Tuesday, July 04, 2006

Rising Low Frequency Response

Hi Rob -- On the WL183 and Low Frequency Response
All the mics I'm now using: NT4, NT1A, WL183 have extended bass response, which is good in comparison to the ECM909 I began with. But I notice that what is recorded with all three mics at lower frequencies is much more pronounced than I hear it myself: distant jet aircraft are more noticeable (a big problem where I live). ECM 959 and 999 don't seem so pronounced at LF.

When I look at the Wavelab FFT analysis screen when editing, I see that response goes down to almost dc levels, with NT1A having a hump from about 9Hz to just under 200Hz with a peak at about 40Hz. WL183 not so obvious but still enhanced.

[Spectrum image at top] is from the WL183s in a SASS setup, recording a few seconds of quiet woodland ambience. Does this show a rising LF repsonse to you, or is it an artefact of the analysis method perhaps?

I recorded a couple of music tracks indoors with theWL183 whilst on holiday - and these don't seem particularly bass heavy. Whereas outdoors, the extended bass is more noticeable. I realise this is all a bit hand-wavingly subjective, but I just thought I'd ask. As it is I tend to EQ out at the bottom end to return things to what I remember in the field.


Allan H


At 3:57 PM, Blogger Rob D. said...

Hi Allan- I've had numerous discussions with other field recordists about what to do with low Hz content. There are many opinions about it and several dependent factors to account for.

The frequency plot for the remote location looks quite familiar. From 400-20 Hz the slope of the line is fairly constant with a ~ 25dB increase. Taking local sound sources and acoustics out, the best explanation I have for this "look" is low frequencies travel far because they have power to do so. The further I get away from man-made sounds, the gentler the slope, but it always rises, substantially. (Again local acoustics and local low Hz aside).

The low end contour of my 183's would probably look about the same. They have less low-end response than my mkhs or mbho's or NT's.

Should one filter the lows in the field or use "roll-off" or high-pass" eq in post? For my surround installation work, I've found both of these practices to be counter-productive. Surprisingly, I rarely filter under 40 Hz except for shaping my LFE channel. I preserve low frequencies that are not unnaturely exaggerated because they are crucial in creating harmonics between 125-700Hz and help define the local acoustics. Given no local source these lows from far "strike" the resonant chord that is the local acoustic. Rob D.

At 1:39 PM, Blogger Curt Olson said...

Rob, Allan-

I'm open to your thinking here, Rob, but for now I'm with Allan. When I'm recording in the field, I often hear a lot of low-end stuff under headphones that my "naked" ears simply don't pick up. I tend to regard it as an unnatural low-end response somewhere in the recording chain. (It also sometimes triggers the "bottoming out" effect we've discussed with the Sony recorders, which I still use.) So I usually switch in the low-cut filter switches on the microphone itself or, in the case of the 183s, I usually run them through my Sound Professionals 9v power supply, with bass roll-off set with a relatively low knee.

Rob says, "I preserve low frequencies that are not unnaturely exaggerated because they are crucial in creating harmonics between 125-700Hz and help define the local acoustics. Given no local source these lows from far "strike" the resonant chord that is the local acoustic."

This is compelling and well worth considering. I'll definitely be pondering it as I record and edit in the weeks ahead.

At 3:24 PM, Blogger Rob D. said...

Hi Curt and Allan When one amplifies quiet location ambient recordings made with full range mics, the botttom end does sound disproportionately loud. I feel this impression is not created by an over-abundance of low-end content in the sound generated by the mics. There is a lot of low Hz energy in these settings from distant sources. Mics and pres have lower sensivity to this content than our ears. I feel the unnaturalness stems from exaggerated tones/pitches that our ears do not hear/produce that have been amplified 20-50 dB in playback. I feel that ambient or "reverberant field" recordings push the limits of mic performance. The exaggerated tones may be related to local acoustics-- standing waves or interference waves, but they are overly simplied into tones or a coarse resonance. [I came to this recognition because I used to have a very poor D-A card that created a consistent harmonic chord in all the sounds I played through it. When I replaced that card I was thrilled to hear differences in locations that I knew were there but could not hear. Slowly, I became aware of a much subtler but similar tonal simplifcation in all quiet location ambient recordings. The tones in these recordings are unique to each setting/situation-- no longer uniform like my old D-A.] I agree the impression is created somehere in the recording/playback chain and I'd really like to know where/how. You can address it without touching anything below 80Hz nost of the time. I find that high pass filtering often colors the tonal balance beyond hope. Kills the healthy bands too. If you are curiuos, try this test. Quickly raise and lower the volume knob while playing back 3 different remore location recordngs-- Don't all of them contain an unnatural low "drones" or harmonic chords? Find and notch the "notes" that create those tones with sharp parametric EQ. Isn't the result "smoother" and more natural than using high pass roll-off? Rob D.

At 6:12 AM, Blogger Sarah Hall said...

According to this graph, I an see that whereas outdoors, the extended bass is more noticeable. Thank you for sharing it with others! Our writing editing services can help you succeed academically with ease!


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