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Posts Tagged ‘noise’

Dead Spots – Sound System or Acoustics?

Posted by jdbsound on April 9, 2018

An unwelcomed guest in any church is Mr. Deadspots. Unfortunately, deadspots in churches are more common than you think. There are two main types of deadspots. Some are frequency related/comb filtering interference and others are dips in sound levels greater that 6dB created by the room.

It is common to see frequency related deadspots in Left/Right sound system regardless if they are Line Arrays or point and shoot speakers. These deadspots are created by interference patterns in a mono speech system as a persons voice is always mono.  These deadspots are where you shift from one foot to another and notice a sound change. In these cases it becomes a problem when on one foot you hear the highs but not the lows. When you shift your position onto the other foot, you hear the lows and the highs disappear. People with hearing aids or early stages of hearing lose notice this the most. People with good hearing notice the change too but learn quickly to put up with it. Some young people think of it as a passive noise filter. If the music is too bright, stand to one side of your seating position. If the music is too boomy, shift to the other side of your seating position. Really! Isn’t that like buying a headset and controlling the sound changes with what angle you tilt your head. It might sound like a great idea until you find yourself with a lot of neck pain. No thank you.

Sound level dips are usually acoustically related. These are created with standing waves, bass building up, hollow walls, room corners, and parallel surfaces that include walls, ceilings and floors and concaved surfaces. In all of these cases, often mid and bass energy build up and the highs are absorbed with carpets, padded pews and people. By the nature of churches and how they are used, carpeted floors and padded seating often represents how the church sounds when it is 50% full. That means that if your attendance is often over 50%, the effects of padded seating and carpeted floors has little to no impact. If church attendance is often over 70% a carpeted floor makes the room more intimate during times of prayer and solemn reverence. In the end, carpets and padded seating is a good thing.

However, because of people in the room, once that room attendance is above 50% the people absorb enough highs that extra mids and bass energy is left behind as is being amplified between parallel surfaces. This excess energy automatically masks the highs. When the highs are masked, speech and music intelligibility drops. The kicker is, if you go around the room with a sound level meter, often the sound levels don’t drop much, even when you stand in a spot where the highs (when you measure just the highs) drop off over 6dB. That is because the excess bass energy is so strong that it fools the sound meter as the meter is reading an average sound level. When you take sound level readings by frequency, then you notice the high number of deadspots in the room. Get a tone generator in a cell phone or computer app and play a constant tone at 55dB at 500 Hertz, 1000 Hertz and 3000 Hertz and then start walking around. At 500 Hertz you shouldn’t notice much of change until you get close to walls. At 1000 Hertz you will notice more changes. At 3000 Hertz, if you are hearing a lot of changes, imagine what 25% of your church audience is experiencing.

Here is a church that had both acoustical and sound system created deadspots, with a central cluster. By nature of a central cluster, in a good room, it gives the best coverage and performance for speech. There is no better way to design a church sound system unless your ceiling is less than 14 ft. high. Choice of speakers, coverage patterns and speaker placement impacts sound too but these are mainly tone qualities and gain before feedback related. It may have up to a 2 or 3% impact on overall intelligibility as well.

corner view pano Ebeneezer Church_s

In this church example, it already has a fairly good quality speaker system in the ideal location for the room. It is designed as a central cluster and by nature, in this setup, it should perform well. However, it didn’t matter if you used the main speaker system or used portable speakers on stands, with any sound amplified you could find deadspots all over the room. On top of that, if you raised your voice in the room, once you were more than 18 feet from someone, understand what was being said was difficult to impossible depending on dictions of the person talking and how good is one’s hearing. When the proper acoustical fix was applied, all of those problems went away and the church didn’t need to upgrade the speaker system.

The church decided to leave the sound system alone as the gain before feedback improved and all of the deadspots disappeared. Since this is a traditional church that has no intentions to do anything contemporary, the acoustical fix was designed to not change the overall reverb time. Before and after reverb time remained about the same. 1.7 seconds.  It was the frequency response of the room that saw a major change. As the graphs shows below, where the mixer for the worship space was located, it was also one of many spots where weird measurements were recorded before. We found dozens of spots where the room measurements went squirrely. This is typical of the results of measuring a Left/Right speaker system, not a cluster system. These weird results were a result of room acoustics and not the sound system.  We used our own test speaker for all room testing.

Sound Booth Before and After

After checking our test equipment for errors, it was then realized that by just moving the mic over a few inches, you would get a very different result. In some places, the sound was perfectly fine but move over a few inches and it was not. Our ears are about 8 inches apart. In one row of seating, the largest distance we could move the test mic between a good spot and bad spot was 14 inches. Pew seating is 18 inches.  Every seating position had both good and bad sound. What we were measuring was sound masking in the mids and lows.  The energy was so strong that it masked the highs.  Not only that but the highs were most likely also being canceled from nearby wall reflection between 1800 to 5000 Hertz.  It gave the impression that there was something wrong with the sound system.

In this church, people marked their seating positions by placing personal pillows in spots where the sound was better. Sure enough, testing these spots showed better sound before the acoustical treatment was applied. After the acoustical system was installed, the sound was the same no matter where you sat including the sound booth.

Deadspots in churches are more often a result of worship space design and not a result of sound system design (unless you have the wrong speaker system design for your room). When a church replaces a well-designed sound within 10 years, and have little to no overall improvement after an upgrade, that should be a BIG RED FLAG that you most likely have a serious acoustical problem and no amount of money spent on the sound system can make those problems go away. Besides, these days, acoustical fixes cost less than sound system fixes. As a caveat, our experience has been this. Churches that have fixed their acoustics and then wanted on to expand their music programs, they were able to upscale their sound system with a much higher budget as they upgraded, it actually lead to better performance rather than an exchange of one set of problems for another.

Bottom line is, get your church properly tested. Have someone who knows how to properly diagnose the data, and then design your church a proper acoustical management system. Install the acoustical system and watch the congregation respond and grow. Don’t be surprised if other churches want to use your church for musical and recording events. Your property value may go up too.

Note – The acoustical system is made up of 8 and 12 inch half round plaster covered foam diffusers on 3 walls.  The side walls use a gradient pattern to maximize room performance. On the back wall there are 24 units of 7 ft. x 2 ft. x 18 inch plaster covered foam diffusers that are hollow which allows for additional passive room equalizing in the near future. Project completed by church members.

By Joseph De Buglio
JdB Sound Acoustics

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Before and After results of a Real Multipurpose Hall

Posted by jdbsound on December 4, 2017

This is a before and after test results of a multipurpose room.  The room is a converter steel factory.  The purpose of the room is for multi use including banquets, acoustical and amplified musical performances, teaching and general meetings.  While the room has a fixed sound system, the  room performs equally well regardless of the orientation of the seating or event layout.

sandbox B-A results

From the graph, it shows the before and after.  Before the room had an average of 2.1 seconds of reverberation.  That said, at around 400 Hertz, the reverb time was 2.85 seconds. This made the room unacceptable for all uses.  It was hard to have a simple conversation with someone only 5 feet away.

The acoustical treatment in this case require 3 different acoustical system.  Tube Radiators were used for controlling sound from 200 to 2000 Hertz.  The tube radiators have only a profile depth of 4 and 6 inches and the idea that something so small can control sound down to 200 Hertz is amazing.  The tube radiators only covers 12% of the total wall space of the room.  

The second system was outround diffuser panels.  They covered another 10% of the available wall space.  These panels were used to manage sound from 100 to 500 Hertz.  By combining these two system with the limited wall space, we were able to cut the reverb time at 400 and 800 Hertz 1.8 seconds.  That is a massive amount considering that 400 Hertz is a wave length of about 33 inches long and 800 Hz is about 17 inches.

The third acoustical was a fiber absorber.  The fiber absorber covered 40% of the ceiling and 10% of the side walls.  The fiber panels covered the outrounds on the side walls.  It is rare to need absorption in such projects but when you have a concrete floor with no carpet, you have to replace the carpet with something similar.  Here is the thing about carpet.  Carpet, which is always within 4 to 7 feet of our ears works very efficiently.  The shallow angle of most sounds we hear in a large room event gives a 1/2 inch of carpet the acoustical performance of 2 inches of a typical wall panel absorber.  Since one of the requirements of this room is to include music that can reach 100dB, it was planned to have a reverb time of 1 second, +/- .2 tenths of a second.  That goal was met and the range it was met is typical of our acoustical fixes.  If you look at the before and after, the room now meets that goal from 150Hz to 4,000Hz.  Before, using the same criteria, the room had a average reverb time of 1.8 second with a +/- of 1.1 second variance.

percent alcons 4 sandbox

The critical question is, how does the room sound for speech, talking and for music.  For speech, the change was from 14% Alcon’s (rated as poor) to 4% Alcon’s.  At 4%, it means that you can talk to someone from end to end of the 55 ft long room with a slightly raise voice.  When you add a properly equalized sound system, you can better the speech intelligibility to 3.5%.  For talking across a table during banquet or social events is easy in this space.  You can talk to someone 15 ft away while the person next to you is talking to someone across the table without having to raise your voice to compete with other conversations.

As for music, so far, for the high energy high SPL events the room has been well received by musician and audience members.  That has meant fewer events with drum shields, fewer events with IEM (in ear monitors) and very little floor monitor spill that degrades the sound for the audience.

At the other end there have been a few recital type performances where the even was all acoustical.  One person who was a graduate of a royal conservatory of music remarked that the room was similar to recital rooms at a well known royal conservatory school in Toronto, Canada.  One violinist said that while she would have liked a longer reverb time, the quality of the sound of her expensive instrument was amazing.  The last time she heard her violin sound so great was at a high end recording studio that was  acoustical treated.  She was also stunned that it didn’t matter where in the room she performed, the violin sounded great.

There is one down side to the new room.  Since there is no carpet, when the room is empty, you do notice the reflection off the floor. This does make the room a little challenging for those who do rehearsals when the room empty and before any table and chairs are set up.  Once tables and chairs are setup, the room behaves well.

In the real world, there are a lot of rooms that are used as multi purposed spaces but perform poorly.  Most facility owners don’t worry about acoustics because they may have the only place in town that can accommodate such events.  That said, if they were to get 10 to 20% more bookings per year, they would recover the cost of investing in an acoustical fix in less than a year.  Furthermore, it would allow the facility owner to charge a hire fee if the place gets too busy.  There is no down side to having an acoustically friendly community center, convention hall, rental hall or banquet facility.

 

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Free Hearing Test with an Apple App

Posted by jdbsound on February 21, 2013

Get your hearing tested for free.  If you have an iPad, iPod or iPhone, you can test your hearing with a free App.  It is good enough to confirm whether if you have a hearing problem and whether you should be getting professional help.

The software is from Unitron and they make hearing aids. So while some people think that this is just a sales marketing tool, and it very well may be, but it matches a hearing test of mine done in a proper lab hours apart. That is good enough to decide who should be behind a mixer and making adjustments during worship.

What I have also done is use this test for people who persistently ask me to turn things up – even when they have sat in different places. When everything is working properly and I know the sound system is passing the STI speech intelligibility test, I will ask the person to take the test.  Be gentle and kind when asking people to take the test.  When people discover that they need help, setting them off in getting their hearing back often gives them back their life too.  The App is called “U-Hear”

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