Weird In room Response drop at 30hz

gentlemaniac · Apr 23, 2019

Hello,

Im having some trouble with my DIY Subwoofer's response and cannot for the life of me work out why my response would drop suddenly at 30hz and then 'level' out until my HPF kicks in at 15hz

the sub is 1 DIY Sealed 18" Dayton HO Driver in about 85-90L cabinet - Room is approx 1500 Cubic Ft

Please could you look and give me your thoughts? - this measurement was with no EQ applied

Many Thanks

John Mulcahy · Apr 23, 2019

Have you made a nearfield measurement of the sub?

gentlemaniac · Apr 23, 2019

Yes, please see attached, i took this measurement today

John Mulcahy · Apr 24, 2019

The scale of that graph is pretty huge, but the response is smooth so the previous measurement looks like the effect of room resonances. For graph scaling a 60 dB range is usually good, for example 45 dB at the bottom of the graph and 105 dB at the top. The Limits button at the top right of the graph lets you set those. Can also attach measurement mdat files.

gentlemaniac · Apr 24, 2019

Im sorry about that, I had the exact same thought after I made the graph but wasnt 100% on how to change it.

Anyhow, as taking measurements around the room would prove (attached), it is a room issue and cannot be sorted by moving the sub or the sofa within my limitations (makes virtually no difference to the large dip at 30hz).

So the question now would be regarding EQ, when I go to the EQ tab in REW and apply the filters to match target, it will not boost any frequencies lower than the target line, it will only cut the ones that are higher, is there any reason for this? Its a very heavy cut so Im reluctant to EQ this way.

I can manually move the filters but I cant help thinking Im missing something or have a setting wrong in the EQ somewhere

John Mulcahy · Apr 24, 2019

REW won't apply automatic boosts below the lowest frequency at which the response drops below the target to avoid the risk of pushing drivers beyond their limits. You can apply any low end boost you feel appropriate with manually entered filters though.

Ultrasonic · Apr 25, 2019

gentlemaniac said:
So the question now would be regarding EQ, when I go to the EQ tab in REW and apply the filters to match target, it will not boost any frequencies lower than the target line, it will only cut the ones that are higher, is there any reason for this?

Have you set the individual and overall max boosts to positive values?

You want to be cautious about using boosts, and note that if you don't attenuate the input signal by an amount corresponding to the boost that you could get signal clipping at the boosted frequency.

gentlemaniac · Apr 25, 2019

John Mulcahy said:
REW won't apply automatic boosts below the lowest frequency at which the response drops below the target to avoid the risk of pushing drivers beyond their limits. You can apply any low end boost you feel appropriate with manually entered filters though.

OK that makes much sense thank you

Ultrasonic said:
Have you set the individual and overall max boosts to positive values?

You want to be cautious about using boosts, and note that if you don't attenuate the input signal by an amount corresponding to the boost that you could get signal clipping at the boosted frequency.

Yes, +3db. Im not sure what you mean by attenuating the signal sorry?

Ultrasonic · Apr 25, 2019

gentlemaniac said:
Yes, +3db. Im not sure what you mean by attenuating the signal sorry?

The issue I was referring to was this. Wherever you are applying your EQ, there will be a particular dynamic range that can be used. If you have an input signal that already uses this full dynamic range (or very close to it) during the loudest parts then an EQ boost would as it were 'try' to increase the level to above the allowed maximum. This can't happen so what does is that the waveform will clip. To prevent this the original signal can be attenuated to ensure the required spare headroom to allow boosts to be applied without clipping occurring.

You're probably familiar, but if not here is a visual representation of what clipping is:

Modern CDs often unfortunately use the full dynamic range far too much. I have no idea what the situation is with movie soundtracks though, which I'd guess is more likely your interest.

Edit: just to be clear, I'm referring to clipping that would occur on the DAC output waveform, not anything that would depend on the headroom of the subsequent amp/sub.

How you are implementing your EQ and what your input signals are will determine to what extent this issue is relevant to you.

gentlemaniac · Apr 25, 2019

OK, Well its a home theatre setup that uses 5.1 speakers through a Denon AVR X3400H AV Receiver, I have the Subwoofer output connected to a MiniDSP 2x4 (for EQ) which then goes directly to the Sub.

My XBOX one & Sky Q are connected to the Denon for viewing purposes

Tonto · Apr 25, 2019

Dips can be difficult, unlike peaks. Adding more energy just makes it worse. You will probably have the results you want with adding another sub...if that is an option. Where do you have the sub in the room?

gentlemaniac · Apr 26, 2019

Tonto said:
Dips can be difficult, unlike peaks. Adding more energy just makes it worse. You will probably have the results you want with adding another sub...if that is an option. Where do you have the sub in the room?

In the Corner, Ive tried moving it about 2 ft away from its position and it makes no difference. Ive also positioned the mic forward of my sofa in several posoitions to see if it would make a difference but it just introduces a massive null/dip around 70 hz ish.

Another sub may be an issue due to WAF, but Im gradually trying to introduce the idea

Ultrasonic · Apr 26, 2019

gentlemaniac said:
OK, Well its a home theatre setup that uses 5.1 speakers through a Denon AVR X3400H AV Receiver, I have the Subwoofer output connected to a MiniDSP 2x4 (for EQ) which then goes directly to the Sub.

I have a miniDSP 2x4 HD. The plug-in for the 2x4 is a little different but I think you can still set an input attenuation if you end up wanting to.

Is it an analogue sub output that you're connecting to the miniDSP? If so what will determine if you have an issue is how the maximum voltage of the input compares to the voltage sensitivty of the input (what voltage is converted to the maximum digital level). With this in mind it's important to note that the 2x4 has two sensitivity settings, one designed for signals up to 0.9 Vrms and the other 2.0 Vrms (as is more standard).

I don't know anything about what the sub output signal from your AV amp will be like though so I can't offer any more specific help I'm afraid.

Personally I just wouldn't use a boost by the way.

Multiple subs can help to even out dips in an in-room response (which are caused by the interference of reflected sound waves) but won't help to improve the low frequency extension that it sounded like you might have been targeting?

Tonto · Apr 26, 2019

One last trick, try turning the sub around. Measure facing each wall and then into the corner. I know looking at the back of the sub is less appealing, but I've seen it measure better.

gentlemaniac · Apr 26, 2019

Ultrasonic said:
I have a miniDSP 2x4 HD. The plug-in for the 2x4 is a little different but I think you can still set an input attenuation if you end up wanting to.

Is it an analogue sub output that you're connecting to the miniDSP? If so what will determine if you have an issue is how the maximum voltage of the input compares to the voltage sensitivty of the input (what voltage is converted to the maximum digital level). With this in mind it's important to note that the 2x4 has two sensitivity settings, one designed for signals up to 0.9 Vrms and the other 2.0 Vrms (as is more standard).

I don't know anything about what the sub output signal from your AV amp will be like though so I can't offer any more specific help I'm afraid.

Personally I just wouldn't use a boost by the way.

Multiple subs can help to even out dips in an in-room response (which are caused by the interference of reflected sound waves) but won't help to improve the low frequency extension that it sounded like you might have been targeting?

The ultimate goal is to get another sub in there somehow, Im just figuring out the best way with limited space. I appreciate all your advice though thanks. Yes its an analogue RCA connection

Tonto said:
One last trick, try turning the sub around. Measure facing each wall and then into the corner. I know looking at the back of the sub is less appealing, but I've seen it measure better.

It is a down firing sub, but just to try things out I faced the sub in all directions (laying it on its side etc) and they all give me virtually the same response, so its just an awful spot unfortunately.

Tonto · Apr 28, 2019

I feel your pain!

Weird In room Response drop at 30hz

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