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bass boom

Neil P said:
A few people mentioned ports (and bunging them) ... my speakers have a rear port. Not sure whether that gives any indication as to how they would perform close/near/away in relation to walls. I notice the HiFi choice review says "No port blocking arrangements are available here, so the A6 should be kept well clear of walls".

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Bass reflex tuning concerns both frequency, amplitude and also Q so it's impossible to say if a speaker's suited to a close to wall location without seeing actual measurements.

Your speakers have the port positioned close to one room boundary (the floor).

My speakers come with two reflex tube lengths, one for free space and one for close to wall positioning.

rTuYoDh.png
 
darrenyeats said:
Helmoltz resonators (2x fridge-freezer size) and membrane traps at the room boundary here made a SMALL difference at 35Hz centre frequency. So I wouldn't go quite as far as saying /impossible/ at 50Hz but yeah very difficult. You can forget foam at such frequencies though!

The biggest levers for improving bass are listener and speaker placements - traps help in finding a slightly better compromise with these (rather than solving the problem, for sure).
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I don’t disagree and my comment was somewhat paraphrased. I also agree regarding your comment on speaker placement, but the reality of most U.K. homes is that ideal speaker placement is seldom possible and often even ‘ideal’ placement does not totally alleviate bass gain.
 
Neil P said:
I'll soon be packing up the system ready for refurb, so won't be going down the REW / mic / room treatment route any time soon. However, are there any likely ways to reduce bass boom in a room? Floor coverings? Book shelves in corners? hanging pictures on bare walls? It's a family room so can't be covered in GIK panels.

The floor is wooden ... possibly suspended, will lift up a floor board in the coming weeks.

Diagrams and detail to follow.
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I had similar problem. Belive or not but gaia acoustics solved problem. Not cheap but works 😄
 
darrenyeats said:
Helmoltz resonators (2x fridge-freezer size) and membrane traps at the room boundary here made a SMALL difference at 35Hz centre frequency. So I wouldn't go quite as far as saying /impossible/ at 50Hz but yeah very difficult. You can forget foam at such frequencies though!

The biggest levers for improving bass are listener and speaker placements - traps help in finding a slightly better compromise with these (rather than solving the problem, for sure).
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I was told 50 Hz to 150 Hz was tunable with Helmholtz resonators by the acoustician that I worked with. The lowest frequency mode in my room is 64 Hz and that was fine with this approach. The acoustician tuned the Auditorium at work and Cadogan Hall (and others) with larger diameter and larger length tubes to reduce resonances in lower frequencies, but this approach, as you have stated, is not practical for lower frequencies in a domestic setting.

I would say that, if, once positioning of the listener and the speakers have been optimised, the room resonances are still intrusive (50 to 150 Hz), then the most effective way to treat them is with Helmholtz resonators. EQ is a static approach and just chops the energy in the request frequency window by a defined amount, whereas room modes are dynamic i.e. varying as a function of time. So using the energy in the frequency range with a passive wave inversion means that you get reduction proportional to the energy in the room. If you want a greater proportional reduction then you need more Helmholtz resonators.

I was going to suggest that there must be an active sub woofer approach using the original signal as the reference. So thanks to @jobseeker for posting a link to just this approach. That sounds like a very good way to do this. And is something that I would like to hear in operation. A lot less hassle than trying to get Helmholtz absorbers installed and tuned etc.
 
ToTo Man said:
That would make your room 2.7 metres long?
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No, it just means that in my room that is the main room mode, however it is built up, as harmonics or reflections etc. I hadn't really thought about how it is generated until you mentioned this, I just used the measurements and what I heard from a bass tone sweep (originally done with a detailed RT60 white noise analysis) to hear what was happening.

Just checked quickly. Velocity of sound in air is around 330 m/s so with a frequency of 64 Hz, the wavelength would be around 5.2m which is the length of the room.
 
IanW said:
No, it just means that in my room that is the main room mode, however it is built up, as harmonics or reflections etc. I hadn't really thought about how it is generated until you mentioned this, I just used the measurements and what I heard from a bass tone sweep (originally done with a detailed RT60 white noise analysis) to hear what was happening.

Just checked quickly. Velocity of sound in air is around 330 m/s so with a frequency of 64 Hz, the wavelength would be around 5.2m which is the length of the room.
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If your room is 5.2m long, your lowest room mode is around 33Hz, which will be at its loudest when you stand against the front and rear walls and at its quietest in the middle of the room. If you cannot hear this room mode then it's likely your speakers begin rolling off at a higher frequency than this. The 64Hz mode you describe is likely to be the 2nd harmonic, which should be at its loudest not only at the front and rear walls but also in the middle of the room. However, the modes associated with the width and height of your room may disrupt this behaviour.

The following diagram shows the behaviour of a 50Hz axial mode in a room, along with its 2nd, 3rd and 4th harmonics:

Figure-4.33-Room-mode.png
 
Good morning all,

Is the AAVA device 'expensive'? I've spent £2025.40 on GIK acoustic panels to date................

Regards

Richard
 
ToTo Man said:
If your room is 5.2m long, your lowest room mode is around 33Hz, which will be at its loudest when you stand against the front and rear walls and at its quietest in the middle of the room. If you cannot hear this room mode then it's likely your speakers begin rolling off at a higher frequency than this. The 64Hz mode you describe is likely to be the 2nd harmonic, which should be at its loudest not only at the front and rear walls but also in the middle of the room. However, the modes associated with the width and height of your room may disrupt this behaviour.

The following diagram shows the behaviour of a 50Hz axial mode in a room, along with its 2nd, 3rd and 4th harmonics:

Figure-4.33-Room-mode.png
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Yes, all I said was that the main room mode was at 64 Hz (music listening, tone listening and then a pro REW type software used by the acoustician for RT60 decay analysis, over 20 years ago now). Using 1/4 wave Helmholtz absorbers is consistent with 64 Hz in a 5.2m room.

My speakers are -6dB by 26 Hz so will have dropped a bit by 32Hz. I can't think of much music that I have that has significant musical content at around 30 Hz and I have not stood at either end or even the middle of the room assess node or anti node locations. And clearly had not thought about node locations. So thanks for the images.

I am going to retune the room after decorating, using REW and so will see what the RT60 looks like then and retune if required.
 
IanW said:
The article focuses on the results of a frequency sweep whereas the unit is really designed to reduce the effect of room modes and hence it is the dynamic room response (RT60) that needs to be assessed. This is also pointed out in one of the early comments below the article.
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But REW measures RT60 with a sweep.
 
tuga said:
But REW measures RT60 with a sweep.
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I assumed that it had a white noise mode? I will investigate further.

Need the software to excite the room so that RT60 can be measured over a period of time.

Thinking about it a little more you need something to excite the room modes and then a means to measure the energy in frequency windows. So I could see a frequency sweep doing that, but it more difficult to measure the effect as the signal to noise ratio will not be as good as a series of tones or a period of time of white noise. Maye there are other approaches to measure this.
 
IanW said:
I assumed that it had a white noise mode? I will investigate further.

Need the software to excite the room so that RT60 can be measured over a period of time.
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This is what you get with REW from a sweep (RT60 Decay tab):

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darrenyeats said:
Agreed re: roll off needs some matching of speaker and room / situation but it can work well when matched IMO. Anechoic flat to DC is for outside e.g. concerts in fields (or at altitude actually!)
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I find group delay to be more audible than amplitude variations at LF. My experiments indicate that lowering the LF roll off sounds tighter and better. The worst case subjectively is high group delay coinciding with the main room mode e.g a port tuned to the main room mode frequency.
 
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