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Monday, January 23, 2017

Q. Is a matched pair of mics necessary for stereo recording?

By Hugh Robjohns
Even small differences in the polar pattern and frequency response of the two mics in an X-Y pair will have an adverse effects on stereo image. Using a matched pair — these are Rode NT5s — is recommended. 
Even small differences in the polar pattern and frequency response of the two mics in an X-Y pair will have an adverse effects on stereo image. Using a matched pair — these are Rode NT5s — is recommended.

Do I really need to use a 'matched pair' of microphones, or even two mics of the same make and model, for stereo recording? I guess professionals need the best quality possible, but for the rest of us surely it doesn't make that much difference. Can't you make good stereo recordings, even if the mics are not of the same make?

SOS Forum Post

Technical Editor Hugh Robjohns replies: Different microphones will sound... well, different! They will have different frequency responses, and more importantly, they will have different polar pickup patterns. What's more, the way in which the two microphones' polar responses vary according to the frequency of the signal will also be different.

If you try to use two different mics for X-Y coincident stereo recording, the inevitable result will be an unstable and ill-defined stereo image that appears to wander about, with different instruments affected in different ways. This isn't a subtle effect, either — it takes very little difference between the two mics to make this a serious problem. This is precisely why the high-end mic manufacturers go to such trouble to maintain tight tolerances in producing matched pairs, and why companies producing more affordable mics offer hand-selected matched pairs.

If you want to check the compatibility of two cardioid (or any other directional pattern) mics, try this simple experiment. You'll need stereo monitoring, a mixer and somewhere to set the mics up where an assistant can walk around them while you listen to the monitoring output in isolation — a studio with a separate control room would be ideal.

Place your two mics above one another with the two capsules facing forward along the same axis and as close together as you can get them. Plug one into the left channel of your mixer and the other into the right channel, pan them hard left and right and match their gains exactly.
High-end mic manufacturers like Microtech Gefell test matched pairs to very tight tolerances. 
High-end mic manufacturers like Microtech Gefell test matched pairs to very tight tolerances.

The easiest way to do this is to get someone to stand in front of the mics and talk. Meanwhile, you select the mono button on your monitoring and reverse the polarity of one of the mics. Fade up the first mic and set the gain to a sensible level to hear the speech. Then as you fade up the second mic some cancellation should occur as the polarity is reversed. Adjust the second mic's gain to achieve the deepest cancellation null. Now you can remove the polarity inversion and cancel the mono monitoring. You should now hear the speech coming from mid-way between the two speakers as a phantom centre image.

Now ask your assistant to walk around the mics, in a circle, keeping roughly the same distance from the capsules and facing them all the time as he or she walks around. If the mics are perfectly matched, the image will not move from the centre line, although the level will obviously vary as the assistant moves around to the rear null of the cardioids, and then back towards the front around the other side.
With two dissimilar (or poorly matched) mics, what you will hear instead is the image wander or flick about, often with different frequency components appearing to come from different places — sibilants from the left and fundamentals from the right, for example. The image will sound unstable and poorly defined.

Exactly the same kind of imaging inaccuracies will occur when the mics are rotated to face left and right as in a conventional X-Y stereo pair, resulting in a poorly defined, blurry image — definitely not a good stereo recording!

Spaced mic arrays aren't quite as critical in terms of mic pattern because level differences caused by the polar pattern don't play as large a role in defining the stereo image. But you will still suffer from strange and unstable imaging if they have different frequency responses.

The only stereo technique that deliberately uses different mics is the coincident M&S array — but that requires that one mic have a figure-of-eight pickup pattern. Even so, it is important that both mics have similar frequency responses if the imaging is to be accurate. In this case, though, poor matching results in unstable width problems, rather than image shifts, which is probably less audible.

Published May 2006

Friday, January 20, 2017

Q. What kind of bass trap do I need?

By Paul White
We have set up a new recording studio at our school. It's not huge, but we do have a separate (very small) control room as well as a live room. We were expecting to need acoustic treatment and have used bass corner traps from Studiospares as well as some general treatment in the control room to reduce reflections from the walls at the mix position. But, although most of the room modes have been cured, we have a humdinger of a resonance at about 58Hz — using the test oscillator in Logic Pro shows it up nicely. I'm at a loss to know what to do next but wondered if a tuned bass trap was the answer, as it is very specific to that frequency. I think the problem is that the height and length of the room are about the same, even though the room is not rectangular (one end wall is angled).
Tim Morris
Editor In Chief Paul White admires his handiwork. 
Editor In Chief Paul White admires his handiwork. 

Technical Editor Hugh Robjohns replies: Small rooms are always difficult to treat well, especially if some of the dimensions are similar, because the modes tend to pile up very close together. A tuned trap might well help, but I would suspect you would actually be better off extending the existing bass trapping.

Commercial foam traps are OK, but you'll get better performance if you build some DIY corner traps using slabs of Rockwool two to four inches thick in simple timber frames covered with fabric.
This diagram shows a cross-section of the bass trap we built in the Studio SOS article in the March 2006 issue. 
This diagram shows a cross-section of the bass trap we built in the Studio SOS article in the March 2006 issue.

Suitable examples of this approach — as well as more complex limp membrane absorbers — have been illustrated in the pages of SOS recently. In the March 2006 issue, for example, we explained in detail how to build just such a trap in the course of a Studio SOS visit. We've included the diagram (right) again for reference, but I'd definitely advise reading the whole article. There's lots of other information on acoustics available on our web site, and there's always plenty of debate on the subject in our DIY Studio Acoustics forum, too.

In a small room you can't really put in too much trapping, so I would suggest treating all four vertical corners, and if possible, go for the wall-ceiling corners too. It might also pay to place Rockwool absorber panels behind the speakers to help reduce any back wall reflections at the bass end.

Published May 2006

Tuesday, January 17, 2017

Q. Should I use my mixer's group outputs or its direct outs for recording?

By Mike Senior
Like other mixers, this Allen Heath GL2400-424 offers both direct outs on channels and group outs. But which should be used and when?

I recently started teaching music technology in a college and was asked to rebuild one of the studios. It uses a 32-channel mixing desk, patchbay and Alesis HD24 hard disk recorder to record to, as well as outboard gear. The desk has eight group busses arranged in four stereo pairs. There are 24 mono group output sockets, three per group buss, so that group 1 goes to outputs 1, 9 and 17, group 2 goes to 2, 10 and 18, and so on. The way it was set up previously was that these 24 group outputs were normalled through the patchbay to the 24 inputs on the HD24. The students were being taught that the signal should come into the desk and then be routed through the relevant group to get to the HD24. For instance, if your mic is plugged into channel 3 and you want to go to track 5, you have to route it to group 5-6, pan it hard left and bring up the channel fader and group fader. However, I changed it so that the direct outs of the first 24 channels are normalled through to the 24 inputs of the HD24, which seems to make more sense. One of the lecturers is kicking up a fuss, so my question is: which practice is most common in professional studios?

Thom Corah

Reviews Editor Mike Senior replies: You're both right after a fashion, but I'm afraid that I think the lecturer is probably more right in this case, as you appear to be using a group desk, rather than an in-line one. Your approach has two main limitations. Firstly, you can only route channel 1 on the desk to channel 1 on the recorder. This is admittedly less of a limitation with a digital recorder, where you can swap tracks digitally, but it's still quicker to do this from the desk than from the recorder.

The second (and more serious) limitation is that you can't record a mix of several channels to the same track on the recorder. Although 24 tracks is quite a lot to work with, you might need to submix a number of microphones to, say, a stereo pair of tracks — for example, when layering up a string quartet a few times to make a composite string sound for a pop production. Another problem is that you can't use the mixer's EQ on the way to the recorder, as direct outputs are often taken from before the EQ circuitry. Also, you couldn't bounce down a group of tracks through the desk in this way without sending them all to a group first, and then patching from the group output to a further channel. So you'll have more flexibility if you do things the lecturer's way.

One reason that you're not completely wrong is that you're implementing a kind of in-line methodology, treating the input stage up to the direct output as the input path and the rest of the channel as the monitor path. However, a group desk isn't really sufficiently well equipped to do this properly, most notably because there is no routing matrix between the input channels and the recorder inputs, as there would be on an SSL desk or similar. There's only one routing matrix per channel on a group desk, and that is situated after the channel fader. There's no real alternative, given the facilities, but to have separate channels for the input and monitor paths. In your case, as you have only 32 mixer channels, this means repatching for mixdown and monitoring purposes, I imagine, but I don't know all the details of your setup.

One situation where you can get away with using an in-line configuration on a group desk, exactly as you have, is where the recorder is actually a computer system. In this case, given the powerful processing facilities a computer offers, there's little advantage these days in pre-processing audio before it reaches the computer, so the lack of input EQ would not really be a problem. Also, there are comprehensive input routing and mixing facilities built into most modern audio-recording packages, so a hardware routing matrix would also be unnecessary. Perhaps you could justify your routing scheme as just being a little ahead of its time? You are simply anticipating the happy day when the college moves to a more flexible computerised system!

At the end of the day, which is the more appropriate arrangement depends on how many tracks you plan to record at one time. The group routing approach is more flexible when it comes to being able to do track bounces and partial submixes, and it is an important way of working to teach students. However, the down side is that you can record no more than eight (different) tracks at a time because there are only eight groups on your mixer.

Taking the direct outs approach allows up to 24 different tracks to be recorded at the same time and is ideal in areas designed purely for tracking, but you are then in for lots of replugging when it's time to mix. In any case, students should definitely be made aware of both techniques and configurations.
One possible solution that you could consider is using the patchbay to normal the group outputs to the recorder inputs, as before, but also send all of the desk's direct outs to patchbays on the row above, so that when you need to patch direct outs straight into recorder tracks it's just a case of plugging in some patch cords.

Published January 2006