IB Frequency response & Fs

[chrisbee]

What is actually happening to produce the frequency response of a typical IB prior to equalisation?

Ignoring room gain just for the moment one would think that Fs would have some direct and important bearing on the roll-off point.

The nearfield response is supposed to be the IB's response without room effects. I shall have to try obtaining an unequalised nearfield response tomorrow using REW and my new RS meter.

What I'm trying at get at is why isn't the response flat down to Fs then rolling off at 12dB/octave?

A very low (claimed) Fs of 16Hz doesn't seem to mean a flat response down to Fs. In fact my IB broadly peaks around 40hz in-room. It is 15dB down an octave lower. Yet WinISD suggests a flat response.

Why do we worry about a low Fs at all if the response doesn't hold up without equalisation?

Here's my raw IB response from the listening position again using my new analogue RS meter. Note the trough at 16Hz. Is this indicative of Fs or just a random room effect? It seems too much of a coincidence. Can one positively confirm Fs this way from a raw IB plot?

[ThomasW]

chrisbee said:

What is actually happening to produce the frequency response of a typical IB prior to equalisation?.......

What I'm trying at get at is why isn't the response flat down to Fs then rolling off at 12dB/octave?

Because no driver has perfect pistonic action throughout it's passband.

Also the AE-IB15's have an incredibly stiff suspension, so they don't function like a driver with a Fs as low as is claimed.

A very low (claimed) Fs of 16Hz doesn't seem to mean a flat response down to Fs. In fact my IB broadly peaks around 40hz in-room. It is 15dB down an octave lower. Yet WinISD suggests a flat response.

The algorithms used to create the computer sims, assume 'perfect' anechoic response.

Why do we worry about a low Fs at all if the response doesn't hold up without equalisation?

Actually I don't worry all that much.... ;D

The primary benefit to having a low Fs is the need for less EQ for the lowest frequencies. IOW, having a really low Fs is 'nice' but not mandatory

Here's my raw IB response from the listening position again using my new analogue RS meter. Note the trough at 16Hz. Is this indicative of Fs or just a random room effect? It seems too much of a coincidence. Can one positively confirm Fs this way from a raw IB plot?

Nope, it's the driver, the baffle, and everything connected.

The only time one can see the 'pure' performance of the driver is with a measurement in an anechoic chamber. And even then it won't be flat for the reasons already mentioned.

If you want performance closer to 'perfect' then look to devices like the DEQX or the even more expensive processors like it.

[ebr]

Here's my nearfield with no EQ. The Fs of my drivers is 22Hz. So, looks like its behaving as you expected.

[chrisbee]

Interesting. Another coincidence?

Does anybody else have an unequalised response of their IB which confirms the Fs trough?

[chrisbee]

Thomas said:

If you want performance closer to 'perfect' then look to devices like the DEQX or the even more expensive processors like it.

Good grief, Thomas! I can't afford the US postage charges let alone the kit. ;D

[ThomasW]

chrisbee said:

Good grief, Thomas! I can't afford the US postage charges let alone the kit. ;D

The DEQX company is in Australia.

Here are unequalized nearfield and 13' distance measurements from my small IB (driver Fs 16.3Hz). These were quick and dirty measurements taken long ago (before Room EQ Wizard) using a ECM8000 mic. These were done simply to illustrate the effects of room-gain (not show off an IB), so I didn't bother to measure the really low stuff.

At some point when I have the new measurement system setup in the family room I'll put up a better plot.