Room Acoustics for Performers: Introduction

Hi Folks,

At the recent L1 conference in Little Switzerland I gave a talk on Room Acoustics for Performers.

We videotaped the presentation and we'll make that available as soon as it's been edited.

I plan to add to the material in that talk here, and eventually I'm hoping we can move the information to the L1 wiki.

I encourage you to ask questions and add your own commentary to these threads. The material gets better and better when you do.

My goal is to provide information about room acoustics that performers can use in their work, in ways that make their experience and that of their audiences better.

Sometimes the material I'll present may seem hard to relate to your work as a performer. My guess is that even when that happens, you will find a way to use the information in a useful way.

I very much look forward to the discussions that will ensue.

With best regards,

Ken
Original Post
This past week I came upon some foam packing material. Sixteen pieces of egg crate type that lock together to make eight perfect pieces 4 X 30 X 36 inches.

As a result of Ken's talk on acoustics at the recent conference I kept this material that I might have otherwise discarded.

I took these pieces to the basement which is highly reflective and has bass killing issues, and placed them about the room.

No particular idea, just spaced relatively uniform on 3 sides of the rehearsal area.

The change in the room was dramatic. Not neccessarily better, as it was totally dead. No reverberation at all.

Bass was not improved, and this type of foam may not work at all for bass traps.

Time will tell if I can actually make an improvement that really stands out as such but, I am encouraged to keep at it because I know now that I can make changes, relatively easy.

So, what did I learn??

Sometimes it doesn't take much to change a room.

If I tinker with this, over time I might learn enough to be able to predict certain things that could improve some performance venues with very little effort on my part.

A small, reverberant, rehearsal space, with feedback issues at performance volume, might be managed with very little investment.

O..
Hi Oldghm,
I am not an acoustician so this is close to a shot-in-the-dark response, based on dim memories from yesteryear.

Try placing the absorbers on only two of the walls. Try this on adjacent walls then on opposite walls. This might bring some life back to the room without the nastyness of a bare room.

YMMV.
I love this.

Acoustics has a reputation as a "black art". With a little knowledge and some experimentation, I think it can be turned into a "science art".

Because this branch of acoustics -- let's call it acoustics for music and speech -- ultimately is "for" human perception, there is a great deal of art that can be applied. But because it is after all physical vibrations in the air that we hear, there is much science that can be applied.

In my talk at the Little Switzerland conference I mentioned about the fact that you can "double down" on absorption by not plastering acoustically absorptive materials flat against a room surface. A rug hung out from the wall is about twice as absorptive as that same rug lying on the floor. Why? Because sound wave can strike both the front of the rug and the back.

So, O, with the material you used, were you able to at least lean it as opposed to fasten it to the wall, thereby making it more effective in absorbing sound?

The bass trap problems you describe are puzzling. What could be absorbing the bass? Can you describe the room thinking a little more about those long, long wavelengths?

And quick! What's a wavelength at 1,000 Hz?

Ken
O,

Really cool observations. I am looking forward to trying out some of my new-found knowledge, but I haven't had any practical chances yet.

Ken,

I panicked when I saw your question, but then I remembered your voice - "A wavelength at 1,000 Hz is 1 foot long." Smile

Tom
quote:
Originally posted by Ken-at-Bose:

And quick! What's a wavelength at 1,000 Hz?

Ken


The speed of sound in dry air at 20° C (68° F) is 343 meters/second or 1129 feet/second.

wavelength = phase velocity / frequency.

so wavelength = speed of sound / frequency


so wavelength at 1000 Hz = 343 meters / 1000
= .343 meters

or in imperial measures

wavelength at 1000 Hz = 1129 feet / 1000
= 1.129 feet

Interestingly as it relates to Bass, the wavelength at 100 Hz = 343 / 100
= 3.43 meters
= 11.29 feet



Ooops - not quick enough - I had to go look stuff up.
quote:
Interestingly as it relates to Bass, the wavelength at 100 Hz = 343 / 100
= 3.43 meters
= 11.29 feet


ST,

You would have really benefited from Ken's talk. The relationship between lengths is easily figured out based on that basic 1000 Hz = 1 foot.

10 Hz = 100 feet
100 Hz = 10 feet
1000 Hz = 1 foot
10000 Hz = .1 foot
quote:
Then how can an approximately 3 foot bass guitar produce a 100 hz vibration??


Hi Gordy,

Perfectly good question.

The 1000 Hz = 1 ft (approximately) is for sound in air.

The string can support the fundamental frequency and the harmonics (2x, 3x, 4x, etc. the fundamental) and all at the same time.

The fundamental depends on the length of the string and the tension of the string and the material of the string.

Ken
Yes Ken, each piece of foam was placed in a manner that allowed both sides to absorb.

My comment about bass was not clear.

I "think" that the basement suffers from standing waves and to better reproduce the low notes accurately I would need bass traps to avoid cancellation of certain low frequencies.

If I save a scene from the basement, that sounds good there, it will always be too bassy in othere venues. That leads me to believe that I am not hearing all that bass while rehearsing.

I also have the same bass problem if I try to mix recordings in the basement. What sounds good there, sounds bad everywhere else.

Mark, I took away half of the foam (4 blocks) and the sound was pretty good. By shear coincidence I did remove from one wall completely. My thinking at the time was "I would at least get one reflection."

Last night eq was closer to flat then ever before. That suggests to me that the room is more "natural" now.

I also keep reflecting on Cliff's band's performance at Geneva Hall. The Vocals were crystal clear. While I know that most of my performances are heard clearly and words are understood, I also know there is room for improvement. Improving the room acoustics will help me get better.

O..
Ken,

That's what I thought it would be, as I have seen larger upright basses, standard electric and acoustic basses, and the mini rubber band basses all producing the same notes with different length strings etc... I assumed it was due to the tension and makeup of the string. Heavier vs lighter, and in the case of the http://www.largesound.com/ rubber band bass a different resonating material altogether.

Thanks for the heads up.
Be VERY careful using packing material in this manner. 100 people died in a nightclub fire in Rhode Island. There were several contributing factors.....one of the big ones was the use of packing material on the walls. When a flame hits this stuff it produces a hot oily liquid. In addition, it burns MUCH faster than acoustic material intended for this purpose.
quote:
The change in the room was dramatic. Not neccessarily better, as it was totally dead. No reverberation at all.

Bass was not improved, and this type of foam may not work at all for bass traps.


For bass traps you need much denser material (e.g. the densest building insulation sheet), ideally suspended a few inches away from the wall, and preferably positioned in corners.

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