What does “Frequency range Free Field” mean and what does it measure?

[PortalKeeper]

I just bought a pair of KEF Q950’s and was wondering what the meaning of the “Frequency range Free Field” specification is. It says it is 38Hz for these speakers in the manual.

Here is a link to the manual:

https://assets.kef.com/uploads/cm/Q_Series_Manual_EN.pdf

It also says in parentheses (-6dB) right next to it. Could you please explain that as well? Thanks.

 

[Elitzur–Vaidman]

"Free Field" means without reflections, or in the context of bass extension, without room gain.
The -6dB means that the speaker measures 6dB down at 38Hz (and likely drops precipitously below 38Hz)

 

[DonH56]

Exactly. Think of a speaker hanging from an infinitely high (and infinitely thin) cable in the middle of an infinitely large field so there are no reflections, no boundary effects. The usual way to measure that scenario is in an anechoic chamber so all reflections are absorbed, so "free field" and "anechoic" response are often interchanged (though technically differ). Most folk interpret "free field" as "in an open field, nothing around it" and fewer know what "anechoic" means, at least IME.

 

[PortalKeeper]

"Free Field" means without reflections, or in the context of bass extension, without room gain.
The -6dB means that the speaker measures 6dB down at 38Hz (and likely drops precipitously below 38Hz)

So then is this speaker able to produce audible frequencies below 38Hz? Is the -6dB relative to the average dB over its audible frequency response above this point at a given voltage level?

 

[Elitzur–Vaidman]

So then is this speaker able to produce audible frequencies below 38Hz? Is the -6dB relative to the average dB over its audible frequency response above this point at a given voltage level?

The -6dB should be relatively close to the average dB SPL over its audible frequency response, but some manufacturers fudge it a little bit. This speaker should be able to produce a bit lower than 38Hz once room gain is factored in, but you'll still probably need a subwoofer for any information below 30hz or so.

 

[PortalKeeper]

The -6dB should be relatively close to the average dB SPL over its audible frequency response, but some manufacturers fudge it a little bit. This speaker should be able to produce a bit lower than 38Hz once room gain is factored in, but you'll still probably need a subwoofer for any information below 30hz or so.

Could you also explain what room gain is? Why does the frequency response say only 44Hz if it can go lower? Is it because of increasing distortion?

 

[Elitzur–Vaidman]

Could you also explain what room gain is? Why does the frequency response say only 44Hz if it can go lower? Is it because of increasing distortion?

Low-frequency wavelengths are very long. When these wavelengths are more than twice as long as a room's longest dimension, their reflections become self-reinforcing, which gives an extra 6dB or so of loudness (room gain increases at lower frequencies, but speaker roll-off still limits maximum extension.

The frequency response giving ±3dB at 44Hz can be interpreted as -3dB at 44Hz, consistent with -6dB at 38hz.

 

[restorer-john]

Exactly. Think of a speaker hanging from an infinitely high (and infinitely thin) cable in the middle of an infinitely large field so there are no reflections, no boundary effects.

So, it was your prototype speaker hanging from the gantry on a skinny kevlar cable at Aricebo that caused the collapse? Shame.

 

[Cote Dazur]

was wondering what the meaning of the “Frequency range Free Field” specification is

Now you know what the “Frequency range Free Field” value is, you also know how it is measured, so now you also understand that it is just an indication and might not be "meaning" anything to you and how you will listen to them.

Where you will place the speakers in your room and the size of the room will have a tremendous impact on what the amount of bass your speakers (yes they both play the bass note) will produce at 38hz. With room bass nodes and depending where you sit, you might not be able to hear anything at 38hz with those speakers.

If measuring those in a room had any value, that is where it would happen and we don't listen to music suspended in the air in free field.

Enjoy your KEF Q950’s, they are great speakers.

 

[DonH56]

So, it was your prototype speaker hanging from the gantry on a skinny kevlar cable at Aricebo that caused the collapse? Shame.

View attachment 265376

Of course not, silly, that was James Bond!

 

[PortalKeeper]

Low-frequency wavelengths are very long. When these wavelengths are more than twice as long as a room's longest dimension, their reflections become self-reinforcing, which gives an extra 6dB or so of loudness (room gain increases at lower frequencies, but speaker roll-off still limits maximum extension.

The frequency response giving ±3dB at 44Hz can be interpreted as -3dB at 44Hz, consistent with -6dB at 38hz.

Does the frequency response at a given frequency change depending on the range of frequencies being sent to the speaker at any given time? Like, if I only sent low frequencies and treated the speaker as a subwoofer, would some magic happen that boosts those frequencies at 38Hz to 0dB since the speaker doesn’t have to deal with mid to high frequencies?

What would be the problem of boosting the bass frequencies with EQ? Would distortion arise? Thanks for the help.

 

[Elitzur–Vaidman]

Does the frequency response at a given frequency change depending on the range of frequencies being sent to the speaker at any given time? Like, if I only sent low frequencies and treated the speaker as a subwoofer, would some magic happen that boosts those frequencies at 38Hz to 0dB since the speaker doesn’t have to deal with mid to high frequencies?

No

What would be the problem of boosting the bass frequencies with EQ? Would distortion arise? Thanks for the help.

Distortion would definitely rise, but the Q950s are large speakers, so you could reasonably EQ a low bass boost to increase the extension so it's linear down to 35-36Hz anechoic. However, this would also come with a significant reduction in the speakers' peak SPL. It's not really an issue if your average listening levels aren't loud, but it is something to consider. Keep in mind that room gain will boost your actual low-frequency spl (With room gain your -6dB at 38Hz free-field could become +1dB in-room).

 

[PortalKeeper]

No

Distortion would definitely rise, but the Q950s are large speakers, so you could reasonably EQ a low bass boost to increase the extension so it's linear down to 35-36Hz anechoic. However, this would also come with a significant reduction in the speakers' peak SPL. It's not really an issue if your average listening levels aren't loud, but it is something to consider. Keep in mind that room gain will boost your actual low-frequency spl (With room gain your -6dB at 38Hz free-field could become +1dB in-room).

I see, could you also explain how exactly the peak SPL is lowered when boosting the low frequencies?

 

[Cote Dazur]

I see, could you also explain how exactly the peak SPL is lowered when boosting the low frequencies?

Before jumping to conclusion and trying to EQ the speakers, you need to understand that there is much more at play when dealing with low frequency than just adding bass with EQ. What the room looks like, where the speakers are and where you sit is key in what kind of bass you will have access to, no matter what speaker you have.
This thread might shed some light, but it is just the beginning.

 

[Elitzur–Vaidman]

I see, could you also explain how exactly the peak SPL is lowered when boosting the low frequencies?

You need to attenuate your overall signal by your highest EQ boost value in order to avoid clipping.
What @Cote Dazur said applies. A cheap measurement microphone will answer more than any amount of theory can though.

 

[PortalKeeper]

Before jumping to conclusion and trying to EQ the speakers, you need to understand that there is much more at play when dealing with low frequency than just adding bass with EQ. What the room looks like, where the speakers are and where you sit is key in what kind of bass you will have access to, no matter what speaker you have.
This thread might shed some light, but it is just the beginning.

Thank you! I've been looking for a post on this topic by Amir.