Amplifier Crossover Frequency And Speaker Frequency Response


I have been trying to figure out and failed, if a Subwoofer with a Frequency Response ranged between 20Hz-130Hz can actually play sound of 20Hz or maybe 25Hz if the Amplifier has a Frequency Response range between 10Hz-50Hz and with a Crossover Frequency between 40Hz-500hz?

Would this mean that the amplifier is going to send sound from 40Hz and up ? In other words would I be able to hear frequencies lower than 40Hz in my speaker ?

Thanks !


Posted 2020-03-08T19:23:22.943

Reputation: 103

1Can you please clarify what the context of the installation is? What sort of subwoofer are we talking about and what sort of amplifier? – Mark – 2020-03-09T12:37:17.700

Questions related to consumer audio are off topic for this stack. – Mark – 2020-03-10T00:08:07.003



This depends on the design of the filters in the crossover.

Filters are defined by a few key characteristics:

  • filter type
  • cutoff frequency: this is the frequency at which the output is attenuated by 3 dB.
  • slope: this is how much attenuation is applied. This is usually specified as 'x dB/octave': when you're one octave below the cutoff frequency, the attenuation will be x db. For every additional octave, you get x dB of additional attenuation. So if the filter is 12 dB/octave, the response will be -12 dB at one octave below the cutoff, -24 dB at 2 octaves etc.


enter image description here

The top green line shows the filter attenuation. This is a 12 dB/oct high-pass filter with a cutoff frequency of 1 kHz. as you can see, the filter is not abrupt. It's very difficult to create a filter that just cuts off everything below the cutoff frequency - any analog filter that does this will have audible side effects. A filter that has a gradual rolloff as shown here will have fewer side effects, but it will allow loud signals below the cutoff frequency to be heard.

So, If your crossover has a 12 dB/oct highpass filter set to 40 Hz and you send a 20 Hz tone through the filter (20/40 Hz is one octave), the output will be at -12 dB compared to the input. Depending on the volume setting on the amplifier, this will still be audible.

If the filter is steeper (say, 48 dB/oct) the output would be much quieter and may not be audible.

If you set the 12 dB/oct filter to a higher frequency (say, 320 Hz), you now have 4 octaves, so 4*12=48 dB of attenuation.


Posted 2020-03-08T19:23:22.943

Reputation: 1 478

Thanks a lot, at this moment I do not fully understand the dB/oct principle but I will do a research and also check the specifications of the amp to see if I figure it out, based on your answer. But since there is a chance of the sound being heard at 20Hz it is satisfying answer for me. – user9349193413 – 2020-03-09T11:45:48.507

1I've added some more info to try and explain that. – Hobbes – 2020-03-09T12:24:03.417

Please delete this. – Mark – 2020-03-10T00:07:21.857

No. Crossovers (and filters in general) are used in both pro audio and consumer stuff, and this answer applies to both. – Hobbes – 2020-03-10T07:14:12.697