"Room Compression"--a factor?

[avare]

It is correct in that the room modes will interact with the drums.

Andre

[Ethan Winer]

Originally Posted by RKrizman

he referred to "room compression", meaning that in the small room the air pressure he was generating from drumming was pressing back and actually affecting the physical response of the drums, "compressing" them, as it were.

Room acoustics are highly linear, so whatever happens at soft volumes happens the same at loud volumes. Same for drums. But our ears aren't linear. A recent issue of Sound on Sound magazine had a fantastic article about how the ear works. It's definitely worth the $1.49 if you don't subscribe:

Hear This

Anyway, reports of rooms "compressing" in the sense of a compressor / limiter are incorrect, and due only to perception.

--Ethan

________________
The Acoustic Treatment Experts

[avare]

Originally Posted by Ethan Winer

Room acoustics are highly linear, so whatever happens at soft volumes happens the same at loud volumes. Same for drums. But our ears aren't linear. A recent issue of Sound on Sound magazine had a fantastic article about how the ear works. It's definitely worth the $1.49 if you don't subscribe:

Hear This

Anyway, reports of rooms "compressing" in the sense of a compressor / limiter are incorrect, and due only to perception.

Thanks for explaining what I did not.

Andre

[art noxon]

Good question....There are a number of "compression" topics being discussed at one time. The general idea is acoustic induced compression of sound making moving surfaces. It's better if we sort them out.

1) Location Compression: The initial question is about drum head compression, where the initial overshoot movement of the drum head is inhibited by the build up of a back pressure which resists the drumhead movement. It is created when the drumhead is located in a cramped area of the room, and the pressure made by the drumhead movement can't quickly escape into the large open volume of the room. This backpressure action is self evident.

I've seen stand up double basses unplayable when played in a hallway, same thing for the tuba. Adding a seriously fast bass trap to the built-up pressure zone does give the sound pressure somewhere to go and lets the moving surfaces move more easily, naturally, and the instrument is more playable.

Similarly, loudspeakers which are not designed for "wall loading" should not be played near a wall for the same reason. Woofers located near a side wall over pressurize the volume of air nearby which pushes back on the driver, slows it down, and changes it's response to the applied voltage.

One of the most sensitive sound makers, to location compression, are huge lightweight sheet type dipole drivers, such as in hifi, Martin Logan and Magniplanar.

Along these lines is something related...ribbon mics seem to weaken as the are backed up to the wall, while condensor mics seem to strengthen .... Kinda like compression, but we diverge. Let's back burner this one for a while, or at least until someone is interested to know more about it...

2) Room Compression: This also exists. Well documented in theoretical acoustic books that deal with room acoustics (Kuttruff, for example).

If we rapidly change the volume of a tone, from quiet to loud and back to quiet and so on, over and over we have a dynamically modulated sound level. Measuring the dB difference between loud and quiet gives us the MTF response curve of the room. Modulation Transfer Function.

Outside, or in an anechoic chamber we might measure with a sound meter and strip chart sound and record level changes of 60 dB within such short times as 1/16 of a second. But in a reverb chamber, with its 15 second reverb time helping to ruin the test we will measure zero dB sound level changes, unless we move the mic up next to the

Note that the human has a limit to how loud it can perceive modulated sound levels, typically at a modulation rate of 8 Hz, to be insensitive to modulation levels that exceed 20 dB.

Notice that we could have a flat frequency response sound system. When we play it outdoors or in the anechoic chamber we still measure a flat response curve along with huge dynamic range. Now in the reverb chamber we again measure a flat frequency response, louder because of all the reverb buildup, but still flat. However we have zero dynamic modulation, again because of all the reverb.

Room reverb reduces modulated dynamic range, or "room speed" as it is sometimes called. Hence room reverb causes "room compression". As a side note...experience shows that people who listen for their living or hobby prefer a lumpy but fast room well over a flat but slow room.

Notice that electronic reverb can also act like compression if it it too loud.

3) Ear Compression. Yes, we all know that the muscles of the inner ear, those that tension and hold those little bones in place, tighten up within maybe 1/10th second of being exposed to a sudden loud noise. This is a muscular nerve reaction, sorta like jerking away... ouch a hot plate, or blink when a light flashes too bright. It's worth about 10 db compression. Unfortunately it does not work fast enough to protect us from rim shots, gunfire or fire crackers.

Usually we can feel the muscles tighten. It feels a little funny, but when standing inside an anechoic chamber, there are zero early or late reflections. This ear compression thing goes to work overtime with every sound we make or someone else makes. And is simply exhausting, irritating. This may be a stretch but I interpret this obnoxious response to an absolutely dead signal as a good sign that dead signals are unnatural signals.....

If you want to know more about what little I know about MTF, google MATT, Musical Articulation Test Tones and you'll find an interesting world of performance testing and standards that relate to high power audio / room acoustics. And it's no surprise that all this is a cousin of speech intelligibility, STI, and the RASTI testing system by B&K RApid Speech Transmission Index.

Thanks for hanging out with me...
Art Noxon
Acoustic Engineer
President of ASC

[avare]

Originally Posted by art noxon

Good question....There are a number of "compression" topics being discussed at one time. The general idea is acoustic induced compression of sound making moving surfaces. It's better if we sort them out...

Great post and welcome to Gearslutz Art!

Andre

[Ethan Winer]

Nice to see you here Art!

--Ethan

[Tom Higgins]

Originally Posted by avare

Great post and welcome to Gearslutz Art!

Andre

+1
Excellent first post

[DanDan]

Welcome Art. As a drummer, sound engineer, and fledgling acoustician, I would like to add a little.

The OP mentioned playing response of the drums. IMO this highlighted in the Kick Drum, strangely as it is through the foot and a pedal.
The way the foot sinks into it and bounces back, or not, can make the groove 'feel' huge, Bonhamesque, or small and tight, like drums on a KD Lang record.

The best feeling seems to be in a decent sized room, positioned in a corner. One feels the resistance to movement created by the high air stiffness in the corner, but the feeling of successfully moving the large volume is wonderful.

Conversely, playing in the centre of a small room, or outdoors, is like hitting Tupperware.

DD

[Tom Higgins]

Thanks DD. I'm going to play around with that next time I have a drummer in