freq range, 50hz-20khz at + -3db please explain

My loudpeakers are quoted as 50hz - 20khz ( + - 3db ) and 40 hz - 20 khz ( + - 6db ) Could someone explain what the plus and minus 3 and 6 db means.

Hi Andrewrona,

db (as you probably have infered) is the loudness value of the speaker output. Each speaker driver, cabinet design, and crossover design does not provide for the exact same volume level at each individual frequency. The +-3db refers to the change in relative volume level within the graph of the speaker output for the frequency range specified.

+-3db is very good. .....6db is regarded as the point where your hearing thinks it sounds as if you turned the volume up or down to twice the sound level, or half the sound level.

The low frequency output of speakers falls off in db very fast; which isn't a bad thing, since rooms suffer from bass loading characteristics which make it tough to get decent bass sometimes.

Best regards,


Hi Andrewrona,

Sorry, I miss stated the 6db point. It is the volume change that is normally just precievable as a change in volume - usually requiring 1/2 or twice the power applied to the speaker.

Duffydawg, there appears to be some confusion in your dB facts.

A 3 dB INCREASE is a DOUBLING of ACOUSTIC POWER [dBw or decibel watts]. A 6dB increase would be a quadrupling of dBw. A 9 dB increase would be an 8 fold increase of dBw.

Now, if we're talking about VOLTAGE, not power [dBv or dB voltage] it takes a 6 dB increase to double the voltage, or a 12 dB increase to quadruple the dBv.

People with good hearing should be able to detect a 1-2 dBw increase in the mid frequencies [not the ultra highs or ultra lows]

The dB readings are a logarithmic progression; someone will provide the formula, no doubt.
It's not as simple as a "loudness value" nor is it a relative volume level comparison between frequencies.

A speaker attempts to reproduce what is on the recording as accuarately as possible. Since we can hear in the range of 20 to 20kHz, most speaker specs cite this range. If the speaker was dead accurate, the "plus/minus dB" spec would be zero. What that means is that all frequencies would be reproduced AS RECORDED, not that all frequencies are the same volume or have no relative loudness between them.

But speakers are not accurate transducers. Some frequencies will not play at the same level as the recording; so they deviate from an "ideal". This ideal level is 0 db reference. So in your case, the +/- 3 db from 50 to 20kHz means that frequencies vary from the ideal by no greater than 3 db. But down to 40 hz, some frequencies vary by 6 db. Just a way of saying that the speaker plays down to 40hz but you may not hear much. In fact speakers play down to well below 20 Hz, it's just that you cannot hear it. The dB value at, say, 15 hZ could be - 18 dB.

The other thing to note is that not all frequencies vary by the +/- db value. One, a hundred or a thousand frequencies can vary. Some vary by that value at both ends. For example, at 200 hz it can drop by (-3 dB) and at 500 Hz can rise by + 3dB. Within spec, but a hell of a 6 dB suckout at the midrange (3 db on each side equals 6 dB). Whereas if only one frequency varied by 6 dB in a 20 to 20 kHz (+/-6dB) range, all the rest of the frequencies would be zero variance and you'd have one of the best speakers in existence. Even though it's +/- 6 dB, it a lot better that one with a +/- 0.5 dB.

Specs are misleading with speakers. The above is one reason why.

Hi Andrewrona,

As you can see, this discussion can become very complex. As Gs5556 has pointed out, the most accurate transducer would reproduce the input energy without deviation (+-0db).

No speaker performs that well. Standard tests graph the performance and deviation with a recorded test at a standard input value, sweeping through the frequencys. This resulting ouput plot is the source information for the manufacturers statement that the speaker performs within +-3db over a specified frequency range.

Here is a site where you can listen to +-6db & +-3db changes in a note (half way down page - "Sound Example"):

Keep in mind that the 3db is the amount of sound level change the manufacturer has measured in your speaker's performance over the frequency range (from one frequency to another).

Here is a site that shows the relationship to db changes and the perception of loudness (Sound Pressure Level graph):

Keep in mind that the frequency where your manufacturer has measured the 3db variation may or may not be easily precieved.

The published specifications are from "on axis" measurments. They don't tell you anything about how the speaker sounds if you are moving in the room, or setting off axis. There will be significant changes off axis. Usually this off axis performance is one of the most variable qualities when comparing different speakers/manufacturers. Proper high frequency dispersion is one of the most crtical elements for good reproduction within the listening area. The specs can't help you to know how the speaker will sound in your room, because the room augments or subtracts sound pressure form various frequencys.

In my opinion if you have a speaker perfoming +-3db from 50-20Khz you have a very flat performance measurement. I would doubt that the db chages would be easily noticed by hearing. I have never seen a measurement with a completely flat line for the entire range.

Whether or not anyone likes the sound of the speaker can not be predicted by this published performance. Strong peaks in the response curve are more noticable than drop outs. And the high freqency dispersion is not indicated within this type of measurement.

Good luck,