We recently had a long conversation about the reasons to set a tower speaker to "small" with a 80Hz crossover allowing a subwoofer to handle the bass below 80Hz versus setting it to "Large" and let it play deep with, or without, a subwoofer.
Part of the logic for setting a tower speaker to large is that the specs may claim a bass response to frequencies lower than 80Hz and it seems a waste to have a big speaker not handling the bass. Additional novice intuition suggests that having the tower speaker play the bass at the same time as the subwoofer must be a good thing because bass plus more speakers must be a good thing, right?
I think a quick consideration of the physics involved with reproducing bass is worth some mental energy.
Just basic physics insists that to produce a given SPL at a frequency half of another frequency requires the cone of the woofer move twice as far as the higher frequency. For example, if the cone moves 1mm at 100Hz to generate X SPL, then it will have to move 2mm at 50Hz to produce X SPL and 4mm at 25Hz to produce X SPL.
However, if a woofer has double the cone area, then the excursion necessary to generate the same SPL is cut in half - so a 6.5 inch woofer with a cone area of 140 cm2 may have to move 1mm to generate X SPL at 100Hz, but a larger 9.5 inch woofer with double the cone area of 280 cm2 only has to move 0.5mm to generate the same SPL at 100Hz.
Obviously, operating two woofers in parallel will accomplish the same thing - reducing the excursion by half to generate a given loudness at a specific frequency.
So, that should be easy to comprehend - a large woofer needs to move less than a small woofer to create bass. Also, lots of woofers producing the same signal will need to move less for a given SPL than a single woofer.
Consider the distortion a woofer generates... most of the distortions are from mechanical nonlinearities and the rest tend to be caused by nonlinearities in the magnetic field the voice coil is passing through as it moves in and out. The more excursion a woofer is forced to take, the greater the distortions will be in relation to the intended signal. In other words, higher SPLs equal higher distortion as a percentage of the signal. This is why you can often hear a woofer reaching its limits before you push it hard enough to blow the voicecoil. Summarizing, more cone motion equals more distortion.
So, if one wants clean, deep, solid, non-distorted bass. adding woofers or increasing the woofer size of the creating the bass is an obvious solution. Also, better woofers can play cleaner at high SPLs than cheaper woofers, even if they are the same size - but it can be very costly to get significantly better output from the same sized cone. That's why you can go to Parts Express and buy a 15" woofer for $20 or another for $500.
Now, in a home system, you may have a tower speaker with two 6.5 inch woofers which operate below 300Hz down to their limits at 40Hz. You may also have a 15 inch subwoofer which can operate well from 120Hz down to below 20Hz. Which woofer would you want producing 60Hz in your room? Well, for me it is a no brainer - go with the 15 inch woofer. However, the 15 inch subwoofer probably isn't designed for great fidelity above 200Hz, so the 6.5 inch woofers are much better in that range. So, given the option, setting the tower with 6.5 inch woofers to "small" with a 80Hz crossover will net the best performance in the frequency range below 100Hz. It'll definitely net you lower distortion and the best controlled output. And, since we generally cannot hear stereo imaging at frequencies below 100Hz in a normal sized room, there is no benefit to the stereo effect by having the towers produce lower bass.
So, in most cases (there are clearly exceptions), it makes sense to set a crossover on main speakers who don't have larger woofers or tons of woofers to something around 80Hz if you also happen to have a good large subwoofer.
Part of the logic for setting a tower speaker to large is that the specs may claim a bass response to frequencies lower than 80Hz and it seems a waste to have a big speaker not handling the bass. Additional novice intuition suggests that having the tower speaker play the bass at the same time as the subwoofer must be a good thing because bass plus more speakers must be a good thing, right?
I think a quick consideration of the physics involved with reproducing bass is worth some mental energy.
Just basic physics insists that to produce a given SPL at a frequency half of another frequency requires the cone of the woofer move twice as far as the higher frequency. For example, if the cone moves 1mm at 100Hz to generate X SPL, then it will have to move 2mm at 50Hz to produce X SPL and 4mm at 25Hz to produce X SPL.
However, if a woofer has double the cone area, then the excursion necessary to generate the same SPL is cut in half - so a 6.5 inch woofer with a cone area of 140 cm2 may have to move 1mm to generate X SPL at 100Hz, but a larger 9.5 inch woofer with double the cone area of 280 cm2 only has to move 0.5mm to generate the same SPL at 100Hz.
Obviously, operating two woofers in parallel will accomplish the same thing - reducing the excursion by half to generate a given loudness at a specific frequency.
So, that should be easy to comprehend - a large woofer needs to move less than a small woofer to create bass. Also, lots of woofers producing the same signal will need to move less for a given SPL than a single woofer.
Consider the distortion a woofer generates... most of the distortions are from mechanical nonlinearities and the rest tend to be caused by nonlinearities in the magnetic field the voice coil is passing through as it moves in and out. The more excursion a woofer is forced to take, the greater the distortions will be in relation to the intended signal. In other words, higher SPLs equal higher distortion as a percentage of the signal. This is why you can often hear a woofer reaching its limits before you push it hard enough to blow the voicecoil. Summarizing, more cone motion equals more distortion.
So, if one wants clean, deep, solid, non-distorted bass. adding woofers or increasing the woofer size of the creating the bass is an obvious solution. Also, better woofers can play cleaner at high SPLs than cheaper woofers, even if they are the same size - but it can be very costly to get significantly better output from the same sized cone. That's why you can go to Parts Express and buy a 15" woofer for $20 or another for $500.
Now, in a home system, you may have a tower speaker with two 6.5 inch woofers which operate below 300Hz down to their limits at 40Hz. You may also have a 15 inch subwoofer which can operate well from 120Hz down to below 20Hz. Which woofer would you want producing 60Hz in your room? Well, for me it is a no brainer - go with the 15 inch woofer. However, the 15 inch subwoofer probably isn't designed for great fidelity above 200Hz, so the 6.5 inch woofers are much better in that range. So, given the option, setting the tower with 6.5 inch woofers to "small" with a 80Hz crossover will net the best performance in the frequency range below 100Hz. It'll definitely net you lower distortion and the best controlled output. And, since we generally cannot hear stereo imaging at frequencies below 100Hz in a normal sized room, there is no benefit to the stereo effect by having the towers produce lower bass.
So, in most cases (there are clearly exceptions), it makes sense to set a crossover on main speakers who don't have larger woofers or tons of woofers to something around 80Hz if you also happen to have a good large subwoofer.
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