What's lurking at the bottom?

Hardly a week goes by without a question about subwoofers, and the joy of extracting that last drop of low frequency information out of a CD, DVD , or LP, but, just what is lurking down at those low frequencies?

What would I miss if my subwoofer only went down to 40Hz? Is there much of anything in the way of music or movie sound effects lurking at 30, or even 20Hz, that makes it an imperative that a speaker or subwoofer goes that low? Off the top of my head, I guess fans of pipe organ music would find something to interest them down at 20Hz, but what about the rest of us?

Thanks,

Glenn
 
Hi Glenn, normal pracice in pop/rock DAW recording and mixdown is to virtually chop any content below 40-50 Hz.
For many such recordings this allows usefull multi-band compression and levelling without objectionable artifacts and wasting/overloading of LF playback system power.
Classical and other genres of course tend to a flatter and more actual LF rec/pb response.
Movie soundtracks are open to any effecting that the sound producer feels like, including very heavy compression/limiting of the LF band.
For typical home HIFI/HT application, so long as you have clean, dynamic and reasonably high power LF capability, you are unlikely to miss very much.
A clean and dynamic LF band system that goes reasonably low, can give a decent impression of low lows, without annoying the neighbours too much.
It takes a more seriously high powered system to reproduce high SPL low lows, so a degree of band limiting can be practically useful.
I believe horn or corner systems can give serious high SPL/low distortion LF replay, but require large dimensions.
NP has threatened to release details of a 20Hz horn CRAW system, but it is still in development it seems - Nelson we are waiting in anticipation ;) .
On my high power, high resoloution workshop system I find a graphic eq to be quite useful for fine tuning of the system sound and compensating for differing recordings.
Sometimes when I feel like it I can patch in a compressor/limiter box to attain clean quite seriously loud SPL without system overload.
Clean, high power, transient capable and non-resonant are the keys to big, punchy, fat and pleasing lows - the downside is that this can be expensive.

Regards, Eric.

Eric.
 
Noisey Neighbours.

Further thoughts - typical modern resonant/bandpass type subwoofers produce rotten/bonky/woofy/single note type bass that actually does not go properly low, and the resonant characteristic produces an irritating type bass sound that travels.
More than once I have telephoned a neighbour across the street and politely infomed him that his movie soundtrack although not playing very loudly, is a PITA at 2.00 AM.
In his lounge room his system does not seem all that loud, but the sound that propagates in the quiet night can be disturbing.
Much better IMO is a full range loudspeaker system that goes low and clean enough, and without resonant augmentation.

Eric.
 

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
Glenn:

Are you building this for home or PA use? It sounds like home use, but that 40 Hz cutoff you are considering makes me wonder.

PA equipment must be more efficient to produce greater sound levels. The sacrifice is bottom end. Most 97 or 98 dB @ 1M/1W PA cabinets of 3 cu ft have a cutoff of 60 Hz, for instance. A home speaker of 3 cubic ft would certainly be expected to go deeper much deeper than that. The payoff for the PA speaker is 8 or 10 dB @ 1W/1M sensitivity.

The lowest note on a bass guitar is 42 Hz. For that reason, a speaker that goes down that low is doing an adequate job.

If you have ever listened, however, to a subwoofer that has high output down around 20 Hz, you will notice that there really is a whole world down there, even in music. A friend had a system that went down below 40 Hz. He added a big subwoofer, and found that even his classical LP's from the 60's had substantial output down below 40 Hz, despite what the magazines told him.

The difference is there, and it is palpable.

40 Hz is good, but there is a reason people make the investment and effort to go lower.
 
kelticwizard said:
The lowest note on a bass guitar is 42 Hz. For that reason, a speaker that goes down that low is doing an adequate job.

If you have ever listened, however, to a subwoofer that has high output down around 20 Hz, you will notice that there really is a whole world down there, even in music. A friend had a system that went down below 40 Hz. He added a big subwoofer, and found that even his classical LP's from the 60's had substantial output down below 40 Hz, despite what the magazines told him.

The difference is there, and it is palpable.

40 Hz is good, but there is a reason people make the investment and effort to go lower.

I simply chose 40Hz as an arbitrary figure, since with the exception of a pipe organ, there are no musical instruments producing anything down there. I figured movie explosions may have extended that low, but wondered what else was lurking below 40Hz.

I always intended to build a subwoofer for my system, for the benefit of movies, if not music, but it's interesting to learn that musically, there is something happening at those low levels.

Thanks

Glenn
 
More below 40Hz

GlennME said:


I simply chose 40Hz as an arbitrary figure, since with the exception of a pipe organ, there are no musical instruments producing anything down there. I figured movie explosions may have extended that low, but wondered what else was lurking below 40Hz.


The fundamental of the A at the bottom of a piano keyboard is 27.5 Hz. Obviously we mostly hear overtones, but that note is there! Think about how much longer those strings are than those on a bass guitar. And don't forget that contrabass sarrusophone, too (whatever the heck that is). :)

Seriously though, harps go below 40 Hz (and play some great stuff there) and some pianos (Bosendorfer Imperial Concert Grand) have 96 keys and go to 16.35 Hz.

Bob's yer uncle.
 

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roddyama

Ex-Moderator
2002-01-19 9:25 am
Michigan
Hi Glenn,

There are two aspects of music that are often over looked. Those are Sub-Harmonics and "Differential" waves (for the lack knowledge of the correct term). These sound components exist in the lower octaves (some below 40Hz) and will be part of the character of the music.

Sub-Harmonics (f/2, f/3,...) of the fundamental frequency exist (at lower levels) as part of the total sound produced by instruments. There's info on the web if you do a search on "Sub-Harmonics". Here's one link:

http://www.phys.unsw.edu.au/~jw/sound.spectrum.html

Differential waves are a form of IM distortion. If two different tones are played from different sources and the frequency difference between them is XHz, where they converge in space, a composite wave will be created that includes both of the original frequencies plus a new tone equal to XHz. This has to happen in performances with multiple instruments adding to the overall character of the performance.
I don't know much about this phenomenon except that it exsist and that there is a weapon that uses it to induce low frequencies in a target person to disable them.

Rodd Yamas***a
 
Hi roddyama,

This is a very interesting phenomenon and is actually used as one of the simplest ways to tune a musical instrument by ear, particularly stringed ones. As any guitar player will tell you, you can tune (most of) the strings in standard tuning relative to each other by playing the harmonic at the fifth fret on one string and then the harmonic on the seventh fret on the next string up (in pitch).

So, if you want to tune E and A relative to each other, assuming E is at the correct pitch to start with, you play the harmonic at the 5th fret on that string. That's the 4th harmonic and the fundamental for concert pitch is E4 which is roughly 330 Hz (equal tempered scale). The 4th harmonic on this is 330 * 4 = 1320 Hz. Similarly you play the 3rd harmonic (7th fret) on the A which is 440 * 3 = 1320 Hz. Bingo !! they both play the same note. Well, yes, that assumes they are already in tune. But this is supposed to be a tuning method, so what happens if they aren't exactly in tune ??

What actually happens is that the two frequencies sum together in the instrument (or in air) giving you the following.

sin X + sin Y = 2 sin((X+Y)/2) cos((X-Y)/2)

Forgetting about the 2 which is just an amplitude scaling factor, you get is the product of a sin and cos at very different frequencies. If we assume that E is spot on at 330 giving 1320 Hz (X above) for it's 4th harmonic and say A is at 430, giving a 3rd harmonic of 1290 Hz (Y above). Thus you get

sin(1320) + sin (1290) = 2 sin(1305) cos(15)

In other words a sin wave at a frequency very close to what you are aiming for is modulated by a low frequency signal. This is what you hear. You hear what is referred to as a "beat frequency" which is a very audible modulation of the harmonic tone that varies as you change the pitch of one of the strings.


To pre-empt somebody saying "What about the product of the two signals ?" Yep, it will happen, given that no complex mechanical system like a stringed instrument is linear, but look at the maths

sin(X) sin(Y) = 1/2 (cos(X-Y) - cos(X+Y))

Again forgetting about the scaling factor and using the numbers above you get two cos waves (90 degree phase shift from the original signals, which makes no difference in this case) at frequencies of 2610 Hz and 30 Hz. Without a reference, you are not going to be able to determine by ear if 2610 Hz is exactly the 2640 Hz you are looking for and an instrument designed to reproduce sound with a lowest note of about 330 Hz isn't going to stand a snowballs chance in hell of reproducing 30 Hz with enough amplitude to be audible.

Ooops, bit of an epic, but there you go,

DocP
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