Improving room acoustics

Hello, did Santa bring you all some new Hi Fi for Christmas?

I've just moved house and I've found my Hi Fi sounds very different in my new living room and not in a good way!

So, to set the scene:

My new living room/lounge is approximately 10m x 5m with a number of alcoves and bay windows in the room. For practicality I've got the speakers firing length ways down the room with the speakers sat in alcoves either side of a chimney breast. The floor is carpeted and I have wooden floorboards with a 4ft high void under the floors.

I've already started to add extra joists/cross bracing in the alcoves in an attempt to stiffen the floor area directly under the speakers. I've only done one side so far so it's too early to say if this has been a success but I figure it can't harm.

Last night I put a camping mattress and some seat upholstery in the alcoves on the side walls and things improved enormously so it looks like some sort of acoustic wall panels are going to be the answer. I've been looking at the options this morning and there are a few different types: profiled foam, flat foam, mineral wool etc. I would appreciate any advice on which materials are most appropriate.

On the positive side it's wonderful having a detached house so I can play what I like, when I like and at whatever volume I like!

 

It would probably be helpful for you to get a microphone and start measuring what is actually going on in the room.

ARTA provide free software as does Room EQ Wizard. You just need a small audio interface to connect your computer to the Hi-Fi and a microphone. Something like an Edirol UA-25 (used) and a Behrigner ECM8000 mic should get you going.

Mineral wool sheets (the compressed stuff, not fluffy) across room corners make good upper-bass traps. The air gap behind them when placed across the corners helps dramatically compared to going flat on the wall.

After you have done your best with passive room treatments get some digital correction as well.

 

Hi Simon, thanks for the advice.

Taking some measurements has to be a good idea.

To begin with I've already decided to install some acoustic panels in the alcoves as my experiments with the camping mattresses have been a great success. I'm going to make my own panels up which I intend to cover in fabric to make them look nice.

Acoustic slab foam looks like the easiest material to work with. Something like this perhaps?

http://www.ebay.co.uk/itm/Advanced-...sical_Acoustic_Treatments&hash=item5405bc49e9

If you read the item description there is some data about the frequency absorption characteristics but I don't really know how to interpret the figures. Does it look suitable?

 

It should work much the same as mineral wool. Mineral wool can be slightly better but it is also messy until backed and covered.

The absorption figures are easy to understand. 1.0 means total absorption. So you can quickly see that at 500 Hz you have a figure of about 0.6 and at 250Hz 0.3 it is already only half as effective. So not much help in the low bass. Bare in mind that converting these figures to attenuation in dB is not intuitive, but we can compare one absorption coefficient to another.

As I said you can improve this by placing the panel across a room corner to give it an air space behind. Backing it with 6mm MDF or Plywood will also help. Bond it nicely across as much surface as possible as the foam damps the wood backing by contact.

 

A quick update on things....

I've purchased two 600x1200mm 3" thick acoustic foam slabs which are mounted onto 6mm MDF. I've been doing some listening with these slabs just resting in the alcoves (see photo) and they make an amazing difference.



Please excuse the dodgy decor and carpet, I haven't redecorated yet!

I got these panels from a company called Advanced Acoustics in Mansfield. They cut them and mounted them onto MDF while I waited. They also offer do 1" and 2" thickness options but I went for the 3" as this has much better absorption specs in the low frequency ranges. They call this material "Acousti-Slab".

I'm going to mount the panels on the wall with a small air gap behind them as recommended by Simon and I bought some nice neutral coloured soft linen cloth to cover them in. I'll post pics when they are finished.

 

Glad you are getting a good result

Just to be clear, I recommend mounting them across the corner, not against the wall.

 

Oops, I wasn't thinking about corner placement when I bought such large flat panels. Still I'm getting a very useful improvement with them against the side wall as pictured.

I suppose I could cut each panel in half lengthways to create narrower panels to mount across the corners but I've already covered the panels in fabric now so I try them as they are to begin with.



After taking this photo I added some more staples and tensioned the fabric a little more to pull out the remaining creases.

I'll post again when I've finished mounting the panels to the walls.

 

The important thing is just to get as large an air gap as possible behind them, this will extend their effect in the low frequencies.

When you mount them on the wall, do so in a way that allows the panel to hang loosely so that the MDF can resonate freely and absorb energy. If you mount it rigidly with a batton on the back or something it will hinder performance.

 

Thanks for the advice, I'll devise a looser fixing system that allows more free movement/resonance of the panel.

How much of an air gap behind the panels is needed to get a useful improvement?

For aesthetic reasons I'd rather keep the panels reasonably close to the wall if possible but I need to balance that with getting the decent performance from the panels. Is a 1" air gap anything like enough or does it need to be like 6"?

 

Anything helps, but I'd say 4" is common.

This will interest you, read from about 1/3 down the page 'OPTIMIZING THE AIR GAP' - http://ethanwiner.com/acoustics.html

 

That's a very informative article!

Last night I mounted the panels to the wall (see pic). The panels hang from nylon cord so they are free to resonate and at the moment they are set up with a 30mm air gap behind.

I'll move them further out from the wall if after some listening I feel I still need more bass absorption.



My next plan is to further improve the situation with the floor under the speakers. Watch this space....

 

If you don't mind me asking given that you have a large lounge can you not move the speakers away from the back wall ? Even as an experiment - it could have a profound effect and can't help thinking in a positive way.

 

Hi Mike,

That's a very good suggestion but.....The room is divided into two with a staircase. With the speakers moved further away from the back wall I feel that the listening position becomes too close to the speakers. I can't move the listening position further back because the stairs get in the way.

The good news is that the acoustic panels have been hugely beneficial and I'm now much happier with how everything sounds.

I've also been busy reinforcing/stiffening the floor under the speakers. I haven't managed to test it yet as I need to wait for a friend to call round to help me move the speakers back into position as they're too heavy to shift by myself (yes really!). I'll post some photos soon.

I've also already downloaded the REW software suggested by Tenson and I plan to buy a microphone and audio interface so that I can measure the acoustics of the room before making any more changes.

 

My effort to stiffen and improve the floor are now finished.



What I've done is based upon an old Linn recommendation I read somewhere with a few extra tweaks. Here's a summary:

• Extra 2"x4" brace beams added at right angles between the joists under the speakers. I didn't take a pic of this as it's too dark under there.
  
• Floorboards screwed down tight nice and tight along joists where possible.
  
• I then cut two sheets of 20mm MDF to size/shape so that they'd fit in the alcoves. These were stuck down using silicone sealant as an adhesive. I was very generous with the silicone and used about 500g of it per side. My thinking was that the viscoelastic (if that's the correct term) material properties of the silicone sealant sandwiched between the floorboards and MDF would have good dampening properties. Once the MDF sheets were in place I weighted each sheet with loads of heavy stuff and left it overnight to set.
  Some wood stain, varnish and a bit of trim along the front keeps it all looking respectable. The floor under the alcoves is now really solid.

 

How is the floor still level at the alcove after adding two 20mm sheets of MDF? Was it lower to start with?

The MDF and silicone is a good idea I'm sure it will help absorb vibration.

 

Sorry, my post was badly worded. There is one sheet of MDF per alcove (two sheets in total) so the new floor is 20mm higher than the underlying floorboards. The carpet and underlay is fairly thick and then the carpet grip-rod strip raises the carpets edge a touch more giving the impression that everything is level.

It has made a substantial improvement.

Simon, this is probably a stupid question but why do I need an audio interface? Can't I just plug a microphone directly into my laptop?

 

My second hand Behringer ECM 8000 arrived today



I just need a suitable audio interface now.

Simon, apart from the Edirol UA-25 are there any others you'd recommend? I've no idea what I'm looking for.

 

There are many that will work. Look for anything that has an xlr mic input (with phantom power, nearly all will if they have an XLR socket) plus a stereo output to feed your hi-fi.

M-Audio have made a lot of 2 input 2 output interfaces. This looks good if it stays below £50- http://www.ebay.co.uk/itm/M-Audio-M...821?pt=LH_DefaultDomain_3&hash=item2c96beb9c5

I think M-Audio and Edirol are the most reliable.

 

Thanks for that.

I had an Amazon gift voucher to use up so I just ordered one of these.

http://www.amazon.co.uk/Lexicon-Alp...&qid=1424168848&sr=8-1&keywords=lexicon+alpha

 

I can't actually see any mention of that having phantom power for the XLR mic input.

Either they just assume it is so obvious as not to mention it, or it actually doesn't have it.

If it's not too late I'd cancel the order and find out.

Edit: Just found out it doesn't have phantom power. Sorry about that! It must be in the very small group of those without.

Can I suggest this ones instead - http://www.amazon.co.uk/Behringer-U...d=1424176838&sr=8-14&keywords=audio+interface