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-   -   Is my frequency response bad? (http://www.diyaudio.com/forums/multi-way/36839-my-frequency-response-bad.html)

SimontY 28th June 2004 02:13 PM

Is my frequency response bad?
 
2 Attachment(s)
Hi folks,

I bought a cheap SPL meter recently, and thought I'd attempt a rough n ready FR plot. So, I made a CD full of sine waves from 22hz to 20khz, in varying steps - not quite logarithmic but useful.

The bass is smoother than expected, but what the heck is going on at 900hz?! Is this severe comb filtering from my rectangular speaker baffles? What can I do about it?

I might try my hand at a sharply tuned panel filter to reduce the peak at 200hz.

Low bass and treble can be ignored as the SPL meter won't be nearly accurate for these, and subjectively they're ok. The meter is +/- 3db accurate for the rest, which is sufficient to highlight my problems.

phase_accurate 28th June 2004 02:18 PM

Some of these irregularities might as well be room interactions.
BTW: have you got a PC with a soundcard ?

Regards

Charles

SimontY 28th June 2004 02:21 PM

Quote:

Originally posted by phase_accurate
Some of these irregularities might as well be room interactions.
BTW: have you got a PC with a soundcard ?

Regards

Charles

Hi Charles,

I do have a PC, but I did it this way as my machine is large and heavy, and difficult to set up in my listening room. Furthermore, I don't yet know how to use it to do a good response plot. It is on my list for the future.

MikeB 28th June 2004 03:14 PM

This curve looks to me like bad roomacoustics, maybe you should
try improving your listeningroom.
specially the downpeaks at 70hz and 115hz might have a massive
impact on bassquality.

I once tried to measure my speakers with the pc (Hobbybox) and
a good micro, but had similar results.
I was able to get a useful measurement by covering the speakers,
a chair with the micro and other stuff with loads of blankets.
Then i had a "linear" curve from 200hz up.
But lower frequencies can't be damped like this...

Damned, seems you can't measure your box simply at home...

Michael

SimontY 28th June 2004 03:26 PM

Quote:

Originally posted by MikeB
This curve looks to me like bad roomacoustics, maybe you should try improving your listeningroom.
specially the downpeaks at 70hz and 115hz might have a massive
impact on bassquality.

I'm sorry, it looks like I missed out one key point - this measurement is from as near lsitening head position as practically possible. It is not a nearfield response!

I am not so interested in measuring my speakers' response at this stage, just finding out what I am currently hearing.

This position is just in front of the rear wall and is about 3 metres away from the speakers.

Another thing - that dip at 115 is drawn wrong - it 'peaks' at 120hz and is on the 50db line ie. bottom of range of SPL meter! I just missed drawing the line to it, probably because it's so whoppingly bad.

SimontY 28th June 2004 03:31 PM

The subjective bass quality is, as you'd expect, variable according to the material being played!

Some tracks sound very impressive and accurate, with little overhang, and good impact. There is generally always good extension. However, some tracks sound dull with weak bass and not enough punch - the dips may be showing themselves up here.

I measured expecting to find some big peaks to try and tame, but instead am worried about my dips :( (actually, there are no audibly terrible peaks)

Can I passively boost my dips? Perhaps some resonator panel or enclosure that takes in bass and sends it out at 70hz or 115hz?

This would be nuts wouldn't it?

MikeB 28th June 2004 03:42 PM

That's a known effect to me... :(

Maybe you should play the material with the weak bass, and walk
through the room, checking if the sound changes. If these dips
are coming from roomacoustics, you will have hot spots. This means
at some point these frequencies are nulled out, at other points they
are boosted. This means, that the dips become at some other point
in the room peaks.
This anwers the question if a passive boost to these frequencies
are nuts... ;)

GM 28th June 2004 03:42 PM

Well, with so little info to work with, my guess is that if the stand's sides are ~7.5" apart, then it's a standing wave issue combining with the standing waves across the baffle face. There will also be one in the long dim, maybe the 400Hz one?

Regardless, I recommend you rethink how you have these spaced up. Adding a front/back and filling them with kitty litter, sterilized sand, or Portland cement to mass load them would be my choice. Mass loading the top is a good idea also. Heavy potted plants work well and damp/diffuse the > -3dB BW somewhat.

GM

sreten 28th June 2004 03:49 PM

You need to use warble tones or centre frequency filtered pink
noise to get a good idea, a set of single tones will never give
you a good idea of whats going on, and ideally you need to
combine measurements from a number of listening positions.

http://www.interstudio.co.uk/sndchk.htm

I use this (with my ears), the warble sweep tones are great for
quick aural checks, the 1/3 octave pink noise good for measurements.

Can't find the thing at the moment.

:) sreten.

phase_accurate 28th June 2004 03:57 PM

Quote:

Can I passively boost my dips?
That depends heavily on what is causing this dip. If a suckout is caused by standing waves for instance it doesn't help much EQing it out. If you feed a suckout of that type with more power, you just use more power to cancel itself out ! What helps in this case is moving the speakers and/or improving the room acoustics.

Regards

Charles


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