Subwoofer Subjectivity

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Brent Venter

Freshman
Mar 31, 2011
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Experience shows that subwoofer low pass filters normally happens around 100 Hz or so, sometimes lower depending on the acoustical crossover but this is a generalisation. My question is this: why do subwoofers sound terrible above 100 Hz (at least to me) even when most of them are specified higher on data sheets by manufacturers.

Is there a technical phenomena to this, maybe box tuning etc?
 
Re: Subwoofer Subjectivity

Back in my days as an engineering student, the saying was "pick the gear in the middle of the chart".

To me this meant simply that things worked best in the middle of their designed operating range. When the driver, box, and everything is designed to optimize performance over a lower range, it makes sense performance suffers at a higher range.

I can ride my 15 speed bike everywhere in 4th gear. That doesn't mean it is effective, efficient, or even enjoyable.
 
Re: Subwoofer Subjectivity

Brent Venter said:
Experience shows that subwoofer low pass filters normally happens around 100 Hz or so, sometimes lower depending on the acoustical crossover but this is a generalisation. My question is this: why do subwoofers sound terrible above 100 Hz (at least to me) even when most of them are specified higher on data sheets by manufacturers.

Is there a technical phenomena to this, maybe box tuning etc?
Click to expand...

Sometimes manufacturers will specify a higher usable frequency on data charts to fudge the output spec. The subs may get louder at a higher frequency, even if they do sound like a rutting moose.
 
Re: Subwoofer Subjectivity

Brent Venter said:
Experience shows that subwoofer low pass filters normally happens around 100 Hz or so, sometimes lower depending on the acoustical crossover but this is a generalisation. My question is this: why do subwoofers sound terrible above 100 Hz (at least to me) even when most of them are specified higher on data sheets by manufacturers.

Is there a technical phenomena to this, maybe box tuning etc?
Click to expand...

I am going to go out on a limb here and suggest the first part of your question can be answered by the enormous output boost we are accustomed to giving our subwoofer feed. +10dB at 30-80Hz is a well loved phenomenon, but at 120Hz nobody wants to hear that shit. The higher specifications on data sheets are probably because there is no basic restriction on the HF output of subwoofers, most 18" woofers go to 1kHz just fine (albeit with nasty resonances and very narrowed pattern), and some applications do need a higher crossover point.
 
Re: Subwoofer Subjectivity

Brent Venter said:
My question is this: why do subwoofers sound terrible above 100 Hz (at least to me) even when most of them are specified higher on data sheets by manufacturers.

Is there a technical phenomena to this, maybe box tuning etc?
Click to expand...
Brent,

Low frequency woofers don't have to sound bad above 100 Hz, it used to be quite common for 15" and 18" to be used as high as 200 Hz and sound quite good at it, but there are many reasons why subwoofers can sound terrible above 100 Hz.

Large diameter woofers do start to have a fair amount of directivity above 100 Hz.
If located on the ground, and tops at normal ear height (or higher) when run above 100 Hz, the lower range of vocals and instruments will sound like they "walk" between the tops and subs, and hearing vocals coming from the ground (or having a variable frequency dependent source) sounds wrong.

Intermodulation distortion also becomes more apparent when it starts affecting the vocal range, which can go well below 100 Hz.
The "gargling" IM sound occurs in vocal reproduction when low frequency tones cause the cone to exceed Xmax, the limit of linear excursion (cone movement).
Excursion increases rapidly above and below box tuning, X max may be exceeded on kick drum hits or bass notes.
Most subs can handle far more power than required to exceed Xmax, even an "underpowered" sub may frequently exceed Xmax.
As the loudspeaker excursion exceeds Xmax, the amount of voice coil that is within the magnetic gap decreases.
As the excessive cone movement reduces the amount of magnetic force controlling the voice coil, distortion increases radically.

To visualize what occurs in IM, think of a speaker playing 40 Hz and 400 Hz tones.
For each in and out motion the 40 Hz tone causes, the cone is also making 10 smaller in and out motions to produce the 400 Hz tone.
When the 40 Hz signal is strong enough to push the coil outside the gap (exceeds Xmax) the 400 Hz signal no longer is reproduced properly at the end of the inner and outer stroke.
The amplitude (SPL) of the 400 Hz tone is actually reduced at either end of the 40 Hz stroke, the period (length) of the 400 Hz amplitude distortion is modulated by the 40 Hz signal.
A lower low note makes for an longer modulation period, and our ears will notice more “gargling” if the modulation tone was 40 or 60 Hz than above 100 Hz.
The objectionable sound of IM distortion in woofers may more accurately be described as "AM" distortion.
 
Re: Subwoofer Subjectivity

Art Welter said:
To visualize what occurs in IM, think of a speaker playing 40 Hz and 400 Hz tones.
For each in and out motion the 40 Hz tone causes, the cone is also making 10 smaller in and out motions to produce the 400 Hz tone.
When the 40 Hz signal is strong enough to push the coil outside the gap (exceeds Xmax) the 400 Hz signal no longer is reproduced properly at the end of the inner and outer stroke.
The amplitude (SPL) of the 400 Hz tone is actually reduced at either end of the 40 Hz stroke, the period (length) of the 400 Hz amplitude distortion is modulated by the 40 Hz signal.
A lower low note makes for an longer modulation period, and our ears will notice more “gargling” if the modulation tone was 40 or 60 Hz than above 100 Hz.
The objectionable sound of IM distortion in woofers may more accurately be described as "AM" distortion.
Click to expand...

Nicely explained, Art. :)
 
Re: Subwoofer Subjectivity

Chris Nixon said:
Art has explained it better than I could, but basically, they could do 300 just fine if they weren't also doing 30.

Chris
Click to expand...
Actually, a low frequency speaker can reproduce 300 Hz just fine while also doing 30 Hz if kept within the driver's linear limitations, that is excursion under Xmax.

Below you can see an example of two cabinets using the same loudspeaker which has 10mm Xmax, a stroke of 20 mm (.79 inch) peak to peak.

The larger cabinet can produce 121 dB at 30 Hz (clean) using only 350 watts.
The small cabinet can produce only 115 dB at 30 Hz using almost four times the power, 1250 watts.

The "underpowered" larger cabinet sounds almost twice as loud at 30 Hz, the smaller cabinet would make any vocals sound like talking through a window fan if you tried to get the same output, as it takes double the excursion to increase level 6 dB.
At 19.2 mm excursion, just short of double the linear excursion, the Kilomax would reach Xlim, it's maximum mechanical limit.
At that excursion it would be flapping uselessly and could tear up the cone, spider, surround, or hammer the voice coil in to the back plate.
That said, in reality once past Xmax, excursion with PA type loudspeakers usually is suspension limited, so the speaker usually just distorts badly until the voice coil is burned, rather than tearing itself up. However, if the Kilomax is put in a horn cabinet to gain another 6 dB output, it could easily kink the cone when driven too hard.

There are 18" with 20mm Xmax now, but few in the PA business are willing to pay for the expense, weight and loss of sensitivity requiring far more power the excursion entails.

It is not that there are "never enough subs", it's just that few want to pay for enough clean subs, and often expect more than can be delivered due to overstated marketing claims.

Art
 

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