Double Bass Get That Cabinet Off The Floor!

[mje]

The notion of AI's woofer as "downfiring" is kind of misleading; the orientation of a 10" woofer has ZERO effect on the radiation pattern for frequencies under 600Hz. That's because a radiator smaller than a half wavelength at a given frequency will radiate ominidirectionally regardless of orientation. (a 600Hz wave in air is 20" long)

The talk about "coupling" to the floor is also a bit misleading and I think a little misunderstood. There are modes of vibration associated with various room dimensions, and placing a speaker close to a surface is more effective at exciting a resonance that is excited parallel to that surface. But the orientation shouldn't matter. So what's going on here? Two things, I think.

One is that being that close to the floor loads the woofer - it's operating in a relatively high-pressure region- which lowers the resonant frequency. I don't know how much of an effect this has but I'm sure there's something there.

The second effect is that getting close to the floor minimizes the kind of complex cancellation and reinforcement you can get from waves that are reflected from the floor mixing with waves arriving direct from the speaker. Just like a boundry microphone, but in reverse. And because you don't have the cancellation of some of the frequency content, you're more likely to excite some room resonance that runs along the floor.

(Disclaimer: It's been 25+ years since I learned acoustics from some B&K seminars, so for all I know I may be full of speaker stuffing)

 

[bolo]

There's an effect called boundary reinforcement - if you put a loudspeaker on the floor the bass energy is increased. This is because in free space the sound wave is a sphere, but if you introduce a boundary, i.e. the floor, you cut the shere in half and the longer wavelengths, i.e. the bass frequencies, reinforce each other and increase in amplitude relative to the shorter treble frequencies. Put the speaker on the floor next to a wall and you get more reinforcement from two boundaries, while on the floor in a corner gives you three boundaries and even more reinforcement.

I found Doug Ring's explanation above in a thread about endpin height in another forum, but I thought it was very helpful. I like the way Doug explained this effect, i.e. floor coupling (and wall coupling ... and corner coupling !! ...).

Thanks Doug!

 

[ToneRanger]

Originally Posted by Doug Ring
There's an effect called boundary reinforcement - if you put a loudspeaker on the floor the bass energy is increased. This is because in free space the sound wave is a sphere, but if you introduce a boundary, i.e. the floor, you cut the shere in half and the longer wavelengths, i.e. the bass frequencies, reinforce each other and increase in amplitude relative to the shorter treble frequencies. Put the speaker on the floor next to a wall and you get more reinforcement from two boundaries, while on the floor in a corner gives you three boundaries and even more reinforcement.

Just to add some numbers to Doug's explanation:
Speaker on floor = +3db increase in low frequencies (relative to speaker in free space)
On floor against wall = +6db
On floor in corner = +9db

That 9db boost is equivalent to 8x the watts, so it can make a big difference to the low/high balance of your sound.

 

[Ric Vice]

Once again ToneRanger, Bolo, and mje come up with great insight into just where to place that cabinet. It's beginning more and more, to look like where you put you're cabinet is just as important as what cabinet you have. I'm assuming that carpet on the floor changes all these figures, because it dampens rather than reflects, it that correct?

Ric
I guess that I'm going to be on the lookout for clubs with
great floors, walls, and corners. To heck with the expensive
amps.

 

[Tbeers]

Mostly what I am after with the Gramma is greater consistency from room to room. If it doesn't deliver on that or make any wonderful difference in amplified sound, then I won't go to the trouble of carrying it around. I will know for sure after my gigs on Saturday.

 

[fdeck]

Just to add some numbers to Doug's explanation:
Speaker on floor = +3db increase in low frequencies (relative to speaker in free space)
On floor against wall = +6db
On floor in corner = +9db

That 9db boost is equivalent to 8x the watts, so it can make a big difference to the low/high balance of your sound.

Now I know what those "speaker corners" are for, in the Parts Express catalog. Each corner gets you an extra +3 dB

 

[bolo]

It's beginning more and more, to look like where you put you're cabinet is just as important as what cabinet you have.

Rick Jones (Acoustic Image) basically said the same thing to me a long time ago. We were talking about getting the best sound out of the Contra of course, but it applies to all speaker cabinets I reckon.

I knew about how floor vs. raised can effect your sound, and having walls behind you, corners, etc. But at least now with Doug Ring's explanation, I kinda feel like I know why it works that way.

 

[bolo]

Mostly what I am after with the Gramma is greater consistency from room to room. If it doesn't deliver on that or make any wonderful difference in amplified sound, then I won't go to the trouble of carrying it around. I will know for sure after my gigs on Saturday.

Tbeers, understood. Thanks.

IME, you will get more consistent sound from room to room with the Gramma if the floor surfaces and stages where you play vary greatly in size, shape and composition.

It is one more thing to carry around like you said though, so I usually leave the Gramma at home unless (1) it's a new place and I don't know what to expect, or (2) I know I'm gonna be setting up on a wooden stage or risers.

I also use it during all my living room tests of old and new gear, to try to create as "neutral" a test environment as possible.

Happy gigging.

 

[DB66]

Last night I put my Acme B1/Eden WT400 on a wooden stool which was right behind me, the speaker was just under my shoulder blades, it was really great, I could really hear myself well and felt much more comfortable about venturing into TP. The club has bad acoustics and this setup seemed to solve all the problems I've been having there previously. I did have to keep turning up the Eden, in fact the master was all the way up! That Acme really eats power, by the end of the last set my amp was distorting a bit. I've got a AI series III Contra on order, it seems a bit strange to put a contra on a stool, with the downfiring 10" going to the chair, I guess I'll just have to experiment.

I am a bit worried about how much power that Acme eats up considering the Eden WT400 was distorting and I had the thing cranked all the way up and the fan was running the whole gig. I hope the Contra and Acme is a good combination.

Part of the problem is that my band, a swing band, for some reason is really loud and I'm always suprised by the end of the gig that we played so loudly, but usually the rooms are full of people and noise.

A stool is a big hassle to take to gigs, except for tomorrow (New Year's Eve) which is a 7.5 hour gig!

 

[mje]

The Doug Ring explanation is intuitively appealing, but I don't believe it's correct. Or at least, it's incomplete, in that it doesn't explain why the distance between the radiator and the boundry surface is so critical.

It's a matter of the reflected waves from the boundry either being in phase with the radiated sound, and thus reinforcing it, or being out of phase, and cancelling out the direct radiation from the radiator/speaker.

For instance: A 1KHz sound wave is 12" long (sound travels roughly 1000 feet per second). A half wavelength is therefore 6". If a speaker is placed an even multiple of a half wave (12", 24") from a boundry, the reflected wave will be in phase, and reinforce the original sound. If it's an odd multiple of a half wave (6", 18") the two waves will cancel each other out.

As you get farther and farther from the boundry, a smaller percentage of the sound is reflected from the boundry so the effect is lessened. And as you get very close to the boundry- less than a half wavelength at the frequencies of interest- there is no cancellation at all.

The woofer in an AI cab is well within a fraction of a wavelength at any frequency it is called upon to reproduce, so there's no direct/reflected cancellation or reinforcement. Practically speaking, everything coming from the woofer is a direct wave. As you raise it, though, you start to get mixing of the direct and reflected wave.

This is a linear effect, so you get intermediate amounts of reincorcement and cancellation at intermediate distances, and since it's frequency dependant, it become a real mess to sort out. So ideally you put all the radiators either as close to the room boundry as possible, or far enough away so the boundry is no longer a factor. Raise a cabinet on a stand and tilt it back and you've removed a lot of the interaction.

Spendor made a HiFi speaker in the 1980s that put the tweeter at the bottom of the front panel, with a little ramp extension, so the tweeter was functionally at a room boundry. It sounded fantastic, and there was nothing in the sound that suggested that the sound was coming from anywhere but straight in front of you. But people wouldn't buy it, because it looked wrong ;-)

 

[ToneRanger]

I'm assuming that carpet on the floor changes all these figures, because it dampens rather than reflects, it that correct?

Carpet has only a small damping effect, and only at high frequencies. It does nothing at all to bass.

 

[Tbeers]

Carpet has only a small damping effect, and only at high frequencies. It does nothing at all to bass.

Oh, it does something.... Ask anyone who has owned an AI Contra.

 

[oystein]

Carpet has only a small damping effect, and only at high frequencies. It does nothing at all to bass.

Do you refer to bass as an instrument or to bass frequency? My experience is that carpet has significant impact on the way my bass sounds, both through my AI Contra and acoustically. Absorption coefficients for different surface materials also show this. Depending on type /thickness etc. carpet absorps considerably down to 120 Hz at least. It's right that the coefficient is higher for higher frequencies, but the bass has overtones quite high in this register. I am just a novice in this area, but have played my bass both acoustically and amplified in too many different rooms to agree that carpet does not impact the bass sound.

oystein

 

[Adrian Cho]

Well yeah that's what happens with the down-firing woofer in the AI cabs. Forward-firing cabinets don't have that problem.

 

[KJung]

Well yeah that's what happens with the down-firing woofer in the AI cabs. Forward-firing cabinets don't have that problem.

I don't think that is correct at all.... the fact that the AI cabs are downfiring does not 'change' the way carpet affects certain frequencies. This was discussed in detail in a different thread a while back. Putting something 'hard' under an AI downfiring cab should have very little impact on the sound, since carpet impacts higher frequencies (which are coming out of the front mounted AI cabs) much, much more than the lowest frequency coming out of the downfiring woofer.

A carpeted room with make the entire instrument sound different, of course, but should have no differential impact on a downfiring vs. front firing speaker.

 

[Doug Ring]

Bolo, I'm glad you found my thumbnail explanation helpful. Thanks to mje for the elaboration as well.

Yes, as you say, mje, my description was incomplete because it was lifted from another thread about whether a bass sounds different on a shorter endpin. I was just trying to give a quick explanation as to why there would be a difference.

So using mje's figures as a starting point, simple arithmetic tells us that a half wavelength at 250Hz is 2 feet, at 125Hz it's 4 feet and at 60Hz it's around 8 feet. Therefore at a typical distance a player would be from his speaker, he's going to hear reinforcement from the boundary coupling at most of the fundamental frequencies of the open strings of a bass.

This assumes that the sound wave is a beam however, but it isn't: it's a sphere. Containing the sphere by floors and walls gives you a very complex pattern of reflections and therefore of reinforcement, but also of attenuation. The results generally tend towards reinforcement however, so it's certainly true that if that reinforcement isn't beneficial to your sound, getting the amp off the floor and away from walls will certainly make the sound wave propagation more sphere-like and therefore more predictable.

 

[mje]

Yes, as you say, mje, my description was incomplete because it was lifted from another thread about whether a bass sounds different on a shorter endpin....

Ah, well, that's different. You're absolutely right about the way sound is propagated, of course. That's why the effect falls off so rapidly as a cabinet is moved away from a room boundry, and why tilting up an AI combo has such a large effect.

 

[SteveC]

2 words...milk crate.

It also works well for load in and out. I put my stands, extension cord reel, etc. in there.

 

[Scot]

Invalid Link Removed an excellent resource regarding acoustics and the different materials used to treat recording and listening enviroments.

 

[bolo]

This assumes that the sound wave is a beam however, but it isn't: it's a sphere. Containing the sphere by floors and walls gives you a very complex pattern of reflections and therefore of reinforcement, but also of attenuation. The results generally tend towards reinforcement however, so it's certainly true that if that reinforcement isn't beneficial to your sound, getting the amp off the floor and away from walls will certainly make the sound wave propagation more sphere-like and therefore more predictable.

Oh my ... sound waves that are sphere like ... That's bending my mind Doug.

Obviously I should stay away from commenting and cutting & pasting on anything remotely technical.

You guys are great BTW.

So when I get to my wedding gig this afternoon, I think I'll try to be more conscious that I'm projecting waves in spheres, not in beams.

Or maybe it will just happen that way automatically ... I dunno.