Physics Question: Why does cabinet volume matter so much to bass response?


If you put the same 8' woofer into a bookshelf speaker or a floorstander, you will get a different frequency response.  Does anyone know what's happening with the air pressure on the inside of the cabinet to cause this to happen?  Does the woofer in the bigger cabinet have greater excursion, and therefore produce more amplitude?  

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A ' driver ' has electro - mechanical - acoustic properties ( TS Parameters ) and the volume relates to ' efficiency ' in a ' system ' ( loudspeaker ) as well as other relational properties.

 

It's a shame manufacturers went the way of small thin cabinets and tiny woofers. They all sound like giant tweeters these days....

 My understanding is that a larger cabinet will support a lower frequency response in the bass. The shape of that frequency response is also shaped by the design of the cabinet, i.e., whether it be sealed box or bass reflex (let's leave out the other designs for the sake of keeping this simple).

  As many of you already know, the sealed box design will make a slower slope in the bass response (although not as deep) than the bass reflex, which goes lower in frequency, yet at a higher slope than the sealed design. Given this, bass response is altered by the physical size of the cabinet. A smaller cabinet simply cannot load the woofer for lower frequencies. Of course, these factors much match the T/S parameters of the driver to begin with to make a reasonable bass response in the first place. A bigger cabinet allows for lower and possibly flatter response in general than trying to make a small woofer and cabinet do what it cannot. A popular trick used for smaller cabinet/woofer sizes is to make use of a bump usually at 100hz. It gives the likeness of more bass, a thump, if you will, but it is not ideal for my tastes. 

   This is the case for passive speakers, but active speakers can use EQ in the signal to try and offset the limitations of the cabinet and driver. In the end, physics will rule up to the limits of the final product.

Hofmann's Iron Law

three parameters that cannot all be had at the same time. They are low-bass reproduction, small (enclosure) size, and high (output) sensitivity." Hofmann stated that designers could pick two of these three parameters, but in doing so, it would compromise the third parameter.