SPL and Audacity

Hello All,

I’m not an audio tech though I have dabbled in home recording, but I’m working for a non-profit and so get handed a lot of things outside my bailiwick. The company is building a studio in one of our buildings and need to get some soundproofing done. That said we don’t want to over do it. I have made a recording of the studio space using a decent HKG mic I have, as is, before we build the inner room and add all the dampening materials. I figured that there must be a way to get some sort of SPL levels out of audacity if I know the profile of the mic so we can get a good idea of just how much dampening will be needed, but can’t find anything. Anyone have any suggestions?

There’s a cyclical, recursive way to do it. You calibrate your microphone against your DB SPL meter and then don’t change anything while you use it.

I don’t know that SPL is going to tell you very much anyway. The goal is to stop echoes inside and stop sound entering from outside.

An old building had a sound room with very little internal soundproofing, but was very effective anyway by having noticeably crooked walls. The room didn’t have any parallel walls including the ceiling which was tilted. I sent multiple sound shoots through that room. It was “clean” without being “Blackness of Space” dead which is what aggressive soundproofing panels could do.

Studio soundproofing is a heavy technical study and I don’t think Audacity is going to be much help.

Somebody is going to have to break down and do the legwork or hire someone.

A million years ago a local radio station put in new studios and everything went mostly according to plan, except the builders considered the oddly placed electrical outlets to be silly and wasteful, so they lined them all up. You could hear through all the studios by yelling through the outlets.

Don’t forget the air handlers. The last building had one conference room that featured, in addition to good sound panels and drapes, padded air conditioning ducts. It was the only room you couldn’t hear during recordings.

Flynwill is a real engineer and he’s an elf here on the forum.

What time zone are you in?


I turned on the bat-signal.


Get a cheap measurement mic like the Behringer ECM8000, the Superlux ECM999 or the Umik B1. Install ARTA* and learn how to measure a room. That will give you an idea about possible problems. You’'l also need one beefy speaker and amp to use as a source.

Room treatment is not isolation, but it is important for the sound. Room treatment is easy enough to do yourself if you have lots of time.

Isolation is a lot harder and in general requires specialist help or you’ll miss important issues like doors, or wall outlets. Ventilation is also a big worry. The difference between a double leaf and a triple leaf construction is the prime example. You’d guess that three layers of isolation would be better than two. In practice, three layers could work out a lot worse than two. And that’s where you need a specialist.

Absolute SPL levels don’t matter at all. So no calibration needed, as you’ll be comparing before/after measurements. All you want is the amount of dampening you can get.

ARTA is affordable and pro. The free alternative REW (Room EQ Wizard) is also worth mentioning.

You need soundproofing because?
(what is the problem you need to fix?)

“Soundproofing” (sound insulation) keeps inside sounds in, and outside sounds out. It does not necessarily create good acoustics for recording within the recording space.

but it does mean that the recording equipment needs to be set up in exactly the same way for each measurement. So for a typical set-up of a measurement mic into a pre-amp, into a computer, the same mic should be in the same place, the same pre-amp with gain set to the exact same level, the same computer with the recording level set to the exact same level and the same sample rate. Probably easier to get SPL levels with a SPL meter.

Except that you would need a real SPL meter. Cheap ones have a usable frequency range around 1 kHz. Say, from 500 Hz to 5 kHz. And that isn’t what you want when doing treatment. You need bass response too. Real SPL meters are very expensive and have limited use. If you have a preamp/interface with digital gain, it’s easy enough to set identical parameters each time. And you take notes/photos to be able to place your mic in the same spot.

When measuring for treatment, you always measure the listening position first. And then you take a lot of other measurements, from different spots, to middle with the first measurement. All these just have one use: to get an acoustic view of the room, to decide where to put absorbers and how big these need to be.

Besides, background noise is usually a bigger problem. That’s why you need more than one measurement.

SPL calibration is only needed when you need to compare your measurements with those of others. If you don’t need that, relative measurements are OK.

Of course, when you’re actually carrying out the treatment, you don’t want to change paramaters, like mic placement, or mic gain as you will be moving speakers or absorbers. Always only change one parameter at a time.

Averaging is really, really needed because the studio will change after treatment. Equipment gets added and moved around, people come in in different numbers. And if you want to be reasonably independent from these changes, you need to measure and graph a lot.

Reading the OPs post it sounds like his primary concern is sound proofing, and in particular having some objective means to verify that contractor and/or methods used meet the requirements.

Unfortunately that isn’t an easy task.

Koz’s suggestion of using an in-expensive SPL meter to calibrate your own microphone/preamp/recorder is a possibility. The main failing of the RS SPL meter is that it’s lowest measurement level is 50 dB SPL. With some care you should be able to measure down to ~6-8dB above the noise floor of your microphone. (But you will need to be aware the frequency response of your test microphone will effect your measurements). If you want certifiable measurements, you’re probably going to need to hire someone with the necessary equipment to make them. If the goal is just to get the room as quiet as the budget will allow then spend some time reading up on the various construction techniques and forge ahead.

And contrary to Koz’s introduction I am not an acoustics engineer, I’m an electrical engineer and most of what I learned about soundproofing came when our architects presented the plans for our new facility and the screening room shared a common wall with the computer room. The wall turned out fine – double-stud construction 2 layers of drywall on one side 3 layers on the other I think was the formula. The screening room was undone when the same architect – citing concerns about insulation in HVAC ducts collecting pollutants – specified that the roof-top ducts for the HVAC system have external insulation. So the air ducts, with their non-absorbing metal inner surfaces, conducted the noise from the HVAC system into the room with near perfect fidelity. The problem was mitigated at considerable expense by installing sound-traps in line with the ducts on the roof.

Thanks for the responses everyone.

Yeah my primary concern at this point is background noise. I was looking for a way to determine approximately how much dampening would be needed to keep outside noises out. Sounds (haha) like there is no easy way to do it though. I have already dealt with some of the obvious stuff, when they started building the room I made sure that the outlets were not lined up as mentioned by kozikowski, and we are getting double doors and looking at heated floors instead of forced air. If we do go with forced air we will probably end up doing something that takes air from the next room through a custom vent instead of being connected up to the central heating system.

I was hoping that I could get a sense of the background noise profile prior to calling in a contractor just so that I know we aren’t being oversold or under built. But between what you’ve said and some research I have been doing, I think that it will be better all around to just find a contractor that seems trustworthy and irritate them by asking lots of questions.

Once that is done then we can move on to setting up the acoustics and calibration of the room which is a whole new issue, but at least there are programs that can help with it…

Anyways thanks again for your input all.

ps: Are any of you in the Greater Vancouver Regional District? If so do you have any reccomendations for soundproofing contractors?

Big mistake!

You can’t view isolation and acoustic treatment separately. At the very least, it will cost a lot extra. It’s a bit like building a bathroom and putting in the plumbing without planning electricity.

If the isolation is done right, very little acoustic treatment will be necessary.

Also, isolation tends to kill ventilation. Floor heating is fine, but people still need air and you need to evacuate “used” air. Again, expensive to correct afterwards…

You need to find a consultant who can measure your space and plan accordingly. And yes, it is possible to do (preliminary) treatment measurements while planning for isolation.

Finding a good consultant isn’t easy. I came across one recently who made an offer without even seeing the room, let alone do measurements. His price was around five times what was spent on this small studio after getting a real pro involved.

Good to know, thanks.

I think that isolation and acoustic treatment are largely separate problems, but both require some forethought and planning.

You haven’t said how big a room you are planning, that will probably significantly effect the suggestions.

Things like making the walls not parallel and making the ceiling not parallel with the floor need to be designed in from the start. That trick alone will go a long way towards limiting the resonances in the room and allow the wall and floor treatments to be much more “live”.

On the ventilation, a duct with internal insulation (or the commonly available insulated flexible duct material) with three to four 90 degree bends will go a long way to damping any noise traveling down the duct. If you are feeding the room from a building system that is already some distance away that will likely suffice. Also the lower the air velocity the better, make the last section of duct and the grill oversize so that the velocity is low. I’ve also seen air inlets that are basically long slits which if done correctly can be very quiet.

“There was obviously a mistake in the blueprints, so we straightened up all the walls and ceiling for you.”


If SPL Meters measure things like manufacturing machinery pounding and screaming and music sound system hearing hazards, how do you measure echoes and background sound leakage?


With a very efficient omni speaker and a 500W amp capable of producing 135 dB…