Frequency Response - Part One

What a frequency response graph can tell you...and what it cannot.
Today, what it CAN tell you.
A frequency response graph is generated using test tones at a wide variety of frequencies but all the same level. Ideally the speaker will reproduce all the tones at the same output level. This doesn't happen in reality however as there will always be some differences in the measured loudness. The smaller these differences the more accurate the speaker is(all else being equal). Often accompanying a frequency response graph is a specification like this 75hz to 20kHz +/- 3dB. What this tell you is there is a maximum of 6dB deviation in the speaker's volume from 75hz to 20khz.
There is one area where "flatter the better" doesn't apply. This is in the deepest bass "rolloff" on subwoofers. The reason for this is all of our home audio rooms have something called "room gain" or "pressure vessel effect". I will detail this more in a future "Tip of the Day". For now, the important note here is...room gain will cause a gradually increase in the bass levels in all but the very largest of rooms.
Here is an example of a frequency response graph WITH room gain being simulated in the same graph.

This is our XS30 subwoofer. The darker trace is the measured response of the XS30 outside, with the microphone 2 meters away. This type of measurement is referred to as "ground plane". The lighter trace is the ground plane measurement COMBINED with a simulation of room gain. You can see how the XS30's shallow rolloff combines with room gain to provide a relatively flat response curve all the way down to 10-15hz. Having a designed in shallow rolloff like this has other benefits as well. Again, that will be covered in a future "Tip of the Day".

So the frequency response is a way to measure the speaker's ability to reproduce all frequencies equally.

Tomorrow's tip..."What a frequency graph can *not* tell you".

Tom V.
Power Sound Audio

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