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Low frequency sound recording (on set)?
  • Hi, I have a question for our sound-guys ;-)

    I have 2 MXL FR-300 mics and a Marantz PMD 661 recorder, which is a nice combo for general audio recording (voice, sound on set, ambient,..). But the MXL mics have a frequency spectrum of only 50Hz-18kHz and there is some sound below 50 Hz. Especially the things you "feel" in a cinema are in the 20-30 Hz range.

    How is that part recorded or is it all done in post? And would you recommend an additional mic like for on set recording?

  • 5 Replies sorted by
  • MXL isn't known for particularly high quality microphones. Low frequency pickup is generally limited by the size of the mic diaphragm with larger being better overall. A LDC or a plate mounted PZM mic would do better on low frequencies than a shotgun mic with a small diaphragm. It's not unusual in the recording world to use speakers wired as mics to pick up things like kick/bass drums in order to get more of the thump of the low frequencies.. Also, it's worth mentioning that frequency response is only one measure of a microphone's quality. There are sensitivity ratings, SPL ratings, distortion ratings, transient ratings, noise ratings, etc. Having a certain frequency response means nothing by itself especially if the mic electronics are slow or noisy, which MXL are well known to do. I'd seriously suggest upgrading your mic before going too far. My first "expensive" mics really opened my eyes to the world of quality mics. It sucks paying large amounts of money for a mic when you've been told by salesmen and friends and advertisements how some cheap mic "is almost as good as (insert quality mic name here)" but I can tell you from experience that they aren't. I've spent a lot of time and money messing around and modifying "cheap" mics, and you can get some hidden gems to polish up nicely, but for the most part they don't compete with the big boys.

    Anyway, lets talk about the details of audio mixing/design a little bit. 20-30hz is felt more than heard but even mics that can pick up this region well it is cut off in post mixing and then "sub-synthesis" is usually done instead. Since the human ear can't differentiate tones in the bass region very well, having a lot of low frequency content causes the eardrum to compress heavily which actually lowers the sensitivity to high frequencies as well. What sound engineers do is cut the frequencies below a certain point and create simple sine or harmonic tones that can excite the ear drum while the high frequency content fools the brain into filling in the missing frequencies. It's called "sub-harmonic synthesis"

    Another trick is to generate harmonics that are above the lower fundamental tone and then cut the fundamental tone. Say you have a lot of 30hz and your audio sounds excessively boomy. You can generate harmonics at 60, 90, 120, 150Hz, etc, and cut the 30Hz completely out. The brain will "hear" the 30Hz tone even though it's not there. This is called "missing fundamental".

    Anyway, chances are that in any modern movie, the audio tracks themselves are entirely dubbed in a studio or the low frequencies are either entirely fake or are augmented heavily to keep clarity of the audio very high, and/or and they use very expensive microphones.

  • Refuse Lowender does a good job at trouser flapping and is cheap at $69

    LFE stuff is added in post 99 %of the time in a mix - in fact pretty much everything you hear in a film is replaced these days (depending on budget/country and style)

  • Lowender is good. I've used it before. I find that the regular sub-synthesis trick works just as well. Take a gate and route a 50hz sine through it. Take the signal that you want to augment and run it to the side chain of the gate. That way, everytime your signal hits, the gate opens and allows the sine wave through. Mix the two so that you can't hear the sine but it enhances the original track. Works really good on kick drums and bass guitars.

  • @Svart Aye good old tricks - the old DBX boombox is nice too if you can still find one. Use Svarts mentioned trick a lot on bass and BD for dance music keep it short and you can make it louder - tightens up the bass and drums too if they're a little sloppy on regular recordings too. You can also tune the sine wave to the bass track if you're feeling creative. Waves Air is on sale too as they prepare to rip us off, erm advance us, into AAX land - does similar but less focussed thing to lowender.

  • @svart Good explanation about getting the brain to "think" it's hearing fundamentals. Back in my organist days that's something you could do by choosing two upper harmonics together - it would generate false bass notes. I'm using this same technique on the harp in a competition this week, to play bass notes that don't physically exist on the harp. It works pretty well!

    Never heard of it being applied in audio post, but it makes huge sense.