo in between the two players, to see if it makes a difference on the tonality of the recording. In some cases having a gobo there makes the recording warmer and better.

Conclusion

It's important to think about boundaries and how they will affect the recording. When choosing mic placement one should always consider the boundaries around the microphone. The worst offender of 2 to 1 rule is a flat wall that runs perpendicular to the angle of the microphone and the sound source. The reason is that the sound wave will bounce right back into the microphone and cause phase issues.

If you have any questions about microphone placement or recording email me at kyle.tronaaudio@gmail.com

Thank you for visiting our blog. Happy listening!!!




 

Studio Monitor Time Alignment

Have you ever wondered what time alignment is? In this article we are going to look into the aspects of time alignment, and how to time align studio monitors. I feel that time alignment is an aspect that people don't know about or understand properly or even care about. For me time alignment is one of the most important aspects for a studio monitor. It helps give depth and brings out the reverb in a recording. It helps with knowing, if the vocals are in the right spot in the mix when dealing with mixing levels and acoustic spacing. I believe that every audio engineer should know and care about time aligning their studio monitors.

What does Time Alignment Mean?

When we look at two speaker drivers on a studio monitor, we will notice that there is a tweeter and larger speaker woofer. Most of these drivers are set up on a flat baffle. The problem with this is not with the flat baffle, but with the drivers themselves. Lets look at the tweeter verses the woofer. When we take the tweeter and the woofer apart we will notice that the tweeter is not as deep as the woofer. Because of this depth difference, we can expect that the tweeter is closer to the listener then the woofer. So what does this mean? It means that the tweeter's sound wave will hit the listener's ears before the woofer's sound wave will hit their ears. It's as if there is a short delay between the tweeter and the woofer.

How short is this delay? It's shorter than you think. Its somewhere around 5-10 micro seconds. But how does 10 micro second delay make a difference in the over all sound of a studio monitor? It's simple, our ears and brain can detect that small of a delay. This small delay is called phase distortion.

Phase Distortion

Phase distortion is when two sounds are not quite in sync with each other, but they are suppose to be. If we take a look at a vocal frequency response, we can see that it starts from around 150 Hz and extends all the way to 8000 Hz. Now lets compare that to the woofer and tweeter's frequency response; we will notice that the lower frequencies of the vocal lives in the woofer, and the upper frequencies live in the tweeter. If there is phase distortion caused by a lack of time alignment, our ears will pick it up and say to our brain, THIS SOUNDS FAKE. It will sound paper thin and flat. The sound coming from the speakers will have no depth and no angle. It will sound 2D and not 3D.  

Time Domain

How do we fix this problem? It's simple, all we have to do is tilt the monitor back on an angle until the tweeter and the woofer are time aligned. This change in distance is referred to as the time domain. The best way to think of time domain is to think about distance and space. The further you are away from a sound source, the longer it takes for the sound source to travel to your ears. Thats why when we tilt the tweeter further away from the listener, it causes the tweeter to take more time to get to the listener. If we tilt it too far the speaker will not be time aligned. There is a fine line to when the speaker is in phase or just slightly out of phase between the tweeter and the woofer.

How to Test Studio Monitors for Time Alignment

The following items are needed to test the time alignment of your studio monitors.

1. Condenser microphone (preferably a reference mic)
2. XLR cable
3. Audio interface
4. DAW digital audio work station (Pro Tools works best)
5. Acoustic baffle (a book works fine or anything that causes an acoustic shadow)
6. Microphone stand
7. Wooden shims or books to prop up the front of the speaker

The following are the steps need to test the time alignment of a studio monitor system. Each monitor will need to be tested separately.

1. Place the microphone on the microphone stand and place the microphone at ear height. Put the microphone where the listener will sit.
2. Plug in the xlr cable into the microphone and plug the other end into the input of the audio interface. Turn the microphone pre-amp up to -12db to -6db when talked into the microphone.
3. Open up the DAW and create 4 tracks. Label the tracks "tweeter left", "tweeter right", "woofer left" and "woofer right".
4. Make sure that the fader on the tweeter and woofer tracks are turned all the way down to infinity. this will insure that there will be no feedback from the mic to the monitors. Arm the "tweeter left" track and turn on the metronome. The metronome is used to gauge the arrival time of the sound waves from the tweeter and woofer.
5. Cover the left woofer with the acoustic baffle. This will help you figure out the tweeter's sound wave. There will be some bleeding from the woofer. Using a low cut filter could change the phase of the speaker (I don't recomend this method). However; if you have a phase-less low cut filter, that will work better than a normal low cut filter. The low cut should be around 1kHz. With all this said, the prefered method is to cover the driver with the acoustic baffle (a book).
6. Hit record and let the metronome sound pulse, ping a few times (ten seconds). Stop the recording.
7. Now cover the tweeter up, or use a phase-less high cut filter around 1kHz. Arm the "left woofer" track and unarm the tweeter track.
8. Hit record and let the woofer ring out with the metronome sound for a few pings. Stop the recording.
9. Now compare the tweeter track to the woofer track. Look at the two tracks extremely zoomed in. The tweeter should lead the woofer. This means that the speaker needs to be tilted back. I like to use small shims or wood to prop up the monitor.
10. Keep testing the monitors and adjusting the tilt until the two waves in the DAW match up. Do the same for the right monitor as well. All 4 drivers should line up if done correctly. If they are not lined up adjust the speakers by moving them forward or backwards or by tilting more or less. This may take some time.

Conclusion

Now that we have the studio monitors time aligned. Go listen to some music that you really know. Listen for depth and a 3D sound. The music should sound wider, more real, deeper and have a live quality to the sound. If you don't hear it at first, thats okay. It takes time to train the brain on how to get use to recognizing time alignment. But once you hear it, it will change the way you hear music and it will help with making better mixing choices.

If you have any questions about time alignment or studio monitors email me at kyle.tronaaudio@gmail.com

Thank you for visiting our blog. Happy listening!!!