Puzzled by my room acoustics - goes way deeper than the dimensions suggest

Loheswaran

Well-Known Member
Dec 19, 2014
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258
I am baffled by something in my room. It measures 5m x 3m. It is in a roof following 'room within a room construction'

I have calibrated and used room eq and sweep after sweep is giving me bass down to 25hz (I have not tried deeper) it's only 10db down with a sweep at 85.

What is this happening?

Are people actually theorising small rooms taking no account of adjoining spaces.

BTW I also have a sharp dip around 180hz - what shall I do?

thanks
 

Tim Link

Well-Known Member
Feb 12, 2019
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Hi.
I can take a stab at this. You should have no problem getting VERY deep bass in a small room if your woofers go that low. The waves are much longer than any dimension of the room so the pressure builds up throughout the room and you get room gain. As for the dip at 180 hz, try moving the speakers and/or listening position. The DSP can't do much about a hard interference cancellation. At 180 hz you are talking about a 75 inch wavelength, which corresponds mostly with typical ceiling height. Changing the height of the speakers might help, if you can do that. Can you show some pictures of your setup?
 

Hipper

Well-Known Member
Jun 12, 2011
68
11
83
Hertfordshire, UK
Are you able to post a graph? Not only an SPL graph but also a Spectogram?

Low frequencies may not only be music but also external noise such as traffic.

If you want to know what the ambient noise is in your room, take an REW measurement but turn off the volume at the amp so the microphone does not hear any REW signal, just ambient noise.
 

Hipper

Well-Known Member
Jun 12, 2011
68
11
83
Hertfordshire, UK
For REW you have to make a screenshot of the graph.

1. Load up the relevant graph in REW.
2. Click the camera icon, top left of the graph, and name and save it somewhere on your PC. That gives you a jpg file.
3. Post on here by clicking the image icon and drag and drop the photo.

EQ Spectogram 9.18.jpg

Here's a copy of a Spectogram of my set up. On the left it shows decay times in milliseconds. Generally the decay time is around 200ms but you can see that in the 40-50Hz range the sound continues indefinitely. It also starts before the REW signal. This means it is likely to be some external noise, which indeed it is. I live on a busy High Street and this is the noise of traffic. If you have amplifier hum for example, as I once had, it shows up like this too, in my case at 150Hz.
 

Hipper

Well-Known Member
Jun 12, 2011
68
11
83
Hertfordshire, UK
From what I see on the Spectogram you don't have problems of external noises. That's good. You also have low decay times which is also good. When I had no treatment in my room I was getting variable decay times of 400-600ms.

Your SPL is also pretty smooth between 35-75Hz which is a good achievement. Again mine was all over the place with no treatment, and with lots of treatment it was still quite a bit all over the place but much smoother overall. Only after adding EQ did I get flat and smooth up to 200Hz.

All room mode calculators seem to be for conventional rectangular rooms. This means if you have a room in the roof with angled ceilings it will give some miscalculations:

https://amcoustics.com/tools/amroc?l=420&w=386&h=240&r60=0.6

What to do? I agree with Tim that moving the speakers and listening chair may get different results. Whether they'll be better is hard to predict. Move then measure then repeat. If your room is symmetrical you could just move and measure one speaker first - arranging for REW to send the signal out of that speaker only. When I did this in my room I found that the best positions were around 'The Thirds' and 'The Fifths'. This is the positioning I currently use:

http://www.barrydiamentaudio.com/monitoring.htm

After that, add room treatment (have you already got some?). Then if you are willing, use DSP or EQ to complete the job.

One thing should be said and that is, do not get obsessed with getting a smooth response. Just do your best but the ultimate goal is of course to make music listening enjoyable. For example, some very narrow dips may not be noticed. What I've done so far is get a smooth response up to 200Hz using positioning, treatment, then careful EQ. I then listened to music and enjoyed it so much that I've not yet done anything above 200Hz!
 

Loheswaran

Well-Known Member
Dec 19, 2014
432
99
258
Hi Hipper

I am lucky to have low background noise - although this is due to the 'room within a room' construction'

that said my room is almost triangular shaped.

I have set my system up as an equilateral triangle as I simple have no other placement options than a near field set up. I am gonna do some movements - I found that putting up diy traps on opposing walls did take a bit from the vicious dip, but it is still there. I need to make up some more traps and see what happens

thanks

Lohan
 

Loheswaran

Well-Known Member
Dec 19, 2014
432
99
258
Can I just say that I still have a really vicious dip which I am not sure a sub will help with because from what I see subs tend to operate in the sub 100hz range primarily

I will try and upload some pics
 

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