Doors with an acoustic rating are being increasingly specified in commercial, healthcare, education and hotel settings. But the performance of an acoustic door is only as good as its seals. But how are these tested and regulated and what can they achieve?

The acoustic performance of a building is covered by Approved Document E (ADE) of the Building Regs but in fact the performance required in ADE isn’t especially stringent and only covers sound insulation requirements in dwelling-houses, flats, rooms for residential use and schools.

But more than just meeting minimum regulatory performance levels, studies have shown that well-designed sound environments in offices, schools, hospitals and hotels help to improve concentration, customer satisfaction and enable better communication. In office environments, privacy is also a key issue driving the acoustic insulation of work spaces. As part of this drive to better acoustic performance, the acoustic performance of doors is increasingly in the spotlight for specifiers.

That’s why we have our own acoustic testing lab in Germany so our engineers can test test and test again to make sure that our acoustic seals not only meet but exceed the relevant standards – sometimes many times over.

How is sound measured?

OK, here’s the science bit. When sound is generated, the ‘loudness’ refers to sound pressure and is measured in decibels (dB). When sound strikes a surface, some will be reflected, some will be absorbed and converted to heat, and some will pass through the structure, losing energy as it travels. 

Acoustic engineers will measure the time it takes for sound pressure to drop by 60dB, and this is called reverberation. The quicker the pressure drops, the ‘deader’ a room will sound so for instance, if the pressure reduces by 60dB in less than 0.3 seconds the room will sound ‘dead’ but if this takes more than five seconds there will be an echo. Optimum reverberation time depends on use of space: one second might be ideal for clear speech in a lecture hall, but 3.5 seconds would provide fuller and richer music sounds. In general, acoustic Engineers will try and ‘tune’ a space to reverb between 1.5 – 2.5 seconds. 


Where do acoustic tests happen?

There’s two ways: a special lab like ours in Germany that has a transmitting and a receiving room where sound is transmitted through the door, to measure the sound reduction between the two. Or an anechoic chamber: items like domestic appliances are sometimes tested in, a special room where acoustic testing of everything from vacuum cleaners to electronic components are tested, and also where radars and antennas are put through their paces. The chamber is a box within a box, both unconnected to stop the transmission of vibration, and fitted out inside with an array of sound deadening foam spikes to absorb noise. Inside that chamber is so quiet that sound can be measured in negative decibels.

To comply with Building Regs, acoustic testing must be conducted by a qualified acoustic engineer accredited to conduct sound testing. In our case that is literally scientists in lab coats!


What is it like for human inside an anechoic chamber?

In one word: disconcerting. Steven Orfield, the founder of ‘the quietest place on earth,’ the anechoic chamber at Orfield Laboratories in Minnesota, says “We challenge people to sit in the chamber in the dark – one person stayed in there for 45 minutes. When it’s quiet, ears will adapt. The quieter the room, the more things you hear. You’ll hear your heart beating, sometimes you can hear your lungs, hear your stomach gurgling loudly. In the anechoic chamber, you become the sound.” He adds: “If you stay in too long then you start to hallucinate. You also need to be sitting down when inside.” This is because one of the key ways we orient our bodies is partly through sound, so a chair is required for anyone going inside for any length of time.


So during a test for acoustic seals, what sound is played?

Some people imagine that the lab technicians play Black Sabbath at full volume. But the truth is a little more prosaic. In fact white noise is played at 150 decibels.

White noise is the noise produced by combining together of all different sound frequencies that a human can hear. It is called white noise in reference to how white light is produced: with every frequency of light combined. In the same way, white noise is a combination of all of the different frequencies like 20,000 pitches all playing at the same time.


What exactly is a decibel and how do they work?

Sound is a pressure wave caused when something vibrates, making particles collide and then move apart, akin to the ripples in a pool. These changes in the spacing of particles causes changes in pressure that can be detected by the human ear and other sound measuring devices. The intensity of a sound is measured on the decibel (dB) scale. Slightly confusingly, the decibel scale is a non-linear, logarithmic scale. As a rule of thumb, an increase of 3DB is a doubling in the sound energy so a sound of 23dB is twice as loud as a sound measured at 20dB.


So How loud is 150 decibels?

Loud! 0dB is the threshold that a normal human ear can start to hear sound and 130 is the usual pain threshold – the sound level that causes pain and can instantly damage hearing.

As a guide, these are the typical noise levels:

Gun shot or fireworks: 140dB

Jet engine at take off: 120 dB

Concert: 110dB

Power tools: 90dB

Alarm clock: 80

Traffic: 70dB

Normal conversation: 60dB

Rainfall: 50dB

Quiet library: 40dB

Whisper: 30dB

Breathing: 20dB

Silent room: 10dB


How do acoustic door seals work?

Sound, unlike air passage can and will escape from the tiniest gaps. Remarkably, a door open 1% will let 50% sound through it. In Norseal’s experience, most soundproofing issues or noise leakage problems are down to either a lack of the correct acoustic doors seal, or badly fitted or damaged door seals which have gone unnoticed. Also it is common to find acoustic door seals fitted to the head and jamb but the threshold, which presents the largest gap often has no acoustic seal fitted at all. 

What lengths do Norseal’s sound engineers go to to test door seals?

Great lengths. Even to the extent of crawling around, with a stethoscope checking meticulously for any tell-tale whistles or drafts that would indicate an escape of sound. That’s how seriously we take acoustic sealing at Norseal.

If you want to find out more contact our team on 01661 830088 or email sales@norseal.co.uk