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After treatment acoustic test showing bass response to 100hz Acoustic Room Test for low bass showing frequency response before acoustic treatment


Benefits of Acoustic Testing and Design :

The only way to be 100% certain of a room’s acoustic response (and therefore what treatment it needs) is to test it using acoustic room testing software and a test microphone + spl meter. Testing is advisable when maximum accuracy & a great sounding space is required, such as in studios doing release quality work, hi-end hi-fi listening rooms and home cinemas.

Acoustic testing is also very useful for establishing exact RT60 times in large offices and meeting rooms / classrooms. From the RT60 times we can calculate the exact amount of acoustic paneling required to properly control the acoustics of a space.

Acoustic Specifications of Studios and Listening Rooms :

There are known ‘good sounding’ and accurate specifications for frequency response, decay times and mid/high reflections (as outlined by the European Broadcasting Union, Dolby for its mix rooms for film and AES papers on studio acoustics etc..).

Because testing gives an extremely detailed picture of the current acoustic situation in a room it also tells us precisely what treatment needs to be installed where in order to bring the room within (or as close to as possible within budget to) the known ‘good sounding’ / accurate specifications.

Meeting these tried and tested acoustic specifications will provide a space that’s accurate, enjoyable / comfortable to work and listen in, and most importantly gives confidence that what you hear in your room is what you’ll hear outside your room (which is crucial for studio control rooms).. or for listening rooms what you hear is what was intended to be heard (ie.. without the room interfering destructively with the source material).

The main acoustic specifications that should be aimed for are :

In some rooms it’s possible to achieve better than this, but in particularly bad sounding spaces this may not be realistic without spending huge amounts of money. However it’s considered that if a frequency response of +/-5db can be achieved that this will provide a space which is capable of producing good results in a studio setting. For home cinema and listening rooms there are no set ‘rules’ - the aim however is to get close to the above specifications, then discuss what the client’s preference is for a lively space (using diffusers) vs more tight and controlled.

Testing is therefore well worth the initial investment in terms of final post treatment results and often final budget for treatment - you may need less or a different type of treatment than calculations or guess work would indicate.

The other benefit of on site testing is that speaker and listening positions can be optimised - this is an often overlooked issue, but bad placement of speakers can lead to massive problems in bass response and poor mid/high imaging.

Figure 1 : Low bass testing for below 100hz (the hardest frequency range to get right) - before room treatment.

This frequency response plot shows the bass response below 100hz as heard at the listening position of an untreated studio control room.

The main issues are the 2 huge 20db dips at 75hz and 85hz, and a 7.5db peak around 38hz. This is a fairly typical bass response in terms of severity of peaks and dips - most room tests look similar to this, if not worse (although the problem frequencies and number of issues varies from room to room - all rooms are different / unique acoustically).

Please note these tests are shown in a very raw format, using 1/24 octave smoothing. Our ears don’t hear in this much detail, but when performing acoustic testing switching between the raw data and how we humans perceive sound is needed to home in on specific issues.

Example Before and After treatment room tests : Bass Testing (click here for detailed mid & high frequency testing results)

Figure 2 : Low bass testing for below 100hz - after treatment.

This frequency response plot shows 15+db improvements at 75hz and 85hz and around 4db improvement at 38hz, which is a difficult frequency area to deal with due to wavelengths involved.

We used BF-1200 corner devices in 2 corners floor to ceiling to achieve this result - if it had been possible to treat all 4 vertical corners the results would’ve been better still, but even so this is a significant improvement!

The After Treatment Response is visibly flatter and truer, which in practice translated into a vast improvement in bass punch, evenness and definition.

Figure 3 Shows this same result with 1/3 octave smoothing, which is a very close approximation of how we perceive sound. As you can see the plot is now flat to within +/-2.5db to 35hz (which is where the speakers dropped off).

Figure 1

Figure 2

Figure 3 : after treatment test with 1/3 Octave Smoothing applied

Acoustic test showing bass response below 100hz in an acoustically treated room with third octave smoothing applied Acoustic test showing waterfall plot decay times below 100hz in an untreated room Acoustic test showing waterfall decay time data below 100hz in an acoustically treated room

Figure 4 : Low bass testing for below 100hz - before room treatment Waterfall Plot

This Waterfall plot shows the decay time data for the same room. As you can see there is a lot of boomy bass around the modal frequency at 38hz, and below this point.

Figure 5 : Low bass testing for below 100hz - after room treatment Waterfall Plot

This is the after treatment Waterfall plot. The bass is far more even (ie.. ‘Solid’, both visually and audibly) and the boomyness below 40hz is brought under control properly. This gave a very tight and controlled bass response whose frequency response is within +/-2.5db of flat. A good result.

Mid and High Frequency Testing :

Acoustic testing for mid and high frequencies mainly involves the use of the Energy Time Curve feature. This plot shows the direct speaker signal reaching the listening position + all individual mid and high frequency reflections as they arrive at the listening spot (both over time and in level compared to the direct signal).

The specification to aim for for guaranteed excellent imaging and accuracy is :

Loud reflections that arrive very soon after the direct signal at the ears combine with the direct signal. This causes comb filtering and peaks and dips in the frequency response.

So we hunt down which surfaces / exact locations these loud individual reflections within the first 20ms are coming from - and either absorb or diffuse them (ie break up the loud reflection into many quieter reflections). This gives a highly targeted mid/high treatment solution.

The Energy Time Curve can be used to tune the acoustics between lively (diffused) and dead (absorbed) to how a client wants to hear the mid and high frequencies.

Figure 6 : Untreated listening room. The ETC shows at least 3 loud reflections within the first 20ms. These caused poor imaging and frequency response issues in mid and high frequencies at the listening position.

Acoustic test showing Energy Time Curve mid and high frequency reflections at the listening spot before acoustic treatment Acoustic test showing Energy Time Curve plot in an acoustically treated room

Figure 7 : After treatment Energy Time Curve plot

Shows that via use of properly placed treatments the loud reflections in Figure 3 have been brought down in level to below -20db of the direct speaker signal. This resulted in clear / proper stereo imaging and a more accurate mid/high frequency response.

To book an acoustic test / site visit call Joe on 07800 952 474 or follow the link on our contact page.