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Author Topic: 120 db at 40 hz?  (Read 1382 times)

Dan Erni

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120 db at 40 hz?
« on: October 05, 2019, 11:52:36 am »

Hello all.  I've got a 30'x50' workshop that I want to equip with a good, loud and relatively low cost (I know, can only have two of the three) sound system.  I've narrowed my search for mains to a pair of 10" or 12" two-way boxes run in stereo, and am looking for a sub (or two if needed) to get me around 120 db at 40 hz.  To be more specific, I want the sub to have fairly flat response to 40 hz or lower.  No much use if it will do 120 db at 40 hz while it's doing 130 db at 60 hz, right, and don't want to force it into unhappy operating range with heavy handed EQ.  I'm avoiding the budget stuff (Peavey, Beringer, etc.), but am not brand loyal/proud - if it will get the job done and stand up to normal use, then it's fine for my project.  This is a fixed installation, so weight/size is not an issue.  To save money I'll be buying used.  All suggestions welcomed.
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Mac Kerr

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Re: 120 db at 40 hz?
« Reply #1 on: October 05, 2019, 12:06:56 pm »

Hello all.  I've got a 30'x50' workshop that I want to equip with a good, loud and relatively low cost (I know, can only have two of the three) sound system.  I've narrowed my search for mains to a pair of 10" or 12" two-way boxes run in stereo, and am looking for a sub (or two if needed) to get me around 120 db at 40 hz.  To be more specific, I want the sub to have fairly flat response to 40 hz or lower.  No much use if it will do 120 db at 40 hz while it's doing 130 db at 60 hz, right, and don't want to force it into unhappy operating range with heavy handed EQ.  I'm avoiding the budget stuff (Peavey, Beringer, etc.), but am not brand loyal/proud - if it will get the job done and stand up to normal use, then it's fine for my project.  This is a fixed installation, so weight/size is not an issue.  To save money I'll be buying used.  All suggestions welcomed.

Hard to know if it will get the job done without knowing what the job is.

Mac
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TJ (Tom) Cornish

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Re: 120 db at 40 hz?
« Reply #2 on: October 05, 2019, 07:18:57 pm »

Hello all.  I've got a 30'x50' workshop that I want to equip with a good, loud and relatively low cost (I know, can only have two of the three) sound system.  I've narrowed my search for mains to a pair of 10" or 12" two-way boxes run in stereo, and am looking for a sub (or two if needed) to get me around 120 db at 40 hz.  To be more specific, I want the sub to have fairly flat response to 40 hz or lower.  No much use if it will do 120 db at 40 hz while it's doing 130 db at 60 hz, right, and don't want to force it into unhappy operating range with heavy handed EQ.  I'm avoiding the budget stuff (Peavey, Beringer, etc.), but am not brand loyal/proud - if it will get the job done and stand up to normal use, then it's fine for my project.  This is a fixed installation, so weight/size is not an issue.  To save money I'll be buying used.  All suggestions welcomed.
Hi Dan - welcome to the forum. I think you are misunderstanding how frequency response works - if a speaker has the capability to do 120dB at one frequency and 130dB at a different frequency, that doesn't mean the frequency response will be out of balance, it just means you don't have to use all of the extra 10dB at the more efficient parts of the speaker's response. This is accomplished by EQ. All speakers require EQ - there is no such thing as a speaker with a naturally flat response.

Any reasonably pro sub should do 120dB of some rating at 40Hz in a small room. You may want to figure out what scale you are referencing - the difference at 40Hz between 120dBA slow and 120dBC fast is really massive - approaching 20dB IIRC. Where did you get that number? Who decides if it is loud enough?

For a specific example, the JBL PRX818XLF claims the following specs:

Maximum SPL Output   134 dB peak
Frequency Range (-10 dB)   30 Hz - 103 Hz
Frequency Response (3 dB)   35 Hz 87 Hz

There are many threads on this forum attempting to take numbers like these and arrive at some meaning. Challenges such as if these are measured vs. calculated, at what distortion level, for what amount of time, for what scale, and at what distance the measurement was taken/calculated all make a big difference in what the speaker does. This is one of Ivan's favorite topics. Searching his posts will send you sufficiently down the rabbit hole.

Putting some of the cynicism aside, you may be able to make the following assumptions from numbers like the above:

- If we assume the "+3dB" of the rating is at 134dB peak, 35Hz is probably a "-3dB" point, meaning the speaker can probably do 128dB peak at 35Hz, and probably slightly better than that at 40Hz.
- This measurement is probably done at 1 meter. Sound falls off at 6dB per doubling of distance (outside anyway - you'll get some of this back indoors), so if we take the 128dB peak number as our 40Hz output at 3', then at 50' (4 doublings or 24dB loss) the output would be around 104dB peak at the extreme end of your room.
- If you wanted 120dB peak at the extreme end of your room, you would need to add additional speakers. Adding a second powered sub adds both more cone space and more electrical drive power, so you should get about 6dB additional output per doubling of speaker quantity. A pair would do 110dB peak, and it would take four of them to do 116dB peak. You'd need 6 of them to get to 119dB peak.
- If you don't want peak and want 120 "real" dB, you would need to double your sub count again and you are now at 12 boxes.

If you choose a different sub like a Danley TH118, the specs are as follows:

- 140dB peak at 1 meter
- 37Hz at -4dB

- Assuming the TH118 is at least as loud at 40Hz as it is at 37Hz, that means we can get 136dB at 40Hz at one meter.
- Subtracting our 24dB due to distance loss at the far end of your room you would end up with 112dB peak at 50'. Adding a second TH118 plus an equal amount of drive power either via a larger amp or a second amp gets you to 118dB peak. You would still need 6 TH118s to get 120 "real DB" at 50'.

All of this math aside, I suspect you have never experienced 120dB of any scale. NIOSH allows 120dB exposure for 7 seconds before hearing impairment happens. OSHA is less conservative giving you 7 1/2 minutes before hearing damage occurs. These are broadband numbers and we are much less sensitive at 40Hz than we are at 4000Hz so if all you listen to is 40Hz you are probably safe, but if you run a full range system, unless you are extraordinarily bass heavy, you're still going to be at 100-110dB for mids and highs. OSHA gives you 30 minutes at 110dB, so you can listen to half an album while you work, though I suspect most of your work time will be filled with replacing tools that continuously vibrate off your workbench.

My serious advice is to start with one single 18" sub of good quality active or passive and see if it's enough. If it's not, then get a second.


* No warranties to my calculations are expressed or implied - this post was typed while watching my team get their butt kicked by the Yankees, so my concentration may have been impaired.
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Frank Koenig

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Re: 120 db at 40 hz?
« Reply #3 on: October 05, 2019, 07:51:57 pm »

If you're measuring output at 40 Hz you had better be on the "C" scale or you won't see anything. A weighting is down ~33 dB at 40 Hz from 1 kHz.

https://en.wikipedia.org/wiki/A-weighting

--Frank

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Chris Grimshaw

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Re: 120 db at 40 hz?
« Reply #4 on: October 06, 2019, 03:26:40 am »

Gents, it's also possible to measure SPLs without any weighting. All you need to do is calibrate your measurement mic to your SPL meter at some midband frequency (preferably with band-limited pink noise) (ie, where the SPL meter will be reading "flat), and then run measurement sweeps. The dB scale on the measurements will then be accurate.

120dB@1m@40Hz is perfectly possible with a single ported 15". I know because I've done it, and measured the results. I'd expect a decent commercial 18" to do it just fine - NB I was using one of the best 15"s available, and dumping quite a lot of power into it.

A KW181 won't manage it - it'll only do 115dB at 40Hz when you hit the limiters. You'll need a step up from there - SRX818 might do it, but I haven't measured those.

Chris
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Jeff Bankston

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Re: 120 db at 40 hz?
« Reply #5 on: October 06, 2019, 04:53:37 am »

FaitalPro 18XL1600
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Luke Geis

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Re: 120 db at 40 hz?
« Reply #6 on: October 06, 2019, 05:58:37 am »

If it is a peak # no problem, if it is a # you actually need at distance, then there is no real way to acquire it without using insane amounts of money and subs.

Any mid-range commercial sub will do what you are asking for a peak output reference. A mid-level sub would be JBL SRX/PRX, QSC KW, EV ETX and so on. If you want 120db at 50', you will have to buy a sub capable of 140+db and in considerable quantities.

SPL is a slippery slope. There are many ways to measure it and cheat the numbers. If you know you need only 120db peak from a speaker at a 1m distance, that is an easy number to get to with commonly available units, at 50', not so much.
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Chris Grimshaw

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Re: 120 db at 40 hz?
« Reply #7 on: October 06, 2019, 07:03:00 am »

Luke, as I mentioned above, the QSC KW181 is only capable of 115dB at 40Hz. Two of them would manage it, though.

Jeff, I found the Faital 15HP1060 did the job in a smallish ported box and plenty of power on-tap. I'd very much like to play with the 18" you mentioned, but it wouldn't be strictly necessary to hit the numbers that Dan is looking for.

Chris
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Dennis Wiggins

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« Last Edit: October 07, 2019, 10:32:58 am by Dennis Wiggins »
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Frank Koenig

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Re: 120 db at 40 hz?
« Reply #9 on: October 06, 2019, 11:38:10 am »

Gents, it's also possible to measure SPLs without any weighting

Of course. I just saw mention of dBA somewhere upthread and thought I'd nip it in the bud. Sorry if I was a little blunt. --Frank
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ProSoundWeb Community

Re: 120 db at 40 hz?
« Reply #9 on: October 06, 2019, 11:38:10 am »


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