Just a thought that hit my mind as I was seated upon the throne

Josh

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"Film is made of silver, video is made of rust"
'nuf said

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Ray Derrick
President/Chief Engineer
Panalogic Corporation Sydney, Australia
Phone: 61 (0)2 9894 6655 Fax: 61 (0)2 9894 6935

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Ray Derrick
President/Chief Engineer
Panalogic Corporation Sydney, Australia
Phone: 61 (0)2 9894 6655 Fax: 61 (0)2 9894 6935

This is something I never heard an explanation for....that is, why would large rooms require high frequencies to be attenuated with that -2db curve? Wouldn't you want frequency response to always be as flat as the equipment and speakers are capable of reproducing? Why would you want to roll-off top end? This roll-off is unique to large rooms, correct? As I said, I never heard a logical explanation for it.

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The sound picked up by our measurement microphones includes not only the direct sound but also the reverberant field, which tends to be rolled off at the high frequency end because high frequencies are absorbed more readily than lower frequencies. Our ears however are capable of differentiating between the spectral response of the direct sound and that of the reverberant field, so unlike our microphones, we hear the response of the direct sound and tend to ignore that of the reverberant field.

So if you were to equalise for a flat response on an RTA it would sound over emphasised at the high frequency end. The effect is more dramatic the bigger the room and hence the correction rolloff should be increased for larger rooms. In studio control rooms we normally equalise flat, with little or no correction, because the reverberant field is carefully controlled and, also because the room is small, usually fairly flat. So the microphones "hear" about the same response as our ears do.

I hope that all makes sense.

But now that I've got you, perhaps you can clear this other littl irsome thing up. Way back somewhere, I recall (barely) a technician telling me that I had to be aware of both the temperature and the humidity in the theatre because both can attenuate the sound. Over the years however, my brain has lost the REST of that information -- I can't remember which is which. Does high temp attenuate the sound level or does cold attenuate it? Does high humidity attenuate or does low humidity? My guess would be that cold would attenuate because it would take more sound pressure to force the waves thru cold air where molecular density is sparce -- in a heated room the molecules already are zipping around with kenetic energy and it should be able to excite them with less acoustic output. But like I said, I am guessing. As for humidity, it's a toss-up; I have no idea at all.

Since most theatres today are have well controlled A/C, I doubt this has much bearing on sound levels, and I pretty much haven't given it a thought over the years, but I am also in charge of the Center's carillon which plays outside across an entire neighborhood. If temperature and humidity significantly affect the sound levels, it would be good idea to make sure I am correcting in the right direction

Thanks in advance for any light you can shed on this.

The observation that sound appears to travel further at different times of the day is due to temperature inversion layers reflecting sound upwards or downwards and has very little relationship with the absolute temperature of the air.

Hope this answers your questions sufficiently.

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Ray Derrick
President/Chief Engineer
Panalogic Corporation Sydney, Australia
Phone: 61 (0)2 9894 6655 Fax: 61 (0)2 9894 6935