Essay: The frequency response

The frequency response is the graph of the frequency output compared to the ‘normal’ level of the sound pressure in decibel. This shows the deviation of the outputted loudness compared to the original loudness. The ‘normal’ level is the sound pressure in decibel of a tone that is inputted and not tuned by the subwoofer. The line represents the difference between the ‘normal’ level, which is the line Y=0 dB, and the sound pressure outputted. When the line is above the normal level the outputted sound is higher than the inputted level of sound. A line 5 dB lower than the inputted level means a sound 5 dB less loud than the input.

The frequency response depends on the volume of the enclosure, whether it has a port or not and the frequency response of the woofer itself.
If the volume is bigger, the frequency response line will reach lower frequencies.
Building in a port will also create the ability to reach lower frequencies with the original sound volume.
Often big woofers have a lower frequency response than the smaller sized woofers.

An example of a frequency response graph with the deviation in dB on the Y-axis and the frequency in Hz on the X-axis.

Why is the frequency response important?

The frequency response shows two important things:
– The effective range
– The deviation between input and output

The effective range exists of the minimum frequency the subwoofer can reproduce and the maximum frequency the subwoofer can reproduce. Frequencies lower than the minimum frequency will not be outputted and frequencies higher than the maximum frequency will not be outputted either. The effective range shows the capability of the subwoofers ability to reproduce very low sounds which is important for experiencing a lot of bass. Most people can hear frequencies till 20 Hz which will make an effective range with a minimum frequency of 20 Hz a very decent range. The effective range also shows the ability to reproduce their highest sound which is useful for crossover systems, which consist out of several speakers that reproduce the sounds that are out of range for the subwoofer itself.

The deviation between input and output is the difference in sound pressure between the original sound pressure recorded by the input and the sound pressure produced by the subwoofer. This shows how good subwoofers can reproduce sounds within their effective range. If the amount of dB differ a lot from the inputted amount of dB the sound might be a lot louder or could barely be heard at all. Causing it to sound rather unnatural. The more consistent the deviation of the sound pressure line is, the more natural it will sound. This is only the case when it is a consistent line on the line dB=0. A line above the 0 dB line means that the subwoofer will be louder at this certain frequency which will have a ‘boosting’ effect on the original sound. This can also be a positive thing based on personal preferences. When bass is found important the outputted dB exceeding the 0 dB line at low frequencies will fill up to the owners desires.

The frequency response is important because it has to do with the important factors: range and deviation. From which you can see the purpose of the subwoofer, for example bass, and the transforming of the sound quality.

The frequency response of a sealed enclosure

Many sealed enclosures, can also be referred to as a closed designs, will have a frequency response graph that will rise till they reached the normal level, turning into a consistent line along the normal level. This will get you a lower sound volume at the lower frequencies, but the same sound volume at higher frequencies.

The frequency response of a vented enclosure

Vented or ported designed enclosures, have a notably higher frequency response at lower frequencies. This is mainly caused by the air being able to travel in and out of the subwoofer through the port, needing a amplifier weaker than a amplifier needed for a sealed enclosure. At a certain frequency the sound volume even has a positive deviation compared to the normal line. This is caused by the resonating of the port. Creating a boomy like effect strengthening the bass.

The difference in frequency response between a enclosure with a port and a sealed enclosure.
The difference in frequency response between a sealed and a vented enclosure is shown in the graph below. The ported enclosure reaches lot higher SPL’s than the sealed enclosure at lower frequencies but will eventually reach the same SPL.

Source: Essay UK - http://doghouse.net/essays/information-technology/essay-the-frequency-response/


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