I’m going to explain the term frequency response as it relates to loudspeakers. As you probably realize, the main purpose of loudspeakers are to convert electrical power into soundwaves. One of the problems, however, is that loudspeakers have a limited ability in transferring electric waves into soundwaves.
The reason for this is because the diaphragm which is generating the soundwaves cannot vibrate at any particular frequency but rather has a limited range. Also, the ratio of soundwaves that is generated versus the amount of electric power which is applied to the diaphragm varies by frequency. There is an upper and lower cut of frequency which is usually defined as the frequency swear the conversion rate drops by more than half.
The frequency response which is specified for loudspeakers and subwoofers is meant to show these to cut-off frequencies. When looking at wireless subwoofers, for example, you will notice that the upper cut-off frequency is only about 100 Hz. That means that the subwoofer can only reproduce a very narrow band of frequencies. However, subwoofers usually are specified with a wattage of several hundred Watts while regular speakers usually have less than 50 Watts. That means that the power density which is the amount of power applied to the speaker versus the frequency range is much larger than that of full-frequency speakers.
Most loudspeakers are designed with several speaker elements. That means there usually is a woofer as well as a tweeter included. The tweeter handles the very high frequency range while the woofer handles the mid range. By using a cross over, the electric energy is divided into different frequencies before being applied to each speaker element.
The enclosure itself of the loudspeaker determines to a large extent speakers frequency response. You will find that speakers that have a fairly bulky enclosure tend to have better frequency response at the very low end. In contrast, speakers which are tiny are not really well at reproducing low frequencies.
That is exactly where subwoofers come in. They complement these satellite speakers by adding the low frequency response. It is not really necessary to have multiple subwoofers because the human hearing is unable to discern low-frequency components in terms of their direction.
When making a frequency response measurement, it should be done in an environment where soundwaves are not being reflected by walls. That is why there are test labs out there which specialize in making these measurements.