Small Room Acoustics

jhbrandt

Acoustic Design
Small Room Acoustics and appropriate treatment.

It has been said that 'You'll never get a home studio room to the point where you can do professional mixes like they do in the big studios'.

This is false.

Today a great many commercial productions are done in small studios & home studios. You see production after production recorded and mixed 'at the private home studio of the producer'.

The job of any Studio Designer is to create an acoustically accurate environment so that YOU, the artist, can work; faster & easier - Get the job done & enjoy it - Knock it out and do the next one - Make more money - Have an edge on your competition. This simple result is what my job in room acoustics and room treatment boils down to.

Amazing productions have been done in the past in horrible rooms because of the talent of the musicians, engineers, & producers - and they continue to this day. I wouldn't tell any of my old friends in the business that they need to change their room because they are doing just fine in their environment.

This may seem an odd thing to say coming from a design professional. But I follow the wise motto (modified for the music business): "If it's not Baroque, Don't FIX it!"

That said, I'll move on to my recommendations for Treating your Space. I would expect and APPRECIATE your comments & questions.

Room size and Ratios:

If you cannot build or move a wall, room mode calculators will only tell you what you've got. Don't try to see if you have a good ratio. It is irrelevant.
It is, however, important that you have good modal distribution, especially if you don't use enough bass trapping. (When I say enough, I mean A WHOLE LOT) :D

Cubes are bad, as are room dimensions with common denominators. (When designing a studio from ground up I will most often use prime numbers for the room dimensions where possible.) For example; if you have a room that is 5 by 10 by 8, you have common denominators of 2 and 5, so 10 would need to change to 11. This works in feet or meters. It is simple mathematics. Of course, you should run it through a room mode calculator like the one on my publications page to be sure. (the link is on my signature) Again, this is only IF you can do anything about the dimensions. Most people can't, and if that's you, it's not the end of the world.

The point of checking your room ratio or dimensions with a mode calculator is; it can tell you where you might have problem areas - plus information like: the lowest axial mode (if your room is concrete), the Schroeder frequency of the room, the ideal RT-60 (target), listening position, etc.

Treatment & Trapping:

I will always design for 'optimal' although, you can always do less. It is important that one knows and accepts the limitations that the real world imposes. ;)
For best results, I recommend a minimum treatment of 20% of 5 surfaces. The floor is excluded and since I am mentioning floors; All studio rooms should have hard floors, EXCEPT for Foley rooms which use thick padded carpet.
Ok, so this means that 20% of four walls and the ceiling will be covered with treatment. Of primary importance are first reflection points (side walls and ceiling) and the back wall. The treatment of the back wall, or lack thereof, will greatly affect the perceived peaks and dips in the frequency response at the mix position, as will:

Speaker position:

The speakers that you use, be they M-Audio or ATCs, will always be the 'Elephant' in the room. In other words, compared to the technological advances of electronics and data storage these 'things' are ancient relics that color, distort, and literally screw-up the recorded sounds as they try to 'reproduce' them in your room. The physics of this energy transfer is complex, so I'm not going to bore you. We are stuck with these 'things' for the time being but I wanted you all to know that there are no perfect speakers. More expensive units are usually much better, but get what you can afford and work with it.
SBIR or Speaker Boundary Interference Response is usually the cause of most monitoring anomalies. - I continue to see folks quoting online references that tell them to keep the speakers at least 'so far' away from the walls. Audiophiles are told to put speakers about 8 feet away from the front wall, etc. This is simply FALSE. I will explain:

First, let me say that if you get the Low Frequency response in your room 'right', you have won 95% of the battle. Low frequency begins folding around objects at the top of the transition region and truly becomes omni-directional at the Schroeder frequency of the room. (Study the information just below the Bonello Distribution graph on the AllModes tab of my room mode calculator)
So, the low frequencies produced by the speakers will fold around the cabinet and expand in a sphere in all directions until they hit an obstruction and reflect back. It is this REFLECTION that causes the problems. When the speaker is further from the room boundary the reflection will be delayed for a longer period of time. Sound travels at 343.54 m/sec @ 20° C or 1129.42 ft/sec. @ 70° F. If your speaker face is 1.22 meters or 4 feet from a boundary the delay will be 3.5 mS. This is half the length of a sound wave at 141 Hz., therefore full trapping is required at this frequency to eliminate the destructive reflection. NOW, push the speaker closer to the boundary so that it is half that distance or 61 cm / 2 feet. The resulting reflection will cause a delay (compared to the source) of 1.7 mS, corresponding to the half-wavelength of 282 Hz.

Now that's quite a bit easier to trap. I'll explain with an example; At zero degrees incidence, a 1/4 wavelength trap is required to be as deep as the speaker is far from the boundary. 61 cm distance = 61 cm deep trap. 122 cm distance (4 feet) = the SAME distance filled with an absorptive trap. -- This is simply TOO MUCH to put in most home studios!

Solution: Put the speakers/monitors as close to the front wall (boundary) as physically possible. This will result in a much higher interference frequency and therefore a much easier treatment solution.

Treatment options & fabrics:

When you build treatment panels, the density or GFR (Gas Flow Resistivity) should follow the thickness of the panels. For example, use higher density/GFR product for thin panels. As the panels go thicker, use lighter/lower GFR product. Rockwool & Fiberglass are made with the same process but with slightly different materials. Rockwool with Basalt and Fiberglass with Silica Sand. Rockwool tends to be crumbly while Fiberglass can be made more rigid and still have a low GFR. BOTH are equally suitable but not perfectly interchangeable. However, it is my opinion that there is too much 'splitting of hairs' over these minute differences. Bottom line: Get what is available to you in your area of the world and make it work. I have never recommended higher density than 60kg/m3 or 4lbs/ft3. (m3 = cubic meters and ft3 = cubic feet)

The fabric that you choose should be breathable. In other words, you should be able to put it across your face and still be able to breathe comfortably. If not, the gas flow resistivity is too high and it will be acoustically reflective especially at glancing angles.

If you are concerned with fibers escaping, you can use a 1/2" (12 mm) layer of Dacron. Dacron is used in dress making and furniture, pillows, etc.

Foam acoustic treatment:

Everything has a place. I have often used foam wedges on top of regular treatment panels that are placed at reflection points in critical listening environments since the fabric used can be (will be) somewhat reflective. If the treatment is very close to the operator, this could be a serious problem. Therefore highly absorbent foam can be used on top of the treatment panels to take care of the upper midrange frequencies that would otherwise be reflected into the sweet spot.

Take care in using ONLY foam treatment in your room. It IS helpful, very useful, and has a place - especially in HOME studios, but it is NOT the Full solution to your room treatment needs. You will need much more trapping than can be obtained with a foam treatment plan. Now, I am not saying that you can't do it with foam. I am saying that proper treatment done with foam will be about 2 to 5 times the price of treatment that you can build with a little plywood, rock wool, glasswool, and fabric. These days, every penny counts and it will certainly add up when you build your home studio. For more information subscribe to my youtube channel.

More to come.

Cheers,
John
 

mjbphotos

What?!?
Thanks for this. John. Someone should make this post into a sticky!

I was always concerned that in my (very small) studio, I can't keep my monitors more than a foot from the front wall, but I can see from what you say that is probably much better for me than trying to move them out into the room further considering I now have full traps on the front wall.

How abut an addendum for those with small rooms WITH carpet (sometimes its not a choice)?
 
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chronicle

New member
Thanks for this. John. Someone should make this post into a sticky!

Wow. Done deal. Sticky it is, and about time. John, I have to congratulate you for being the FIRST Pro Studio designer to post advice in a STICKY on HR! Fantastic. In that regard, I too am getting ready to start a series of Sketchup illustrations on various construction and treatment subjects, and have been offered a chance to make it a sticky as well. I have tons of illustrations I have modeled over the years, but frankly, if you don't mind, I may need you to "verify" the validity of each. But first, I have to organize them into a series of common sense sequences. And that will take me some time. In the meantime..Happy New Year!
 

Chili

Site Moderator
Thanks for putting this up, John. It's a great benefit to have one post to refer our new members to.

In that regard, I too am getting ready to start a series of Sketchup illustrations on various construction and treatment subjects, and have been offered a chance to make it a sticky as well....
In the meantime..Happy New Year!

Looking forward to your post, Rick. Let me know when you're ready.
 

mixsit

Well-known member
I would like to add a thank you as well. This is going to be a good addition to the HR site!
 

chronicle

New member
Looking forward to your post, Rick. Let me know when you're ready.

OK Chile, will do. Right now I'm helping my step daughter move...again.:facepalm: After 50 times over the years , this is it.:mad::D
 

DrewPeterson7

Sage of the Order
Awesome thread, and a very interesting (and sensible!) argument for getting your speakers close to the wall - that's the sort of very common-sense problem solving approach you don't often see applied to the home studio environment. I'll be reading this closely.
 

Jens Eklund

New member
Hi John


Nice post and even if I haven’t read it that closely, I´ll bet I´ll agree with most of it … except this:

If your speaker face is 1.22 meters or 4 feet from a boundary the delay will be 3.5 mS. This is half the length of a sound wave at 141 Hz., therefore full trapping is required at this frequency to eliminate the destructive reflection. NOW, push the speaker closer to the boundary so that it is half that distance or 61 cm / 2 feet. The resulting reflection will cause a delay (compared to the source) of 1.7 mS, corresponding to the half-wavelength of 282 Hz.

Assuming we talk about the reflection from the front wall (0 degree incident angle relative to wall) and receiver right in front of the speaker (if not; we need exact Cartesian coordinate positions relative to boundaries in order to calculate the path length for the reflection(s) and direct sound, in order to figure out the difference in path length between them):

If the distance from speaker face to wall (behind the speaker) is 1,22 meters, the path difference between reflection and direct, is 2,44 meters thus the delay will be about 7 ms and thus the first cancellation frequency (only taking the front wall into consideration) will be around 70 Hz.


Sincerely Jens Eklund
 

jhbrandt

Acoustic Design
Hi John


Nice post and even if I haven’t read it that closely, I´ll bet I´ll agree with most of it … except this:



Assuming we talk about the reflection from the front wall (0 degree incident angle relative to wall) and receiver right in front of the speaker (if not; we need exact Cartesian coordinate positions relative to boundaries in order to calculate the path length for the reflection(s) and direct sound, in order to figure out the difference in path length between them):

If the distance from speaker face to wall (behind the speaker) is 1,22 meters, the path difference between reflection and direct, is 2,44 meters thus the delay will be about 7 ms and thus the first cancellation frequency (only taking the front wall into consideration) will be around 70 Hz.


Sincerely Jens Eklund

Jens,

LOL! It's not a matter of agreement! There is no disagreement in math unless it's wrong!. And yes, in my 'quick explanation' I did indeed forget to add the distances. Therefore, the actual, direct (0 degree incident angle relative to the wall behind the speaker) reflection at 4 feet or 1.22 meters will be delayed by 7.1 milliseconds which corresponds to half the length of 70.5 Hz which will be the lowest frequency cancellation caused by this bounce.

The point in my example is still valid showing the shift upwards in frequency as the sound source nears a boundary. The higher frequency reflection is easier to treat.

Cheers,
John

PS. More to come soon. I had been away since we were having flooding problems in our area. But I'm back home now with good internet again. :D
 

Jens Eklund

New member
SBIR calculations can be a brain twister indeed, and often subject for confusion (thinking of the big SBIR-thread on Gearslutz, I actually found the excel sheet I put together very useful).

Sorry to hear about the flooding problems, nothing serious I hope.
 

Robert27191

New member
Hi John,

Thanks a lot for this thread. I have a question regarding mic sound reflection filters.
Do you believe they can improve recording (vocals) in an untreated room - as I have heard views both agreeing and disagreeing, and which would you recommend?
Many thanks,
Robert
 

jhbrandt

Acoustic Design
Robert,

It is important to eliminate destructive reflections into the microphone when recording, however panels placed behind a cardioid pattern microphone will do little for room reflections. If the recording is important, the room should be treated so that the frequency response at the mic is uniform and uncolored as it is very difficult to remove the room coloration without affecting the original sound of the voice or instrument. Although today's software IS very good and filters can do the job, that's not the way a good record is made.
Panels around the vocalist CAN improve the recording in a TREATED ROOM. They can HELP in an otherwise untreated room, but small panels that are mounted on the mic stand are, In my Opinion, far too small to have much of an effect - especially in an Untreated room.

For absorption panels to work properly in an otherwise untreated room, you will need to place them AROUND the vocalist & microphone so that they; 1. absorb the initial sound waves from the vocalist and reduce the sound available to reflect off of surfaces IN the room. and 2. absorb any reflections that 'get through' and keep them from entering the microphone at interference levels (higher than -20 dB relative to the source sound).

Panels will do next to nothing for frequencies below 250 Hz. The frequency range of 80 - 300 Hz is the most problematic for voice / vocal recording... That's why trapping is so very important for a vocal booth.

Cheers,
John
 

jimmys69

MOODerator
Robert,

It is important to eliminate destructive reflections into the microphone when recording, however panels placed behind a cardioid pattern microphone will do little for room reflections. If the recording is important, the room should be treated so that the frequency response at the mic is uniform and uncolored as it is very difficult to remove the room coloration without affecting the original sound of the voice or instrument. Although today's software IS very good and filters can do the job, that's not the way a good record is made.
Panels around the vocalist CAN improve the recording in a TREATED ROOM. They can HELP in an otherwise untreated room, but small panels that are mounted on the mic stand are, In my Opinion, far too small to have much of an effect - especially in an Untreated room.

For absorption panels to work properly in an otherwise untreated room, you will need to place them AROUND the vocalist & microphone so that they; 1. absorb the initial sound waves from the vocalist and reduce the sound available to reflect off of surfaces IN the room. and 2. absorb any reflections that 'get through' and keep them from entering the microphone at interference levels (higher than -20 dB relative to the source sound).

Panels will do next to nothing for frequencies below 250 Hz. The frequency range of 80 - 300 Hz is the most problematic for voice / vocal recording... That's why trapping is so very important for a vocal booth.

Cheers,
John

That is something that I always found in theory to be ineffective with the 'reflection filters'. It does nothing for the room itself. Yes, it will help a bit with some reflections to a degree, but the big issues are not addressed. For one user to say it works, does not mean that it will effectively work well in another users environment. Correct me if I am wrong, but as little as 1 foot of length or width of a room can make a drastic change in the modes that cause issues right?

Seems that in any situation where at least basic room treatment (the good kind) isn't applied, it is just guessing whether a 'reflection filter' will be useful. Not to say that it will not help, but It just seems to me that money would be well spent in more effective areas.

It seems to me that $200 in bass traps, and a few blankets would do much more than just one of these devices.

I do not have experience with these devices so I am just speculating here.
 

ashcat_lt

Well-known member
I've read over and over that thing about how the listening position should be at 38% (or whatever) the length of the room. Can you comment at all on that, and specifically how it relates to your points about the distance from monitors to front wall?
 

Robert27191

New member
Thanks for all the responses. I'm trying to piece together the best advice and suggestions.
John, I understand what you say about the direction the mic is facing. How would view using a SR Filter, to cover the rear and side of a mic, with blankets put up as a wall behind the singer, in the direction that the mic faces?
 

jimmys69

MOODerator
I know you asked John, but blankets will help some. Obviously not the best choice, but you gotta deal with what you resources you have. Some get away with hanging blankets over boom stands or pvc pipe frames around singer and get decent results. The basic idea is to hinder the reflections of the room. You will still have low end issues because the blankets will not absorb them, but your sound will be a bit more focused.
 
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