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AntM 12th June 2006 01:42 PM

Strange dip in bass reflex response
 
I've found a strange problem with a bass reflex design that I have been measuring.

On doing the frequency sweep, the frequency response has two peaks, one at 37Hz, and the other at 60Hz, with a dip in the middle. The dip is noticeable on playing music - on some recordings the speaker appears to have very little bass, and on others it appears to have too much.

The design uses a Seas L18 H1224, in a 20litre cabinet. The cabinet is filled with BAF wadding to minimise reflections at higher frequencies. I designed the tuning with Unibox - nothing unusual showed up in the simulated responses.

I checked the room nodes, and nothing obvious turns up at these frequencies.

Is this a known problem with some bass refex designs, and is there a solution?

ShinOBIWAN 12th June 2006 03:23 PM

It most likely a room-speaker interaction but I wouldn't rule out cancellation from port and driver output.

The quickest most simple test would be to block up the port and remeasure. Another quick test would be to move the speaker to an obviously difference position in the room and then remeasure.

If you fancy a little more work then you could use gated measurement if you insist on measuring in room or alternatively you could setup in the backgarden and measure their - the weather is certainly nice enough these days!

My own gut feeling is the room.

Do you have any more specifics?

richie00boy 12th June 2006 03:38 PM

Agree on all accounts.

Also I would remove some BAF wadding as vented cabinets should not be filled, merely lined.

AntM 12th June 2006 04:11 PM

Thanks for your comments.
ShinOBIWAN: How can cancellation from port and driver be avoided?

A few more specifics:

Room dims are L=5m, W=2.4m, H=3.2m
Speakers at front of bay window, 1.75m apart, 0.66m from rear-wall, 0.56m from side of bay, and 1.2m from side of room.
Listening height is 1m, speaker bass unit is 49cm from ground.
I x

I couldn't see any obvious antinodes at the dip frequency (just about 50Hz), as the 3.2m

Port has 5.5cm int diameter, rear mounted, two lengths tried: 12cm and 18cm. 18cm tube has more pronounced dip, but 12cm give too much upper bass.

Can give you cabinet dims if that helps.

AntM 12th June 2006 04:20 PM

Sorry about the gobbledegook. Pressed send by mistake!
I was going to say that if I've done my maths correctly, I couldn't see any obvious room interaction, as the 3.2m room height would give a 53Hz dip in the response for a source or ear at half the room height (1.6m), rather than at the 1m listening/0.5m speaker position. The other room dimensions, don't seem to give standing waves at the correct frequencies.

But as you say, maybe annoying the neighbours in the back garden is another option...

Is it true that reflex cabinets shouldn't be filled with wadding, even if the bass driver goes up to frequencies (2kHz in this case)where cabinet reflections can occur?

sreten 12th June 2006 05:53 PM

Hi,

its possible the peaks and dips are due to an amplifier (soundcard ?)
with a high output impedance as what you describe seems to be
similar to bass reflex impedance behaviour.

Shouldn't be a problem with a normal amplifier.

Generally speaking the air space near the port should be kept clear
for maximum port Q, though ports still work but more damped for
light BAF stuffing.

Generally line a reflex with foam and lightly stuff the area behind
the driver for more damping if require , lightly stuff all for even more.

If no stuffing a double layer of foam behind the driver helps.

:)/sreten.

Svante 12th June 2006 07:06 PM

Could you show us the response? How deep is the dip? Where was the microphone placed during the measurement?

A measurement in the close field of the driver would present a dip at the vent tuning frequency.

AntM 13th June 2006 01:20 PM

Two tests were performed:

1. Near field with the microphone between the woofer and tweeter

2. Far field with the microphone at the listening position

Both tests showed the dip (sorry, can't get the response to you just yet, as its sitting on a mate's laptop...)


Interesting idea about the amplifier, however I'm sure this is not the problem. The amp is a rega amplifier, which should have a low output impedance. The speaker is nominally 8 ohms, dipping to no less than 6 ohms, so there is not an unusually low impedance to drive.

I read another post a while back which suggested that tuning the box too low could cause this problem,( though my Unibox simulations suggested otherwise. ). Interestingly, the simulations did show that if you put more stuffing in the box, you need a shorter port to get the same box resonant frequency.

richie00boy 13th June 2006 02:07 PM

Have you actually measured the T-S parameters of the drive unit yourself, or are you relying on supplied ones?

AntM 13th June 2006 02:09 PM

I have relied on the published parameters. I've heard reports that production control is pretty good with Seas units.

What are you thinking?


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