'INSULATION' IN SPEAKER CABS

[brucew]

Just wondered what peoples opinions are on cabs that have some form of insulation on the inside , either foam or the stuff that looks like what you put in your loft ! whats the theory behind it? does it work ? ive got two cabs, the oldest has nothing in it, the newest has, for what its worth i prefer the sound of the older cab.

[lurksalot]

From what I recall the 'insulation' acts as a space enhancer , it slows the sound waves enough to make the space inside the cab seem bigger. This is done to allow the cab to provide the volume to suit the driver without needing too big a cab ......I think

[nikon F]

I had a 1 x 15 cab that was a bare box that sounded awful ,,,tacked duvet lining on the inside and put a wooden brace in ,,and it sounded so much better . sounding better is a personal opinion though .

[pmjos]

Acoustic wadding as it I known is used to reduce standing waves inside a cab. You can't just use yer mums old cardie . It has to have the right properties. It reduces reflections, resonant humps and rattle. Its complicated.

[TimR]

Loft insulation can also be used as acoustic wadding, it's used as acoustic insulation in buildings.

You're effectively stopping the energetic short wavelength, high frequencies. The long wavelength low frequencies require a large mass (heavy box) to damp them.

.

Edited July 4, 2017 by TimR

[NancyJohnson]

I'm baffled.

I thank you and I'll be here all week.

[Phil Starr]

I can do a week's worth of reading

It is moderately complex and because there isn't a good mathematical model of cabinet damping you'll get some disagreement even amongst experts. Even the word damping is confusing because it is both a properly used English word and also a mathematical term used to describe the nature of a resonance. I'll keep away from that for the time being.

As a musician I'll guess you know a little about resonances. Assuming your bass is in tune you'll know that if you play the E string fretted on the fifth fret then the A string will start to resonate by itself. Well in a speaker cab if you play all the frequencies some notes will jump out louder than others. All the bits of the cabs have their own resonances, frequencies which they naturally vibrate at and they will wobble away if you play those frequencies, just like your A string. Sometimes the vibration of the cab can be as loud as the speaker itself making a muddy, distorted sound. Stuffing the cab is an attempt to kill those resonances and make the sound cleaner, as is a lot of panel bracing.

There are two sources of resonances, the air and the panels of the cab and each of these can resonate in a number of ways, so stuffing is used differently to kill different resonances. I'll deal with three of these.

The first is the air resonances often referred to as standing waves. The speaker sets up a pressure wave in the cab which reaches the rear panel and is reflected back to the speaker. Now if you soften the back panel with some wadding the sound won't reflect so well and the wave is damped. The conventional thing is to put a bit of wadding on three panels, the back, one of the sides and the top or bottom. This damps all three standing waves. Unfortunately the thin white flimsy pillow stuffing isn't very good at this, and that's what a lot of people use. Some experts (Martin Colloms) recommend the wadding hung in the middle of the cab to break up standing waves there. The egg crate stuff is usually there to do the same thing. It's denser and the shape is meant to absorb sound and to reflect it randomly. Stating the obvious it can't reflect and absorb at the same time and the dimples are the wrong size to do much good.

The second technique is to fill the cab with wadding. This helps kill standing waves but also affects the behaviour of the sound in the box, done properly it can increse the effective volume of the box but you are talking about stuffing with densities of 1lb per cubic ft and upwards (16kg/cu m) and at least half of the volume filled. I've never seen that in a commercial pro audio cab. You do see it frequently in sealed hi fi cabs.

The third technique is heavy wadding used to damp the panel resonances. The black sticky stuff you see on your car door panels. In speakers it's usually some sort of rubber loaded with sand or stone to make it heavier. The problem with this is obvious, the most effective system is to use something which has the same mass as the panel you are damping, so you end up doubling the weight of your cab, fine in hi fi cabs but not if you are carrying cabs to and from a gig.

You'll have detected that I'm sceptical about a lot of commercial cabs approach to damping. They put it in so that reviewers can say 'the cab is well damped' but without extensive test gear it is hard to say if it is effective or just decorative.

Edited July 4, 2017 by Phil Starr

[SpondonBassed]

I always thought bracing opposite panels was more effective for reducing resonance.

[taunton-hobbit]

I've always used old-fashioned carpet felt - cheap & good.........